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1 -----------------------------------------------------------------------------
2 This file contains a concatenation of the PCRE man pages, converted to plain
3 text format for ease of searching with a text editor, or for use on systems
4 that do not have a man page processor. The small individual files that give
5 synopses of each function in the library have not been included. There are
6 separate text files for the pcregrep and pcretest commands.
7 -----------------------------------------------------------------------------
8
9
10 PCRE(3) PCRE(3)
11
12
13 NAME
14 PCRE - Perl-compatible regular expressions
15
16
17 INTRODUCTION
18
19 The PCRE library is a set of functions that implement regular expres-
20 sion pattern matching using the same syntax and semantics as Perl, with
21 just a few differences. (Certain features that appeared in Python and
22 PCRE before they appeared in Perl are also available using the Python
23 syntax.)
24
25 The current implementation of PCRE (release 7.x) corresponds approxi-
26 mately with Perl 5.10, including support for UTF-8 encoded strings and
27 Unicode general category properties. However, UTF-8 and Unicode support
28 has to be explicitly enabled; it is not the default. The Unicode tables
29 correspond to Unicode release 5.0.0.
30
31 In addition to the Perl-compatible matching function, PCRE contains an
32 alternative matching function that matches the same compiled patterns
33 in a different way. In certain circumstances, the alternative function
34 has some advantages. For a discussion of the two matching algorithms,
35 see the pcrematching page.
36
37 PCRE is written in C and released as a C library. A number of people
38 have written wrappers and interfaces of various kinds. In particular,
39 Google Inc. have provided a comprehensive C++ wrapper. This is now
40 included as part of the PCRE distribution. The pcrecpp page has details
41 of this interface. Other people's contributions can be found in the
42 Contrib directory at the primary FTP site, which is:
43
44 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
45
46 Details of exactly which Perl regular expression features are and are
47 not supported by PCRE are given in separate documents. See the pcrepat-
48 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
49 page.
50
51 Some features of PCRE can be included, excluded, or changed when the
52 library is built. The pcre_config() function makes it possible for a
53 client to discover which features are available. The features them-
54 selves are described in the pcrebuild page. Documentation about build-
55 ing PCRE for various operating systems can be found in the README file
56 in the source distribution.
57
58 The library contains a number of undocumented internal functions and
59 data tables that are used by more than one of the exported external
60 functions, but which are not intended for use by external callers.
61 Their names all begin with "_pcre_", which hopefully will not provoke
62 any name clashes. In some environments, it is possible to control which
63 external symbols are exported when a shared library is built, and in
64 these cases the undocumented symbols are not exported.
65
66
67 USER DOCUMENTATION
68
69 The user documentation for PCRE comprises a number of different sec-
70 tions. In the "man" format, each of these is a separate "man page". In
71 the HTML format, each is a separate page, linked from the index page.
72 In the plain text format, all the sections are concatenated, for ease
73 of searching. The sections are as follows:
74
75 pcre this document
76 pcre-config show PCRE installation configuration information
77 pcreapi details of PCRE's native C API
78 pcrebuild options for building PCRE
79 pcrecallout details of the callout feature
80 pcrecompat discussion of Perl compatibility
81 pcrecpp details of the C++ wrapper
82 pcregrep description of the pcregrep command
83 pcrematching discussion of the two matching algorithms
84 pcrepartial details of the partial matching facility
85 pcrepattern syntax and semantics of supported
86 regular expressions
87 pcresyntax quick syntax reference
88 pcreperform discussion of performance issues
89 pcreposix the POSIX-compatible C API
90 pcreprecompile details of saving and re-using precompiled patterns
91 pcresample discussion of the sample program
92 pcrestack discussion of stack usage
93 pcretest description of the pcretest testing command
94
95 In addition, in the "man" and HTML formats, there is a short page for
96 each C library function, listing its arguments and results.
97
98
99 LIMITATIONS
100
101 There are some size limitations in PCRE but it is hoped that they will
102 never in practice be relevant.
103
104 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
105 is compiled with the default internal linkage size of 2. If you want to
106 process regular expressions that are truly enormous, you can compile
107 PCRE with an internal linkage size of 3 or 4 (see the README file in
108 the source distribution and the pcrebuild documentation for details).
109 In these cases the limit is substantially larger. However, the speed
110 of execution is slower.
111
112 All values in repeating quantifiers must be less than 65536.
113
114 There is no limit to the number of parenthesized subpatterns, but there
115 can be no more than 65535 capturing subpatterns.
116
117 The maximum length of name for a named subpattern is 32 characters, and
118 the maximum number of named subpatterns is 10000.
119
120 The maximum length of a subject string is the largest positive number
121 that an integer variable can hold. However, when using the traditional
122 matching function, PCRE uses recursion to handle subpatterns and indef-
123 inite repetition. This means that the available stack space may limit
124 the size of a subject string that can be processed by certain patterns.
125 For a discussion of stack issues, see the pcrestack documentation.
126
127
128 UTF-8 AND UNICODE PROPERTY SUPPORT
129
130 From release 3.3, PCRE has had some support for character strings
131 encoded in the UTF-8 format. For release 4.0 this was greatly extended
132 to cover most common requirements, and in release 5.0 additional sup-
133 port for Unicode general category properties was added.
134
135 In order process UTF-8 strings, you must build PCRE to include UTF-8
136 support in the code, and, in addition, you must call pcre_compile()
137 with the PCRE_UTF8 option flag. When you do this, both the pattern and
138 any subject strings that are matched against it are treated as UTF-8
139 strings instead of just strings of bytes.
140
141 If you compile PCRE with UTF-8 support, but do not use it at run time,
142 the library will be a bit bigger, but the additional run time overhead
143 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
144 very big.
145
146 If PCRE is built with Unicode character property support (which implies
147 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
148 ported. The available properties that can be tested are limited to the
149 general category properties such as Lu for an upper case letter or Nd
150 for a decimal number, the Unicode script names such as Arabic or Han,
151 and the derived properties Any and L&. A full list is given in the
152 pcrepattern documentation. Only the short names for properties are sup-
153 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
154 ter}, is not supported. Furthermore, in Perl, many properties may
155 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
156 does not support this.
157
158 Validity of UTF-8 strings
159
160 When you set the PCRE_UTF8 flag, the strings passed as patterns and
161 subjects are (by default) checked for validity on entry to the relevant
162 functions. From release 7.3 of PCRE, the check is according the rules
163 of RFC 3629, which are themselves derived from the Unicode specifica-
164 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
165 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
166 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
167 to U+DFFF.
168
169 The excluded code points are the "Low Surrogate Area" of Unicode, of
170 which the Unicode Standard says this: "The Low Surrogate Area does not
171 contain any character assignments, consequently no character code
172 charts or namelists are provided for this area. Surrogates are reserved
173 for use with UTF-16 and then must be used in pairs." The code points
174 that are encoded by UTF-16 pairs are available as independent code
175 points in the UTF-8 encoding. (In other words, the whole surrogate
176 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
177
178 If an invalid UTF-8 string is passed to PCRE, an error return
179 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
180 that your strings are valid, and therefore want to skip these checks in
181 order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
182 compile time or at run time, PCRE assumes that the pattern or subject
183 it is given (respectively) contains only valid UTF-8 codes. In this
184 case, it does not diagnose an invalid UTF-8 string.
185
186 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
187 what happens depends on why the string is invalid. If the string con-
188 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
189 string of characters in the range 0 to 0x7FFFFFFF. In other words,
190 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
191 strings according to the more liberal rules of RFC 2279. However, if
192 the string does not even conform to RFC 2279, the result is undefined.
193 Your program may crash.
194
195 If you want to process strings of values in the full range 0 to
196 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
197 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
198 this situation, you will have to apply your own validity check.
199
200 General comments about UTF-8 mode
201
202 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
203 two-byte UTF-8 character if the value is greater than 127.
204
205 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
206 characters for values greater than \177.
207
208 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
209 vidual bytes, for example: \x{100}{3}.
210
211 4. The dot metacharacter matches one UTF-8 character instead of a sin-
212 gle byte.
213
214 5. The escape sequence \C can be used to match a single byte in UTF-8
215 mode, but its use can lead to some strange effects. This facility is
216 not available in the alternative matching function, pcre_dfa_exec().
217
218 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
219 test characters of any code value, but the characters that PCRE recog-
220 nizes as digits, spaces, or word characters remain the same set as
221 before, all with values less than 256. This remains true even when PCRE
222 includes Unicode property support, because to do otherwise would slow
223 down PCRE in many common cases. If you really want to test for a wider
224 sense of, say, "digit", you must use Unicode property tests such as
225 \p{Nd}.
226
227 7. Similarly, characters that match the POSIX named character classes
228 are all low-valued characters.
229
230 8. However, the Perl 5.10 horizontal and vertical whitespace matching
231 escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
232 acters.
233
234 9. Case-insensitive matching applies only to characters whose values
235 are less than 128, unless PCRE is built with Unicode property support.
236 Even when Unicode property support is available, PCRE still uses its
237 own character tables when checking the case of low-valued characters,
238 so as not to degrade performance. The Unicode property information is
239 used only for characters with higher values. Even when Unicode property
240 support is available, PCRE supports case-insensitive matching only when
241 there is a one-to-one mapping between a letter's cases. There are a
242 small number of many-to-one mappings in Unicode; these are not sup-
243 ported by PCRE.
244
245
246 AUTHOR
247
248 Philip Hazel
249 University Computing Service
250 Cambridge CB2 3QH, England.
251
252 Putting an actual email address here seems to have been a spam magnet,
253 so I've taken it away. If you want to email me, use my two initials,
254 followed by the two digits 10, at the domain cam.ac.uk.
255
256
257 REVISION
258
259 Last updated: 09 August 2007
260 Copyright (c) 1997-2007 University of Cambridge.
261 ------------------------------------------------------------------------------
262
263
264 PCREBUILD(3) PCREBUILD(3)
265
266
267 NAME
268 PCRE - Perl-compatible regular expressions
269
270
271 PCRE BUILD-TIME OPTIONS
272
273 This document describes the optional features of PCRE that can be
274 selected when the library is compiled. It assumes use of the configure
275 script, where the optional features are selected or deselected by pro-
276 viding options to configure before running the make command. However,
277 the same options can be selected in both Unix-like and non-Unix-like
278 environments using the GUI facility of CMakeSetup if you are using
279 CMake instead of configure to build PCRE.
280
281 The complete list of options for configure (which includes the standard
282 ones such as the selection of the installation directory) can be
283 obtained by running
284
285 ./configure --help
286
287 The following sections include descriptions of options whose names
288 begin with --enable or --disable. These settings specify changes to the
289 defaults for the configure command. Because of the way that configure
290 works, --enable and --disable always come in pairs, so the complemen-
291 tary option always exists as well, but as it specifies the default, it
292 is not described.
293
294
295 C++ SUPPORT
296
297 By default, the configure script will search for a C++ compiler and C++
298 header files. If it finds them, it automatically builds the C++ wrapper
299 library for PCRE. You can disable this by adding
300
301 --disable-cpp
302
303 to the configure command.
304
305
306 UTF-8 SUPPORT
307
308 To build PCRE with support for UTF-8 character strings, add
309
310 --enable-utf8
311
312 to the configure command. Of itself, this does not make PCRE treat
313 strings as UTF-8. As well as compiling PCRE with this option, you also
314 have have to set the PCRE_UTF8 option when you call the pcre_compile()
315 function.
316
317
318 UNICODE CHARACTER PROPERTY SUPPORT
319
320 UTF-8 support allows PCRE to process character values greater than 255
321 in the strings that it handles. On its own, however, it does not pro-
322 vide any facilities for accessing the properties of such characters. If
323 you want to be able to use the pattern escapes \P, \p, and \X, which
324 refer to Unicode character properties, you must add
325
326 --enable-unicode-properties
327
328 to the configure command. This implies UTF-8 support, even if you have
329 not explicitly requested it.
330
331 Including Unicode property support adds around 30K of tables to the
332 PCRE library. Only the general category properties such as Lu and Nd
333 are supported. Details are given in the pcrepattern documentation.
334
335
336 CODE VALUE OF NEWLINE
337
338 By default, PCRE interprets character 10 (linefeed, LF) as indicating
339 the end of a line. This is the normal newline character on Unix-like
340 systems. You can compile PCRE to use character 13 (carriage return, CR)
341 instead, by adding
342
343 --enable-newline-is-cr
344
345 to the configure command. There is also a --enable-newline-is-lf
346 option, which explicitly specifies linefeed as the newline character.
347
348 Alternatively, you can specify that line endings are to be indicated by
349 the two character sequence CRLF. If you want this, add
350
351 --enable-newline-is-crlf
352
353 to the configure command. There is a fourth option, specified by
354
355 --enable-newline-is-anycrlf
356
357 which causes PCRE to recognize any of the three sequences CR, LF, or
358 CRLF as indicating a line ending. Finally, a fifth option, specified by
359
360 --enable-newline-is-any
361
362 causes PCRE to recognize any Unicode newline sequence.
363
364 Whatever line ending convention is selected when PCRE is built can be
365 overridden when the library functions are called. At build time it is
366 conventional to use the standard for your operating system.
367
368
369 WHAT \R MATCHES
370
371 By default, the sequence \R in a pattern matches any Unicode newline
372 sequence, whatever has been selected as the line ending sequence. If
373 you specify
374
375 --enable-bsr-anycrlf
376
377 the default is changed so that \R matches only CR, LF, or CRLF. What-
378 ever is selected when PCRE is built can be overridden when the library
379 functions are called.
380
381
382 BUILDING SHARED AND STATIC LIBRARIES
383
384 The PCRE building process uses libtool to build both shared and static
385 Unix libraries by default. You can suppress one of these by adding one
386 of
387
388 --disable-shared
389 --disable-static
390
391 to the configure command, as required.
392
393
394 POSIX MALLOC USAGE
395
396 When PCRE is called through the POSIX interface (see the pcreposix doc-
397 umentation), additional working storage is required for holding the
398 pointers to capturing substrings, because PCRE requires three integers
399 per substring, whereas the POSIX interface provides only two. If the
400 number of expected substrings is small, the wrapper function uses space
401 on the stack, because this is faster than using malloc() for each call.
402 The default threshold above which the stack is no longer used is 10; it
403 can be changed by adding a setting such as
404
405 --with-posix-malloc-threshold=20
406
407 to the configure command.
408
409
410 HANDLING VERY LARGE PATTERNS
411
412 Within a compiled pattern, offset values are used to point from one
413 part to another (for example, from an opening parenthesis to an alter-
414 nation metacharacter). By default, two-byte values are used for these
415 offsets, leading to a maximum size for a compiled pattern of around
416 64K. This is sufficient to handle all but the most gigantic patterns.
417 Nevertheless, some people do want to process enormous patterns, so it
418 is possible to compile PCRE to use three-byte or four-byte offsets by
419 adding a setting such as
420
421 --with-link-size=3
422
423 to the configure command. The value given must be 2, 3, or 4. Using
424 longer offsets slows down the operation of PCRE because it has to load
425 additional bytes when handling them.
426
427
428 AVOIDING EXCESSIVE STACK USAGE
429
430 When matching with the pcre_exec() function, PCRE implements backtrack-
431 ing by making recursive calls to an internal function called match().
432 In environments where the size of the stack is limited, this can se-
433 verely limit PCRE's operation. (The Unix environment does not usually
434 suffer from this problem, but it may sometimes be necessary to increase
435 the maximum stack size. There is a discussion in the pcrestack docu-
436 mentation.) An alternative approach to recursion that uses memory from
437 the heap to remember data, instead of using recursive function calls,
438 has been implemented to work round the problem of limited stack size.
439 If you want to build a version of PCRE that works this way, add
440
441 --disable-stack-for-recursion
442
443 to the configure command. With this configuration, PCRE will use the
444 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
445 ment functions. By default these point to malloc() and free(), but you
446 can replace the pointers so that your own functions are used.
447
448 Separate functions are provided rather than using pcre_malloc and
449 pcre_free because the usage is very predictable: the block sizes
450 requested are always the same, and the blocks are always freed in
451 reverse order. A calling program might be able to implement optimized
452 functions that perform better than malloc() and free(). PCRE runs
453 noticeably more slowly when built in this way. This option affects only
454 the pcre_exec() function; it is not relevant for the the
455 pcre_dfa_exec() function.
456
457
458 LIMITING PCRE RESOURCE USAGE
459
460 Internally, PCRE has a function called match(), which it calls repeat-
461 edly (sometimes recursively) when matching a pattern with the
462 pcre_exec() function. By controlling the maximum number of times this
463 function may be called during a single matching operation, a limit can
464 be placed on the resources used by a single call to pcre_exec(). The
465 limit can be changed at run time, as described in the pcreapi documen-
466 tation. The default is 10 million, but this can be changed by adding a
467 setting such as
468
469 --with-match-limit=500000
470
471 to the configure command. This setting has no effect on the
472 pcre_dfa_exec() matching function.
473
474 In some environments it is desirable to limit the depth of recursive
475 calls of match() more strictly than the total number of calls, in order
476 to restrict the maximum amount of stack (or heap, if --disable-stack-
477 for-recursion is specified) that is used. A second limit controls this;
478 it defaults to the value that is set for --with-match-limit, which
479 imposes no additional constraints. However, you can set a lower limit
480 by adding, for example,
481
482 --with-match-limit-recursion=10000
483
484 to the configure command. This value can also be overridden at run
485 time.
486
487
488 CREATING CHARACTER TABLES AT BUILD TIME
489
490 PCRE uses fixed tables for processing characters whose code values are
491 less than 256. By default, PCRE is built with a set of tables that are
492 distributed in the file pcre_chartables.c.dist. These tables are for
493 ASCII codes only. If you add
494
495 --enable-rebuild-chartables
496
497 to the configure command, the distributed tables are no longer used.
498 Instead, a program called dftables is compiled and run. This outputs
499 the source for new set of tables, created in the default locale of your
500 C runtime system. (This method of replacing the tables does not work if
501 you are cross compiling, because dftables is run on the local host. If
502 you need to create alternative tables when cross compiling, you will
503 have to do so "by hand".)
504
505
506 USING EBCDIC CODE
507
508 PCRE assumes by default that it will run in an environment where the
509 character code is ASCII (or Unicode, which is a superset of ASCII).
510 This is the case for most computer operating systems. PCRE can, how-
511 ever, be compiled to run in an EBCDIC environment by adding
512
513 --enable-ebcdic
514
515 to the configure command. This setting implies --enable-rebuild-charta-
516 bles. You should only use it if you know that you are in an EBCDIC
517 environment (for example, an IBM mainframe operating system).
518
519
520 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
521
522 By default, pcregrep reads all files as plain text. You can build it so
523 that it recognizes files whose names end in .gz or .bz2, and reads them
524 with libz or libbz2, respectively, by adding one or both of
525
526 --enable-pcregrep-libz
527 --enable-pcregrep-libbz2
528
529 to the configure command. These options naturally require that the rel-
530 evant libraries are installed on your system. Configuration will fail
531 if they are not.
532
533
534 PCRETEST OPTION FOR LIBREADLINE SUPPORT
535
536 If you add
537
538 --enable-pcretest-libreadline
539
540 to the configure command, pcretest is linked with the libreadline
541 library, and when its input is from a terminal, it reads it using the
542 readline() function. This provides line-editing and history facilities.
543 Note that libreadline is GPL-licenced, so if you distribute a binary of
544 pcretest linked in this way, there may be licensing issues.
545
546
547 SEE ALSO
548
549 pcreapi(3), pcre_config(3).
550
551
552 AUTHOR
553
554 Philip Hazel
555 University Computing Service
556 Cambridge CB2 3QH, England.
557
558
559 REVISION
560
561 Last updated: 18 December 2007
562 Copyright (c) 1997-2007 University of Cambridge.
563 ------------------------------------------------------------------------------
564
565
566 PCREMATCHING(3) PCREMATCHING(3)
567
568
569 NAME
570 PCRE - Perl-compatible regular expressions
571
572
573 PCRE MATCHING ALGORITHMS
574
575 This document describes the two different algorithms that are available
576 in PCRE for matching a compiled regular expression against a given sub-
577 ject string. The "standard" algorithm is the one provided by the
578 pcre_exec() function. This works in the same was as Perl's matching
579 function, and provides a Perl-compatible matching operation.
580
581 An alternative algorithm is provided by the pcre_dfa_exec() function;
582 this operates in a different way, and is not Perl-compatible. It has
583 advantages and disadvantages compared with the standard algorithm, and
584 these are described below.
585
586 When there is only one possible way in which a given subject string can
587 match a pattern, the two algorithms give the same answer. A difference
588 arises, however, when there are multiple possibilities. For example, if
589 the pattern
590
591 ^<.*>
592
593 is matched against the string
594
595 <something> <something else> <something further>
596
597 there are three possible answers. The standard algorithm finds only one
598 of them, whereas the alternative algorithm finds all three.
599
600
601 REGULAR EXPRESSIONS AS TREES
602
603 The set of strings that are matched by a regular expression can be rep-
604 resented as a tree structure. An unlimited repetition in the pattern
605 makes the tree of infinite size, but it is still a tree. Matching the
606 pattern to a given subject string (from a given starting point) can be
607 thought of as a search of the tree. There are two ways to search a
608 tree: depth-first and breadth-first, and these correspond to the two
609 matching algorithms provided by PCRE.
610
611
612 THE STANDARD MATCHING ALGORITHM
613
614 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
615 sions", the standard algorithm is an "NFA algorithm". It conducts a
616 depth-first search of the pattern tree. That is, it proceeds along a
617 single path through the tree, checking that the subject matches what is
618 required. When there is a mismatch, the algorithm tries any alterna-
619 tives at the current point, and if they all fail, it backs up to the
620 previous branch point in the tree, and tries the next alternative
621 branch at that level. This often involves backing up (moving to the
622 left) in the subject string as well. The order in which repetition
623 branches are tried is controlled by the greedy or ungreedy nature of
624 the quantifier.
625
626 If a leaf node is reached, a matching string has been found, and at
627 that point the algorithm stops. Thus, if there is more than one possi-
628 ble match, this algorithm returns the first one that it finds. Whether
629 this is the shortest, the longest, or some intermediate length depends
630 on the way the greedy and ungreedy repetition quantifiers are specified
631 in the pattern.
632
633 Because it ends up with a single path through the tree, it is rela-
634 tively straightforward for this algorithm to keep track of the sub-
635 strings that are matched by portions of the pattern in parentheses.
636 This provides support for capturing parentheses and back references.
637
638
639 THE ALTERNATIVE MATCHING ALGORITHM
640
641 This algorithm conducts a breadth-first search of the tree. Starting
642 from the first matching point in the subject, it scans the subject
643 string from left to right, once, character by character, and as it does
644 this, it remembers all the paths through the tree that represent valid
645 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
646 though it is not implemented as a traditional finite state machine (it
647 keeps multiple states active simultaneously).
648
649 The scan continues until either the end of the subject is reached, or
650 there are no more unterminated paths. At this point, terminated paths
651 represent the different matching possibilities (if there are none, the
652 match has failed). Thus, if there is more than one possible match,
653 this algorithm finds all of them, and in particular, it finds the long-
654 est. In PCRE, there is an option to stop the algorithm after the first
655 match (which is necessarily the shortest) has been found.
656
657 Note that all the matches that are found start at the same point in the
658 subject. If the pattern
659
660 cat(er(pillar)?)
661
662 is matched against the string "the caterpillar catchment", the result
663 will be the three strings "cat", "cater", and "caterpillar" that start
664 at the fourth character of the subject. The algorithm does not automat-
665 ically move on to find matches that start at later positions.
666
667 There are a number of features of PCRE regular expressions that are not
668 supported by the alternative matching algorithm. They are as follows:
669
670 1. Because the algorithm finds all possible matches, the greedy or
671 ungreedy nature of repetition quantifiers is not relevant. Greedy and
672 ungreedy quantifiers are treated in exactly the same way. However, pos-
673 sessive quantifiers can make a difference when what follows could also
674 match what is quantified, for example in a pattern like this:
675
676 ^a++\w!
677
678 This pattern matches "aaab!" but not "aaa!", which would be matched by
679 a non-possessive quantifier. Similarly, if an atomic group is present,
680 it is matched as if it were a standalone pattern at the current point,
681 and the longest match is then "locked in" for the rest of the overall
682 pattern.
683
684 2. When dealing with multiple paths through the tree simultaneously, it
685 is not straightforward to keep track of captured substrings for the
686 different matching possibilities, and PCRE's implementation of this
687 algorithm does not attempt to do this. This means that no captured sub-
688 strings are available.
689
690 3. Because no substrings are captured, back references within the pat-
691 tern are not supported, and cause errors if encountered.
692
693 4. For the same reason, conditional expressions that use a backrefer-
694 ence as the condition or test for a specific group recursion are not
695 supported.
696
697 5. Because many paths through the tree may be active, the \K escape
698 sequence, which resets the start of the match when encountered (but may
699 be on some paths and not on others), is not supported. It causes an
700 error if encountered.
701
702 6. Callouts are supported, but the value of the capture_top field is
703 always 1, and the value of the capture_last field is always -1.
704
705 7. The \C escape sequence, which (in the standard algorithm) matches a
706 single byte, even in UTF-8 mode, is not supported because the alterna-
707 tive algorithm moves through the subject string one character at a
708 time, for all active paths through the tree.
709
710 8. None of the backtracking control verbs such as (*PRUNE) are sup-
711 ported.
712
713
714 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
715
716 Using the alternative matching algorithm provides the following advan-
717 tages:
718
719 1. All possible matches (at a single point in the subject) are automat-
720 ically found, and in particular, the longest match is found. To find
721 more than one match using the standard algorithm, you have to do kludgy
722 things with callouts.
723
724 2. There is much better support for partial matching. The restrictions
725 on the content of the pattern that apply when using the standard algo-
726 rithm for partial matching do not apply to the alternative algorithm.
727 For non-anchored patterns, the starting position of a partial match is
728 available.
729
730 3. Because the alternative algorithm scans the subject string just
731 once, and never needs to backtrack, it is possible to pass very long
732 subject strings to the matching function in several pieces, checking
733 for partial matching each time.
734
735
736 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
737
738 The alternative algorithm suffers from a number of disadvantages:
739
740 1. It is substantially slower than the standard algorithm. This is
741 partly because it has to search for all possible matches, but is also
742 because it is less susceptible to optimization.
743
744 2. Capturing parentheses and back references are not supported.
745
746 3. Although atomic groups are supported, their use does not provide the
747 performance advantage that it does for the standard algorithm.
748
749
750 AUTHOR
751
752 Philip Hazel
753 University Computing Service
754 Cambridge CB2 3QH, England.
755
756
757 REVISION
758
759 Last updated: 08 August 2007
760 Copyright (c) 1997-2007 University of Cambridge.
761 ------------------------------------------------------------------------------
762
763
764 PCREAPI(3) PCREAPI(3)
765
766
767 NAME
768 PCRE - Perl-compatible regular expressions
769
770
771 PCRE NATIVE API
772
773 #include <pcre.h>
774
775 pcre *pcre_compile(const char *pattern, int options,
776 const char **errptr, int *erroffset,
777 const unsigned char *tableptr);
778
779 pcre *pcre_compile2(const char *pattern, int options,
780 int *errorcodeptr,
781 const char **errptr, int *erroffset,
782 const unsigned char *tableptr);
783
784 pcre_extra *pcre_study(const pcre *code, int options,
785 const char **errptr);
786
787 int pcre_exec(const pcre *code, const pcre_extra *extra,
788 const char *subject, int length, int startoffset,
789 int options, int *ovector, int ovecsize);
790
791 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
792 const char *subject, int length, int startoffset,
793 int options, int *ovector, int ovecsize,
794 int *workspace, int wscount);
795
796 int pcre_copy_named_substring(const pcre *code,
797 const char *subject, int *ovector,
798 int stringcount, const char *stringname,
799 char *buffer, int buffersize);
800
801 int pcre_copy_substring(const char *subject, int *ovector,
802 int stringcount, int stringnumber, char *buffer,
803 int buffersize);
804
805 int pcre_get_named_substring(const pcre *code,
806 const char *subject, int *ovector,
807 int stringcount, const char *stringname,
808 const char **stringptr);
809
810 int pcre_get_stringnumber(const pcre *code,
811 const char *name);
812
813 int pcre_get_stringtable_entries(const pcre *code,
814 const char *name, char **first, char **last);
815
816 int pcre_get_substring(const char *subject, int *ovector,
817 int stringcount, int stringnumber,
818 const char **stringptr);
819
820 int pcre_get_substring_list(const char *subject,
821 int *ovector, int stringcount, const char ***listptr);
822
823 void pcre_free_substring(const char *stringptr);
824
825 void pcre_free_substring_list(const char **stringptr);
826
827 const unsigned char *pcre_maketables(void);
828
829 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
830 int what, void *where);
831
832 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
833
834 int pcre_refcount(pcre *code, int adjust);
835
836 int pcre_config(int what, void *where);
837
838 char *pcre_version(void);
839
840 void *(*pcre_malloc)(size_t);
841
842 void (*pcre_free)(void *);
843
844 void *(*pcre_stack_malloc)(size_t);
845
846 void (*pcre_stack_free)(void *);
847
848 int (*pcre_callout)(pcre_callout_block *);
849
850
851 PCRE API OVERVIEW
852
853 PCRE has its own native API, which is described in this document. There
854 are also some wrapper functions that correspond to the POSIX regular
855 expression API. These are described in the pcreposix documentation.
856 Both of these APIs define a set of C function calls. A C++ wrapper is
857 distributed with PCRE. It is documented in the pcrecpp page.
858
859 The native API C function prototypes are defined in the header file
860 pcre.h, and on Unix systems the library itself is called libpcre. It
861 can normally be accessed by adding -lpcre to the command for linking an
862 application that uses PCRE. The header file defines the macros
863 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
864 bers for the library. Applications can use these to include support
865 for different releases of PCRE.
866
867 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
868 pcre_exec() are used for compiling and matching regular expressions in
869 a Perl-compatible manner. A sample program that demonstrates the sim-
870 plest way of using them is provided in the file called pcredemo.c in
871 the source distribution. The pcresample documentation describes how to
872 compile and run it.
873
874 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
875 ble, is also provided. This uses a different algorithm for the match-
876 ing. The alternative algorithm finds all possible matches (at a given
877 point in the subject), and scans the subject just once. However, this
878 algorithm does not return captured substrings. A description of the two
879 matching algorithms and their advantages and disadvantages is given in
880 the pcrematching documentation.
881
882 In addition to the main compiling and matching functions, there are
883 convenience functions for extracting captured substrings from a subject
884 string that is matched by pcre_exec(). They are:
885
886 pcre_copy_substring()
887 pcre_copy_named_substring()
888 pcre_get_substring()
889 pcre_get_named_substring()
890 pcre_get_substring_list()
891 pcre_get_stringnumber()
892 pcre_get_stringtable_entries()
893
894 pcre_free_substring() and pcre_free_substring_list() are also provided,
895 to free the memory used for extracted strings.
896
897 The function pcre_maketables() is used to build a set of character
898 tables in the current locale for passing to pcre_compile(),
899 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
900 provided for specialist use. Most commonly, no special tables are
901 passed, in which case internal tables that are generated when PCRE is
902 built are used.
903
904 The function pcre_fullinfo() is used to find out information about a
905 compiled pattern; pcre_info() is an obsolete version that returns only
906 some of the available information, but is retained for backwards com-
907 patibility. The function pcre_version() returns a pointer to a string
908 containing the version of PCRE and its date of release.
909
910 The function pcre_refcount() maintains a reference count in a data
911 block containing a compiled pattern. This is provided for the benefit
912 of object-oriented applications.
913
914 The global variables pcre_malloc and pcre_free initially contain the
915 entry points of the standard malloc() and free() functions, respec-
916 tively. PCRE calls the memory management functions via these variables,
917 so a calling program can replace them if it wishes to intercept the
918 calls. This should be done before calling any PCRE functions.
919
920 The global variables pcre_stack_malloc and pcre_stack_free are also
921 indirections to memory management functions. These special functions
922 are used only when PCRE is compiled to use the heap for remembering
923 data, instead of recursive function calls, when running the pcre_exec()
924 function. See the pcrebuild documentation for details of how to do
925 this. It is a non-standard way of building PCRE, for use in environ-
926 ments that have limited stacks. Because of the greater use of memory
927 management, it runs more slowly. Separate functions are provided so
928 that special-purpose external code can be used for this case. When
929 used, these functions are always called in a stack-like manner (last
930 obtained, first freed), and always for memory blocks of the same size.
931 There is a discussion about PCRE's stack usage in the pcrestack docu-
932 mentation.
933
934 The global variable pcre_callout initially contains NULL. It can be set
935 by the caller to a "callout" function, which PCRE will then call at
936 specified points during a matching operation. Details are given in the
937 pcrecallout documentation.
938
939
940 NEWLINES
941
942 PCRE supports five different conventions for indicating line breaks in
943 strings: a single CR (carriage return) character, a single LF (line-
944 feed) character, the two-character sequence CRLF, any of the three pre-
945 ceding, or any Unicode newline sequence. The Unicode newline sequences
946 are the three just mentioned, plus the single characters VT (vertical
947 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
948 separator, U+2028), and PS (paragraph separator, U+2029).
949
950 Each of the first three conventions is used by at least one operating
951 system as its standard newline sequence. When PCRE is built, a default
952 can be specified. The default default is LF, which is the Unix stan-
953 dard. When PCRE is run, the default can be overridden, either when a
954 pattern is compiled, or when it is matched.
955
956 At compile time, the newline convention can be specified by the options
957 argument of pcre_compile(), or it can be specified by special text at
958 the start of the pattern itself; this overrides any other settings. See
959 the pcrepattern page for details of the special character sequences.
960
961 In the PCRE documentation the word "newline" is used to mean "the char-
962 acter or pair of characters that indicate a line break". The choice of
963 newline convention affects the handling of the dot, circumflex, and
964 dollar metacharacters, the handling of #-comments in /x mode, and, when
965 CRLF is a recognized line ending sequence, the match position advance-
966 ment for a non-anchored pattern. There is more detail about this in the
967 section on pcre_exec() options below.
968
969 The choice of newline convention does not affect the interpretation of
970 the \n or \r escape sequences, nor does it affect what \R matches,
971 which is controlled in a similar way, but by separate options.
972
973
974 MULTITHREADING
975
976 The PCRE functions can be used in multi-threading applications, with
977 the proviso that the memory management functions pointed to by
978 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
979 callout function pointed to by pcre_callout, are shared by all threads.
980
981 The compiled form of a regular expression is not altered during match-
982 ing, so the same compiled pattern can safely be used by several threads
983 at once.
984
985
986 SAVING PRECOMPILED PATTERNS FOR LATER USE
987
988 The compiled form of a regular expression can be saved and re-used at a
989 later time, possibly by a different program, and even on a host other
990 than the one on which it was compiled. Details are given in the
991 pcreprecompile documentation. However, compiling a regular expression
992 with one version of PCRE for use with a different version is not guar-
993 anteed to work and may cause crashes.
994
995
996 CHECKING BUILD-TIME OPTIONS
997
998 int pcre_config(int what, void *where);
999
1000 The function pcre_config() makes it possible for a PCRE client to dis-
1001 cover which optional features have been compiled into the PCRE library.
1002 The pcrebuild documentation has more details about these optional fea-
1003 tures.
1004
1005 The first argument for pcre_config() is an integer, specifying which
1006 information is required; the second argument is a pointer to a variable
1007 into which the information is placed. The following information is
1008 available:
1009
1010 PCRE_CONFIG_UTF8
1011
1012 The output is an integer that is set to one if UTF-8 support is avail-
1013 able; otherwise it is set to zero.
1014
1015 PCRE_CONFIG_UNICODE_PROPERTIES
1016
1017 The output is an integer that is set to one if support for Unicode
1018 character properties is available; otherwise it is set to zero.
1019
1020 PCRE_CONFIG_NEWLINE
1021
1022 The output is an integer whose value specifies the default character
1023 sequence that is recognized as meaning "newline". The four values that
1024 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1025 and -1 for ANY. The default should normally be the standard sequence
1026 for your operating system.
1027
1028 PCRE_CONFIG_BSR
1029
1030 The output is an integer whose value indicates what character sequences
1031 the \R escape sequence matches by default. A value of 0 means that \R
1032 matches any Unicode line ending sequence; a value of 1 means that \R
1033 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1034 tern is compiled or matched.
1035
1036 PCRE_CONFIG_LINK_SIZE
1037
1038 The output is an integer that contains the number of bytes used for
1039 internal linkage in compiled regular expressions. The value is 2, 3, or
1040 4. Larger values allow larger regular expressions to be compiled, at
1041 the expense of slower matching. The default value of 2 is sufficient
1042 for all but the most massive patterns, since it allows the compiled
1043 pattern to be up to 64K in size.
1044
1045 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1046
1047 The output is an integer that contains the threshold above which the
1048 POSIX interface uses malloc() for output vectors. Further details are
1049 given in the pcreposix documentation.
1050
1051 PCRE_CONFIG_MATCH_LIMIT
1052
1053 The output is an integer that gives the default limit for the number of
1054 internal matching function calls in a pcre_exec() execution. Further
1055 details are given with pcre_exec() below.
1056
1057 PCRE_CONFIG_MATCH_LIMIT_RECURSION
1058
1059 The output is an integer that gives the default limit for the depth of
1060 recursion when calling the internal matching function in a pcre_exec()
1061 execution. Further details are given with pcre_exec() below.
1062
1063 PCRE_CONFIG_STACKRECURSE
1064
1065 The output is an integer that is set to one if internal recursion when
1066 running pcre_exec() is implemented by recursive function calls that use
1067 the stack to remember their state. This is the usual way that PCRE is
1068 compiled. The output is zero if PCRE was compiled to use blocks of data
1069 on the heap instead of recursive function calls. In this case,
1070 pcre_stack_malloc and pcre_stack_free are called to manage memory
1071 blocks on the heap, thus avoiding the use of the stack.
1072
1073
1074 COMPILING A PATTERN
1075
1076 pcre *pcre_compile(const char *pattern, int options,
1077 const char **errptr, int *erroffset,
1078 const unsigned char *tableptr);
1079
1080 pcre *pcre_compile2(const char *pattern, int options,
1081 int *errorcodeptr,
1082 const char **errptr, int *erroffset,
1083 const unsigned char *tableptr);
1084
1085 Either of the functions pcre_compile() or pcre_compile2() can be called
1086 to compile a pattern into an internal form. The only difference between
1087 the two interfaces is that pcre_compile2() has an additional argument,
1088 errorcodeptr, via which a numerical error code can be returned.
1089
1090 The pattern is a C string terminated by a binary zero, and is passed in
1091 the pattern argument. A pointer to a single block of memory that is
1092 obtained via pcre_malloc is returned. This contains the compiled code
1093 and related data. The pcre type is defined for the returned block; this
1094 is a typedef for a structure whose contents are not externally defined.
1095 It is up to the caller to free the memory (via pcre_free) when it is no
1096 longer required.
1097
1098 Although the compiled code of a PCRE regex is relocatable, that is, it
1099 does not depend on memory location, the complete pcre data block is not
1100 fully relocatable, because it may contain a copy of the tableptr argu-
1101 ment, which is an address (see below).
1102
1103 The options argument contains various bit settings that affect the com-
1104 pilation. It should be zero if no options are required. The available
1105 options are described below. Some of them, in particular, those that
1106 are compatible with Perl, can also be set and unset from within the
1107 pattern (see the detailed description in the pcrepattern documenta-
1108 tion). For these options, the contents of the options argument speci-
1109 fies their initial settings at the start of compilation and execution.
1110 The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
1111 of matching as well as at compile time.
1112
1113 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1114 if compilation of a pattern fails, pcre_compile() returns NULL, and
1115 sets the variable pointed to by errptr to point to a textual error mes-
1116 sage. This is a static string that is part of the library. You must not
1117 try to free it. The offset from the start of the pattern to the charac-
1118 ter where the error was discovered is placed in the variable pointed to
1119 by erroffset, which must not be NULL. If it is, an immediate error is
1120 given.
1121
1122 If pcre_compile2() is used instead of pcre_compile(), and the error-
1123 codeptr argument is not NULL, a non-zero error code number is returned
1124 via this argument in the event of an error. This is in addition to the
1125 textual error message. Error codes and messages are listed below.
1126
1127 If the final argument, tableptr, is NULL, PCRE uses a default set of
1128 character tables that are built when PCRE is compiled, using the
1129 default C locale. Otherwise, tableptr must be an address that is the
1130 result of a call to pcre_maketables(). This value is stored with the
1131 compiled pattern, and used again by pcre_exec(), unless another table
1132 pointer is passed to it. For more discussion, see the section on locale
1133 support below.
1134
1135 This code fragment shows a typical straightforward call to pcre_com-
1136 pile():
1137
1138 pcre *re;
1139 const char *error;
1140 int erroffset;
1141 re = pcre_compile(
1142 "^A.*Z", /* the pattern */
1143 0, /* default options */
1144 &error, /* for error message */
1145 &erroffset, /* for error offset */
1146 NULL); /* use default character tables */
1147
1148 The following names for option bits are defined in the pcre.h header
1149 file:
1150
1151 PCRE_ANCHORED
1152
1153 If this bit is set, the pattern is forced to be "anchored", that is, it
1154 is constrained to match only at the first matching point in the string
1155 that is being searched (the "subject string"). This effect can also be
1156 achieved by appropriate constructs in the pattern itself, which is the
1157 only way to do it in Perl.
1158
1159 PCRE_AUTO_CALLOUT
1160
1161 If this bit is set, pcre_compile() automatically inserts callout items,
1162 all with number 255, before each pattern item. For discussion of the
1163 callout facility, see the pcrecallout documentation.
1164
1165 PCRE_BSR_ANYCRLF
1166 PCRE_BSR_UNICODE
1167
1168 These options (which are mutually exclusive) control what the \R escape
1169 sequence matches. The choice is either to match only CR, LF, or CRLF,
1170 or to match any Unicode newline sequence. The default is specified when
1171 PCRE is built. It can be overridden from within the pattern, or by set-
1172 ting an option when a compiled pattern is matched.
1173
1174 PCRE_CASELESS
1175
1176 If this bit is set, letters in the pattern match both upper and lower
1177 case letters. It is equivalent to Perl's /i option, and it can be
1178 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1179 always understands the concept of case for characters whose values are
1180 less than 128, so caseless matching is always possible. For characters
1181 with higher values, the concept of case is supported if PCRE is com-
1182 piled with Unicode property support, but not otherwise. If you want to
1183 use caseless matching for characters 128 and above, you must ensure
1184 that PCRE is compiled with Unicode property support as well as with
1185 UTF-8 support.
1186
1187 PCRE_DOLLAR_ENDONLY
1188
1189 If this bit is set, a dollar metacharacter in the pattern matches only
1190 at the end of the subject string. Without this option, a dollar also
1191 matches immediately before a newline at the end of the string (but not
1192 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1193 if PCRE_MULTILINE is set. There is no equivalent to this option in
1194 Perl, and no way to set it within a pattern.
1195
1196 PCRE_DOTALL
1197
1198 If this bit is set, a dot metacharater in the pattern matches all char-
1199 acters, including those that indicate newline. Without it, a dot does
1200 not match when the current position is at a newline. This option is
1201 equivalent to Perl's /s option, and it can be changed within a pattern
1202 by a (?s) option setting. A negative class such as [^a] always matches
1203 newline characters, independent of the setting of this option.
1204
1205 PCRE_DUPNAMES
1206
1207 If this bit is set, names used to identify capturing subpatterns need
1208 not be unique. This can be helpful for certain types of pattern when it
1209 is known that only one instance of the named subpattern can ever be
1210 matched. There are more details of named subpatterns below; see also
1211 the pcrepattern documentation.
1212
1213 PCRE_EXTENDED
1214
1215 If this bit is set, whitespace data characters in the pattern are
1216 totally ignored except when escaped or inside a character class. White-
1217 space does not include the VT character (code 11). In addition, charac-
1218 ters between an unescaped # outside a character class and the next new-
1219 line, inclusive, are also ignored. This is equivalent to Perl's /x
1220 option, and it can be changed within a pattern by a (?x) option set-
1221 ting.
1222
1223 This option makes it possible to include comments inside complicated
1224 patterns. Note, however, that this applies only to data characters.
1225 Whitespace characters may never appear within special character
1226 sequences in a pattern, for example within the sequence (?( which
1227 introduces a conditional subpattern.
1228
1229 PCRE_EXTRA
1230
1231 This option was invented in order to turn on additional functionality
1232 of PCRE that is incompatible with Perl, but it is currently of very
1233 little use. When set, any backslash in a pattern that is followed by a
1234 letter that has no special meaning causes an error, thus reserving
1235 these combinations for future expansion. By default, as in Perl, a
1236 backslash followed by a letter with no special meaning is treated as a
1237 literal. (Perl can, however, be persuaded to give a warning for this.)
1238 There are at present no other features controlled by this option. It
1239 can also be set by a (?X) option setting within a pattern.
1240
1241 PCRE_FIRSTLINE
1242
1243 If this option is set, an unanchored pattern is required to match
1244 before or at the first newline in the subject string, though the
1245 matched text may continue over the newline.
1246
1247 PCRE_MULTILINE
1248
1249 By default, PCRE treats the subject string as consisting of a single
1250 line of characters (even if it actually contains newlines). The "start
1251 of line" metacharacter (^) matches only at the start of the string,
1252 while the "end of line" metacharacter ($) matches only at the end of
1253 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1254 is set). This is the same as Perl.
1255
1256 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1257 constructs match immediately following or immediately before internal
1258 newlines in the subject string, respectively, as well as at the very
1259 start and end. This is equivalent to Perl's /m option, and it can be
1260 changed within a pattern by a (?m) option setting. If there are no new-
1261 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1262 setting PCRE_MULTILINE has no effect.
1263
1264 PCRE_NEWLINE_CR
1265 PCRE_NEWLINE_LF
1266 PCRE_NEWLINE_CRLF
1267 PCRE_NEWLINE_ANYCRLF
1268 PCRE_NEWLINE_ANY
1269
1270 These options override the default newline definition that was chosen
1271 when PCRE was built. Setting the first or the second specifies that a
1272 newline is indicated by a single character (CR or LF, respectively).
1273 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1274 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1275 that any of the three preceding sequences should be recognized. Setting
1276 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1277 recognized. The Unicode newline sequences are the three just mentioned,
1278 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1279 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1280 (paragraph separator, U+2029). The last two are recognized only in
1281 UTF-8 mode.
1282
1283 The newline setting in the options word uses three bits that are
1284 treated as a number, giving eight possibilities. Currently only six are
1285 used (default plus the five values above). This means that if you set
1286 more than one newline option, the combination may or may not be sensi-
1287 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1288 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1289 cause an error.
1290
1291 The only time that a line break is specially recognized when compiling
1292 a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1293 character class is encountered. This indicates a comment that lasts
1294 until after the next line break sequence. In other circumstances, line
1295 break sequences are treated as literal data, except that in
1296 PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1297 and are therefore ignored.
1298
1299 The newline option that is set at compile time becomes the default that
1300 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1301
1302 PCRE_NO_AUTO_CAPTURE
1303
1304 If this option is set, it disables the use of numbered capturing paren-
1305 theses in the pattern. Any opening parenthesis that is not followed by
1306 ? behaves as if it were followed by ?: but named parentheses can still
1307 be used for capturing (and they acquire numbers in the usual way).
1308 There is no equivalent of this option in Perl.
1309
1310 PCRE_UNGREEDY
1311
1312 This option inverts the "greediness" of the quantifiers so that they
1313 are not greedy by default, but become greedy if followed by "?". It is
1314 not compatible with Perl. It can also be set by a (?U) option setting
1315 within the pattern.
1316
1317 PCRE_UTF8
1318
1319 This option causes PCRE to regard both the pattern and the subject as
1320 strings of UTF-8 characters instead of single-byte character strings.
1321 However, it is available only when PCRE is built to include UTF-8 sup-
1322 port. If not, the use of this option provokes an error. Details of how
1323 this option changes the behaviour of PCRE are given in the section on
1324 UTF-8 support in the main pcre page.
1325
1326 PCRE_NO_UTF8_CHECK
1327
1328 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1329 automatically checked. There is a discussion about the validity of
1330 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1331 bytes is found, pcre_compile() returns an error. If you already know
1332 that your pattern is valid, and you want to skip this check for perfor-
1333 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1334 set, the effect of passing an invalid UTF-8 string as a pattern is
1335 undefined. It may cause your program to crash. Note that this option
1336 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1337 UTF-8 validity checking of subject strings.
1338
1339
1340 COMPILATION ERROR CODES
1341
1342 The following table lists the error codes than may be returned by
1343 pcre_compile2(), along with the error messages that may be returned by
1344 both compiling functions. As PCRE has developed, some error codes have
1345 fallen out of use. To avoid confusion, they have not been re-used.
1346
1347 0 no error
1348 1 \ at end of pattern
1349 2 \c at end of pattern
1350 3 unrecognized character follows \
1351 4 numbers out of order in {} quantifier
1352 5 number too big in {} quantifier
1353 6 missing terminating ] for character class
1354 7 invalid escape sequence in character class
1355 8 range out of order in character class
1356 9 nothing to repeat
1357 10 [this code is not in use]
1358 11 internal error: unexpected repeat
1359 12 unrecognized character after (? or (?-
1360 13 POSIX named classes are supported only within a class
1361 14 missing )
1362 15 reference to non-existent subpattern
1363 16 erroffset passed as NULL
1364 17 unknown option bit(s) set
1365 18 missing ) after comment
1366 19 [this code is not in use]
1367 20 regular expression is too large
1368 21 failed to get memory
1369 22 unmatched parentheses
1370 23 internal error: code overflow
1371 24 unrecognized character after (?<
1372 25 lookbehind assertion is not fixed length
1373 26 malformed number or name after (?(
1374 27 conditional group contains more than two branches
1375 28 assertion expected after (?(
1376 29 (?R or (?[+-]digits must be followed by )
1377 30 unknown POSIX class name
1378 31 POSIX collating elements are not supported
1379 32 this version of PCRE is not compiled with PCRE_UTF8 support
1380 33 [this code is not in use]
1381 34 character value in \x{...} sequence is too large
1382 35 invalid condition (?(0)
1383 36 \C not allowed in lookbehind assertion
1384 37 PCRE does not support \L, \l, \N, \U, or \u
1385 38 number after (?C is > 255
1386 39 closing ) for (?C expected
1387 40 recursive call could loop indefinitely
1388 41 unrecognized character after (?P
1389 42 syntax error in subpattern name (missing terminator)
1390 43 two named subpatterns have the same name
1391 44 invalid UTF-8 string
1392 45 support for \P, \p, and \X has not been compiled
1393 46 malformed \P or \p sequence
1394 47 unknown property name after \P or \p
1395 48 subpattern name is too long (maximum 32 characters)
1396 49 too many named subpatterns (maximum 10000)
1397 50 [this code is not in use]
1398 51 octal value is greater than \377 (not in UTF-8 mode)
1399 52 internal error: overran compiling workspace
1400 53 internal error: previously-checked referenced subpattern not
1401 found
1402 54 DEFINE group contains more than one branch
1403 55 repeating a DEFINE group is not allowed
1404 56 inconsistent NEWLINE options
1405 57 \g is not followed by a braced name or an optionally braced
1406 non-zero number
1407 58 (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
1408 59 (*VERB) with an argument is not supported
1409 60 (*VERB) not recognized
1410 61 number is too big
1411 62 subpattern name expected
1412 63 digit expected after (?+
1413
1414 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1415 values may be used if the limits were changed when PCRE was built.
1416
1417
1418 STUDYING A PATTERN
1419
1420 pcre_extra *pcre_study(const pcre *code, int options
1421 const char **errptr);
1422
1423 If a compiled pattern is going to be used several times, it is worth
1424 spending more time analyzing it in order to speed up the time taken for
1425 matching. The function pcre_study() takes a pointer to a compiled pat-
1426 tern as its first argument. If studying the pattern produces additional
1427 information that will help speed up matching, pcre_study() returns a
1428 pointer to a pcre_extra block, in which the study_data field points to
1429 the results of the study.
1430
1431 The returned value from pcre_study() can be passed directly to
1432 pcre_exec(). However, a pcre_extra block also contains other fields
1433 that can be set by the caller before the block is passed; these are
1434 described below in the section on matching a pattern.
1435
1436 If studying the pattern does not produce any additional information
1437 pcre_study() returns NULL. In that circumstance, if the calling program
1438 wants to pass any of the other fields to pcre_exec(), it must set up
1439 its own pcre_extra block.
1440
1441 The second argument of pcre_study() contains option bits. At present,
1442 no options are defined, and this argument should always be zero.
1443
1444 The third argument for pcre_study() is a pointer for an error message.
1445 If studying succeeds (even if no data is returned), the variable it
1446 points to is set to NULL. Otherwise it is set to point to a textual
1447 error message. This is a static string that is part of the library. You
1448 must not try to free it. You should test the error pointer for NULL
1449 after calling pcre_study(), to be sure that it has run successfully.
1450
1451 This is a typical call to pcre_study():
1452
1453 pcre_extra *pe;
1454 pe = pcre_study(
1455 re, /* result of pcre_compile() */
1456 0, /* no options exist */
1457 &error); /* set to NULL or points to a message */
1458
1459 At present, studying a pattern is useful only for non-anchored patterns
1460 that do not have a single fixed starting character. A bitmap of possi-
1461 ble starting bytes is created.
1462
1463
1464 LOCALE SUPPORT
1465
1466 PCRE handles caseless matching, and determines whether characters are
1467 letters, digits, or whatever, by reference to a set of tables, indexed
1468 by character value. When running in UTF-8 mode, this applies only to
1469 characters with codes less than 128. Higher-valued codes never match
1470 escapes such as \w or \d, but can be tested with \p if PCRE is built
1471 with Unicode character property support. The use of locales with Uni-
1472 code is discouraged. If you are handling characters with codes greater
1473 than 128, you should either use UTF-8 and Unicode, or use locales, but
1474 not try to mix the two.
1475
1476 PCRE contains an internal set of tables that are used when the final
1477 argument of pcre_compile() is NULL. These are sufficient for many
1478 applications. Normally, the internal tables recognize only ASCII char-
1479 acters. However, when PCRE is built, it is possible to cause the inter-
1480 nal tables to be rebuilt in the default "C" locale of the local system,
1481 which may cause them to be different.
1482
1483 The internal tables can always be overridden by tables supplied by the
1484 application that calls PCRE. These may be created in a different locale
1485 from the default. As more and more applications change to using Uni-
1486 code, the need for this locale support is expected to die away.
1487
1488 External tables are built by calling the pcre_maketables() function,
1489 which has no arguments, in the relevant locale. The result can then be
1490 passed to pcre_compile() or pcre_exec() as often as necessary. For
1491 example, to build and use tables that are appropriate for the French
1492 locale (where accented characters with values greater than 128 are
1493 treated as letters), the following code could be used:
1494
1495 setlocale(LC_CTYPE, "fr_FR");
1496 tables = pcre_maketables();
1497 re = pcre_compile(..., tables);
1498
1499 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1500 if you are using Windows, the name for the French locale is "french".
1501
1502 When pcre_maketables() runs, the tables are built in memory that is
1503 obtained via pcre_malloc. It is the caller's responsibility to ensure
1504 that the memory containing the tables remains available for as long as
1505 it is needed.
1506
1507 The pointer that is passed to pcre_compile() is saved with the compiled
1508 pattern, and the same tables are used via this pointer by pcre_study()
1509 and normally also by pcre_exec(). Thus, by default, for any single pat-
1510 tern, compilation, studying and matching all happen in the same locale,
1511 but different patterns can be compiled in different locales.
1512
1513 It is possible to pass a table pointer or NULL (indicating the use of
1514 the internal tables) to pcre_exec(). Although not intended for this
1515 purpose, this facility could be used to match a pattern in a different
1516 locale from the one in which it was compiled. Passing table pointers at
1517 run time is discussed below in the section on matching a pattern.
1518
1519
1520 INFORMATION ABOUT A PATTERN
1521
1522 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1523 int what, void *where);
1524
1525 The pcre_fullinfo() function returns information about a compiled pat-
1526 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1527 less retained for backwards compability (and is documented below).
1528
1529 The first argument for pcre_fullinfo() is a pointer to the compiled
1530 pattern. The second argument is the result of pcre_study(), or NULL if
1531 the pattern was not studied. The third argument specifies which piece
1532 of information is required, and the fourth argument is a pointer to a
1533 variable to receive the data. The yield of the function is zero for
1534 success, or one of the following negative numbers:
1535
1536 PCRE_ERROR_NULL the argument code was NULL
1537 the argument where was NULL
1538 PCRE_ERROR_BADMAGIC the "magic number" was not found
1539 PCRE_ERROR_BADOPTION the value of what was invalid
1540
1541 The "magic number" is placed at the start of each compiled pattern as
1542 an simple check against passing an arbitrary memory pointer. Here is a
1543 typical call of pcre_fullinfo(), to obtain the length of the compiled
1544 pattern:
1545
1546 int rc;
1547 size_t length;
1548 rc = pcre_fullinfo(
1549 re, /* result of pcre_compile() */
1550 pe, /* result of pcre_study(), or NULL */
1551 PCRE_INFO_SIZE, /* what is required */
1552 &length); /* where to put the data */
1553
1554 The possible values for the third argument are defined in pcre.h, and
1555 are as follows:
1556
1557 PCRE_INFO_BACKREFMAX
1558
1559 Return the number of the highest back reference in the pattern. The
1560 fourth argument should point to an int variable. Zero is returned if
1561 there are no back references.
1562
1563 PCRE_INFO_CAPTURECOUNT
1564
1565 Return the number of capturing subpatterns in the pattern. The fourth
1566 argument should point to an int variable.
1567
1568 PCRE_INFO_DEFAULT_TABLES
1569
1570 Return a pointer to the internal default character tables within PCRE.
1571 The fourth argument should point to an unsigned char * variable. This
1572 information call is provided for internal use by the pcre_study() func-
1573 tion. External callers can cause PCRE to use its internal tables by
1574 passing a NULL table pointer.
1575
1576 PCRE_INFO_FIRSTBYTE
1577
1578 Return information about the first byte of any matched string, for a
1579 non-anchored pattern. The fourth argument should point to an int vari-
1580 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1581 is still recognized for backwards compatibility.)
1582
1583 If there is a fixed first byte, for example, from a pattern such as
1584 (cat|cow|coyote), its value is returned. Otherwise, if either
1585
1586 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1587 branch starts with "^", or
1588
1589 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1590 set (if it were set, the pattern would be anchored),
1591
1592 -1 is returned, indicating that the pattern matches only at the start
1593 of a subject string or after any newline within the string. Otherwise
1594 -2 is returned. For anchored patterns, -2 is returned.
1595
1596 PCRE_INFO_FIRSTTABLE
1597
1598 If the pattern was studied, and this resulted in the construction of a
1599 256-bit table indicating a fixed set of bytes for the first byte in any
1600 matching string, a pointer to the table is returned. Otherwise NULL is
1601 returned. The fourth argument should point to an unsigned char * vari-
1602 able.
1603
1604 PCRE_INFO_HASCRORLF
1605
1606 Return 1 if the pattern contains any explicit matches for CR or LF
1607 characters, otherwise 0. The fourth argument should point to an int
1608 variable. An explicit match is either a literal CR or LF character, or
1609 \r or \n.
1610
1611 PCRE_INFO_JCHANGED
1612
1613 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
1614 otherwise 0. The fourth argument should point to an int variable. (?J)
1615 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1616
1617 PCRE_INFO_LASTLITERAL
1618
1619 Return the value of the rightmost literal byte that must exist in any
1620 matched string, other than at its start, if such a byte has been
1621 recorded. The fourth argument should point to an int variable. If there
1622 is no such byte, -1 is returned. For anchored patterns, a last literal
1623 byte is recorded only if it follows something of variable length. For
1624 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1625 /^a\dz\d/ the returned value is -1.
1626
1627 PCRE_INFO_NAMECOUNT
1628 PCRE_INFO_NAMEENTRYSIZE
1629 PCRE_INFO_NAMETABLE
1630
1631 PCRE supports the use of named as well as numbered capturing parenthe-
1632 ses. The names are just an additional way of identifying the parenthe-
1633 ses, which still acquire numbers. Several convenience functions such as
1634 pcre_get_named_substring() are provided for extracting captured sub-
1635 strings by name. It is also possible to extract the data directly, by
1636 first converting the name to a number in order to access the correct
1637 pointers in the output vector (described with pcre_exec() below). To do
1638 the conversion, you need to use the name-to-number map, which is
1639 described by these three values.
1640
1641 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1642 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1643 of each entry; both of these return an int value. The entry size
1644 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1645 a pointer to the first entry of the table (a pointer to char). The
1646 first two bytes of each entry are the number of the capturing parenthe-
1647 sis, most significant byte first. The rest of the entry is the corre-
1648 sponding name, zero terminated. The names are in alphabetical order.
1649 When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1650 theses numbers. For example, consider the following pattern (assume
1651 PCRE_EXTENDED is set, so white space - including newlines - is
1652 ignored):
1653
1654 (?<date> (?<year>(\d\d)?\d\d) -
1655 (?<month>\d\d) - (?<day>\d\d) )
1656
1657 There are four named subpatterns, so the table has four entries, and
1658 each entry in the table is eight bytes long. The table is as follows,
1659 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1660 as ??:
1661
1662 00 01 d a t e 00 ??
1663 00 05 d a y 00 ?? ??
1664 00 04 m o n t h 00
1665 00 02 y e a r 00 ??
1666
1667 When writing code to extract data from named subpatterns using the
1668 name-to-number map, remember that the length of the entries is likely
1669 to be different for each compiled pattern.
1670
1671 PCRE_INFO_OKPARTIAL
1672
1673 Return 1 if the pattern can be used for partial matching, otherwise 0.
1674 The fourth argument should point to an int variable. The pcrepartial
1675 documentation lists the restrictions that apply to patterns when par-
1676 tial matching is used.
1677
1678 PCRE_INFO_OPTIONS
1679
1680 Return a copy of the options with which the pattern was compiled. The
1681 fourth argument should point to an unsigned long int variable. These
1682 option bits are those specified in the call to pcre_compile(), modified
1683 by any top-level option settings at the start of the pattern itself. In
1684 other words, they are the options that will be in force when matching
1685 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1686 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1687 and PCRE_EXTENDED.
1688
1689 A pattern is automatically anchored by PCRE if all of its top-level
1690 alternatives begin with one of the following:
1691
1692 ^ unless PCRE_MULTILINE is set
1693 \A always
1694 \G always
1695 .* if PCRE_DOTALL is set and there are no back
1696 references to the subpattern in which .* appears
1697
1698 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1699 by pcre_fullinfo().
1700
1701 PCRE_INFO_SIZE
1702
1703 Return the size of the compiled pattern, that is, the value that was
1704 passed as the argument to pcre_malloc() when PCRE was getting memory in
1705 which to place the compiled data. The fourth argument should point to a
1706 size_t variable.
1707
1708 PCRE_INFO_STUDYSIZE
1709
1710 Return the size of the data block pointed to by the study_data field in
1711 a pcre_extra block. That is, it is the value that was passed to
1712 pcre_malloc() when PCRE was getting memory into which to place the data
1713 created by pcre_study(). The fourth argument should point to a size_t
1714 variable.
1715
1716
1717 OBSOLETE INFO FUNCTION
1718
1719 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1720
1721 The pcre_info() function is now obsolete because its interface is too
1722 restrictive to return all the available data about a compiled pattern.
1723 New programs should use pcre_fullinfo() instead. The yield of
1724 pcre_info() is the number of capturing subpatterns, or one of the fol-
1725 lowing negative numbers:
1726
1727 PCRE_ERROR_NULL the argument code was NULL
1728 PCRE_ERROR_BADMAGIC the "magic number" was not found
1729
1730 If the optptr argument is not NULL, a copy of the options with which
1731 the pattern was compiled is placed in the integer it points to (see
1732 PCRE_INFO_OPTIONS above).
1733
1734 If the pattern is not anchored and the firstcharptr argument is not
1735 NULL, it is used to pass back information about the first character of
1736 any matched string (see PCRE_INFO_FIRSTBYTE above).
1737
1738
1739 REFERENCE COUNTS
1740
1741 int pcre_refcount(pcre *code, int adjust);
1742
1743 The pcre_refcount() function is used to maintain a reference count in
1744 the data block that contains a compiled pattern. It is provided for the
1745 benefit of applications that operate in an object-oriented manner,
1746 where different parts of the application may be using the same compiled
1747 pattern, but you want to free the block when they are all done.
1748
1749 When a pattern is compiled, the reference count field is initialized to
1750 zero. It is changed only by calling this function, whose action is to
1751 add the adjust value (which may be positive or negative) to it. The
1752 yield of the function is the new value. However, the value of the count
1753 is constrained to lie between 0 and 65535, inclusive. If the new value
1754 is outside these limits, it is forced to the appropriate limit value.
1755
1756 Except when it is zero, the reference count is not correctly preserved
1757 if a pattern is compiled on one host and then transferred to a host
1758 whose byte-order is different. (This seems a highly unlikely scenario.)
1759
1760
1761 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1762
1763 int pcre_exec(const pcre *code, const pcre_extra *extra,
1764 const char *subject, int length, int startoffset,
1765 int options, int *ovector, int ovecsize);
1766
1767 The function pcre_exec() is called to match a subject string against a
1768 compiled pattern, which is passed in the code argument. If the pattern
1769 has been studied, the result of the study should be passed in the extra
1770 argument. This function is the main matching facility of the library,
1771 and it operates in a Perl-like manner. For specialist use there is also
1772 an alternative matching function, which is described below in the sec-
1773 tion about the pcre_dfa_exec() function.
1774
1775 In most applications, the pattern will have been compiled (and option-
1776 ally studied) in the same process that calls pcre_exec(). However, it
1777 is possible to save compiled patterns and study data, and then use them
1778 later in different processes, possibly even on different hosts. For a
1779 discussion about this, see the pcreprecompile documentation.
1780
1781 Here is an example of a simple call to pcre_exec():
1782
1783 int rc;
1784 int ovector[30];
1785 rc = pcre_exec(
1786 re, /* result of pcre_compile() */
1787 NULL, /* we didn't study the pattern */
1788 "some string", /* the subject string */
1789 11, /* the length of the subject string */
1790 0, /* start at offset 0 in the subject */
1791 0, /* default options */
1792 ovector, /* vector of integers for substring information */
1793 30); /* number of elements (NOT size in bytes) */
1794
1795 Extra data for pcre_exec()
1796
1797 If the extra argument is not NULL, it must point to a pcre_extra data
1798 block. The pcre_study() function returns such a block (when it doesn't
1799 return NULL), but you can also create one for yourself, and pass addi-
1800 tional information in it. The pcre_extra block contains the following
1801 fields (not necessarily in this order):
1802
1803 unsigned long int flags;
1804 void *study_data;
1805 unsigned long int match_limit;
1806 unsigned long int match_limit_recursion;
1807 void *callout_data;
1808 const unsigned char *tables;
1809
1810 The flags field is a bitmap that specifies which of the other fields
1811 are set. The flag bits are:
1812
1813 PCRE_EXTRA_STUDY_DATA
1814 PCRE_EXTRA_MATCH_LIMIT
1815 PCRE_EXTRA_MATCH_LIMIT_RECURSION
1816 PCRE_EXTRA_CALLOUT_DATA
1817 PCRE_EXTRA_TABLES
1818
1819 Other flag bits should be set to zero. The study_data field is set in
1820 the pcre_extra block that is returned by pcre_study(), together with
1821 the appropriate flag bit. You should not set this yourself, but you may
1822 add to the block by setting the other fields and their corresponding
1823 flag bits.
1824
1825 The match_limit field provides a means of preventing PCRE from using up
1826 a vast amount of resources when running patterns that are not going to
1827 match, but which have a very large number of possibilities in their
1828 search trees. The classic example is the use of nested unlimited
1829 repeats.
1830
1831 Internally, PCRE uses a function called match() which it calls repeat-
1832 edly (sometimes recursively). The limit set by match_limit is imposed
1833 on the number of times this function is called during a match, which
1834 has the effect of limiting the amount of backtracking that can take
1835 place. For patterns that are not anchored, the count restarts from zero
1836 for each position in the subject string.
1837
1838 The default value for the limit can be set when PCRE is built; the
1839 default default is 10 million, which handles all but the most extreme
1840 cases. You can override the default by suppling pcre_exec() with a
1841 pcre_extra block in which match_limit is set, and
1842 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1843 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1844
1845 The match_limit_recursion field is similar to match_limit, but instead
1846 of limiting the total number of times that match() is called, it limits
1847 the depth of recursion. The recursion depth is a smaller number than
1848 the total number of calls, because not all calls to match() are recur-
1849 sive. This limit is of use only if it is set smaller than match_limit.
1850
1851 Limiting the recursion depth limits the amount of stack that can be
1852 used, or, when PCRE has been compiled to use memory on the heap instead
1853 of the stack, the amount of heap memory that can be used.
1854
1855 The default value for match_limit_recursion can be set when PCRE is
1856 built; the default default is the same value as the default for
1857 match_limit. You can override the default by suppling pcre_exec() with
1858 a pcre_extra block in which match_limit_recursion is set, and
1859 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1860 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1861
1862 The pcre_callout field is used in conjunction with the "callout" fea-
1863 ture, which is described in the pcrecallout documentation.
1864
1865 The tables field is used to pass a character tables pointer to
1866 pcre_exec(); this overrides the value that is stored with the compiled
1867 pattern. A non-NULL value is stored with the compiled pattern only if
1868 custom tables were supplied to pcre_compile() via its tableptr argu-
1869 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1870 PCRE's internal tables to be used. This facility is helpful when re-
1871 using patterns that have been saved after compiling with an external
1872 set of tables, because the external tables might be at a different
1873 address when pcre_exec() is called. See the pcreprecompile documenta-
1874 tion for a discussion of saving compiled patterns for later use.
1875
1876 Option bits for pcre_exec()
1877
1878 The unused bits of the options argument for pcre_exec() must be zero.
1879 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1880 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
1881 PCRE_PARTIAL.
1882
1883 PCRE_ANCHORED
1884
1885 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
1886 matching position. If a pattern was compiled with PCRE_ANCHORED, or
1887 turned out to be anchored by virtue of its contents, it cannot be made
1888 unachored at matching time.
1889
1890 PCRE_BSR_ANYCRLF
1891 PCRE_BSR_UNICODE
1892
1893 These options (which are mutually exclusive) control what the \R escape
1894 sequence matches. The choice is either to match only CR, LF, or CRLF,
1895 or to match any Unicode newline sequence. These options override the
1896 choice that was made or defaulted when the pattern was compiled.
1897
1898 PCRE_NEWLINE_CR
1899 PCRE_NEWLINE_LF
1900 PCRE_NEWLINE_CRLF
1901 PCRE_NEWLINE_ANYCRLF
1902 PCRE_NEWLINE_ANY
1903
1904 These options override the newline definition that was chosen or
1905 defaulted when the pattern was compiled. For details, see the descrip-
1906 tion of pcre_compile() above. During matching, the newline choice
1907 affects the behaviour of the dot, circumflex, and dollar metacharac-
1908 ters. It may also alter the way the match position is advanced after a
1909 match failure for an unanchored pattern.
1910
1911 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
1912 set, and a match attempt for an unanchored pattern fails when the cur-
1913 rent position is at a CRLF sequence, and the pattern contains no
1914 explicit matches for CR or LF characters, the match position is
1915 advanced by two characters instead of one, in other words, to after the
1916 CRLF.
1917
1918 The above rule is a compromise that makes the most common cases work as
1919 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
1920 option is not set), it does not match the string "\r\nA" because, after
1921 failing at the start, it skips both the CR and the LF before retrying.
1922 However, the pattern [\r\n]A does match that string, because it con-
1923 tains an explicit CR or LF reference, and so advances only by one char-
1924 acter after the first failure.
1925
1926 An explicit match for CR of LF is either a literal appearance of one of
1927 those characters, or one of the \r or \n escape sequences. Implicit
1928 matches such as [^X] do not count, nor does \s (which includes CR and
1929 LF in the characters that it matches).
1930
1931 Notwithstanding the above, anomalous effects may still occur when CRLF
1932 is a valid newline sequence and explicit \r or \n escapes appear in the
1933 pattern.
1934
1935 PCRE_NOTBOL
1936
1937 This option specifies that first character of the subject string is not
1938 the beginning of a line, so the circumflex metacharacter should not
1939 match before it. Setting this without PCRE_MULTILINE (at compile time)
1940 causes circumflex never to match. This option affects only the behav-
1941 iour of the circumflex metacharacter. It does not affect \A.
1942
1943 PCRE_NOTEOL
1944
1945 This option specifies that the end of the subject string is not the end
1946 of a line, so the dollar metacharacter should not match it nor (except
1947 in multiline mode) a newline immediately before it. Setting this with-
1948 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1949 option affects only the behaviour of the dollar metacharacter. It does
1950 not affect \Z or \z.
1951
1952 PCRE_NOTEMPTY
1953
1954 An empty string is not considered to be a valid match if this option is
1955 set. If there are alternatives in the pattern, they are tried. If all
1956 the alternatives match the empty string, the entire match fails. For
1957 example, if the pattern
1958
1959 a?b?
1960
1961 is applied to a string not beginning with "a" or "b", it matches the
1962 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
1963 match is not valid, so PCRE searches further into the string for occur-
1964 rences of "a" or "b".
1965
1966 Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1967 cial case of a pattern match of the empty string within its split()
1968 function, and when using the /g modifier. It is possible to emulate
1969 Perl's behaviour after matching a null string by first trying the match
1970 again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1971 if that fails by advancing the starting offset (see below) and trying
1972 an ordinary match again. There is some code that demonstrates how to do
1973 this in the pcredemo.c sample program.
1974
1975 PCRE_NO_UTF8_CHECK
1976
1977 When PCRE_UTF8 is set at compile time, the validity of the subject as a
1978 UTF-8 string is automatically checked when pcre_exec() is subsequently
1979 called. The value of startoffset is also checked to ensure that it
1980 points to the start of a UTF-8 character. There is a discussion about
1981 the validity of UTF-8 strings in the section on UTF-8 support in the
1982 main pcre page. If an invalid UTF-8 sequence of bytes is found,
1983 pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con-
1984 tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
1985
1986 If you already know that your subject is valid, and you want to skip
1987 these checks for performance reasons, you can set the
1988 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
1989 do this for the second and subsequent calls to pcre_exec() if you are
1990 making repeated calls to find all the matches in a single subject
1991 string. However, you should be sure that the value of startoffset
1992 points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1993 set, the effect of passing an invalid UTF-8 string as a subject, or a
1994 value of startoffset that does not point to the start of a UTF-8 char-
1995 acter, is undefined. Your program may crash.
1996
1997 PCRE_PARTIAL
1998
1999 This option turns on the partial matching feature. If the subject
2000 string fails to match the pattern, but at some point during the match-
2001 ing process the end of the subject was reached (that is, the subject
2002 partially matches the pattern and the failure to match occurred only
2003 because there were not enough subject characters), pcre_exec() returns
2004 PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
2005 used, there are restrictions on what may appear in the pattern. These
2006 are discussed in the pcrepartial documentation.
2007
2008 The string to be matched by pcre_exec()
2009
2010 The subject string is passed to pcre_exec() as a pointer in subject, a
2011 length in length, and a starting byte offset in startoffset. In UTF-8
2012 mode, the byte offset must point to the start of a UTF-8 character.
2013 Unlike the pattern string, the subject may contain binary zero bytes.
2014 When the starting offset is zero, the search for a match starts at the
2015 beginning of the subject, and this is by far the most common case.
2016
2017 A non-zero starting offset is useful when searching for another match
2018 in the same subject by calling pcre_exec() again after a previous suc-
2019 cess. Setting startoffset differs from just passing over a shortened
2020 string and setting PCRE_NOTBOL in the case of a pattern that begins
2021 with any kind of lookbehind. For example, consider the pattern
2022
2023 \Biss\B
2024
2025 which finds occurrences of "iss" in the middle of words. (\B matches
2026 only if the current position in the subject is not a word boundary.)
2027 When applied to the string "Mississipi" the first call to pcre_exec()
2028 finds the first occurrence. If pcre_exec() is called again with just
2029 the remainder of the subject, namely "issipi", it does not match,
2030 because \B is always false at the start of the subject, which is deemed
2031 to be a word boundary. However, if pcre_exec() is passed the entire
2032 string again, but with startoffset set to 4, it finds the second occur-
2033 rence of "iss" because it is able to look behind the starting point to
2034 discover that it is preceded by a letter.
2035
2036 If a non-zero starting offset is passed when the pattern is anchored,
2037 one attempt to match at the given offset is made. This can only succeed
2038 if the pattern does not require the match to be at the start of the
2039 subject.
2040
2041 How pcre_exec() returns captured substrings
2042
2043 In general, a pattern matches a certain portion of the subject, and in
2044 addition, further substrings from the subject may be picked out by
2045 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2046 this is called "capturing" in what follows, and the phrase "capturing
2047 subpattern" is used for a fragment of a pattern that picks out a sub-
2048 string. PCRE supports several other kinds of parenthesized subpattern
2049 that do not cause substrings to be captured.
2050
2051 Captured substrings are returned to the caller via a vector of integer
2052 offsets whose address is passed in ovector. The number of elements in
2053 the vector is passed in ovecsize, which must be a non-negative number.
2054 Note: this argument is NOT the size of ovector in bytes.
2055
2056 The first two-thirds of the vector is used to pass back captured sub-
2057 strings, each substring using a pair of integers. The remaining third
2058 of the vector is used as workspace by pcre_exec() while matching cap-
2059 turing subpatterns, and is not available for passing back information.
2060 The length passed in ovecsize should always be a multiple of three. If
2061 it is not, it is rounded down.
2062
2063 When a match is successful, information about captured substrings is
2064 returned in pairs of integers, starting at the beginning of ovector,
2065 and continuing up to two-thirds of its length at the most. The first
2066 element of a pair is set to the offset of the first character in a sub-
2067 string, and the second is set to the offset of the first character
2068 after the end of a substring. The first pair, ovector[0] and ovec-
2069 tor[1], identify the portion of the subject string matched by the
2070 entire pattern. The next pair is used for the first capturing subpat-
2071 tern, and so on. The value returned by pcre_exec() is one more than the
2072 highest numbered pair that has been set. For example, if two substrings
2073 have been captured, the returned value is 3. If there are no capturing
2074 subpatterns, the return value from a successful match is 1, indicating
2075 that just the first pair of offsets has been set.
2076
2077 If a capturing subpattern is matched repeatedly, it is the last portion
2078 of the string that it matched that is returned.
2079
2080 If the vector is too small to hold all the captured substring offsets,
2081 it is used as far as possible (up to two-thirds of its length), and the
2082 function returns a value of zero. In particular, if the substring off-
2083 sets are not of interest, pcre_exec() may be called with ovector passed
2084 as NULL and ovecsize as zero. However, if the pattern contains back
2085 references and the ovector is not big enough to remember the related
2086 substrings, PCRE has to get additional memory for use during matching.
2087 Thus it is usually advisable to supply an ovector.
2088
2089 The pcre_info() function can be used to find out how many capturing
2090 subpatterns there are in a compiled pattern. The smallest size for
2091 ovector that will allow for n captured substrings, in addition to the
2092 offsets of the substring matched by the whole pattern, is (n+1)*3.
2093
2094 It is possible for capturing subpattern number n+1 to match some part
2095 of the subject when subpattern n has not been used at all. For example,
2096 if the string "abc" is matched against the pattern (a|(z))(bc) the
2097 return from the function is 4, and subpatterns 1 and 3 are matched, but
2098 2 is not. When this happens, both values in the offset pairs corre-
2099 sponding to unused subpatterns are set to -1.
2100
2101 Offset values that correspond to unused subpatterns at the end of the
2102 expression are also set to -1. For example, if the string "abc" is
2103 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2104 matched. The return from the function is 2, because the highest used
2105 capturing subpattern number is 1. However, you can refer to the offsets
2106 for the second and third capturing subpatterns if you wish (assuming
2107 the vector is large enough, of course).
2108
2109 Some convenience functions are provided for extracting the captured
2110 substrings as separate strings. These are described below.
2111
2112 Error return values from pcre_exec()
2113
2114 If pcre_exec() fails, it returns a negative number. The following are
2115 defined in the header file:
2116
2117 PCRE_ERROR_NOMATCH (-1)
2118
2119 The subject string did not match the pattern.
2120
2121 PCRE_ERROR_NULL (-2)
2122
2123 Either code or subject was passed as NULL, or ovector was NULL and
2124 ovecsize was not zero.
2125
2126 PCRE_ERROR_BADOPTION (-3)
2127
2128 An unrecognized bit was set in the options argument.
2129
2130 PCRE_ERROR_BADMAGIC (-4)
2131
2132 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2133 to catch the case when it is passed a junk pointer and to detect when a
2134 pattern that was compiled in an environment of one endianness is run in
2135 an environment with the other endianness. This is the error that PCRE
2136 gives when the magic number is not present.
2137
2138 PCRE_ERROR_UNKNOWN_OPCODE (-5)
2139
2140 While running the pattern match, an unknown item was encountered in the
2141 compiled pattern. This error could be caused by a bug in PCRE or by
2142 overwriting of the compiled pattern.
2143
2144 PCRE_ERROR_NOMEMORY (-6)
2145
2146 If a pattern contains back references, but the ovector that is passed
2147 to pcre_exec() is not big enough to remember the referenced substrings,
2148 PCRE gets a block of memory at the start of matching to use for this
2149 purpose. If the call via pcre_malloc() fails, this error is given. The
2150 memory is automatically freed at the end of matching.
2151
2152 PCRE_ERROR_NOSUBSTRING (-7)
2153
2154 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2155 and pcre_get_substring_list() functions (see below). It is never
2156 returned by pcre_exec().
2157
2158 PCRE_ERROR_MATCHLIMIT (-8)
2159
2160 The backtracking limit, as specified by the match_limit field in a
2161 pcre_extra structure (or defaulted) was reached. See the description
2162 above.
2163
2164 PCRE_ERROR_CALLOUT (-9)
2165
2166 This error is never generated by pcre_exec() itself. It is provided for
2167 use by callout functions that want to yield a distinctive error code.
2168 See the pcrecallout documentation for details.
2169
2170 PCRE_ERROR_BADUTF8 (-10)
2171
2172 A string that contains an invalid UTF-8 byte sequence was passed as a
2173 subject.
2174
2175 PCRE_ERROR_BADUTF8_OFFSET (-11)
2176
2177 The UTF-8 byte sequence that was passed as a subject was valid, but the
2178 value of startoffset did not point to the beginning of a UTF-8 charac-
2179 ter.
2180
2181 PCRE_ERROR_PARTIAL (-12)
2182
2183 The subject string did not match, but it did match partially. See the
2184 pcrepartial documentation for details of partial matching.
2185
2186 PCRE_ERROR_BADPARTIAL (-13)
2187
2188 The PCRE_PARTIAL option was used with a compiled pattern containing
2189 items that are not supported for partial matching. See the pcrepartial
2190 documentation for details of partial matching.
2191
2192 PCRE_ERROR_INTERNAL (-14)
2193
2194 An unexpected internal error has occurred. This error could be caused
2195 by a bug in PCRE or by overwriting of the compiled pattern.
2196
2197 PCRE_ERROR_BADCOUNT (-15)
2198
2199 This error is given if the value of the ovecsize argument is negative.
2200
2201 PCRE_ERROR_RECURSIONLIMIT (-21)
2202
2203 The internal recursion limit, as specified by the match_limit_recursion
2204 field in a pcre_extra structure (or defaulted) was reached. See the
2205 description above.
2206
2207 PCRE_ERROR_BADNEWLINE (-23)
2208
2209 An invalid combination of PCRE_NEWLINE_xxx options was given.
2210
2211 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2212
2213
2214 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2215
2216 int pcre_copy_substring(const char *subject, int *ovector,
2217 int stringcount, int stringnumber, char *buffer,
2218 int buffersize);
2219
2220 int pcre_get_substring(const char *subject, int *ovector,
2221 int stringcount, int stringnumber,
2222 const char **stringptr);
2223
2224 int pcre_get_substring_list(const char *subject,
2225 int *ovector, int stringcount, const char ***listptr);
2226
2227 Captured substrings can be accessed directly by using the offsets
2228 returned by pcre_exec() in ovector. For convenience, the functions
2229 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2230 string_list() are provided for extracting captured substrings as new,
2231 separate, zero-terminated strings. These functions identify substrings
2232 by number. The next section describes functions for extracting named
2233 substrings.
2234
2235 A substring that contains a binary zero is correctly extracted and has
2236 a further zero added on the end, but the result is not, of course, a C
2237 string. However, you can process such a string by referring to the
2238 length that is returned by pcre_copy_substring() and pcre_get_sub-
2239 string(). Unfortunately, the interface to pcre_get_substring_list() is
2240 not adequate for handling strings containing binary zeros, because the
2241 end of the final string is not independently indicated.
2242
2243 The first three arguments are the same for all three of these func-
2244 tions: subject is the subject string that has just been successfully
2245 matched, ovector is a pointer to the vector of integer offsets that was
2246 passed to pcre_exec(), and stringcount is the number of substrings that
2247 were captured by the match, including the substring that matched the
2248 entire regular expression. This is the value returned by pcre_exec() if
2249 it is greater than zero. If pcre_exec() returned zero, indicating that
2250 it ran out of space in ovector, the value passed as stringcount should
2251 be the number of elements in the vector divided by three.
2252
2253 The functions pcre_copy_substring() and pcre_get_substring() extract a
2254 single substring, whose number is given as stringnumber. A value of
2255 zero extracts the substring that matched the entire pattern, whereas
2256 higher values extract the captured substrings. For pcre_copy_sub-
2257 string(), the string is placed in buffer, whose length is given by
2258 buffersize, while for pcre_get_substring() a new block of memory is
2259 obtained via pcre_malloc, and its address is returned via stringptr.
2260 The yield of the function is the length of the string, not including
2261 the terminating zero, or one of these error codes:
2262
2263 PCRE_ERROR_NOMEMORY (-6)
2264
2265 The buffer was too small for pcre_copy_substring(), or the attempt to
2266 get memory failed for pcre_get_substring().
2267
2268 PCRE_ERROR_NOSUBSTRING (-7)
2269
2270 There is no substring whose number is stringnumber.
2271
2272 The pcre_get_substring_list() function extracts all available sub-
2273 strings and builds a list of pointers to them. All this is done in a
2274 single block of memory that is obtained via pcre_malloc. The address of
2275 the memory block is returned via listptr, which is also the start of
2276 the list of string pointers. The end of the list is marked by a NULL
2277 pointer. The yield of the function is zero if all went well, or the
2278 error code
2279
2280 PCRE_ERROR_NOMEMORY (-6)
2281
2282 if the attempt to get the memory block failed.
2283
2284 When any of these functions encounter a substring that is unset, which
2285 can happen when capturing subpattern number n+1 matches some part of
2286 the subject, but subpattern n has not been used at all, they return an
2287 empty string. This can be distinguished from a genuine zero-length sub-
2288 string by inspecting the appropriate offset in ovector, which is nega-
2289 tive for unset substrings.
2290
2291 The two convenience functions pcre_free_substring() and pcre_free_sub-
2292 string_list() can be used to free the memory returned by a previous
2293 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2294 tively. They do nothing more than call the function pointed to by
2295 pcre_free, which of course could be called directly from a C program.
2296 However, PCRE is used in some situations where it is linked via a spe-
2297 cial interface to another programming language that cannot use
2298 pcre_free directly; it is for these cases that the functions are pro-
2299 vided.
2300
2301
2302 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2303
2304 int pcre_get_stringnumber(const pcre *code,
2305 const char *name);
2306
2307 int pcre_copy_named_substring(const pcre *code,
2308 const char *subject, int *ovector,
2309 int stringcount, const char *stringname,
2310 char *buffer, int buffersize);
2311
2312 int pcre_get_named_substring(const pcre *code,
2313 const char *subject, int *ovector,
2314 int stringcount, const char *stringname,
2315 const char **stringptr);
2316
2317 To extract a substring by name, you first have to find associated num-
2318 ber. For example, for this pattern
2319
2320 (a+)b(?<xxx>\d+)...
2321
2322 the number of the subpattern called "xxx" is 2. If the name is known to
2323 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2324 name by calling pcre_get_stringnumber(). The first argument is the com-
2325 piled pattern, and the second is the name. The yield of the function is
2326 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2327 subpattern of that name.
2328
2329 Given the number, you can extract the substring directly, or use one of
2330 the functions described in the previous section. For convenience, there
2331 are also two functions that do the whole job.
2332
2333 Most of the arguments of pcre_copy_named_substring() and
2334 pcre_get_named_substring() are the same as those for the similarly
2335 named functions that extract by number. As these are described in the
2336 previous section, they are not re-described here. There are just two
2337 differences:
2338
2339 First, instead of a substring number, a substring name is given. Sec-
2340 ond, there is an extra argument, given at the start, which is a pointer
2341 to the compiled pattern. This is needed in order to gain access to the
2342 name-to-number translation table.
2343
2344 These functions call pcre_get_stringnumber(), and if it succeeds, they
2345 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2346 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2347 behaviour may not be what you want (see the next section).
2348
2349
2350 DUPLICATE SUBPATTERN NAMES
2351
2352 int pcre_get_stringtable_entries(const pcre *code,
2353 const char *name, char **first, char **last);
2354
2355 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2356 subpatterns are not required to be unique. Normally, patterns with
2357 duplicate names are such that in any one match, only one of the named
2358 subpatterns participates. An example is shown in the pcrepattern docu-
2359 mentation.
2360
2361 When duplicates are present, pcre_copy_named_substring() and
2362 pcre_get_named_substring() return the first substring corresponding to
2363 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2364 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2365 function returns one of the numbers that are associated with the name,
2366 but it is not defined which it is.
2367
2368 If you want to get full details of all captured substrings for a given
2369 name, you must use the pcre_get_stringtable_entries() function. The
2370 first argument is the compiled pattern, and the second is the name. The
2371 third and fourth are pointers to variables which are updated by the
2372 function. After it has run, they point to the first and last entries in
2373 the name-to-number table for the given name. The function itself
2374 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2375 there are none. The format of the table is described above in the sec-
2376 tion entitled Information about a pattern. Given all the relevant
2377 entries for the name, you can extract each of their numbers, and hence
2378 the captured data, if any.
2379
2380
2381 FINDING ALL POSSIBLE MATCHES
2382
2383 The traditional matching function uses a similar algorithm to Perl,
2384 which stops when it finds the first match, starting at a given point in
2385 the subject. If you want to find all possible matches, or the longest
2386 possible match, consider using the alternative matching function (see
2387 below) instead. If you cannot use the alternative function, but still
2388 need to find all possible matches, you can kludge it up by making use
2389 of the callout facility, which is described in the pcrecallout documen-
2390 tation.
2391
2392 What you have to do is to insert a callout right at the end of the pat-
2393 tern. When your callout function is called, extract and save the cur-
2394 rent matched substring. Then return 1, which forces pcre_exec() to
2395 backtrack and try other alternatives. Ultimately, when it runs out of
2396 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2397
2398
2399 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2400
2401 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2402 const char *subject, int length, int startoffset,
2403 int options, int *ovector, int ovecsize,
2404 int *workspace, int wscount);
2405
2406 The function pcre_dfa_exec() is called to match a subject string
2407 against a compiled pattern, using a matching algorithm that scans the
2408 subject string just once, and does not backtrack. This has different
2409 characteristics to the normal algorithm, and is not compatible with
2410 Perl. Some of the features of PCRE patterns are not supported. Never-
2411 theless, there are times when this kind of matching can be useful. For
2412 a discussion of the two matching algorithms, see the pcrematching docu-
2413 mentation.
2414
2415 The arguments for the pcre_dfa_exec() function are the same as for
2416 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2417 ent way, and this is described below. The other common arguments are
2418 used in the same way as for pcre_exec(), so their description is not
2419 repeated here.
2420
2421 The two additional arguments provide workspace for the function. The
2422 workspace vector should contain at least 20 elements. It is used for
2423 keeping track of multiple paths through the pattern tree. More
2424 workspace will be needed for patterns and subjects where there are a
2425 lot of potential matches.
2426
2427 Here is an example of a simple call to pcre_dfa_exec():
2428
2429 int rc;
2430 int ovector[10];
2431 int wspace[20];
2432 rc = pcre_dfa_exec(
2433 re, /* result of pcre_compile() */
2434 NULL, /* we didn't study the pattern */
2435 "some string", /* the subject string */
2436 11, /* the length of the subject string */
2437 0, /* start at offset 0 in the subject */
2438 0, /* default options */
2439 ovector, /* vector of integers for substring information */
2440 10, /* number of elements (NOT size in bytes) */
2441 wspace, /* working space vector */
2442 20); /* number of elements (NOT size in bytes) */
2443
2444 Option bits for pcre_dfa_exec()
2445
2446 The unused bits of the options argument for pcre_dfa_exec() must be
2447 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2448 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2449 PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2450 three of these are the same as for pcre_exec(), so their description is
2451 not repeated here.
2452
2453 PCRE_PARTIAL
2454
2455 This has the same general effect as it does for pcre_exec(), but the
2456 details are slightly different. When PCRE_PARTIAL is set for
2457 pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
2458 PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
2459 been no complete matches, but there is still at least one matching pos-
2460 sibility. The portion of the string that provided the partial match is
2461 set as the first matching string.
2462
2463 PCRE_DFA_SHORTEST
2464
2465 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2466 stop as soon as it has found one match. Because of the way the alterna-
2467 tive algorithm works, this is necessarily the shortest possible match
2468 at the first possible matching point in the subject string.
2469
2470 PCRE_DFA_RESTART
2471
2472 When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
2473 returns a partial match, it is possible to call it again, with addi-
2474 tional subject characters, and have it continue with the same match.
2475 The PCRE_DFA_RESTART option requests this action; when it is set, the
2476 workspace and wscount options must reference the same vector as before
2477 because data about the match so far is left in them after a partial
2478 match. There is more discussion of this facility in the pcrepartial
2479 documentation.
2480
2481 Successful returns from pcre_dfa_exec()
2482
2483 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2484 string in the subject. Note, however, that all the matches from one run
2485 of the function start at the same point in the subject. The shorter
2486 matches are all initial substrings of the longer matches. For example,
2487 if the pattern
2488
2489 <.*>
2490
2491 is matched against the string
2492
2493 This is <something> <something else> <something further> no more
2494
2495 the three matched strings are
2496
2497 <something>
2498 <something> <something else>
2499 <something> <something else> <something further>
2500
2501 On success, the yield of the function is a number greater than zero,
2502 which is the number of matched substrings. The substrings themselves
2503 are returned in ovector. Each string uses two elements; the first is
2504 the offset to the start, and the second is the offset to the end. In
2505 fact, all the strings have the same start offset. (Space could have
2506 been saved by giving this only once, but it was decided to retain some
2507 compatibility with the way pcre_exec() returns data, even though the
2508 meaning of the strings is different.)
2509
2510 The strings are returned in reverse order of length; that is, the long-
2511 est matching string is given first. If there were too many matches to
2512 fit into ovector, the yield of the function is zero, and the vector is
2513 filled with the longest matches.
2514
2515 Error returns from pcre_dfa_exec()
2516
2517 The pcre_dfa_exec() function returns a negative number when it fails.
2518 Many of the errors are the same as for pcre_exec(), and these are
2519 described above. There are in addition the following errors that are
2520 specific to pcre_dfa_exec():
2521
2522 PCRE_ERROR_DFA_UITEM (-16)
2523
2524 This return is given if pcre_dfa_exec() encounters an item in the pat-
2525 tern that it does not support, for instance, the use of \C or a back
2526 reference.
2527
2528 PCRE_ERROR_DFA_UCOND (-17)
2529
2530 This return is given if pcre_dfa_exec() encounters a condition item
2531 that uses a back reference for the condition, or a test for recursion
2532 in a specific group. These are not supported.
2533
2534 PCRE_ERROR_DFA_UMLIMIT (-18)
2535
2536 This return is given if pcre_dfa_exec() is called with an extra block
2537 that contains a setting of the match_limit field. This is not supported
2538 (it is meaningless).
2539
2540 PCRE_ERROR_DFA_WSSIZE (-19)
2541
2542 This return is given if pcre_dfa_exec() runs out of space in the
2543 workspace vector.
2544
2545 PCRE_ERROR_DFA_RECURSE (-20)
2546
2547 When a recursive subpattern is processed, the matching function calls
2548 itself recursively, using private vectors for ovector and workspace.
2549 This error is given if the output vector is not large enough. This
2550 should be extremely rare, as a vector of size 1000 is used.
2551
2552
2553 SEE ALSO
2554
2555 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2556 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2557
2558
2559 AUTHOR
2560
2561 Philip Hazel
2562 University Computing Service
2563 Cambridge CB2 3QH, England.
2564
2565
2566 REVISION
2567
2568 Last updated: 23 January 2008
2569 Copyright (c) 1997-2008 University of Cambridge.
2570 ------------------------------------------------------------------------------
2571
2572
2573 PCRECALLOUT(3) PCRECALLOUT(3)
2574
2575
2576 NAME
2577 PCRE - Perl-compatible regular expressions
2578
2579
2580 PCRE CALLOUTS
2581
2582 int (*pcre_callout)(pcre_callout_block *);
2583
2584 PCRE provides a feature called "callout", which is a means of temporar-
2585 ily passing control to the caller of PCRE in the middle of pattern
2586 matching. The caller of PCRE provides an external function by putting
2587 its entry point in the global variable pcre_callout. By default, this
2588 variable contains NULL, which disables all calling out.
2589
2590 Within a regular expression, (?C) indicates the points at which the
2591 external function is to be called. Different callout points can be
2592 identified by putting a number less than 256 after the letter C. The
2593 default value is zero. For example, this pattern has two callout
2594 points:
2595
2596 (?C1)abc(?C2)def
2597
2598 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
2599 called, PCRE automatically inserts callouts, all with number 255,
2600 before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2601 used with the pattern
2602
2603 A(\d{2}|--)
2604
2605 it is processed as if it were
2606
2607 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2608
2609 Notice that there is a callout before and after each parenthesis and
2610 alternation bar. Automatic callouts can be used for tracking the
2611 progress of pattern matching. The pcretest command has an option that
2612 sets automatic callouts; when it is used, the output indicates how the
2613 pattern is matched. This is useful information when you are trying to
2614 optimize the performance of a particular pattern.
2615
2616
2617 MISSING CALLOUTS
2618
2619 You should be aware that, because of optimizations in the way PCRE
2620 matches patterns, callouts sometimes do not happen. For example, if the
2621 pattern is
2622
2623 ab(?C4)cd
2624
2625 PCRE knows that any matching string must contain the letter "d". If the
2626 subject string is "abyz", the lack of "d" means that matching doesn't
2627 ever start, and the callout is never reached. However, with "abyd",
2628 though the result is still no match, the callout is obeyed.
2629
2630
2631 THE CALLOUT INTERFACE
2632
2633 During matching, when PCRE reaches a callout point, the external func-
2634 tion defined by pcre_callout is called (if it is set). This applies to
2635 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2636 only argument to the callout function is a pointer to a pcre_callout
2637 block. This structure contains the following fields:
2638
2639 int version;
2640 int callout_number;
2641 int *offset_vector;
2642 const char *subject;
2643 int subject_length;
2644 int start_match;
2645 int current_position;
2646 int capture_top;
2647 int capture_last;
2648 void *callout_data;
2649 int pattern_position;
2650 int next_item_length;
2651
2652 The version field is an integer containing the version number of the
2653 block format. The initial version was 0; the current version is 1. The
2654 version number will change again in future if additional fields are
2655 added, but the intention is never to remove any of the existing fields.
2656
2657 The callout_number field contains the number of the callout, as com-
2658 piled into the pattern (that is, the number after ?C for manual call-
2659 outs, and 255 for automatically generated callouts).
2660
2661 The offset_vector field is a pointer to the vector of offsets that was
2662 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2663 pcre_exec() is used, the contents can be inspected in order to extract
2664 substrings that have been matched so far, in the same way as for
2665 extracting substrings after a match has completed. For pcre_dfa_exec()
2666 this field is not useful.
2667
2668 The subject and subject_length fields contain copies of the values that
2669 were passed to pcre_exec().
2670
2671 The start_match field normally contains the offset within the subject
2672 at which the current match attempt started. However, if the escape
2673 sequence \K has been encountered, this value is changed to reflect the
2674 modified starting point. If the pattern is not anchored, the callout
2675 function may be called several times from the same point in the pattern
2676 for different starting points in the subject.
2677
2678 The current_position field contains the offset within the subject of
2679 the current match pointer.
2680
2681 When the pcre_exec() function is used, the capture_top field contains
2682 one more than the number of the highest numbered captured substring so
2683 far. If no substrings have been captured, the value of capture_top is
2684 one. This is always the case when pcre_dfa_exec() is used, because it
2685 does not support captured substrings.
2686
2687 The capture_last field contains the number of the most recently cap-
2688 tured substring. If no substrings have been captured, its value is -1.
2689 This is always the case when pcre_dfa_exec() is used.
2690
2691 The callout_data field contains a value that is passed to pcre_exec()
2692 or pcre_dfa_exec() specifically so that it can be passed back in call-
2693 outs. It is passed in the pcre_callout field of the pcre_extra data
2694 structure. If no such data was passed, the value of callout_data in a
2695 pcre_callout block is NULL. There is a description of the pcre_extra
2696 structure in the pcreapi documentation.
2697
2698 The pattern_position field is present from version 1 of the pcre_call-
2699 out structure. It contains the offset to the next item to be matched in
2700 the pattern string.
2701
2702 The next_item_length field is present from version 1 of the pcre_call-
2703 out structure. It contains the length of the next item to be matched in
2704 the pattern string. When the callout immediately precedes an alterna-
2705 tion bar, a closing parenthesis, or the end of the pattern, the length
2706 is zero. When the callout precedes an opening parenthesis, the length
2707 is that of the entire subpattern.
2708
2709 The pattern_position and next_item_length fields are intended to help
2710 in distinguishing between different automatic callouts, which all have
2711 the same callout number. However, they are set for all callouts.
2712
2713
2714 RETURN VALUES
2715
2716 The external callout function returns an integer to PCRE. If the value
2717 is zero, matching proceeds as normal. If the value is greater than
2718 zero, matching fails at the current point, but the testing of other
2719 matching possibilities goes ahead, just as if a lookahead assertion had
2720 failed. If the value is less than zero, the match is abandoned, and
2721 pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2722
2723 Negative values should normally be chosen from the set of
2724 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2725 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2726 reserved for use by callout functions; it will never be used by PCRE
2727 itself.
2728
2729
2730 AUTHOR
2731
2732 Philip Hazel
2733 University Computing Service
2734 Cambridge CB2 3QH, England.
2735
2736
2737 REVISION
2738
2739 Last updated: 29 May 2007
2740 Copyright (c) 1997-2007 University of Cambridge.
2741 ------------------------------------------------------------------------------
2742
2743
2744 PCRECOMPAT(3) PCRECOMPAT(3)
2745
2746
2747 NAME
2748 PCRE - Perl-compatible regular expressions
2749
2750
2751 DIFFERENCES BETWEEN PCRE AND PERL
2752
2753 This document describes the differences in the ways that PCRE and Perl
2754 handle regular expressions. The differences described here are mainly
2755 with respect to Perl 5.8, though PCRE versions 7.0 and later contain
2756 some features that are expected to be in the forthcoming Perl 5.10.
2757
2758 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2759 of what it does have are given in the section on UTF-8 support in the
2760 main pcre page.
2761
2762 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2763 permits them, but they do not mean what you might think. For example,
2764 (?!a){3} does not assert that the next three characters are not "a". It
2765 just asserts that the next character is not "a" three times.
2766
2767 3. Capturing subpatterns that occur inside negative lookahead asser-
2768 tions are counted, but their entries in the offsets vector are never
2769 set. Perl sets its numerical variables from any such patterns that are
2770 matched before the assertion fails to match something (thereby succeed-
2771 ing), but only if the negative lookahead assertion contains just one
2772 branch.
2773
2774 4. Though binary zero characters are supported in the subject string,
2775 they are not allowed in a pattern string because it is passed as a nor-
2776 mal C string, terminated by zero. The escape sequence \0 can be used in
2777 the pattern to represent a binary zero.
2778
2779 5. The following Perl escape sequences are not supported: \l, \u, \L,
2780 \U, and \N. In fact these are implemented by Perl's general string-han-
2781 dling and are not part of its pattern matching engine. If any of these
2782 are encountered by PCRE, an error is generated.
2783
2784 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2785 is built with Unicode character property support. The properties that
2786 can be tested with \p and \P are limited to the general category prop-
2787 erties such as Lu and Nd, script names such as Greek or Han, and the
2788 derived properties Any and L&.
2789
2790 7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2791 ters in between are treated as literals. This is slightly different
2792 from Perl in that $ and @ are also handled as literals inside the
2793 quotes. In Perl, they cause variable interpolation (but of course PCRE
2794 does not have variables). Note the following examples:
2795
2796 Pattern PCRE matches Perl matches
2797
2798 \Qabc$xyz\E abc$xyz abc followed by the
2799 contents of $xyz
2800 \Qabc\$xyz\E abc\$xyz abc\$xyz
2801 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2802
2803 The \Q...\E sequence is recognized both inside and outside character
2804 classes.
2805
2806 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2807 constructions. However, there is support for recursive patterns. This
2808 is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2809 "callout" feature allows an external function to be called during pat-
2810 tern matching. See the pcrecallout documentation for details.
2811
2812 9. Subpatterns that are called recursively or as "subroutines" are
2813 always treated as atomic groups in PCRE. This is like Python, but
2814 unlike Perl.
2815
2816 10. There are some differences that are concerned with the settings of
2817 captured strings when part of a pattern is repeated. For example,
2818 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
2819 unset, but in PCRE it is set to "b".
2820
2821 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT),
2822 (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
2823 the forms without an argument. PCRE does not support (*MARK). If
2824 (*ACCEPT) is within capturing parentheses, PCRE does not set that cap-
2825 ture group; this is different to Perl.
2826
2827 12. PCRE provides some extensions to the Perl regular expression facil-
2828 ities. Perl 5.10 will include new features that are not in earlier
2829 versions, some of which (such as named parentheses) have been in PCRE
2830 for some time. This list is with respect to Perl 5.10:
2831
2832 (a) Although lookbehind assertions must match fixed length strings,
2833 each alternative branch of a lookbehind assertion can match a different
2834 length of string. Perl requires them all to have the same length.
2835
2836 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2837 meta-character matches only at the very end of the string.
2838
2839 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2840 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
2841 ignored. (Perl can be made to issue a warning.)
2842
2843 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
2844 fiers is inverted, that is, by default they are not greedy, but if fol-
2845 lowed by a question mark they are.
2846
2847 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2848 tried only at the first matching position in the subject string.
2849
2850 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2851 TURE options for pcre_exec() have no Perl equivalents.
2852
2853 (g) The \R escape sequence can be restricted to match only CR, LF, or
2854 CRLF by the PCRE_BSR_ANYCRLF option.
2855
2856 (h) The callout facility is PCRE-specific.
2857
2858 (i) The partial matching facility is PCRE-specific.
2859
2860 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
2861 even on different hosts that have the other endianness.
2862
2863 (k) The alternative matching function (pcre_dfa_exec()) matches in a
2864 different way and is not Perl-compatible.
2865
2866 (l) PCRE recognizes some special sequences such as (*CR) at the start
2867 of a pattern that set overall options that cannot be changed within the
2868 pattern.
2869
2870
2871 AUTHOR
2872
2873 Philip Hazel
2874 University Computing Service
2875 Cambridge CB2 3QH, England.
2876
2877
2878 REVISION
2879
2880 Last updated: 11 September 2007
2881 Copyright (c) 1997-2007 University of Cambridge.
2882 ------------------------------------------------------------------------------
2883
2884
2885 PCREPATTERN(3) PCREPATTERN(3)
2886
2887
2888 NAME
2889 PCRE - Perl-compatible regular expressions
2890
2891
2892 PCRE REGULAR EXPRESSION DETAILS
2893
2894 The syntax and semantics of the regular expressions that are supported
2895 by PCRE are described in detail below. There is a quick-reference syn-
2896 tax summary in the pcresyntax page. Perl's regular expressions are
2897 described in its own documentation, and regular expressions in general
2898 are covered in a number of books, some of which have copious examples.
2899 Jeffrey Friedl's "Mastering Regular Expressions", published by
2900 O'Reilly, covers regular expressions in great detail. This description
2901 of PCRE's regular expressions is intended as reference material.
2902
2903 The original operation of PCRE was on strings of one-byte characters.
2904 However, there is now also support for UTF-8 character strings. To use
2905 this, you must build PCRE to include UTF-8 support, and then call
2906 pcre_compile() with the PCRE_UTF8 option. How this affects pattern
2907 matching is mentioned in several places below. There is also a summary
2908 of UTF-8 features in the section on UTF-8 support in the main pcre
2909 page.
2910
2911 The remainder of this document discusses the patterns that are sup-
2912 ported by PCRE when its main matching function, pcre_exec(), is used.
2913 From release 6.0, PCRE offers a second matching function,
2914 pcre_dfa_exec(), which matches using a different algorithm that is not
2915 Perl-compatible. Some of the features discussed below are not available
2916 when pcre_dfa_exec() is used. The advantages and disadvantages of the
2917 alternative function, and how it differs from the normal function, are
2918 discussed in the pcrematching page.
2919
2920
2921 NEWLINE CONVENTIONS
2922
2923 PCRE supports five different conventions for indicating line breaks in
2924 strings: a single CR (carriage return) character, a single LF (line-
2925 feed) character, the two-character sequence CRLF, any of the three pre-
2926 ceding, or any Unicode newline sequence. The pcreapi page has further
2927 discussion about newlines, and shows how to set the newline convention
2928 in the options arguments for the compiling and matching functions.
2929
2930 It is also possible to specify a newline convention by starting a pat-
2931 tern string with one of the following five sequences:
2932
2933 (*CR) carriage return
2934 (*LF) linefeed
2935 (*CRLF) carriage return, followed by linefeed
2936 (*ANYCRLF) any of the three above
2937 (*ANY) all Unicode newline sequences
2938
2939 These override the default and the options given to pcre_compile(). For
2940 example, on a Unix system where LF is the default newline sequence, the
2941 pattern
2942
2943 (*CR)a.b
2944
2945 changes the convention to CR. That pattern matches "a\nb" because LF is
2946 no longer a newline. Note that these special settings, which are not
2947 Perl-compatible, are recognized only at the very start of a pattern,
2948 and that they must be in upper case. If more than one of them is
2949 present, the last one is used.
2950
2951 The newline convention does not affect what the \R escape sequence
2952 matches. By default, this is any Unicode newline sequence, for Perl
2953 compatibility. However, this can be changed; see the description of \R
2954 in the section entitled "Newline sequences" below. A change of \R set-
2955 ting can be combined with a change of newline convention.
2956
2957
2958 CHARACTERS AND METACHARACTERS
2959
2960 A regular expression is a pattern that is matched against a subject
2961 string from left to right. Most characters stand for themselves in a
2962 pattern, and match the corresponding characters in the subject. As a
2963 trivial example, the pattern
2964
2965 The quick brown fox
2966
2967 matches a portion of a subject string that is identical to itself. When
2968 caseless matching is specified (the PCRE_CASELESS option), letters are
2969 matched independently of case. In UTF-8 mode, PCRE always understands
2970 the concept of case for characters whose values are less than 128, so
2971 caseless matching is always possible. For characters with higher val-
2972 ues, the concept of case is supported if PCRE is compiled with Unicode
2973 property support, but not otherwise. If you want to use caseless
2974 matching for characters 128 and above, you must ensure that PCRE is
2975 compiled with Unicode property support as well as with UTF-8 support.
2976
2977 The power of regular expressions comes from the ability to include
2978 alternatives and repetitions in the pattern. These are encoded in the
2979 pattern by the use of metacharacters, which do not stand for themselves
2980 but instead are interpreted in some special way.
2981
2982 There are two different sets of metacharacters: those that are recog-
2983 nized anywhere in the pattern except within square brackets, and those
2984 that are recognized within square brackets. Outside square brackets,
2985 the metacharacters are as follows:
2986
2987 \ general escape character with several uses
2988 ^ assert start of string (or line, in multiline mode)
2989 $ assert end of string (or line, in multiline mode)
2990 . match any character except newline (by default)
2991 [ start character class definition
2992 | start of alternative branch
2993 ( start subpattern
2994 ) end subpattern
2995 ? extends the meaning of (
2996 also 0 or 1 quantifier
2997 also quantifier minimizer
2998 * 0 or more quantifier
2999 + 1 or more quantifier
3000 also "possessive quantifier"
3001 { start min/max quantifier
3002
3003 Part of a pattern that is in square brackets is called a "character
3004 class". In a character class the only metacharacters are:
3005
3006 \ general escape character
3007 ^ negate the class, but only if the first character
3008 - indicates character range
3009 [ POSIX character class (only if followed by POSIX
3010 syntax)
3011 ] terminates the character class
3012
3013 The following sections describe the use of each of the metacharacters.
3014
3015
3016 BACKSLASH
3017
3018 The backslash character has several uses. Firstly, if it is followed by
3019 a non-alphanumeric character, it takes away any special meaning that
3020 character may have. This use of backslash as an escape character
3021 applies both inside and outside character classes.
3022
3023 For example, if you want to match a * character, you write \* in the
3024 pattern. This escaping action applies whether or not the following
3025 character would otherwise be interpreted as a metacharacter, so it is
3026 always safe to precede a non-alphanumeric with backslash to specify
3027 that it stands for itself. In particular, if you want to match a back-
3028 slash, you write \\.
3029
3030 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
3031 the pattern (other than in a character class) and characters between a
3032 # outside a character class and the next newline are ignored. An escap-
3033 ing backslash can be used to include a whitespace or # character as
3034 part of the pattern.
3035
3036 If you want to remove the special meaning from a sequence of charac-
3037 ters, you can do so by putting them between \Q and \E. This is differ-
3038 ent from Perl in that $ and @ are handled as literals in \Q...\E
3039 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
3040 tion. Note the following examples:
3041
3042 Pattern PCRE matches Perl matches
3043
3044 \Qabc$xyz\E abc$xyz abc followed by the
3045 contents of $xyz
3046 \Qabc\$xyz\E abc\$xyz abc\$xyz
3047 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3048
3049 The \Q...\E sequence is recognized both inside and outside character
3050 classes.
3051
3052 Non-printing characters
3053
3054 A second use of backslash provides a way of encoding non-printing char-
3055 acters in patterns in a visible manner. There is no restriction on the
3056 appearance of non-printing characters, apart from the binary zero that
3057 terminates a pattern, but when a pattern is being prepared by text
3058 editing, it is usually easier to use one of the following escape
3059 sequences than the binary character it represents:
3060
3061 \a alarm, that is, the BEL character (hex 07)
3062 \cx "control-x", where x is any character
3063 \e escape (hex 1B)
3064 \f formfeed (hex 0C)
3065 \n linefeed (hex 0A)
3066 \r carriage return (hex 0D)
3067 \t tab (hex 09)
3068 \ddd character with octal code ddd, or backreference
3069 \xhh character with hex code hh
3070 \x{hhh..} character with hex code hhh..
3071
3072 The precise effect of \cx is as follows: if x is a lower case letter,
3073 it is converted to upper case. Then bit 6 of the character (hex 40) is
3074 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
3075 becomes hex 7B.
3076
3077 After \x, from zero to two hexadecimal digits are read (letters can be
3078 in upper or lower case). Any number of hexadecimal digits may appear
3079 between \x{ and }, but the value of the character code must be less
3080 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3081 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3082 than the largest Unicode code point, which is 10FFFF.
3083
3084 If characters other than hexadecimal digits appear between \x{ and },
3085 or if there is no terminating }, this form of escape is not recognized.
3086 Instead, the initial \x will be interpreted as a basic hexadecimal
3087 escape, with no following digits, giving a character whose value is
3088 zero.
3089
3090 Characters whose value is less than 256 can be defined by either of the
3091 two syntaxes for \x. There is no difference in the way they are han-
3092 dled. For example, \xdc is exactly the same as \x{dc}.
3093
3094 After \0 up to two further octal digits are read. If there are fewer
3095 than two digits, just those that are present are used. Thus the
3096 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3097 (code value 7). Make sure you supply two digits after the initial zero
3098 if the pattern character that follows is itself an octal digit.
3099
3100 The handling of a backslash followed by a digit other than 0 is compli-
3101 cated. Outside a character class, PCRE reads it and any following dig-
3102 its as a decimal number. If the number is less than 10, or if there
3103 have been at least that many previous capturing left parentheses in the
3104 expression, the entire sequence is taken as a back reference. A
3105 description of how this works is given later, following the discussion
3106 of parenthesized subpatterns.
3107
3108 Inside a character class, or if the decimal number is greater than 9
3109 and there have not been that many capturing subpatterns, PCRE re-reads
3110 up to three octal digits following the backslash, and uses them to gen-
3111 erate a data character. Any subsequent digits stand for themselves. In
3112 non-UTF-8 mode, the value of a character specified in octal must be
3113 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3114 example:
3115
3116 \040 is another way of writing a space
3117 \40 is the same, provided there are fewer than 40
3118 previous capturing subpatterns
3119 \7 is always a back reference
3120 \11 might be a back reference, or another way of
3121 writing a tab
3122 \011 is always a tab
3123 \0113 is a tab followed by the character "3"
3124 \113 might be a back reference, otherwise the
3125 character with octal code 113
3126 \377 might be a back reference, otherwise
3127 the byte consisting entirely of 1 bits
3128 \81 is either a back reference, or a binary zero
3129 followed by the two characters "8" and "1"
3130
3131 Note that octal values of 100 or greater must not be introduced by a
3132 leading zero, because no more than three octal digits are ever read.
3133
3134 All the sequences that define a single character value can be used both
3135 inside and outside character classes. In addition, inside a character
3136 class, the sequence \b is interpreted as the backspace character (hex
3137 08), and the sequences \R and \X are interpreted as the characters "R"
3138 and "X", respectively. Outside a character class, these sequences have
3139 different meanings (see below).
3140
3141 Absolute and relative back references
3142
3143 The sequence \g followed by an unsigned or a negative number, option-
3144 ally enclosed in braces, is an absolute or relative back reference. A
3145 named back reference can be coded as \g{name}. Back references are dis-
3146 cussed later, following the discussion of parenthesized subpatterns.
3147
3148 Generic character types
3149
3150 Another use of backslash is for specifying generic character types. The
3151 following are always recognized:
3152
3153 \d any decimal digit
3154 \D any character that is not a decimal digit
3155 \h any horizontal whitespace character
3156 \H any character that is not a horizontal whitespace character
3157 \s any whitespace character
3158 \S any character that is not a whitespace character
3159 \v any vertical whitespace character
3160 \V any character that is not a vertical whitespace character
3161 \w any "word" character
3162 \W any "non-word" character
3163
3164 Each pair of escape sequences partitions the complete set of characters
3165 into two disjoint sets. Any given character matches one, and only one,
3166 of each pair.
3167
3168 These character type sequences can appear both inside and outside char-
3169 acter classes. They each match one character of the appropriate type.
3170 If the current matching point is at the end of the subject string, all
3171 of them fail, since there is no character to match.
3172
3173 For compatibility with Perl, \s does not match the VT character (code
3174 11). This makes it different from the the POSIX "space" class. The \s
3175 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3176 "use locale;" is included in a Perl script, \s may match the VT charac-
3177 ter. In PCRE, it never does.
3178
3179 In UTF-8 mode, characters with values greater than 128 never match \d,
3180 \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3181 code character property support is available. These sequences retain
3182 their original meanings from before UTF-8 support was available, mainly
3183 for efficiency reasons.
3184
3185 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3186 the other sequences, these do match certain high-valued codepoints in
3187 UTF-8 mode. The horizontal space characters are:
3188
3189 U+0009 Horizontal tab
3190 U+0020 Space
3191 U+00A0 Non-break space
3192 U+1680 Ogham space mark
3193 U+180E Mongolian vowel separator
3194 U+2000 En quad
3195 U+2001 Em quad
3196 U+2002 En space
3197 U+2003 Em space
3198 U+2004 Three-per-em space
3199 U+2005 Four-per-em space
3200 U+2006 Six-per-em space
3201 U+2007 Figure space
3202 U+2008 Punctuation space
3203 U+2009 Thin space
3204 U+200A Hair space
3205 U+202F Narrow no-break space
3206 U+205F Medium mathematical space
3207 U+3000 Ideographic space
3208
3209 The vertical space characters are:
3210
3211 U+000A Linefeed
3212 U+000B Vertical tab
3213 U+000C Formfeed
3214 U+000D Carriage return
3215 U+0085 Next line
3216 U+2028 Line separator
3217 U+2029 Paragraph separator
3218
3219 A "word" character is an underscore or any character less than 256 that
3220 is a letter or digit. The definition of letters and digits is con-
3221 trolled by PCRE's low-valued character tables, and may vary if locale-
3222 specific matching is taking place (see "Locale support" in the pcreapi
3223 page). For example, in a French locale such as "fr_FR" in Unix-like
3224 systems, or "french" in Windows, some character codes greater than 128
3225 are used for accented letters, and these are matched by \w. The use of
3226 locales with Unicode is discouraged.
3227
3228 Newline sequences
3229
3230 Outside a character class, by default, the escape sequence \R matches
3231 any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3232 mode \R is equivalent to the following:
3233
3234 (?>\r\n|\n|\x0b|\f|\r|\x85)
3235
3236 This is an example of an "atomic group", details of which are given
3237 below. This particular group matches either the two-character sequence
3238 CR followed by LF, or one of the single characters LF (linefeed,
3239 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3240 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3241 is treated as a single unit that cannot be split.
3242
3243 In UTF-8 mode, two additional characters whose codepoints are greater
3244 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3245 rator, U+2029). Unicode character property support is not needed for
3246 these characters to be recognized.
3247
3248 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3249 the complete set of Unicode line endings) by setting the option
3250 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3251 (BSR is an abbrevation for "backslash R".) This can be made the default
3252 when PCRE is built; if this is the case, the other behaviour can be
3253 requested via the PCRE_BSR_UNICODE option. It is also possible to
3254 specify these settings by starting a pattern string with one of the
3255 following sequences:
3256
3257 (*BSR_ANYCRLF) CR, LF, or CRLF only
3258 (*BSR_UNICODE) any Unicode newline sequence
3259
3260 These override the default and the options given to pcre_compile(), but
3261 they can be overridden by options given to pcre_exec(). Note that these
3262 special settings, which are not Perl-compatible, are recognized only at
3263 the very start of a pattern, and that they must be in upper case. If
3264 more than one of them is present, the last one is used. They can be
3265 combined with a change of newline convention, for example, a pattern
3266 can start with:
3267
3268 (*ANY)(*BSR_ANYCRLF)
3269
3270 Inside a character class, \R matches the letter "R".
3271
3272 Unicode character properties
3273
3274 When PCRE is built with Unicode character property support, three addi-
3275 tional escape sequences that match characters with specific properties
3276 are available. When not in UTF-8 mode, these sequences are of course
3277 limited to testing characters whose codepoints are less than 256, but
3278 they do work in this mode. The extra escape sequences are:
3279
3280 \p{xx} a character with the xx property
3281 \P{xx} a character without the xx property
3282 \X an extended Unicode sequence
3283
3284 The property names represented by xx above are limited to the Unicode
3285 script names, the general category properties, and "Any", which matches
3286 any character (including newline). Other properties such as "InMusical-
3287 Symbols" are not currently supported by PCRE. Note that \P{Any} does
3288 not match any characters, so always causes a match failure.
3289
3290 Sets of Unicode characters are defined as belonging to certain scripts.
3291 A character from one of these sets can be matched using a script name.
3292 For example:
3293
3294 \p{Greek}
3295 \P{Han}
3296
3297 Those that are not part of an identified script are lumped together as
3298 "Common". The current list of scripts is:
3299
3300 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
3301 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
3302 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3303 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
3304 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
3305 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
3306 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
3307 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
3308 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3309
3310 Each character has exactly one general category property, specified by
3311 a two-letter abbreviation. For compatibility with Perl, negation can be
3312 specified by including a circumflex between the opening brace and the
3313 property name. For example, \p{^Lu} is the same as \P{Lu}.
3314
3315 If only one letter is specified with \p or \P, it includes all the gen-
3316 eral category properties that start with that letter. In this case, in
3317 the absence of negation, the curly brackets in the escape sequence are
3318 optional; these two examples have the same effect:
3319
3320 \p{L}
3321 \pL
3322
3323 The following general category property codes are supported:
3324
3325 C Other
3326 Cc Control
3327 Cf Format
3328 Cn Unassigned
3329 Co Private use
3330 Cs Surrogate
3331
3332 L Letter
3333 Ll Lower case letter
3334 Lm Modifier letter
3335 Lo Other letter
3336 Lt Title case letter
3337 Lu Upper case letter
3338
3339 M Mark
3340 Mc Spacing mark
3341 Me Enclosing mark
3342 Mn Non-spacing mark
3343
3344 N Number
3345 Nd Decimal number
3346 Nl Letter number
3347 No Other number
3348
3349 P Punctuation
3350 Pc Connector punctuation
3351 Pd Dash punctuation
3352 Pe Close punctuation
3353 Pf Final punctuation
3354 Pi Initial punctuation
3355 Po Other punctuation
3356 Ps Open punctuation
3357
3358 S Symbol
3359 Sc Currency symbol
3360 Sk Modifier symbol
3361 Sm Mathematical symbol
3362 So Other symbol
3363
3364 Z Separator
3365 Zl Line separator
3366 Zp Paragraph separator
3367 Zs Space separator
3368
3369 The special property L& is also supported: it matches a character that
3370 has the Lu, Ll, or Lt property, in other words, a letter that is not
3371 classified as a modifier or "other".
3372
3373 The Cs (Surrogate) property applies only to characters in the range
3374 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3375 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3376 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3377 the pcreapi page).
3378
3379 The long synonyms for these properties that Perl supports (such as
3380 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3381 any of these properties with "Is".
3382
3383 No character that is in the Unicode table has the Cn (unassigned) prop-
3384 erty. Instead, this property is assumed for any code point that is not
3385 in the Unicode table.
3386
3387 Specifying caseless matching does not affect these escape sequences.
3388 For example, \p{Lu} always matches only upper case letters.
3389
3390 The \X escape matches any number of Unicode characters that form an
3391 extended Unicode sequence. \X is equivalent to
3392
3393 (?>\PM\pM*)
3394
3395 That is, it matches a character without the "mark" property, followed
3396 by zero or more characters with the "mark" property, and treats the
3397 sequence as an atomic group (see below). Characters with the "mark"
3398 property are typically accents that affect the preceding character.
3399 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3400 matches any one character.
3401
3402 Matching characters by Unicode property is not fast, because PCRE has
3403 to search a structure that contains data for over fifteen thousand
3404 characters. That is why the traditional escape sequences such as \d and
3405 \w do not use Unicode properties in PCRE.
3406
3407 Resetting the match start
3408
3409 The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3410 ously matched characters not to be included in the final matched
3411 sequence. For example, the pattern:
3412
3413 foo\Kbar
3414
3415 matches "foobar", but reports that it has matched "bar". This feature
3416 is similar to a lookbehind assertion (described below). However, in
3417 this case, the part of the subject before the real match does not have
3418 to be of fixed length, as lookbehind assertions do. The use of \K does
3419 not interfere with the setting of captured substrings. For example,
3420 when the pattern
3421
3422 (foo)\Kbar
3423
3424 matches "foobar", the first substring is still set to "foo".
3425
3426 Simple assertions
3427
3428 The final use of backslash is for certain simple assertions. An asser-
3429 tion specifies a condition that has to be met at a particular point in
3430 a match, without consuming any characters from the subject string. The
3431 use of subpatterns for more complicated assertions is described below.
3432 The backslashed assertions are:
3433
3434 \b matches at a word boundary
3435 \B matches when not at a word boundary
3436 \A matches at the start of the subject
3437 \Z matches at the end of the subject
3438 also matches before a newline at the end of the subject
3439 \z matches only at the end of the subject
3440 \G matches at the first matching position in the subject
3441
3442 These assertions may not appear in character classes (but note that \b
3443 has a different meaning, namely the backspace character, inside a char-
3444 acter class).
3445
3446 A word boundary is a position in the subject string where the current
3447 character and the previous character do not both match \w or \W (i.e.
3448 one matches \w and the other matches \W), or the start or end of the
3449 string if the first or last character matches \w, respectively.
3450
3451 The \A, \Z, and \z assertions differ from the traditional circumflex
3452 and dollar (described in the next section) in that they only ever match
3453 at the very start and end of the subject string, whatever options are
3454 set. Thus, they are independent of multiline mode. These three asser-
3455 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3456 affect only the behaviour of the circumflex and dollar metacharacters.
3457 However, if the startoffset argument of pcre_exec() is non-zero, indi-
3458 cating that matching is to start at a point other than the beginning of
3459 the subject, \A can never match. The difference between \Z and \z is
3460 that \Z matches before a newline at the end of the string as well as at
3461 the very end, whereas \z matches only at the end.
3462
3463 The \G assertion is true only when the current matching position is at
3464 the start point of the match, as specified by the startoffset argument
3465 of pcre_exec(). It differs from \A when the value of startoffset is
3466 non-zero. By calling pcre_exec() multiple times with appropriate argu-
3467 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3468 mentation where \G can be useful.
3469
3470 Note, however, that PCRE's interpretation of \G, as the start of the
3471 current match, is subtly different from Perl's, which defines it as the
3472 end of the previous match. In Perl, these can be different when the
3473 previously matched string was empty. Because PCRE does just one match
3474 at a time, it cannot reproduce this behaviour.
3475
3476 If all the alternatives of a pattern begin with \G, the expression is
3477 anchored to the starting match position, and the "anchored" flag is set
3478 in the compiled regular expression.
3479
3480
3481 CIRCUMFLEX AND DOLLAR
3482
3483 Outside a character class, in the default matching mode, the circumflex
3484 character is an assertion that is true only if the current matching
3485 point is at the start of the subject string. If the startoffset argu-
3486 ment of pcre_exec() is non-zero, circumflex can never match if the
3487 PCRE_MULTILINE option is unset. Inside a character class, circumflex
3488 has an entirely different meaning (see below).
3489
3490 Circumflex need not be the first character of the pattern if a number
3491 of alternatives are involved, but it should be the first thing in each
3492 alternative in which it appears if the pattern is ever to match that
3493 branch. If all possible alternatives start with a circumflex, that is,
3494 if the pattern is constrained to match only at the start of the sub-
3495 ject, it is said to be an "anchored" pattern. (There are also other
3496 constructs that can cause a pattern to be anchored.)
3497
3498 A dollar character is an assertion that is true only if the current
3499 matching point is at the end of the subject string, or immediately
3500 before a newline at the end of the string (by default). Dollar need not
3501 be the last character of the pattern if a number of alternatives are
3502 involved, but it should be the last item in any branch in which it
3503 appears. Dollar has no special meaning in a character class.
3504
3505 The meaning of dollar can be changed so that it matches only at the
3506 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3507 compile time. This does not affect the \Z assertion.
3508
3509 The meanings of the circumflex and dollar characters are changed if the
3510 PCRE_MULTILINE option is set. When this is the case, a circumflex
3511 matches immediately after internal newlines as well as at the start of
3512 the subject string. It does not match after a newline that ends the
3513 string. A dollar matches before any newlines in the string, as well as
3514 at the very end, when PCRE_MULTILINE is set. When newline is specified
3515 as the two-character sequence CRLF, isolated CR and LF characters do
3516 not indicate newlines.
3517
3518 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3519 (where \n represents a newline) in multiline mode, but not otherwise.
3520 Consequently, patterns that are anchored in single line mode because
3521 all branches start with ^ are not anchored in multiline mode, and a
3522 match for circumflex is possible when the startoffset argument of
3523 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3524 PCRE_MULTILINE is set.
3525
3526 Note that the sequences \A, \Z, and \z can be used to match the start
3527 and end of the subject in both modes, and if all branches of a pattern
3528 start with \A it is always anchored, whether or not PCRE_MULTILINE is
3529 set.
3530
3531
3532 FULL STOP (PERIOD, DOT)
3533
3534 Outside a character class, a dot in the pattern matches any one charac-
3535 ter in the subject string except (by default) a character that signi-
3536 fies the end of a line. In UTF-8 mode, the matched character may be
3537 more than one byte long.
3538
3539 When a line ending is defined as a single character, dot never matches
3540 that character; when the two-character sequence CRLF is used, dot does
3541 not match CR if it is immediately followed by LF, but otherwise it
3542 matches all characters (including isolated CRs and LFs). When any Uni-
3543 code line endings are being recognized, dot does not match CR or LF or
3544 any of the other line ending characters.
3545
3546 The behaviour of dot with regard to newlines can be changed. If the
3547 PCRE_DOTALL option is set, a dot matches any one character, without
3548 exception. If the two-character sequence CRLF is present in the subject
3549 string, it takes two dots to match it.
3550
3551 The handling of dot is entirely independent of the handling of circum-
3552 flex and dollar, the only relationship being that they both involve
3553 newlines. Dot has no special meaning in a character class.
3554
3555
3556 MATCHING A SINGLE BYTE
3557
3558 Outside a character class, the escape sequence \C matches any one byte,
3559 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3560 line-ending characters. The feature is provided in Perl in order to
3561 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3562 acters into individual bytes, what remains in the string may be a mal-
3563 formed UTF-8 string. For this reason, the \C escape sequence is best
3564 avoided.
3565
3566 PCRE does not allow \C to appear in lookbehind assertions (described
3567 below), because in UTF-8 mode this would make it impossible to calcu-
3568 late the length of the lookbehind.
3569
3570
3571 SQUARE BRACKETS AND CHARACTER CLASSES
3572
3573 An opening square bracket introduces a character class, terminated by a
3574 closing square bracket. A closing square bracket on its own is not spe-
3575 cial. If a closing square bracket is required as a member of the class,
3576 it should be the first data character in the class (after an initial
3577 circumflex, if present) or escaped with a backslash.
3578
3579 A character class matches a single character in the subject. In UTF-8
3580 mode, the character may occupy more than one byte. A matched character
3581 must be in the set of characters defined by the class, unless the first
3582 character in the class definition is a circumflex, in which case the
3583 subject character must not be in the set defined by the class. If a
3584 circumflex is actually required as a member of the class, ensure it is
3585 not the first character, or escape it with a backslash.
3586
3587 For example, the character class [aeiou] matches any lower case vowel,
3588 while [^aeiou] matches any character that is not a lower case vowel.
3589 Note that a circumflex is just a convenient notation for specifying the
3590 characters that are in the class by enumerating those that are not. A
3591 class that starts with a circumflex is not an assertion: it still con-
3592 sumes a character from the subject string, and therefore it fails if
3593 the current pointer is at the end of the string.
3594
3595 In UTF-8 mode, characters with values greater than 255 can be included
3596 in a class as a literal string of bytes, or by using the \x{ escaping
3597 mechanism.
3598
3599 When caseless matching is set, any letters in a class represent both
3600 their upper case and lower case versions, so for example, a caseless
3601 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3602 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3603 understands the concept of case for characters whose values are less
3604 than 128, so caseless matching is always possible. For characters with
3605 higher values, the concept of case is supported if PCRE is compiled
3606 with Unicode property support, but not otherwise. If you want to use
3607 caseless matching for characters 128 and above, you must ensure that
3608 PCRE is compiled with Unicode property support as well as with UTF-8
3609 support.
3610
3611 Characters that might indicate line breaks are never treated in any
3612 special way when matching character classes, whatever line-ending
3613 sequence is in use, and whatever setting of the PCRE_DOTALL and
3614 PCRE_MULTILINE options is used. A class such as [^a] always matches one
3615 of these characters.
3616
3617 The minus (hyphen) character can be used to specify a range of charac-
3618 ters in a character class. For example, [d-m] matches any letter
3619 between d and m, inclusive. If a minus character is required in a
3620 class, it must be escaped with a backslash or appear in a position
3621 where it cannot be interpreted as indicating a range, typically as the
3622 first or last character in the class.
3623
3624 It is not possible to have the literal character "]" as the end charac-
3625 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3626 two characters ("W" and "-") followed by a literal string "46]", so it
3627 would match "W46]" or "-46]". However, if the "]" is escaped with a
3628 backslash it is interpreted as the end of range, so [W-\]46] is inter-
3629 preted as a class containing a range followed by two other characters.
3630 The octal or hexadecimal representation of "]" can also be used to end
3631 a range.
3632
3633 Ranges operate in the collating sequence of character values. They can
3634 also be used for characters specified numerically, for example
3635 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3636 are greater than 255, for example [\x{100}-\x{2ff}].
3637
3638 If a range that includes letters is used when caseless matching is set,
3639 it matches the letters in either case. For example, [W-c] is equivalent
3640 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3641 character tables for a French locale are in use, [\xc8-\xcb] matches
3642 accented E characters in both cases. In UTF-8 mode, PCRE supports the
3643 concept of case for characters with values greater than 128 only when
3644 it is compiled with Unicode property support.
3645
3646 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3647 in a character class, and add the characters that they match to the
3648 class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3649 flex can conveniently be used with the upper case character types to
3650 specify a more restricted set of characters than the matching lower
3651 case type. For example, the class [^\W_] matches any letter or digit,
3652 but not underscore.
3653
3654 The only metacharacters that are recognized in character classes are
3655 backslash, hyphen (only where it can be interpreted as specifying a
3656 range), circumflex (only at the start), opening square bracket (only
3657 when it can be interpreted as introducing a POSIX class name - see the
3658 next section), and the terminating closing square bracket. However,
3659 escaping other non-alphanumeric characters does no harm.
3660
3661
3662 POSIX CHARACTER CLASSES
3663
3664 Perl supports the POSIX notation for character classes. This uses names
3665 enclosed by [: and :] within the enclosing square brackets. PCRE also
3666 supports this notation. For example,
3667
3668 [01[:alpha:]%]
3669
3670 matches "0", "1", any alphabetic character, or "%". The supported class
3671 names are
3672
3673 alnum letters and digits
3674 alpha letters
3675 ascii character codes 0 - 127
3676 blank space or tab only
3677 cntrl control characters
3678 digit decimal digits (same as \d)
3679 graph printing characters, excluding space
3680 lower lower case letters
3681 print printing characters, including space
3682 punct printing characters, excluding letters and digits
3683 space white space (not quite the same as \s)
3684 upper upper case letters
3685 word "word" characters (same as \w)
3686 xdigit hexadecimal digits
3687
3688 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3689 and space (32). Notice that this list includes the VT character (code
3690 11). This makes "space" different to \s, which does not include VT (for
3691 Perl compatibility).
3692
3693 The name "word" is a Perl extension, and "blank" is a GNU extension
3694 from Perl 5.8. Another Perl extension is negation, which is indicated
3695 by a ^ character after the colon. For example,
3696
3697 [12[:^digit:]]
3698
3699 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3700 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3701 these are not supported, and an error is given if they are encountered.
3702
3703 In UTF-8 mode, characters with values greater than 128 do not match any
3704 of the POSIX character classes.
3705
3706
3707 VERTICAL BAR
3708
3709 Vertical bar characters are used to separate alternative patterns. For
3710 example, the pattern
3711
3712 gilbert|sullivan
3713
3714 matches either "gilbert" or "sullivan". Any number of alternatives may
3715 appear, and an empty alternative is permitted (matching the empty
3716 string). The matching process tries each alternative in turn, from left
3717 to right, and the first one that succeeds is used. If the alternatives
3718 are within a subpattern (defined below), "succeeds" means matching the
3719 rest of the main pattern as well as the alternative in the subpattern.
3720
3721
3722 INTERNAL OPTION SETTING
3723
3724 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3725 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
3726 within the pattern by a sequence of Perl option letters enclosed
3727 between "(?" and ")". The option letters are
3728
3729 i for PCRE_CASELESS
3730 m for PCRE_MULTILINE
3731 s for PCRE_DOTALL
3732 x for PCRE_EXTENDED
3733
3734 For example, (?im) sets caseless, multiline matching. It is also possi-
3735 ble to unset these options by preceding the letter with a hyphen, and a
3736 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3737 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3738 is also permitted. If a letter appears both before and after the
3739 hyphen, the option is unset.
3740
3741 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3742 can be changed in the same way as the Perl-compatible options by using
3743 the characters J, U and X respectively.
3744
3745 When an option change occurs at top level (that is, not inside subpat-
3746 tern parentheses), the change applies to the remainder of the pattern
3747 that follows. If the change is placed right at the start of a pattern,
3748 PCRE extracts it into the global options (and it will therefore show up
3749 in data extracted by the pcre_fullinfo() function).
3750
3751 An option change within a subpattern (see below for a description of
3752 subpatterns) affects only that part of the current pattern that follows
3753 it, so
3754
3755 (a(?i)b)c
3756
3757 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3758 used). By this means, options can be made to have different settings
3759 in different parts of the pattern. Any changes made in one alternative
3760 do carry on into subsequent branches within the same subpattern. For
3761 example,
3762
3763 (a(?i)b|c)
3764
3765 matches "ab", "aB", "c", and "C", even though when matching "C" the
3766 first branch is abandoned before the option setting. This is because
3767 the effects of option settings happen at compile time. There would be
3768 some very weird behaviour otherwise.
3769
3770 Note: There are other PCRE-specific options that can be set by the
3771 application when the compile or match functions are called. In some
3772 cases the pattern can contain special leading sequences to override
3773 what the application has set or what has been defaulted. Details are
3774 given in the section entitled "Newline sequences" above.
3775
3776
3777 SUBPATTERNS
3778
3779 Subpatterns are delimited by parentheses (round brackets), which can be
3780 nested. Turning part of a pattern into a subpattern does two things:
3781
3782 1. It localizes a set of alternatives. For example, the pattern
3783
3784 cat(aract|erpillar|)
3785
3786 matches one of the words "cat", "cataract", or "caterpillar". Without
3787 the parentheses, it would match "cataract", "erpillar" or an empty
3788 string.
3789
3790 2. It sets up the subpattern as a capturing subpattern. This means
3791 that, when the whole pattern matches, that portion of the subject
3792 string that matched the subpattern is passed back to the caller via the
3793 ovector argument of pcre_exec(). Opening parentheses are counted from
3794 left to right (starting from 1) to obtain numbers for the capturing
3795 subpatterns.
3796
3797 For example, if the string "the red king" is matched against the pat-
3798 tern
3799
3800 the ((red|white) (king|queen))
3801
3802 the captured substrings are "red king", "red", and "king", and are num-
3803 bered 1, 2, and 3, respectively.
3804
3805 The fact that plain parentheses fulfil two functions is not always
3806 helpful. There are often times when a grouping subpattern is required
3807 without a capturing requirement. If an opening parenthesis is followed
3808 by a question mark and a colon, the subpattern does not do any captur-
3809 ing, and is not counted when computing the number of any subsequent
3810 capturing subpatterns. For example, if the string "the white queen" is
3811 matched against the pattern
3812
3813 the ((?:red|white) (king|queen))
3814
3815 the captured substrings are "white queen" and "queen", and are numbered
3816 1 and 2. The maximum number of capturing subpatterns is 65535.
3817
3818 As a convenient shorthand, if any option settings are required at the
3819 start of a non-capturing subpattern, the option letters may appear
3820 between the "?" and the ":". Thus the two patterns
3821
3822 (?i:saturday|sunday)
3823 (?:(?i)saturday|sunday)
3824
3825 match exactly the same set of strings. Because alternative branches are
3826 tried from left to right, and options are not reset until the end of
3827 the subpattern is reached, an option setting in one branch does affect
3828 subsequent branches, so the above patterns match "SUNDAY" as well as
3829 "Saturday".
3830
3831
3832 DUPLICATE SUBPATTERN NUMBERS
3833
3834 Perl 5.10 introduced a feature whereby each alternative in a subpattern
3835 uses the same numbers for its capturing parentheses. Such a subpattern
3836 starts with (?| and is itself a non-capturing subpattern. For example,
3837 consider this pattern:
3838
3839 (?|(Sat)ur|(Sun))day
3840
3841 Because the two alternatives are inside a (?| group, both sets of cap-
3842 turing parentheses are numbered one. Thus, when the pattern matches,
3843 you can look at captured substring number one, whichever alternative
3844 matched. This construct is useful when you want to capture part, but
3845 not all, of one of a number of alternatives. Inside a (?| group, paren-
3846 theses are numbered as usual, but the number is reset at the start of
3847 each branch. The numbers of any capturing buffers that follow the sub-
3848 pattern start after the highest number used in any branch. The follow-
3849 ing example is taken from the Perl documentation. The numbers under-
3850 neath show in which buffer the captured content will be stored.
3851
3852 # before ---------------branch-reset----------- after
3853 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3854 # 1 2 2 3 2 3 4
3855
3856 A backreference or a recursive call to a numbered subpattern always
3857 refers to the first one in the pattern with the given number.
3858
3859 An alternative approach to using this "branch reset" feature is to use
3860 duplicate named subpatterns, as described in the next section.
3861
3862
3863 NAMED SUBPATTERNS
3864
3865 Identifying capturing parentheses by number is simple, but it can be
3866 very hard to keep track of the numbers in complicated regular expres-
3867 sions. Furthermore, if an expression is modified, the numbers may
3868 change. To help with this difficulty, PCRE supports the naming of sub-
3869 patterns. This feature was not added to Perl until release 5.10. Python
3870 had the feature earlier, and PCRE introduced it at release 4.0, using
3871 the Python syntax. PCRE now supports both the Perl and the Python syn-
3872 tax.
3873
3874 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
3875 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
3876 to capturing parentheses from other parts of the pattern, such as back-
3877 references, recursion, and conditions, can be made by name as well as
3878 by number.
3879
3880 Names consist of up to 32 alphanumeric characters and underscores.
3881 Named capturing parentheses are still allocated numbers as well as
3882 names, exactly as if the names were not present. The PCRE API provides
3883 function calls for extracting the name-to-number translation table from
3884 a compiled pattern. There is also a convenience function for extracting
3885 a captured substring by name.
3886
3887 By default, a name must be unique within a pattern, but it is possible
3888 to relax this constraint by setting the PCRE_DUPNAMES option at compile
3889 time. This can be useful for patterns where only one instance of the
3890 named parentheses can match. Suppose you want to match the name of a
3891 weekday, either as a 3-letter abbreviation or as the full name, and in
3892 both cases you want to extract the abbreviation. This pattern (ignoring
3893 the line breaks) does the job:
3894
3895 (?<DN>Mon|Fri|Sun)(?:day)?|
3896 (?<DN>Tue)(?:sday)?|
3897 (?<DN>Wed)(?:nesday)?|
3898 (?<DN>Thu)(?:rsday)?|
3899 (?<DN>Sat)(?:urday)?
3900
3901 There are five capturing substrings, but only one is ever set after a
3902 match. (An alternative way of solving this problem is to use a "branch
3903 reset" subpattern, as described in the previous section.)
3904
3905 The convenience function for extracting the data by name returns the
3906 substring for the first (and in this example, the only) subpattern of
3907 that name that matched. This saves searching to find which numbered
3908 subpattern it was. If you make a reference to a non-unique named sub-
3909 pattern from elsewhere in the pattern, the one that corresponds to the
3910 lowest number is used. For further details of the interfaces for han-
3911 dling named subpatterns, see the pcreapi documentation.
3912
3913
3914 REPETITION
3915
3916 Repetition is specified by quantifiers, which can follow any of the
3917 following items:
3918
3919 a literal data character
3920 the dot metacharacter
3921 the \C escape sequence
3922 the \X escape sequence (in UTF-8 mode with Unicode properties)
3923 the \R escape sequence
3924 an escape such as \d that matches a single character
3925 a character class
3926 a back reference (see next section)
3927 a parenthesized subpattern (unless it is an assertion)
3928
3929 The general repetition quantifier specifies a minimum and maximum num-
3930 ber of permitted matches, by giving the two numbers in curly brackets
3931 (braces), separated by a comma. The numbers must be less than 65536,
3932 and the first must be less than or equal to the second. For example:
3933
3934 z{2,4}
3935
3936 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
3937 special character. If the second number is omitted, but the comma is
3938 present, there is no upper limit; if the second number and the comma
3939 are both omitted, the quantifier specifies an exact number of required
3940 matches. Thus
3941
3942 [aeiou]{3,}
3943
3944 matches at least 3 successive vowels, but may match many more, while
3945
3946 \d{8}
3947
3948 matches exactly 8 digits. An opening curly bracket that appears in a
3949 position where a quantifier is not allowed, or one that does not match
3950 the syntax of a quantifier, is taken as a literal character. For exam-
3951 ple, {,6} is not a quantifier, but a literal string of four characters.
3952
3953 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
3954 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
3955 acters, each of which is represented by a two-byte sequence. Similarly,
3956 when Unicode property support is available, \X{3} matches three Unicode
3957 extended sequences, each of which may be several bytes long (and they
3958 may be of different lengths).
3959
3960 The quantifier {0} is permitted, causing the expression to behave as if
3961 the previous item and the quantifier were not present.
3962
3963 For convenience, the three most common quantifiers have single-charac-
3964 ter abbreviations:
3965
3966 * is equivalent to {0,}
3967 + is equivalent to {1,}
3968 ? is equivalent to {0,1}
3969
3970 It is possible to construct infinite loops by following a subpattern
3971 that can match no characters with a quantifier that has no upper limit,
3972 for example:
3973
3974 (a?)*
3975
3976 Earlier versions of Perl and PCRE used to give an error at compile time
3977 for such patterns. However, because there are cases where this can be
3978 useful, such patterns are now accepted, but if any repetition of the
3979 subpattern does in fact match no characters, the loop is forcibly bro-
3980 ken.
3981
3982 By default, the quantifiers are "greedy", that is, they match as much
3983 as possible (up to the maximum number of permitted times), without
3984 causing the rest of the pattern to fail. The classic example of where
3985 this gives problems is in trying to match comments in C programs. These
3986 appear between /* and */ and within the comment, individual * and /
3987 characters may appear. An attempt to match C comments by applying the
3988 pattern
3989
3990 /\*.*\*/
3991
3992 to the string
3993
3994 /* first comment */ not comment /* second comment */
3995
3996 fails, because it matches the entire string owing to the greediness of
3997 the .* item.
3998
3999 However, if a quantifier is followed by a question mark, it ceases to
4000 be greedy, and instead matches the minimum number of times possible, so
4001 the pattern
4002
4003 /\*.*?\*/
4004
4005 does the right thing with the C comments. The meaning of the various
4006 quantifiers is not otherwise changed, just the preferred number of
4007 matches. Do not confuse this use of question mark with its use as a
4008 quantifier in its own right. Because it has two uses, it can sometimes
4009 appear doubled, as in
4010
4011 \d??\d
4012
4013 which matches one digit by preference, but can match two if that is the
4014 only way the rest of the pattern matches.
4015
4016 If the PCRE_UNGREEDY option is set (an option that is not available in
4017 Perl), the quantifiers are not greedy by default, but individual ones
4018 can be made greedy by following them with a question mark. In other
4019 words, it inverts the default behaviour.
4020
4021 When a parenthesized subpattern is quantified with a minimum repeat
4022 count that is greater than 1 or with a limited maximum, more memory is
4023 required for the compiled pattern, in proportion to the size of the
4024 minimum or maximum.
4025
4026 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4027 alent to Perl's /s) is set, thus allowing the dot to match newlines,
4028 the pattern is implicitly anchored, because whatever follows will be
4029 tried against every character position in the subject string, so there
4030 is no point in retrying the overall match at any position after the
4031 first. PCRE normally treats such a pattern as though it were preceded
4032 by \A.
4033
4034 In cases where it is known that the subject string contains no new-
4035 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
4036 mization, or alternatively using ^ to indicate anchoring explicitly.
4037
4038 However, there is one situation where the optimization cannot be used.
4039 When .* is inside capturing parentheses that are the subject of a
4040 backreference elsewhere in the pattern, a match at the start may fail
4041 where a later one succeeds. Consider, for example:
4042
4043 (.*)abc\1
4044
4045 If the subject is "xyz123abc123" the match point is the fourth charac-
4046 ter. For this reason, such a pattern is not implicitly anchored.
4047
4048 When a capturing subpattern is repeated, the value captured is the sub-
4049 string that matched the final iteration. For example, after
4050
4051 (tweedle[dume]{3}\s*)+
4052
4053 has matched "tweedledum tweedledee" the value of the captured substring
4054 is "tweedledee". However, if there are nested capturing subpatterns,
4055 the corresponding captured values may have been set in previous itera-
4056 tions. For example, after
4057
4058 /(a|(b))+/
4059
4060 matches "aba" the value of the second captured substring is "b".
4061
4062
4063 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4064
4065 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4066 repetition, failure of what follows normally causes the repeated item
4067 to be re-evaluated to see if a different number of repeats allows the
4068 rest of the pattern to match. Sometimes it is useful to prevent this,
4069 either to change the nature of the match, or to cause it fail earlier
4070 than it otherwise might, when the author of the pattern knows there is
4071 no point in carrying on.
4072
4073 Consider, for example, the pattern \d+foo when applied to the subject
4074 line
4075
4076 123456bar
4077
4078 After matching all 6 digits and then failing to match "foo", the normal
4079 action of the matcher is to try again with only 5 digits matching the
4080 \d+ item, and then with 4, and so on, before ultimately failing.
4081 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4082 the means for specifying that once a subpattern has matched, it is not
4083 to be re-evaluated in this way.
4084
4085 If we use atomic grouping for the previous example, the matcher gives
4086 up immediately on failing to match "foo" the first time. The notation
4087 is a kind of special parenthesis, starting with (?> as in this example:
4088
4089 (?>\d+)foo
4090
4091 This kind of parenthesis "locks up" the part of the pattern it con-
4092 tains once it has matched, and a failure further into the pattern is
4093 prevented from backtracking into it. Backtracking past it to previous
4094 items, however, works as normal.
4095
4096 An alternative description is that a subpattern of this type matches
4097 the string of characters that an identical standalone pattern would
4098 match, if anchored at the current point in the subject string.
4099
4100 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4101 such as the above example can be thought of as a maximizing repeat that
4102 must swallow everything it can. So, while both \d+ and \d+? are pre-
4103 pared to adjust the number of digits they match in order to make the
4104 rest of the pattern match, (?>\d+) can only match an entire sequence of
4105 digits.
4106
4107 Atomic groups in general can of course contain arbitrarily complicated
4108 subpatterns, and can be nested. However, when the subpattern for an
4109 atomic group is just a single repeated item, as in the example above, a
4110 simpler notation, called a "possessive quantifier" can be used. This
4111 consists of an additional + character following a quantifier. Using
4112 this notation, the previous example can be rewritten as
4113
4114 \d++foo
4115
4116 Note that a possessive quantifier can be used with an entire group, for
4117 example:
4118
4119 (abc|xyz){2,3}+
4120
4121 Possessive quantifiers are always greedy; the setting of the
4122 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4123 simpler forms of atomic group. However, there is no difference in the
4124 meaning of a possessive quantifier and the equivalent atomic group,
4125 though there may be a performance difference; possessive quantifiers
4126 should be slightly faster.
4127
4128 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4129 tax. Jeffrey Friedl originated the idea (and the name) in the first
4130 edition of his book. Mike McCloskey liked it, so implemented it when he
4131 built Sun's Java package, and PCRE copied it from there. It ultimately
4132 found its way into Perl at release 5.10.
4133
4134 PCRE has an optimization that automatically "possessifies" certain sim-
4135 ple pattern constructs. For example, the sequence A+B is treated as
4136 A++B because there is no point in backtracking into a sequence of A's
4137 when B must follow.
4138
4139 When a pattern contains an unlimited repeat inside a subpattern that
4140 can itself be repeated an unlimited number of times, the use of an
4141 atomic group is the only way to avoid some failing matches taking a
4142 very long time indeed. The pattern
4143
4144 (\D+|<\d+>)*[!?]
4145
4146 matches an unlimited number of substrings that either consist of non-
4147 digits, or digits enclosed in <>, followed by either ! or ?. When it
4148 matches, it runs quickly. However, if it is applied to
4149
4150 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4151
4152 it takes a long time before reporting failure. This is because the
4153 string can be divided between the internal \D+ repeat and the external
4154 * repeat in a large number of ways, and all have to be tried. (The
4155 example uses [!?] rather than a single character at the end, because
4156 both PCRE and Perl have an optimization that allows for fast failure
4157 when a single character is used. They remember the last single charac-
4158 ter that is required for a match, and fail early if it is not present
4159 in the string.) If the pattern is changed so that it uses an atomic
4160 group, like this:
4161
4162 ((?>\D+)|<\d+>)*[!?]
4163
4164 sequences of non-digits cannot be broken, and failure happens quickly.
4165
4166
4167 BACK REFERENCES
4168
4169 Outside a character class, a backslash followed by a digit greater than
4170 0 (and possibly further digits) is a back reference to a capturing sub-
4171 pattern earlier (that is, to its left) in the pattern, provided there
4172 have been that many previous capturing left parentheses.
4173
4174 However, if the decimal number following the backslash is less than 10,
4175 it is always taken as a back reference, and causes an error only if
4176 there are not that many capturing left parentheses in the entire pat-
4177 tern. In other words, the parentheses that are referenced need not be
4178 to the left of the reference for numbers less than 10. A "forward back
4179 reference" of this type can make sense when a repetition is involved
4180 and the subpattern to the right has participated in an earlier itera-
4181 tion.
4182
4183 It is not possible to have a numerical "forward back reference" to a
4184 subpattern whose number is 10 or more using this syntax because a
4185 sequence such as \50 is interpreted as a character defined in octal.
4186 See the subsection entitled "Non-printing characters" above for further
4187 details of the handling of digits following a backslash. There is no
4188 such problem when named parentheses are used. A back reference to any
4189 subpattern is possible using named parentheses (see below).
4190
4191 Another way of avoiding the ambiguity inherent in the use of digits
4192 following a backslash is to use the \g escape sequence, which is a fea-
4193 ture introduced in Perl 5.10. This escape must be followed by an
4194 unsigned number or a negative number, optionally enclosed in braces.
4195 These examples are all identical:
4196
4197 (ring), \1
4198 (ring), \g1
4199 (ring), \g{1}
4200
4201 An unsigned number specifies an absolute reference without the ambigu-
4202 ity that is present in the older syntax. It is also useful when literal
4203 digits follow the reference. A negative number is a relative reference.
4204 Consider this example:
4205
4206 (abc(def)ghi)\g{-1}
4207
4208 The sequence \g{-1} is a reference to the most recently started captur-
4209 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
4210 \g{-2} would be equivalent to \1. The use of relative references can be
4211 helpful in long patterns, and also in patterns that are created by
4212 joining together fragments that contain references within themselves.
4213
4214 A back reference matches whatever actually matched the capturing sub-
4215 pattern in the current subject string, rather than anything matching
4216 the subpattern itself (see "Subpatterns as subroutines" below for a way
4217 of doing that). So the pattern
4218
4219 (sens|respons)e and \1ibility
4220
4221 matches "sense and sensibility" and "response and responsibility", but
4222 not "sense and responsibility". If caseful matching is in force at the
4223 time of the back reference, the case of letters is relevant. For exam-
4224 ple,
4225
4226 ((?i)rah)\s+\1
4227
4228 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4229 original capturing subpattern is matched caselessly.
4230
4231 There are several different ways of writing back references to named
4232 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4233 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4234 unified back reference syntax, in which \g can be used for both numeric
4235 and named references, is also supported. We could rewrite the above
4236 example in any of the following ways:
4237
4238 (?<p1>(?i)rah)\s+\k<p1>
4239 (?'p1'(?i)rah)\s+\k{p1}
4240 (?P<p1>(?i)rah)\s+(?P=p1)
4241 (?<p1>(?i)rah)\s+\g{p1}
4242
4243 A subpattern that is referenced by name may appear in the pattern
4244 before or after the reference.
4245
4246 There may be more than one back reference to the same subpattern. If a
4247 subpattern has not actually been used in a particular match, any back
4248 references to it always fail. For example, the pattern
4249
4250 (a|(bc))\2
4251
4252 always fails if it starts to match "a" rather than "bc". Because there
4253 may be many capturing parentheses in a pattern, all digits following
4254 the backslash are taken as part of a potential back reference number.
4255 If the pattern continues with a digit character, some delimiter must be
4256 used to terminate the back reference. If the PCRE_EXTENDED option is
4257 set, this can be whitespace. Otherwise an empty comment (see "Com-
4258 ments" below) can be used.
4259
4260 A back reference that occurs inside the parentheses to which it refers
4261 fails when the subpattern is first used, so, for example, (a\1) never
4262 matches. However, such references can be useful inside repeated sub-
4263 patterns. For example, the pattern
4264
4265 (a|b\1)+
4266
4267 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4268 ation of the subpattern, the back reference matches the character
4269 string corresponding to the previous iteration. In order for this to
4270 work, the pattern must be such that the first iteration does not need
4271 to match the back reference. This can be done using alternation, as in
4272 the example above, or by a quantifier with a minimum of zero.
4273
4274
4275 ASSERTIONS
4276
4277 An assertion is a test on the characters following or preceding the
4278 current matching point that does not actually consume any characters.
4279 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
4280 described above.
4281
4282 More complicated assertions are coded as subpatterns. There are two
4283 kinds: those that look ahead of the current position in the subject
4284 string, and those that look behind it. An assertion subpattern is
4285 matched in the normal way, except that it does not cause the current
4286 matching position to be changed.
4287
4288 Assertion subpatterns are not capturing subpatterns, and may not be
4289 repeated, because it makes no sense to assert the same thing several
4290 times. If any kind of assertion contains capturing subpatterns within
4291 it, these are counted for the purposes of numbering the capturing sub-
4292 patterns in the whole pattern. However, substring capturing is carried
4293 out only for positive assertions, because it does not make sense for
4294 negative assertions.
4295
4296 Lookahead assertions
4297
4298 Lookahead assertions start with (?= for positive assertions and (?! for
4299 negative assertions. For example,
4300
4301 \w+(?=;)
4302
4303 matches a word followed by a semicolon, but does not include the semi-
4304 colon in the match, and
4305
4306 foo(?!bar)
4307
4308 matches any occurrence of "foo" that is not followed by "bar". Note
4309 that the apparently similar pattern
4310
4311 (?!foo)bar
4312
4313 does not find an occurrence of "bar" that is preceded by something
4314 other than "foo"; it finds any occurrence of "bar" whatsoever, because
4315 the assertion (?!foo) is always true when the next three characters are
4316 "bar". A lookbehind assertion is needed to achieve the other effect.
4317
4318 If you want to force a matching failure at some point in a pattern, the
4319 most convenient way to do it is with (?!) because an empty string
4320 always matches, so an assertion that requires there not to be an empty
4321 string must always fail.
4322
4323 Lookbehind assertions
4324
4325 Lookbehind assertions start with (?<= for positive assertions and (?<!
4326 for negative assertions. For example,
4327
4328 (?<!foo)bar
4329
4330 does find an occurrence of "bar" that is not preceded by "foo". The
4331 contents of a lookbehind assertion are restricted such that all the
4332 strings it matches must have a fixed length. However, if there are sev-
4333 eral top-level alternatives, they do not all have to have the same
4334 fixed length. Thus
4335
4336 (?<=bullock|donkey)
4337
4338 is permitted, but
4339
4340 (?<!dogs?|cats?)
4341
4342 causes an error at compile time. Branches that match different length
4343 strings are permitted only at the top level of a lookbehind assertion.
4344 This is an extension compared with Perl (at least for 5.8), which
4345 requires all branches to match the same length of string. An assertion
4346 such as
4347
4348 (?<=ab(c|de))
4349
4350 is not permitted, because its single top-level branch can match two
4351 different lengths, but it is acceptable if rewritten to use two top-
4352 level branches:
4353
4354 (?<=abc|abde)
4355
4356 In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4357 instead of a lookbehind assertion; this is not restricted to a fixed-
4358 length.
4359
4360 The implementation of lookbehind assertions is, for each alternative,
4361 to temporarily move the current position back by the fixed length and
4362 then try to match. If there are insufficient characters before the cur-
4363 rent position, the assertion fails.
4364
4365 PCRE does not allow the \C escape (which matches a single byte in UTF-8
4366 mode) to appear in lookbehind assertions, because it makes it impossi-
4367 ble to calculate the length of the lookbehind. The \X and \R escapes,
4368 which can match different numbers of bytes, are also not permitted.
4369
4370 Possessive quantifiers can be used in conjunction with lookbehind
4371 assertions to specify efficient matching at the end of the subject
4372 string. Consider a simple pattern such as
4373
4374 abcd$
4375
4376 when applied to a long string that does not match. Because matching
4377 proceeds from left to right, PCRE will look for each "a" in the subject
4378 and then see if what follows matches the rest of the pattern. If the
4379 pattern is specified as
4380
4381 ^.*abcd$
4382
4383 the initial .* matches the entire string at first, but when this fails
4384 (because there is no following "a"), it backtracks to match all but the
4385 last character, then all but the last two characters, and so on. Once
4386 again the search for "a" covers the entire string, from right to left,
4387 so we are no better off. However, if the pattern is written as
4388
4389 ^.*+(?<=abcd)
4390
4391 there can be no backtracking for the .*+ item; it can match only the
4392 entire string. The subsequent lookbehind assertion does a single test
4393 on the last four characters. If it fails, the match fails immediately.
4394 For long strings, this approach makes a significant difference to the
4395 processing time.
4396
4397 Using multiple assertions
4398
4399 Several assertions (of any sort) may occur in succession. For example,
4400
4401 (?<=\d{3})(?<!999)foo
4402
4403 matches "foo" preceded by three digits that are not "999". Notice that
4404 each of the assertions is applied independently at the same point in
4405 the subject string. First there is a check that the previous three
4406 characters are all digits, and then there is a check that the same
4407 three characters are not "999". This pattern does not match "foo" pre-
4408 ceded by six characters, the first of which are digits and the last
4409 three of which are not "999". For example, it doesn't match "123abc-
4410 foo". A pattern to do that is
4411
4412 (?<=\d{3}...)(?<!999)foo
4413
4414 This time the first assertion looks at the preceding six characters,
4415 checking that the first three are digits, and then the second assertion
4416 checks that the preceding three characters are not "999".
4417
4418 Assertions can be nested in any combination. For example,
4419
4420 (?<=(?<!foo)bar)baz
4421
4422 matches an occurrence of "baz" that is preceded by "bar" which in turn
4423 is not preceded by "foo", while
4424
4425 (?<=\d{3}(?!999)...)foo
4426
4427 is another pattern that matches "foo" preceded by three digits and any
4428 three characters that are not "999".
4429
4430
4431 CONDITIONAL SUBPATTERNS
4432
4433 It is possible to cause the matching process to obey a subpattern con-
4434 ditionally or to choose between two alternative subpatterns, depending
4435 on the result of an assertion, or whether a previous capturing subpat-
4436 tern matched or not. The two possible forms of conditional subpattern
4437 are
4438
4439 (?(condition)yes-pattern)
4440 (?(condition)yes-pattern|no-pattern)
4441
4442 If the condition is satisfied, the yes-pattern is used; otherwise the
4443 no-pattern (if present) is used. If there are more than two alterna-
4444 tives in the subpattern, a compile-time error occurs.
4445
4446 There are four kinds of condition: references to subpatterns, refer-
4447 ences to recursion, a pseudo-condition called DEFINE, and assertions.
4448
4449 Checking for a used subpattern by number
4450
4451 If the text between the parentheses consists of a sequence of digits,
4452 the condition is true if the capturing subpattern of that number has
4453 previously matched. An alternative notation is to precede the digits
4454 with a plus or minus sign. In this case, the subpattern number is rela-
4455 tive rather than absolute. The most recently opened parentheses can be
4456 referenced by (?(-1), the next most recent by (?(-2), and so on. In
4457 looping constructs it can also make sense to refer to subsequent groups
4458 with constructs such as (?(+2).
4459
4460 Consider the following pattern, which contains non-significant white
4461 space to make it more readable (assume the PCRE_EXTENDED option) and to
4462 divide it into three parts for ease of discussion:
4463
4464 ( \( )? [^()]+ (?(1) \) )
4465
4466 The first part matches an optional opening parenthesis, and if that
4467 character is present, sets it as the first captured substring. The sec-
4468 ond part matches one or more characters that are not parentheses. The
4469 third part is a conditional subpattern that tests whether the first set
4470 of parentheses matched or not. If they did, that is, if subject started
4471 with an opening parenthesis, the condition is true, and so the yes-pat-
4472 tern is executed and a closing parenthesis is required. Otherwise,
4473 since no-pattern is not present, the subpattern matches nothing. In
4474 other words, this pattern matches a sequence of non-parentheses,
4475 optionally enclosed in parentheses.
4476
4477 If you were embedding this pattern in a larger one, you could use a
4478 relative reference:
4479
4480 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
4481
4482 This makes the fragment independent of the parentheses in the larger
4483 pattern.
4484
4485 Checking for a used subpattern by name
4486
4487 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
4488 used subpattern by name. For compatibility with earlier versions of
4489 PCRE, which had this facility before Perl, the syntax (?(name)...) is
4490 also recognized. However, there is a possible ambiguity with this syn-
4491 tax, because subpattern names may consist entirely of digits. PCRE
4492 looks first for a named subpattern; if it cannot find one and the name
4493 consists entirely of digits, PCRE looks for a subpattern of that num-
4494 ber, which must be greater than zero. Using subpattern names that con-
4495 sist entirely of digits is not recommended.
4496
4497 Rewriting the above example to use a named subpattern gives this:
4498
4499 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
4500
4501
4502 Checking for pattern recursion
4503
4504 If the condition is the string (R), and there is no subpattern with the
4505 name R, the condition is true if a recursive call to the whole pattern
4506 or any subpattern has been made. If digits or a name preceded by amper-
4507 sand follow the letter R, for example:
4508
4509 (?(R3)...) or (?(R&name)...)
4510
4511 the condition is true if the most recent recursion is into the subpat-
4512 tern whose number or name is given. This condition does not check the
4513 entire recursion stack.
4514
4515 At "top level", all these recursion test conditions are false. Recur-
4516 sive patterns are described below.
4517
4518 Defining subpatterns for use by reference only
4519
4520 If the condition is the string (DEFINE), and there is no subpattern
4521 with the name DEFINE, the condition is always false. In this case,
4522 there may be only one alternative in the subpattern. It is always
4523 skipped if control reaches this point in the pattern; the idea of
4524 DEFINE is that it can be used to define "subroutines" that can be ref-
4525 erenced from elsewhere. (The use of "subroutines" is described below.)
4526 For example, a pattern to match an IPv4 address could be written like
4527 this (ignore whitespace and line breaks):
4528
4529 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4530 \b (?&byte) (\.(?&byte)){3} \b
4531
4532 The first part of the pattern is a DEFINE group inside which a another
4533 group named "byte" is defined. This matches an individual component of
4534 an IPv4 address (a number less than 256). When matching takes place,
4535 this part of the pattern is skipped because DEFINE acts like a false
4536 condition.
4537
4538 The rest of the pattern uses references to the named group to match the
4539 four dot-separated components of an IPv4 address, insisting on a word
4540 boundary at each end.
4541
4542 Assertion conditions
4543
4544 If the condition is not in any of the above formats, it must be an
4545 assertion. This may be a positive or negative lookahead or lookbehind
4546 assertion. Consider this pattern, again containing non-significant
4547 white space, and with the two alternatives on the second line:
4548
4549 (?(?=[^a-z]*[a-z])
4550 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
4551
4552 The condition is a positive lookahead assertion that matches an
4553 optional sequence of non-letters followed by a letter. In other words,
4554 it tests for the presence of at least one letter in the subject. If a
4555 letter is found, the subject is matched against the first alternative;
4556 otherwise it is matched against the second. This pattern matches
4557 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4558 letters and dd are digits.
4559
4560
4561 COMMENTS
4562
4563 The sequence (?# marks the start of a comment that continues up to the
4564 next closing parenthesis. Nested parentheses are not permitted. The
4565 characters that make up a comment play no part in the pattern matching
4566 at all.
4567
4568 If the PCRE_EXTENDED option is set, an unescaped # character outside a
4569 character class introduces a comment that continues to immediately
4570 after the next newline in the pattern.
4571
4572
4573 RECURSIVE PATTERNS
4574
4575 Consider the problem of matching a string in parentheses, allowing for
4576 unlimited nested parentheses. Without the use of recursion, the best
4577 that can be done is to use a pattern that matches up to some fixed
4578 depth of nesting. It is not possible to handle an arbitrary nesting
4579 depth.
4580
4581 For some time, Perl has provided a facility that allows regular expres-
4582 sions to recurse (amongst other things). It does this by interpolating
4583 Perl code in the expression at run time, and the code can refer to the
4584 expression itself. A Perl pattern using code interpolation to solve the
4585 parentheses problem can be created like this:
4586
4587 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4588
4589 The (?p{...}) item interpolates Perl code at run time, and in this case
4590 refers recursively to the pattern in which it appears.
4591
4592 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4593 it supports special syntax for recursion of the entire pattern, and
4594 also for individual subpattern recursion. After its introduction in
4595 PCRE and Python, this kind of recursion was introduced into Perl at
4596 release 5.10.
4597
4598 A special item that consists of (? followed by a number greater than
4599 zero and a closing parenthesis is a recursive call of the subpattern of
4600 the given number, provided that it occurs inside that subpattern. (If
4601 not, it is a "subroutine" call, which is described in the next sec-
4602 tion.) The special item (?R) or (?0) is a recursive call of the entire
4603 regular expression.
4604
4605 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
4606 always treated as an atomic group. That is, once it has matched some of
4607 the subject string, it is never re-entered, even if it contains untried
4608 alternatives and there is a subsequent matching failure.
4609
4610 This PCRE pattern solves the nested parentheses problem (assume the
4611 PCRE_EXTENDED option is set so that white space is ignored):
4612
4613 \( ( (?>[^()]+) | (?R) )* \)
4614
4615 First it matches an opening parenthesis. Then it matches any number of
4616 substrings which can either be a sequence of non-parentheses, or a
4617 recursive match of the pattern itself (that is, a correctly parenthe-
4618 sized substring). Finally there is a closing parenthesis.
4619
4620 If this were part of a larger pattern, you would not want to recurse
4621 the entire pattern, so instead you could use this:
4622
4623 ( \( ( (?>[^()]+) | (?1) )* \) )
4624
4625 We have put the pattern into parentheses, and caused the recursion to
4626 refer to them instead of the whole pattern.
4627
4628 In a larger pattern, keeping track of parenthesis numbers can be
4629 tricky. This is made easier by the use of relative references. (A Perl
4630 5.10 feature.) Instead of (?1) in the pattern above you can write
4631 (?-2) to refer to the second most recently opened parentheses preceding
4632 the recursion. In other words, a negative number counts capturing
4633 parentheses leftwards from the point at which it is encountered.
4634
4635 It is also possible to refer to subsequently opened parentheses, by
4636 writing references such as (?+2). However, these cannot be recursive
4637 because the reference is not inside the parentheses that are refer-
4638 enced. They are always "subroutine" calls, as described in the next
4639 section.
4640
4641 An alternative approach is to use named parentheses instead. The Perl
4642 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
4643 supported. We could rewrite the above example as follows:
4644
4645 (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4646
4647 If there is more than one subpattern with the same name, the earliest
4648 one is used.
4649
4650 This particular example pattern that we have been looking at contains
4651 nested unlimited repeats, and so the use of atomic grouping for match-
4652 ing strings of non-parentheses is important when applying the pattern
4653 to strings that do not match. For example, when this pattern is applied
4654 to
4655
4656 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4657
4658 it yields "no match" quickly. However, if atomic grouping is not used,
4659 the match runs for a very long time indeed because there are so many
4660 different ways the + and * repeats can carve up the subject, and all
4661 have to be tested before failure can be reported.
4662
4663 At the end of a match, the values set for any capturing subpatterns are
4664 those from the outermost level of the recursion at which the subpattern
4665 value is set. If you want to obtain intermediate values, a callout
4666 function can be used (see below and the pcrecallout documentation). If
4667 the pattern above is matched against
4668
4669 (ab(cd)ef)
4670
4671 the value for the capturing parentheses is "ef", which is the last
4672 value taken on at the top level. If additional parentheses are added,
4673 giving
4674
4675 \( ( ( (?>[^()]+) | (?R) )* ) \)
4676 ^ ^
4677 ^ ^
4678
4679 the string they capture is "ab(cd)ef", the contents of the top level
4680 parentheses. If there are more than 15 capturing parentheses in a pat-
4681 tern, PCRE has to obtain extra memory to store data during a recursion,
4682 which it does by using pcre_malloc, freeing it via pcre_free after-
4683 wards. If no memory can be obtained, the match fails with the
4684 PCRE_ERROR_NOMEMORY error.
4685
4686 Do not confuse the (?R) item with the condition (R), which tests for
4687 recursion. Consider this pattern, which matches text in angle brack-
4688 ets, allowing for arbitrary nesting. Only digits are allowed in nested
4689 brackets (that is, when recursing), whereas any characters are permit-
4690 ted at the outer level.
4691
4692 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
4693
4694 In this pattern, (?(R) is the start of a conditional subpattern, with
4695 two different alternatives for the recursive and non-recursive cases.
4696 The (?R) item is the actual recursive call.
4697
4698
4699 SUBPATTERNS AS SUBROUTINES
4700
4701 If the syntax for a recursive subpattern reference (either by number or
4702 by name) is used outside the parentheses to which it refers, it oper-
4703 ates like a subroutine in a programming language. The "called" subpat-
4704 tern may be defined before or after the reference. A numbered reference
4705 can be absolute or relative, as in these examples:
4706
4707 (...(absolute)...)...(?2)...
4708 (...(relative)...)...(?-1)...
4709 (...(?+1)...(relative)...
4710
4711 An earlier example pointed out that the pattern
4712
4713 (sens|respons)e and \1ibility
4714
4715 matches "sense and sensibility" and "response and responsibility", but
4716 not "sense and responsibility". If instead the pattern
4717
4718 (sens|respons)e and (?1)ibility
4719
4720 is used, it does match "sense and responsibility" as well as the other
4721 two strings. Another example is given in the discussion of DEFINE
4722 above.
4723
4724 Like recursive subpatterns, a "subroutine" call is always treated as an
4725 atomic group. That is, once it has matched some of the subject string,
4726 it is never re-entered, even if it contains untried alternatives and
4727 there is a subsequent matching failure.
4728
4729 When a subpattern is used as a subroutine, processing options such as
4730 case-independence are fixed when the subpattern is defined. They cannot
4731 be changed for different calls. For example, consider this pattern:
4732
4733 (abc)(?i:(?-1))
4734
4735 It matches "abcabc". It does not match "abcABC" because the change of
4736 processing option does not affect the called subpattern.
4737
4738
4739 CALLOUTS
4740
4741 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4742 Perl code to be obeyed in the middle of matching a regular expression.
4743 This makes it possible, amongst other things, to extract different sub-
4744 strings that match the same pair of parentheses when there is a repeti-
4745 tion.
4746
4747 PCRE provides a similar feature, but of course it cannot obey arbitrary
4748 Perl code. The feature is called "callout". The caller of PCRE provides
4749 an external function by putting its entry point in the global variable
4750 pcre_callout. By default, this variable contains NULL, which disables
4751 all calling out.
4752
4753 Within a regular expression, (?C) indicates the points at which the
4754 external function is to be called. If you want to identify different
4755 callout points, you can put a number less than 256 after the letter C.
4756 The default value is zero. For example, this pattern has two callout
4757 points:
4758
4759 (?C1)abc(?C2)def
4760
4761 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4762 automatically installed before each item in the pattern. They are all
4763 numbered 255.
4764
4765 During matching, when PCRE reaches a callout point (and pcre_callout is
4766 set), the external function is called. It is provided with the number
4767 of the callout, the position in the pattern, and, optionally, one item
4768 of data originally supplied by the caller of pcre_exec(). The callout
4769 function may cause matching to proceed, to backtrack, or to fail alto-
4770 gether. A complete description of the interface to the callout function
4771 is given in the pcrecallout documentation.
4772
4773
4774 BACKTRACKING CONTROL
4775
4776 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
4777 which are described in the Perl documentation as "experimental and sub-
4778 ject to change or removal in a future version of Perl". It goes on to
4779 say: "Their usage in production code should be noted to avoid problems
4780 during upgrades." The same remarks apply to the PCRE features described
4781 in this section.
4782
4783 Since these verbs are specifically related to backtracking, they can be
4784 used only when the pattern is to be matched using pcre_exec(), which
4785 uses a backtracking algorithm. They cause an error if encountered by
4786 pcre_dfa_exec().
4787
4788 The new verbs make use of what was previously invalid syntax: an open-
4789 ing parenthesis followed by an asterisk. In Perl, they are generally of
4790 the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4791 its general form is just (*VERB). Any number of these verbs may occur
4792 in a pattern. There are two kinds:
4793
4794 Verbs that act immediately
4795
4796 The following verbs act as soon as they are encountered:
4797
4798 (*ACCEPT)
4799
4800 This verb causes the match to end successfully, skipping the remainder
4801 of the pattern. When inside a recursion, only the innermost pattern is
4802 ended immediately. PCRE differs from Perl in what happens if the
4803 (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is
4804 captured: in PCRE no data is captured. For example:
4805
4806 A(A|B(*ACCEPT)|C)D
4807
4808 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data
4809 is captured.
4810
4811 (*FAIL) or (*F)
4812
4813 This verb causes the match to fail, forcing backtracking to occur. It
4814 is equivalent to (?!) but easier to read. The Perl documentation notes
4815 that it is probably useful only when combined with (?{}) or (??{}).
4816 Those are, of course, Perl features that are not present in PCRE. The
4817 nearest equivalent is the callout feature, as for example in this pat-
4818 tern:
4819
4820 a+(?C)(*FAIL)
4821
4822 A match with the string "aaaa" always fails, but the callout is taken
4823 before each backtrack happens (in this example, 10 times).
4824
4825 Verbs that act after backtracking
4826
4827 The following verbs do nothing when they are encountered. Matching con-
4828 tinues with what follows, but if there is no subsequent match, a fail-
4829 ure is forced. The verbs differ in exactly what kind of failure
4830 occurs.
4831
4832 (*COMMIT)
4833
4834 This verb causes the whole match to fail outright if the rest of the
4835 pattern does not match. Even if the pattern is unanchored, no further
4836 attempts to find a match by advancing the start point take place. Once
4837 (*COMMIT) has been passed, pcre_exec() is committed to finding a match
4838 at the current starting point, or not at all. For example:
4839
4840 a+(*COMMIT)b
4841
4842 This matches "xxaab" but not "aacaab". It can be thought of as a kind
4843 of dynamic anchor, or "I've started, so I must finish."
4844
4845 (*PRUNE)
4846
4847 This verb causes the match to fail at the current position if the rest
4848 of the pattern does not match. If the pattern is unanchored, the normal
4849 "bumpalong" advance to the next starting character then happens. Back-
4850 tracking can occur as usual to the left of (*PRUNE), or when matching
4851 to the right of (*PRUNE), but if there is no match to the right, back-
4852 tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
4853 is just an alternative to an atomic group or possessive quantifier, but
4854 there are some uses of (*PRUNE) that cannot be expressed in any other
4855 way.
4856
4857 (*SKIP)
4858
4859 This verb is like (*PRUNE), except that if the pattern is unanchored,
4860 the "bumpalong" advance is not to the next character, but to the posi-
4861 tion in the subject where (*SKIP) was encountered. (*SKIP) signifies
4862 that whatever text was matched leading up to it cannot be part of a
4863 successful match. Consider:
4864
4865 a+(*SKIP)b
4866
4867 If the subject is "aaaac...", after the first match attempt fails
4868 (starting at the first character in the string), the starting point
4869 skips on to start the next attempt at "c". Note that a possessive quan-
4870 tifer does not have the same effect in this example; although it would
4871 suppress backtracking during the first match attempt, the second
4872 attempt would start at the second character instead of skipping on to
4873 "c".
4874
4875 (*THEN)
4876
4877 This verb causes a skip to the next alternation if the rest of the pat-
4878 tern does not match. That is, it cancels pending backtracking, but only
4879 within the current alternation. Its name comes from the observation
4880 that it can be used for a pattern-based if-then-else block:
4881
4882 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
4883
4884 If the COND1 pattern matches, FOO is tried (and possibly further items
4885 after the end of the group if FOO succeeds); on failure the matcher
4886 skips to the second alternative and tries COND2, without backtracking
4887 into COND1. If (*THEN) is used outside of any alternation, it acts
4888 exactly like (*PRUNE).
4889
4890
4891 SEE ALSO
4892
4893 pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
4894
4895
4896 AUTHOR
4897
4898 Philip Hazel
4899 University Computing Service
4900 Cambridge CB2 3QH, England.
4901
4902
4903 REVISION
4904
4905 Last updated: 17 September 2007
4906 Copyright (c) 1997-2007 University of Cambridge.
4907 ------------------------------------------------------------------------------
4908
4909
4910 PCRESYNTAX(3) PCRESYNTAX(3)
4911
4912
4913 NAME
4914 PCRE - Perl-compatible regular expressions
4915
4916
4917 PCRE REGULAR EXPRESSION SYNTAX SUMMARY
4918
4919 The full syntax and semantics of the regular expressions that are sup-
4920 ported by PCRE are described in the pcrepattern documentation. This
4921 document contains just a quick-reference summary of the syntax.
4922
4923
4924 QUOTING
4925
4926 \x where x is non-alphanumeric is a literal x
4927 \Q...\E treat enclosed characters as literal
4928
4929
4930 CHARACTERS
4931
4932 \a alarm, that is, the BEL character (hex 07)
4933 \cx "control-x", where x is any character
4934 \e escape (hex 1B)
4935 \f formfeed (hex 0C)
4936 \n newline (hex 0A)
4937 \r carriage return (hex 0D)
4938 \t tab (hex 09)
4939 \ddd character with octal code ddd, or backreference
4940 \xhh character with hex code hh
4941 \x{hhh..} character with hex code hhh..
4942
4943
4944 CHARACTER TYPES
4945
4946 . any character except newline;
4947 in dotall mode, any character whatsoever
4948 \C one byte, even in UTF-8 mode (best avoided)
4949 \d a decimal digit
4950 \D a character that is not a decimal digit
4951 \h a horizontal whitespace character
4952 \H a character that is not a horizontal whitespace character
4953 \p{xx} a character with the xx property
4954 \P{xx} a character without the xx property
4955 \R a newline sequence
4956 \s a whitespace character
4957 \S a character that is not a whitespace character
4958 \v a vertical whitespace character
4959 \V a character that is not a vertical whitespace character
4960 \w a "word" character
4961 \W a "non-word" character
4962 \X an extended Unicode sequence
4963
4964 In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.
4965
4966
4967 GENERAL CATEGORY PROPERTY CODES FOR \p and \P
4968
4969 C Other
4970 Cc Control
4971 Cf Format
4972 Cn Unassigned
4973 Co Private use
4974 Cs Surrogate
4975
4976 L Letter
4977 Ll Lower case letter
4978 Lm Modifier letter
4979 Lo Other letter
4980 Lt Title case letter
4981 Lu Upper case letter
4982 L& Ll, Lu, or Lt
4983
4984 M Mark
4985 Mc Spacing mark
4986 Me Enclosing mark
4987 Mn Non-spacing mark
4988
4989 N Number
4990 Nd Decimal number
4991 Nl Letter number
4992 No Other number
4993
4994 P Punctuation
4995 Pc Connector punctuation
4996 Pd Dash punctuation
4997 Pe Close punctuation
4998 Pf Final punctuation
4999 Pi Initial punctuation
5000 Po Other punctuation
5001 Ps Open punctuation
5002
5003 S Symbol
5004 Sc Currency symbol
5005 Sk Modifier symbol
5006 Sm Mathematical symbol
5007 So Other symbol
5008
5009 Z Separator
5010 Zl Line separator
5011 Zp Paragraph separator
5012 Zs Space separator
5013
5014
5015 SCRIPT NAMES FOR \p AND \P
5016
5017 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
5018 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
5019 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
5020 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
5021 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
5022 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
5023 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
5024 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
5025 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
5026
5027
5028 CHARACTER CLASSES
5029
5030 [...] positive character class
5031 [^...] negative character class
5032 [x-y] range (can be used for hex characters)
5033 [[:xxx:]] positive POSIX named set
5034 [[:^xxx:]] negative POSIX named set
5035
5036 alnum alphanumeric
5037 alpha alphabetic
5038 ascii 0-127
5039 blank space or tab
5040 cntrl control character
5041 digit decimal digit
5042 graph printing, excluding space
5043 lower lower case letter
5044 print printing, including space
5045 punct printing, excluding alphanumeric
5046 space whitespace
5047 upper upper case letter
5048 word same as \w
5049 xdigit hexadecimal digit
5050
5051 In PCRE, POSIX character set names recognize only ASCII characters. You
5052 can use \Q...\E inside a character class.
5053
5054
5055 QUANTIFIERS
5056
5057 ? 0 or 1, greedy
5058 ?+ 0 or 1, possessive
5059 ?? 0 or 1, lazy
5060 * 0 or more, greedy
5061 *+ 0 or more, possessive
5062 *? 0 or more, lazy
5063 + 1 or more, greedy
5064 ++ 1 or more, possessive
5065 +? 1 or more, lazy
5066 {n} exactly n
5067 {n,m} at least n, no more than m, greedy
5068 {n,m}+ at least n, no more than m, possessive
5069 {n,m}? at least n, no more than m, lazy
5070 {n,} n or more, greedy
5071 {n,}+ n or more, possessive
5072 {n,}? n or more, lazy
5073
5074
5075 ANCHORS AND SIMPLE ASSERTIONS
5076
5077 \b word boundary
5078 \B not a word boundary
5079 ^ start of subject
5080 also after internal newline in multiline mode
5081 \A start of subject
5082 $ end of subject
5083 also before newline at end of subject
5084 also before internal newline in multiline mode
5085 \Z end of subject
5086 also before newline at end of subject
5087 \z end of subject
5088 \G first matching position in subject
5089
5090
5091 MATCH POINT RESET
5092
5093 \K reset start of match
5094
5095
5096 ALTERNATION
5097
5098 expr|expr|expr...
5099
5100
5101 CAPTURING
5102
5103 (...) capturing group
5104 (?<name>...) named capturing group (Perl)
5105 (?'name'...) named capturing group (Perl)
5106 (?P<name>...) named capturing group (Python)
5107 (?:...) non-capturing group
5108 (?|...) non-capturing group; reset group numbers for
5109 capturing groups in each alternative
5110
5111
5112 ATOMIC GROUPS
5113
5114 (?>...) atomic, non-capturing group
5115
5116
5117 COMMENT
5118
5119 (?#....) comment (not nestable)
5120
5121
5122 OPTION SETTING
5123
5124 (?i) caseless
5125 (?J) allow duplicate names
5126 (?m) multiline
5127 (?s) single line (dotall)
5128 (?U) default ungreedy (lazy)
5129 (?x) extended (ignore white space)
5130 (?-...) unset option(s)
5131
5132
5133 LOOKAHEAD AND LOOKBEHIND ASSERTIONS
5134
5135 (?=...) positive look ahead
5136 (?!...) negative look ahead
5137 (?<=...) positive look behind
5138 (?<!...) negative look behind
5139
5140 Each top-level branch of a look behind must be of a fixed length.
5141
5142
5143 BACKREFERENCES
5144
5145 \n reference by number (can be ambiguous)
5146 \gn reference by number
5147 \g{n} reference by number
5148 \g{-n} relative reference by number
5149 \k<name> reference by name (Perl)
5150 \k'name' reference by name (Perl)
5151 \g{name} reference by name (Perl)
5152 \k{name} reference by name (.NET)
5153 (?P=name) reference by name (Python)
5154
5155
5156 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
5157
5158 (?R) recurse whole pattern
5159 (?n) call subpattern by absolute number
5160 (?+n) call subpattern by relative number
5161 (?-n) call subpattern by relative number
5162 (?&name) call subpattern by name (Perl)
5163 (?P>name) call subpattern by name (Python)
5164
5165
5166 CONDITIONAL PATTERNS
5167
5168 (?(condition)yes-pattern)
5169 (?(condition)yes-pattern|no-pattern)
5170
5171 (?(n)... absolute reference condition
5172 (?(+n)... relative reference condition
5173 (?(-n)... relative reference condition
5174 (?(<name>)... named reference condition (Perl)
5175 (?('name')... named reference condition (Perl)
5176 (?(name)... named reference condition (PCRE)
5177 (?(R)... overall recursion condition
5178 (?(Rn)... specific group recursion condition
5179 (?(R&name)... specific recursion condition
5180 (?(DEFINE)... define subpattern for reference
5181 (?(assert)... assertion condition
5182
5183
5184 BACKTRACKING CONTROL
5185
5186 The following act immediately they are reached:
5187
5188 (*ACCEPT) force successful match
5189 (*FAIL) force backtrack; synonym (*F)
5190
5191 The following act only when a subsequent match failure causes a back-
5192 track to reach them. They all force a match failure, but they differ in
5193 what happens afterwards. Those that advance the start-of-match point do
5194 so only if the pattern is not anchored.
5195
5196 (*COMMIT) overall failure, no advance of starting point
5197 (*PRUNE) advance to next starting character
5198 (*SKIP) advance start to current matching position
5199 (*THEN) local failure, backtrack to next alternation
5200
5201
5202 NEWLINE CONVENTIONS
5203
5204 These are recognized only at the very start of the pattern or after a
5205 (*BSR_...) option.
5206
5207 (*CR)
5208 (*LF)
5209 (*CRLF)
5210 (*ANYCRLF)
5211 (*ANY)
5212
5213
5214 WHAT \R MATCHES
5215
5216 These are recognized only at the very start of the pattern or after a
5217 (*...) option that sets the newline convention.
5218
5219 (*BSR_ANYCRLF)
5220 (*BSR_UNICODE)
5221
5222
5223 CALLOUTS
5224
5225 (?C) callout
5226 (?Cn) callout with data n
5227
5228
5229 SEE ALSO
5230
5231 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
5232
5233
5234 AUTHOR
5235
5236 Philip Hazel
5237 University Computing Service
5238 Cambridge CB2 3QH, England.
5239
5240
5241 REVISION
5242
5243 Last updated: 14 November 2007
5244 Copyright (c) 1997-2007 University of Cambridge.
5245 ------------------------------------------------------------------------------
5246
5247
5248 PCREPARTIAL(3) PCREPARTIAL(3)
5249
5250
5251 NAME
5252 PCRE - Perl-compatible regular expressions
5253
5254
5255 PARTIAL MATCHING IN PCRE
5256
5257 In normal use of PCRE, if the subject string that is passed to
5258 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
5259 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
5260 There are circumstances where it might be helpful to distinguish this
5261 case from other cases in which there is no match.
5262
5263 Consider, for example, an application where a human is required to type
5264 in data for a field with specific formatting requirements. An example
5265 might be a date in the form ddmmmyy, defined by this pattern:
5266
5267 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
5268
5269 If the application sees the user's keystrokes one by one, and can check
5270 that what has been typed so far is potentially valid, it is able to
5271 raise an error as soon as a mistake is made, possibly beeping and not
5272 reflecting the character that has been typed. This immediate feedback
5273 is likely to be a better user interface than a check that is delayed
5274 until the entire string has been entered.
5275
5276 PCRE supports the concept of partial matching by means of the PCRE_PAR-
5277 TIAL option, which can be set when calling pcre_exec() or
5278 pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
5279 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
5280 during the matching process the last part of the subject string matched
5281 part of the pattern. Unfortunately, for non-anchored matching, it is
5282 not possible to obtain the position of the start of the partial match.
5283 No captured data is set when PCRE_ERROR_PARTIAL is returned.
5284
5285 When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code
5286 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of
5287 the subject is reached, there have been no complete matches, but there
5288 is still at least one matching possibility. The portion of the string
5289 that provided the partial match is set as the first matching string.
5290
5291 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
5292 the last literal byte in a pattern, and abandons matching immediately
5293 if such a byte is not present in the subject string. This optimization
5294 cannot be used for a subject string that might match only partially.
5295
5296
5297 RESTRICTED PATTERNS FOR PCRE_PARTIAL
5298
5299 Because of the way certain internal optimizations are implemented in
5300 the pcre_exec() function, the PCRE_PARTIAL option cannot be used with
5301 all patterns. These restrictions do not apply when pcre_dfa_exec() is
5302 used. For pcre_exec(), repeated single characters such as
5303
5304 a{2,4}
5305
5306 and repeated single metasequences such as
5307
5308 \d+
5309
5310 are not permitted if the maximum number of occurrences is greater than
5311 one. Optional items such as \d? (where the maximum is one) are permit-
5312 ted. Quantifiers with any values are permitted after parentheses, so
5313 the invalid examples above can be coded thus:
5314
5315 (a){2,4}
5316 (\d)+
5317
5318 These constructions run more slowly, but for the kinds of application
5319 that are envisaged for this facility, this is not felt to be a major
5320 restriction.
5321
5322 If PCRE_PARTIAL is set for a pattern that does not conform to the
5323 restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
5324 (-13). You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to
5325 find out if a compiled pattern can be used for partial matching.
5326
5327
5328 EXAMPLE OF PARTIAL MATCHING USING PCRETEST
5329
5330 If the escape sequence \P is present in a pcretest data line, the
5331 PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
5332 uses the date example quoted above:
5333
5334 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5335 data> 25jun04\P
5336 0: 25jun04
5337 1: jun
5338 data> 25dec3\P
5339 Partial match
5340 data> 3ju\P
5341 Partial match
5342 data> 3juj\P
5343 No match
5344 data> j\P
5345 No match
5346
5347 The first data string is matched completely, so pcretest shows the
5348 matched substrings. The remaining four strings do not match the com-
5349 plete pattern, but the first two are partial matches. The same test,
5350 using pcre_dfa_exec() matching (by means of the \D escape sequence),
5351 produces the following output:
5352
5353 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5354 data> 25jun04\P\D
5355 0: 25jun04
5356 data> 23dec3\P\D
5357 Partial match: 23dec3
5358 data> 3ju\P\D
5359 Partial match: 3ju
5360 data> 3juj\P\D
5361 No match
5362 data> j\P\D
5363 No match
5364
5365 Notice that in this case the portion of the string that was matched is
5366 made available.
5367
5368
5369 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
5370
5371 When a partial match has been found using pcre_dfa_exec(), it is possi-
5372 ble to continue the match by providing additional subject data and
5373 calling pcre_dfa_exec() again with the same compiled regular expres-
5374 sion, this time setting the PCRE_DFA_RESTART option. You must also pass
5375 the same working space as before, because this is where details of the
5376 previous partial match are stored. Here is an example using pcretest,
5377 using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
5378 \D are as above):
5379
5380 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5381 data> 23ja\P\D
5382 Partial match: 23ja
5383 data> n05\R\D
5384 0: n05
5385
5386 The first call has "23ja" as the subject, and requests partial match-
5387 ing; the second call has "n05" as the subject for the continued
5388 (restarted) match. Notice that when the match is complete, only the
5389 last part is shown; PCRE does not retain the previously partially-
5390 matched string. It is up to the calling program to do that if it needs
5391 to.
5392
5393 You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial
5394 matching over multiple segments. This facility can be used to pass very
5395 long subject strings to pcre_dfa_exec(). However, some care is needed
5396 for certain types of pattern.
5397
5398 1. If the pattern contains tests for the beginning or end of a line,
5399 you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
5400 ate, when the subject string for any call does not contain the begin-
5401 ning or end of a line.
5402
5403 2. If the pattern contains backward assertions (including \b or \B),
5404 you need to arrange for some overlap in the subject strings to allow
5405 for this. For example, you could pass the subject in chunks that are
5406 500 bytes long, but in a buffer of 700 bytes, with the starting offset
5407 set to 200 and the previous 200 bytes at the start of the buffer.
5408
5409 3. Matching a subject string that is split into multiple segments does
5410 not always produce exactly the same result as matching over one single
5411 long string. The difference arises when there are multiple matching
5412 possibilities, because a partial match result is given only when there
5413 are no completed matches in a call to pcre_dfa_exec(). This means that
5414 as soon as the shortest match has been found, continuation to a new
5415 subject segment is no longer possible. Consider this pcretest example:
5416
5417 re> /dog(sbody)?/
5418 data> do\P\D
5419 Partial match: do
5420 data> gsb\R\P\D
5421 0: g
5422 data> dogsbody\D
5423 0: dogsbody
5424 1: dog
5425
5426 The pattern matches the words "dog" or "dogsbody". When the subject is
5427 presented in several parts ("do" and "gsb" being the first two) the
5428 match stops when "dog" has been found, and it is not possible to con-
5429 tinue. On the other hand, if "dogsbody" is presented as a single
5430 string, both matches are found.
5431
5432 Because of this phenomenon, it does not usually make sense to end a
5433 pattern that is going to be matched in this way with a variable repeat.
5434
5435 4. Patterns that contain alternatives at the top level which do not all
5436 start with the same pattern item may not work as expected. For example,
5437 consider this pattern:
5438
5439 1234|3789
5440
5441 If the first part of the subject is "ABC123", a partial match of the
5442 first alternative is found at offset 3. There is no partial match for
5443 the second alternative, because such a match does not start at the same
5444 point in the subject string. Attempting to continue with the string
5445 "789" does not yield a match because only those alternatives that match
5446 at one point in the subject are remembered. The problem arises because
5447 the start of the second alternative matches within the first alterna-
5448 tive. There is no problem with anchored patterns or patterns such as:
5449
5450 1234|ABCD
5451
5452 where no string can be a partial match for both alternatives.
5453
5454
5455 AUTHOR
5456
5457 Philip Hazel
5458 University Computing Service
5459 Cambridge CB2 3QH, England.
5460
5461
5462 REVISION
5463
5464 Last updated: 04 June 2007
5465 Copyright (c) 1997-2007 University of Cambridge.
5466 ------------------------------------------------------------------------------
5467
5468
5469 PCREPRECOMPILE(3) PCREPRECOMPILE(3)
5470
5471
5472 NAME
5473 PCRE - Perl-compatible regular expressions
5474
5475
5476 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
5477
5478 If you are running an application that uses a large number of regular
5479 expression patterns, it may be useful to store them in a precompiled
5480 form instead of having to compile them every time the application is
5481 run. If you are not using any private character tables (see the
5482 pcre_maketables() documentation), this is relatively straightforward.
5483 If you are using private tables, it is a little bit more complicated.
5484
5485 If you save compiled patterns to a file, you can copy them to a differ-
5486 ent host and run them there. This works even if the new host has the
5487 opposite endianness to the one on which the patterns were compiled.
5488 There may be a small performance penalty, but it should be insignifi-
5489 cant. However, compiling regular expressions with one version of PCRE
5490 for use with a different version is not guaranteed to work and may
5491 cause crashes.
5492
5493
5494 SAVING A COMPILED PATTERN
5495 The value returned by pcre_compile() points to a single block of memory
5496 that holds the compiled pattern and associated data. You can find the
5497 length of this block in bytes by calling pcre_fullinfo() with an argu-
5498 ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
5499 manner. Here is sample code that compiles a pattern and writes it to a
5500 file. It assumes that the variable fd refers to a file that is open for
5501 output:
5502
5503 int erroroffset, rc, size;
5504 char *error;
5505 pcre *re;
5506
5507 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
5508 if (re == NULL) { ... handle errors ... }
5509 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
5510 if (rc < 0) { ... handle errors ... }
5511 rc = fwrite(re, 1, size, fd);
5512 if (rc != size) { ... handle errors ... }
5513
5514 In this example, the bytes that comprise the compiled pattern are
5515 copied exactly. Note that this is binary data that may contain any of
5516 the 256 possible byte values. On systems that make a distinction
5517 between binary and non-binary data, be sure that the file is opened for
5518 binary output.
5519
5520 If you want to write more than one pattern to a file, you will have to
5521 devise a way of separating them. For binary data, preceding each pat-
5522 tern with its length is probably the most straightforward approach.
5523 Another possibility is to write out the data in hexadecimal instead of
5524 binary, one pattern to a line.
5525
5526 Saving compiled patterns in a file is only one possible way of storing
5527 them for later use. They could equally well be saved in a database, or
5528 in the memory of some daemon process that passes them via sockets to
5529 the processes that want them.
5530
5531 If the pattern has been studied, it is also possible to save the study
5532 data in a similar way to the compiled pattern itself. When studying
5533 generates additional information, pcre_study() returns a pointer to a
5534 pcre_extra data block. Its format is defined in the section on matching
5535 a pattern in the pcreapi documentation. The study_data field points to
5536 the binary study data, and this is what you must save (not the
5537 pcre_extra block itself). The length of the study data can be obtained
5538 by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
5539 Remember to check that pcre_study() did return a non-NULL value before
5540 trying to save the study data.
5541
5542
5543 RE-USING A PRECOMPILED PATTERN
5544
5545 Re-using a precompiled pattern is straightforward. Having reloaded it
5546 into main memory, you pass its pointer to pcre_exec() or
5547 pcre_dfa_exec() in the usual way. This should work even on another
5548 host, and even if that host has the opposite endianness to the one
5549 where the pattern was compiled.
5550
5551 However, if you passed a pointer to custom character tables when the
5552 pattern was compiled (the tableptr argument of pcre_compile()), you
5553 must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
5554 because the value saved with the compiled pattern will obviously be
5555 nonsense. A field in a pcre_extra() block is used to pass this data, as
5556 described in the section on matching a pattern in the pcreapi documen-
5557 tation.
5558
5559 If you did not provide custom character tables when the pattern was
5560 compiled, the pointer in the compiled pattern is NULL, which causes
5561 pcre_exec() to use PCRE's internal tables. Thus, you do not need to
5562 take any special action at run time in this case.
5563
5564 If you saved study data with the compiled pattern, you need to create
5565 your own pcre_extra data block and set the study_data field to point to
5566 the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
5567 bit in the flags field to indicate that study data is present. Then
5568 pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
5569 usual way.
5570
5571
5572 COMPATIBILITY WITH DIFFERENT PCRE RELEASES
5573
5574 In general, it is safest to recompile all saved patterns when you
5575 update to a new PCRE release, though not all updates actually require
5576 this. Recompiling is definitely needed for release 7.2.
5577
5578
5579 AUTHOR
5580
5581 Philip Hazel
5582 University Computing Service
5583 Cambridge CB2 3QH, England.
5584
5585
5586 REVISION
5587
5588 Last updated: 13 June 2007
5589 Copyright (c) 1997-2007 University of Cambridge.
5590 ------------------------------------------------------------------------------
5591
5592
5593 PCREPERFORM(3) PCREPERFORM(3)
5594
5595
5596 NAME
5597 PCRE - Perl-compatible regular expressions
5598
5599
5600 PCRE PERFORMANCE
5601
5602 Two aspects of performance are discussed below: memory usage and pro-
5603 cessing time. The way you express your pattern as a regular expression
5604 can affect both of them.
5605
5606
5607 MEMORY USAGE
5608
5609 Patterns are compiled by PCRE into a reasonably efficient byte code, so
5610 that most simple patterns do not use much memory. However, there is one
5611 case where memory usage can be unexpectedly large. When a parenthesized
5612 subpattern has a quantifier with a minimum greater than 1 and/or a lim-
5613 ited maximum, the whole subpattern is repeated in the compiled code.
5614 For example, the pattern
5615
5616 (abc|def){2,4}
5617
5618 is compiled as if it were
5619
5620 (abc|def)(abc|def)((abc|def)(abc|def)?)?
5621
5622 (Technical aside: It is done this way so that backtrack points within
5623 each of the repetitions can be independently maintained.)
5624
5625 For regular expressions whose quantifiers use only small numbers, this
5626 is not usually a problem. However, if the numbers are large, and par-
5627 ticularly if such repetitions are nested, the memory usage can become
5628 an embarrassment. For example, the very simple pattern
5629
5630 ((ab){1,1000}c){1,3}
5631
5632 uses 51K bytes when compiled. When PCRE is compiled with its default
5633 internal pointer size of two bytes, the size limit on a compiled pat-
5634 tern is 64K, and this is reached with the above pattern if the outer
5635 repetition is increased from 3 to 4. PCRE can be compiled to use larger
5636 internal pointers and thus handle larger compiled patterns, but it is
5637 better to try to rewrite your pattern to use less memory if you can.
5638
5639 One way of reducing the memory usage for such patterns is to make use
5640 of PCRE's "subroutine" facility. Re-writing the above pattern as
5641
5642 ((ab)(?2){0,999}c)(?1){0,2}
5643
5644 reduces the memory requirements to 18K, and indeed it remains under 20K
5645 even with the outer repetition increased to 100. However, this pattern
5646 is not exactly equivalent, because the "subroutine" calls are treated
5647 as atomic groups into which there can be no backtracking if there is a
5648 subsequent matching failure. Therefore, PCRE cannot do this kind of
5649 rewriting automatically. Furthermore, there is a noticeable loss of
5650 speed when executing the modified pattern. Nevertheless, if the atomic
5651 grouping is not a problem and the loss of speed is acceptable, this
5652 kind of rewriting will allow you to process patterns that PCRE cannot
5653 otherwise handle.
5654
5655
5656 PROCESSING TIME
5657
5658 Certain items in regular expression patterns are processed more effi-
5659 ciently than others. It is more efficient to use a character class like
5660 [aeiou] than a set of single-character alternatives such as
5661 (a|e|i|o|u). In general, the simplest construction that provides the
5662 required behaviour is usually the most efficient. Jeffrey Friedl's book
5663 contains a lot of useful general discussion about optimizing regular
5664 expressions for efficient performance. This document contains a few
5665 observations about PCRE.
5666
5667 Using Unicode character properties (the \p, \P, and \X escapes) is
5668 slow, because PCRE has to scan a structure that contains data for over
5669 fifteen thousand characters whenever it needs a character's property.
5670 If you can find an alternative pattern that does not use character
5671 properties, it will probably be faster.
5672
5673 When a pattern begins with .* not in parentheses, or in parentheses
5674 that are not the subject of a backreference, and the PCRE_DOTALL option
5675 is set, the pattern is implicitly anchored by PCRE, since it can match
5676 only at the start of a subject string. However, if PCRE_DOTALL is not
5677 set, PCRE cannot make this optimization, because the . metacharacter
5678 does not then match a newline, and if the subject string contains new-
5679 lines, the pattern may match from the character immediately following
5680 one of them instead of from the very start. For example, the pattern
5681
5682 .*second
5683
5684 matches the subject "first\nand second" (where \n stands for a newline
5685 character), with the match starting at the seventh character. In order
5686 to do this, PCRE has to retry the match starting after every newline in
5687 the subject.
5688
5689 If you are using such a pattern with subject strings that do not con-
5690 tain newlines, the best performance is obtained by setting PCRE_DOTALL,
5691 or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
5692 ing. That saves PCRE from having to scan along the subject looking for
5693 a newline to restart at.
5694
5695 Beware of patterns that contain nested indefinite repeats. These can
5696 take a long time to run when applied to a string that does not match.
5697 Consider the pattern fragment
5698
5699 ^(a+)*
5700
5701 This can match "aaaa" in 16 different ways, and this number increases
5702 very rapidly as the string gets longer. (The * repeat can match 0, 1,
5703 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
5704 repeats can match different numbers of times.) When the remainder of
5705 the pattern is such that the entire match is going to fail, PCRE has in
5706 principle to try every possible variation, and this can take an
5707 extremely long time, even for relatively short strings.
5708
5709 An optimization catches some of the more simple cases such as
5710
5711 (a+)*b
5712
5713 where a literal character follows. Before embarking on the standard
5714 matching procedure, PCRE checks that there is a "b" later in the sub-
5715 ject string, and if there is not, it fails the match immediately. How-
5716 ever, when there is no following literal this optimization cannot be
5717 used. You can see the difference by comparing the behaviour of
5718
5719 (a+)*\d
5720
5721 with the pattern above. The former gives a failure almost instantly
5722 when applied to a whole line of "a" characters, whereas the latter
5723 takes an appreciable time with strings longer than about 20 characters.
5724
5725 In many cases, the solution to this kind of performance issue is to use
5726 an atomic group or a possessive quantifier.
5727
5728
5729 AUTHOR
5730
5731 Philip Hazel
5732 University Computing Service
5733 Cambridge CB2 3QH, England.
5734
5735
5736 REVISION
5737
5738 Last updated: 06 March 2007
5739 Copyright (c) 1997-2007 University of Cambridge.
5740 ------------------------------------------------------------------------------
5741
5742
5743 PCREPOSIX(3) PCREPOSIX(3)
5744
5745
5746 NAME
5747 PCRE - Perl-compatible regular expressions.
5748
5749
5750 SYNOPSIS OF POSIX API
5751
5752 #include <pcreposix.h>
5753
5754 int regcomp(regex_t *preg, const char *pattern,
5755 int cflags);
5756
5757 int regexec(regex_t *preg, const char *string,
5758 size_t nmatch, regmatch_t pmatch[], int eflags);
5759
5760 size_t regerror(int errcode, const regex_t *preg,
5761 char *errbuf, size_t errbuf_size);
5762
5763 void regfree(regex_t *preg);
5764
5765
5766 DESCRIPTION
5767
5768 This set of functions provides a POSIX-style API to the PCRE regular
5769 expression package. See the pcreapi documentation for a description of
5770 PCRE's native API, which contains much additional functionality.
5771
5772 The functions described here are just wrapper functions that ultimately
5773 call the PCRE native API. Their prototypes are defined in the
5774 pcreposix.h header file, and on Unix systems the library itself is
5775 called pcreposix.a, so can be accessed by adding -lpcreposix to the
5776 command for linking an application that uses them. Because the POSIX
5777 functions call the native ones, it is also necessary to add -lpcre.
5778
5779 I have implemented only those option bits that can be reasonably mapped
5780 to PCRE native options. In addition, the option REG_EXTENDED is defined
5781 with the value zero. This has no effect, but since programs that are
5782 written to the POSIX interface often use it, this makes it easier to
5783 slot in PCRE as a replacement library. Other POSIX options are not even
5784 defined.
5785
5786 When PCRE is called via these functions, it is only the API that is
5787 POSIX-like in style. The syntax and semantics of the regular expres-
5788 sions themselves are still those of Perl, subject to the setting of
5789 various PCRE options, as described below. "POSIX-like in style" means
5790 that the API approximates to the POSIX definition; it is not fully
5791 POSIX-compatible, and in multi-byte encoding domains it is probably
5792 even less compatible.
5793
5794 The header for these functions is supplied as pcreposix.h to avoid any
5795 potential clash with other POSIX libraries. It can, of course, be
5796 renamed or aliased as regex.h, which is the "correct" name. It provides
5797 two structure types, regex_t for compiled internal forms, and reg-
5798 match_t for returning captured substrings. It also defines some con-
5799 stants whose names start with "REG_"; these are used for setting
5800 options and identifying error codes.
5801
5802
5803 COMPILING A PATTERN
5804
5805 The function regcomp() is called to compile a pattern into an internal
5806 form. The pattern is a C string terminated by a binary zero, and is
5807 passed in the argument pattern. The preg argument is a pointer to a
5808 regex_t structure that is used as a base for storing information about
5809 the compiled regular expression.
5810
5811 The argument cflags is either zero, or contains one or more of the bits
5812 defined by the following macros:
5813
5814 REG_DOTALL
5815
5816 The PCRE_DOTALL option is set when the regular expression is passed for
5817 compilation to the native function. Note that REG_DOTALL is not part of
5818 the POSIX standard.
5819
5820 REG_ICASE
5821
5822 The PCRE_CASELESS option is set when the regular expression is passed
5823 for compilation to the native function.
5824
5825 REG_NEWLINE
5826
5827 The PCRE_MULTILINE option is set when the regular expression is passed
5828 for compilation to the native function. Note that this does not mimic
5829 the defined POSIX behaviour for REG_NEWLINE (see the following sec-
5830 tion).
5831
5832 REG_NOSUB
5833
5834 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
5835 passed for compilation to the native function. In addition, when a pat-
5836 tern that is compiled with this flag is passed to regexec() for match-
5837 ing, the nmatch and pmatch arguments are ignored, and no captured
5838 strings are returned.
5839
5840 REG_UTF8
5841
5842 The PCRE_UTF8 option is set when the regular expression is passed for
5843 compilation to the native function. This causes the pattern itself and
5844 all data strings used for matching it to be treated as UTF-8 strings.
5845 Note that REG_UTF8 is not part of the POSIX standard.
5846
5847 In the absence of these flags, no options are passed to the native
5848 function. This means the the regex is compiled with PCRE default
5849 semantics. In particular, the way it handles newline characters in the
5850 subject string is the Perl way, not the POSIX way. Note that setting
5851 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
5852 It does not affect the way newlines are matched by . (they aren't) or
5853 by a negative class such as [^a] (they are).
5854
5855 The yield of regcomp() is zero on success, and non-zero otherwise. The
5856 preg structure is filled in on success, and one member of the structure
5857 is public: re_nsub contains the number of capturing subpatterns in the
5858 regular expression. Various error codes are defined in the header file.
5859
5860
5861 MATCHING NEWLINE CHARACTERS
5862
5863 This area is not simple, because POSIX and Perl take different views of
5864 things. It is not possible to get PCRE to obey POSIX semantics, but
5865 then PCRE was never intended to be a POSIX engine. The following table
5866 lists the different possibilities for matching newline characters in
5867 PCRE:
5868
5869 Default Change with
5870
5871 . matches newline no PCRE_DOTALL
5872 newline matches [^a] yes not changeable
5873 $ matches \n at end yes PCRE_DOLLARENDONLY
5874 $ matches \n in middle no PCRE_MULTILINE
5875 ^ matches \n in middle no PCRE_MULTILINE
5876
5877 This is the equivalent table for POSIX:
5878
5879 Default Change with
5880
5881 . matches newline yes REG_NEWLINE
5882 newline matches [^a] yes REG_NEWLINE
5883 $ matches \n at end no REG_NEWLINE
5884 $ matches \n in middle no REG_NEWLINE
5885 ^ matches \n in middle no REG_NEWLINE
5886
5887 PCRE's behaviour is the same as Perl's, except that there is no equiva-
5888 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
5889 no way to stop newline from matching [^a].
5890
5891 The default POSIX newline handling can be obtained by setting
5892 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
5893 behave exactly as for the REG_NEWLINE action.
5894
5895
5896 MATCHING A PATTERN
5897
5898 The function regexec() is called to match a compiled pattern preg
5899 against a given string, which is terminated by a zero byte, subject to
5900 the options in eflags. These can be:
5901
5902 REG_NOTBOL
5903
5904 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
5905 function.
5906
5907 REG_NOTEOL
5908
5909 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
5910 function.
5911
5912 If the pattern was compiled with the REG_NOSUB flag, no data about any
5913 matched strings is returned. The nmatch and pmatch arguments of
5914 regexec() are ignored.
5915
5916 Otherwise,the portion of the string that was matched, and also any cap-
5917 tured substrings, are returned via the pmatch argument, which points to
5918 an array of nmatch structures of type regmatch_t, containing the mem-
5919 bers rm_so and rm_eo. These contain the offset to the first character
5920 of each substring and the offset to the first character after the end
5921 of each substring, respectively. The 0th element of the vector relates
5922 to the entire portion of string that was matched; subsequent elements
5923 relate to the capturing subpatterns of the regular expression. Unused
5924 entries in the array have both structure members set to -1.
5925
5926 A successful match yields a zero return; various error codes are
5927 defined in the header file, of which REG_NOMATCH is the "expected"
5928 failure code.
5929
5930
5931 ERROR MESSAGES
5932
5933 The regerror() function maps a non-zero errorcode from either regcomp()
5934 or regexec() to a printable message. If preg is not NULL, the error
5935 should have arisen from the use of that structure. A message terminated
5936 by a binary zero is placed in errbuf. The length of the message,
5937 including the zero, is limited to errbuf_size. The yield of the func-
5938 tion is the size of buffer needed to hold the whole message.
5939
5940
5941 MEMORY USAGE
5942
5943 Compiling a regular expression causes memory to be allocated and asso-
5944 ciated with the preg structure. The function regfree() frees all such
5945 memory, after which preg may no longer be used as a compiled expres-
5946 sion.
5947
5948
5949 AUTHOR
5950
5951 Philip Hazel
5952 University Computing Service
5953 Cambridge CB2 3QH, England.
5954
5955
5956 REVISION
5957
5958 Last updated: 06 March 2007
5959 Copyright (c) 1997-2007 University of Cambridge.
5960 ------------------------------------------------------------------------------
5961
5962
5963 PCRECPP(3) PCRECPP(3)
5964
5965
5966 NAME
5967 PCRE - Perl-compatible regular expressions.
5968
5969
5970 SYNOPSIS OF C++ WRAPPER
5971
5972 #include <pcrecpp.h>
5973
5974
5975 DESCRIPTION
5976
5977 The C++ wrapper for PCRE was provided by Google Inc. Some additional
5978 functionality was added by Giuseppe Maxia. This brief man page was con-
5979 structed from the notes in the pcrecpp.h file, which should be con-
5980 sulted for further details.
5981
5982
5983 MATCHING INTERFACE
5984
5985 The "FullMatch" operation checks that supplied text matches a supplied
5986 pattern exactly. If pointer arguments are supplied, it copies matched
5987 sub-strings that match sub-patterns into them.
5988
5989 Example: successful match
5990 pcrecpp::RE re("h.*o");
5991 re.FullMatch("hello");
5992
5993 Example: unsuccessful match (requires full match):
5994 pcrecpp::RE re("e");
5995 !re.FullMatch("hello");
5996
5997 Example: creating a temporary RE object:
5998 pcrecpp::RE("h.*o").FullMatch("hello");
5999
6000 You can pass in a "const char*" or a "string" for "text". The examples
6001 below tend to use a const char*. You can, as in the different examples
6002 above, store the RE object explicitly in a variable or use a temporary
6003 RE object. The examples below use one mode or the other arbitrarily.
6004 Either could correctly be used for any of these examples.
6005
6006 You must supply extra pointer arguments to extract matched subpieces.
6007
6008 Example: extracts "ruby" into "s" and 1234 into "i"
6009 int i;
6010 string s;
6011 pcrecpp::RE re("(\\w+):(\\d+)");
6012 re.FullMatch("ruby:1234", &s, &i);
6013
6014 Example: does not try to extract any extra sub-patterns
6015 re.FullMatch("ruby:1234", &s);
6016
6017 Example: does not try to extract into NULL
6018 re.FullMatch("ruby:1234", NULL, &i);
6019
6020 Example: integer overflow causes failure
6021 !re.FullMatch("ruby:1234567891234", NULL, &i);
6022
6023 Example: fails because there aren't enough sub-patterns:
6024 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
6025
6026 Example: fails because string cannot be stored in integer
6027 !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
6028
6029 The provided pointer arguments can be pointers to any scalar numeric
6030 type, or one of:
6031
6032 string (matched piece is copied to string)
6033 StringPiece (StringPiece is mutated to point to matched piece)
6034 T (where "bool T::ParseFrom(const char*, int)" exists)
6035 NULL (the corresponding matched sub-pattern is not copied)
6036
6037 The function returns true iff all of the following conditions are sat-
6038 isfied:
6039
6040 a. "text" matches "pattern" exactly;
6041
6042 b. The number of matched sub-patterns is >= number of supplied
6043 pointers;
6044
6045 c. The "i"th argument has a suitable type for holding the
6046 string captured as the "i"th sub-pattern. If you pass in
6047 void * NULL for the "i"th argument, or a non-void * NULL
6048 of the correct type, or pass fewer arguments than the
6049 number of sub-patterns, "i"th captured sub-pattern is
6050 ignored.
6051
6052 CAVEAT: An optional sub-pattern that does not exist in the matched
6053 string is assigned the empty string. Therefore, the following will
6054 return false (because the empty string is not a valid number):
6055
6056 int number;
6057 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
6058
6059 The matching interface supports at most 16 arguments per call. If you
6060 need more, consider using the more general interface
6061 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
6062
6063
6064 QUOTING METACHARACTERS
6065
6066 You can use the "QuoteMeta" operation to insert backslashes before all
6067 potentially meaningful characters in a string. The returned string,
6068 used as a regular expression, will exactly match the original string.
6069
6070 Example:
6071 string quoted = RE::QuoteMeta(unquoted);
6072
6073 Note that it's legal to escape a character even if it has no special
6074 meaning in a regular expression -- so this function does that. (This
6075 also makes it identical to the perl function of the same name; see
6076 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
6077 "1\.5\-2\.0\?".
6078
6079
6080 PARTIAL MATCHES
6081
6082 You can use the "PartialMatch" operation when you want the pattern to
6083 match any substring of the text.
6084
6085 Example: simple search for a string:
6086 pcrecpp::RE("ell").PartialMatch("hello");
6087
6088 Example: find first number in a string:
6089 int number;
6090 pcrecpp::RE re("(\\d+)");
6091 re.PartialMatch("x*100 + 20", &number);
6092 assert(number == 100);
6093
6094
6095 UTF-8 AND THE MATCHING INTERFACE
6096
6097 By default, pattern and text are plain text, one byte per character.
6098 The UTF8 flag, passed to the constructor, causes both pattern and
6099 string to be treated as UTF-8 text, still a byte stream but potentially
6100 multiple bytes per character. In practice, the text is likelier to be
6101 UTF-8 than the pattern, but the match returned may depend on the UTF8
6102 flag, so always use it when matching UTF8 text. For example, "." will
6103 match one byte normally but with UTF8 set may match up to three bytes
6104 of a multi-byte character.
6105
6106 Example:
6107 pcrecpp::RE_Options options;
6108 options.set_utf8();
6109