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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.
49
50 Some features of PCRE can be included, excluded, or changed when the
51 library is built. The pcre_config() function makes it possible for a
52 client to discover which features are available. The features them-
53 selves are described in the pcrebuild page. Documentation about build-
54 ing PCRE for various operating systems can be found in the README file
55 in the source distribution.
56
57 The library contains a number of undocumented internal functions and
58 data tables that are used by more than one of the exported external
59 functions, but which are not intended for use by external callers.
60 Their names all begin with "_pcre_", which hopefully will not provoke
61 any name clashes. In some environments, it is possible to control which
62 external symbols are exported when a shared library is built, and in
63 these cases the undocumented symbols are not exported.
64
65
66 USER DOCUMENTATION
67
68 The user documentation for PCRE comprises a number of different sec-
69 tions. In the "man" format, each of these is a separate "man page". In
70 the HTML format, each is a separate page, linked from the index page.
71 In the plain text format, all the sections are concatenated, for ease
72 of searching. The sections are as follows:
73
74 pcre this document
75 pcreapi details of PCRE's native C API
76 pcrebuild options for building PCRE
77 pcrecallout details of the callout feature
78 pcrecompat discussion of Perl compatibility
79 pcrecpp details of the C++ wrapper
80 pcregrep description of the pcregrep command
81 pcrematching discussion of the two matching algorithms
82 pcrepartial details of the partial matching facility
83 pcrepattern syntax and semantics of supported
84 regular expressions
85 pcreperform discussion of performance issues
86 pcreposix the POSIX-compatible C API
87 pcreprecompile details of saving and re-using precompiled patterns
88 pcresample discussion of the sample program
89 pcrestack discussion of stack usage
90 pcretest description of the pcretest testing command
91
92 In addition, in the "man" and HTML formats, there is a short page for
93 each C library function, listing its arguments and results.
94
95
96 LIMITATIONS
97
98 There are some size limitations in PCRE but it is hoped that they will
99 never in practice be relevant.
100
101 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
102 is compiled with the default internal linkage size of 2. If you want to
103 process regular expressions that are truly enormous, you can compile
104 PCRE with an internal linkage size of 3 or 4 (see the README file in
105 the source distribution and the pcrebuild documentation for details).
106 In these cases the limit is substantially larger. However, the speed
107 of execution is slower.
108
109 All values in repeating quantifiers must be less than 65536. The maxi-
110 mum compiled length of subpattern with an explicit repeat count is
111 30000 bytes. The maximum number of capturing subpatterns is 65535.
112
113 There is no limit to the number of parenthesized subpatterns, but there
114 can be no more than 65535 capturing subpatterns.
115
116 The maximum length of name for a named subpattern is 32 characters, and
117 the maximum number of named subpatterns is 10000.
118
119 The maximum length of a subject string is the largest positive number
120 that an integer variable can hold. However, when using the traditional
121 matching function, PCRE uses recursion to handle subpatterns and indef-
122 inite repetition. This means that the available stack space may limit
123 the size of a subject string that can be processed by certain patterns.
124 For a discussion of stack issues, see the pcrestack documentation.
125
126
127 UTF-8 AND UNICODE PROPERTY SUPPORT
128
129 From release 3.3, PCRE has had some support for character strings
130 encoded in the UTF-8 format. For release 4.0 this was greatly extended
131 to cover most common requirements, and in release 5.0 additional sup-
132 port for Unicode general category properties was added.
133
134 In order process UTF-8 strings, you must build PCRE to include UTF-8
135 support in the code, and, in addition, you must call pcre_compile()
136 with the PCRE_UTF8 option flag. When you do this, both the pattern and
137 any subject strings that are matched against it are treated as UTF-8
138 strings instead of just strings of bytes.
139
140 If you compile PCRE with UTF-8 support, but do not use it at run time,
141 the library will be a bit bigger, but the additional run time overhead
142 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
143 very big.
144
145 If PCRE is built with Unicode character property support (which implies
146 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
147 ported. The available properties that can be tested are limited to the
148 general category properties such as Lu for an upper case letter or Nd
149 for a decimal number, the Unicode script names such as Arabic or Han,
150 and the derived properties Any and L&. A full list is given in the
151 pcrepattern documentation. Only the short names for properties are sup-
152 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
153 ter}, is not supported. Furthermore, in Perl, many properties may
154 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
155 does not support this.
156
157 The following comments apply when PCRE is running in UTF-8 mode:
158
159 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and
160 subjects are checked for validity on entry to the relevant functions.
161 If an invalid UTF-8 string is passed, an error return is given. In some
162 situations, you may already know that your strings are valid, and
163 therefore want to skip these checks in order to improve performance. If
164 you set the PCRE_NO_UTF8_CHECK flag at compile time or at run time,
165 PCRE assumes that the pattern or subject it is given (respectively)
166 contains only valid UTF-8 codes. In this case, it does not diagnose an
167 invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
168 PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program may
169 crash.
170
171 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a
172 two-byte UTF-8 character if the value is greater than 127.
173
174 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
175 characters for values greater than \177.
176
177 4. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
178 vidual bytes, for example: \x{100}{3}.
179
180 5. The dot metacharacter matches one UTF-8 character instead of a sin-
181 gle byte.
182
183 6. The escape sequence \C can be used to match a single byte in UTF-8
184 mode, but its use can lead to some strange effects. This facility is
185 not available in the alternative matching function, pcre_dfa_exec().
186
187 7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
188 test characters of any code value, but the characters that PCRE recog-
189 nizes as digits, spaces, or word characters remain the same set as
190 before, all with values less than 256. This remains true even when PCRE
191 includes Unicode property support, because to do otherwise would slow
192 down PCRE in many common cases. If you really want to test for a wider
193 sense of, say, "digit", you must use Unicode property tests such as
194 \p{Nd}.
195
196 8. Similarly, characters that match the POSIX named character classes
197 are all low-valued characters.
198
199 9. Case-insensitive matching applies only to characters whose values
200 are less than 128, unless PCRE is built with Unicode property support.
201 Even when Unicode property support is available, PCRE still uses its
202 own character tables when checking the case of low-valued characters,
203 so as not to degrade performance. The Unicode property information is
204 used only for characters with higher values. Even when Unicode property
205 support is available, PCRE supports case-insensitive matching only when
206 there is a one-to-one mapping between a letter's cases. There are a
207 small number of many-to-one mappings in Unicode; these are not sup-
208 ported by PCRE.
209
210
211 AUTHOR
212
213 Philip Hazel
214 University Computing Service
215 Cambridge CB2 3QH, England.
216
217 Putting an actual email address here seems to have been a spam magnet,
218 so I've taken it away. If you want to email me, use my initial and sur-
219 name, separated by a dot, at the domain ucs.cam.ac.uk.
220
221
222 REVISION
223
224 Last updated: 06 March 2007
225 Copyright (c) 1997-2007 University of Cambridge.
226 ------------------------------------------------------------------------------
227
228
229 PCREBUILD(3) PCREBUILD(3)
230
231
232 NAME
233 PCRE - Perl-compatible regular expressions
234
235
236 PCRE BUILD-TIME OPTIONS
237
238 This document describes the optional features of PCRE that can be
239 selected when the library is compiled. They are all selected, or dese-
240 lected, by providing options to the configure script that is run before
241 the make command. The complete list of options for configure (which
242 includes the standard ones such as the selection of the installation
243 directory) can be obtained by running
244
245 ./configure --help
246
247 The following sections include descriptions of options whose names
248 begin with --enable or --disable. These settings specify changes to the
249 defaults for the configure command. Because of the way that configure
250 works, --enable and --disable always come in pairs, so the complemen-
251 tary option always exists as well, but as it specifies the default, it
252 is not described.
253
254
255 C++ SUPPORT
256
257 By default, the configure script will search for a C++ compiler and C++
258 header files. If it finds them, it automatically builds the C++ wrapper
259 library for PCRE. You can disable this by adding
260
261 --disable-cpp
262
263 to the configure command.
264
265
266 UTF-8 SUPPORT
267
268 To build PCRE with support for UTF-8 character strings, add
269
270 --enable-utf8
271
272 to the configure command. Of itself, this does not make PCRE treat
273 strings as UTF-8. As well as compiling PCRE with this option, you also
274 have have to set the PCRE_UTF8 option when you call the pcre_compile()
275 function.
276
277
278 UNICODE CHARACTER PROPERTY SUPPORT
279
280 UTF-8 support allows PCRE to process character values greater than 255
281 in the strings that it handles. On its own, however, it does not pro-
282 vide any facilities for accessing the properties of such characters. If
283 you want to be able to use the pattern escapes \P, \p, and \X, which
284 refer to Unicode character properties, you must add
285
286 --enable-unicode-properties
287
288 to the configure command. This implies UTF-8 support, even if you have
289 not explicitly requested it.
290
291 Including Unicode property support adds around 30K of tables to the
292 PCRE library. Only the general category properties such as Lu and Nd
293 are supported. Details are given in the pcrepattern documentation.
294
295
296 CODE VALUE OF NEWLINE
297
298 By default, PCRE interprets character 10 (linefeed, LF) as indicating
299 the end of a line. This is the normal newline character on Unix-like
300 systems. You can compile PCRE to use character 13 (carriage return, CR)
301 instead, by adding
302
303 --enable-newline-is-cr
304
305 to the configure command. There is also a --enable-newline-is-lf
306 option, which explicitly specifies linefeed as the newline character.
307
308 Alternatively, you can specify that line endings are to be indicated by
309 the two character sequence CRLF. If you want this, add
310
311 --enable-newline-is-crlf
312
313 to the configure command. There is a fourth option, specified by
314
315 --enable-newline-is-any
316
317 which causes PCRE to recognize any Unicode newline sequence.
318
319 Whatever line ending convention is selected when PCRE is built can be
320 overridden when the library functions are called. At build time it is
321 conventional to use the standard for your operating system.
322
323
324 BUILDING SHARED AND STATIC LIBRARIES
325
326 The PCRE building process uses libtool to build both shared and static
327 Unix libraries by default. You can suppress one of these by adding one
328 of
329
330 --disable-shared
331 --disable-static
332
333 to the configure command, as required.
334
335
336 POSIX MALLOC USAGE
337
338 When PCRE is called through the POSIX interface (see the pcreposix doc-
339 umentation), additional working storage is required for holding the
340 pointers to capturing substrings, because PCRE requires three integers
341 per substring, whereas the POSIX interface provides only two. If the
342 number of expected substrings is small, the wrapper function uses space
343 on the stack, because this is faster than using malloc() for each call.
344 The default threshold above which the stack is no longer used is 10; it
345 can be changed by adding a setting such as
346
347 --with-posix-malloc-threshold=20
348
349 to the configure command.
350
351
352 HANDLING VERY LARGE PATTERNS
353
354 Within a compiled pattern, offset values are used to point from one
355 part to another (for example, from an opening parenthesis to an alter-
356 nation metacharacter). By default, two-byte values are used for these
357 offsets, leading to a maximum size for a compiled pattern of around
358 64K. This is sufficient to handle all but the most gigantic patterns.
359 Nevertheless, some people do want to process enormous patterns, so it
360 is possible to compile PCRE to use three-byte or four-byte offsets by
361 adding a setting such as
362
363 --with-link-size=3
364
365 to the configure command. The value given must be 2, 3, or 4. Using
366 longer offsets slows down the operation of PCRE because it has to load
367 additional bytes when handling them.
368
369
370 AVOIDING EXCESSIVE STACK USAGE
371
372 When matching with the pcre_exec() function, PCRE implements backtrack-
373 ing by making recursive calls to an internal function called match().
374 In environments where the size of the stack is limited, this can se-
375 verely limit PCRE's operation. (The Unix environment does not usually
376 suffer from this problem, but it may sometimes be necessary to increase
377 the maximum stack size. There is a discussion in the pcrestack docu-
378 mentation.) An alternative approach to recursion that uses memory from
379 the heap to remember data, instead of using recursive function calls,
380 has been implemented to work round the problem of limited stack size.
381 If you want to build a version of PCRE that works this way, add
382
383 --disable-stack-for-recursion
384
385 to the configure command. With this configuration, PCRE will use the
386 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
387 ment functions. Separate functions are provided because the usage is
388 very predictable: the block sizes requested are always the same, and
389 the blocks are always freed in reverse order. A calling program might
390 be able to implement optimized functions that perform better than the
391 standard malloc() and free() functions. PCRE runs noticeably more
392 slowly when built in this way. This option affects only the pcre_exec()
393 function; it is not relevant for the the pcre_dfa_exec() function.
394
395
396 LIMITING PCRE RESOURCE USAGE
397
398 Internally, PCRE has a function called match(), which it calls repeat-
399 edly (sometimes recursively) when matching a pattern with the
400 pcre_exec() function. By controlling the maximum number of times this
401 function may be called during a single matching operation, a limit can
402 be placed on the resources used by a single call to pcre_exec(). The
403 limit can be changed at run time, as described in the pcreapi documen-
404 tation. The default is 10 million, but this can be changed by adding a
405 setting such as
406
407 --with-match-limit=500000
408
409 to the configure command. This setting has no effect on the
410 pcre_dfa_exec() matching function.
411
412 In some environments it is desirable to limit the depth of recursive
413 calls of match() more strictly than the total number of calls, in order
414 to restrict the maximum amount of stack (or heap, if --disable-stack-
415 for-recursion is specified) that is used. A second limit controls this;
416 it defaults to the value that is set for --with-match-limit, which
417 imposes no additional constraints. However, you can set a lower limit
418 by adding, for example,
419
420 --with-match-limit-recursion=10000
421
422 to the configure command. This value can also be overridden at run
423 time.
424
425
426 CREATING CHARACTER TABLES AT BUILD TIME
427
428 PCRE uses fixed tables for processing characters whose code values are
429 less than 256. By default, PCRE is built with a set of tables that are
430 distributed in the file pcre_chartables.c.dist. These tables are for
431 ASCII codes only. If you add
432
433 --enable-rebuild-chartables
434
435 to the configure command, the distributed tables are no longer used.
436 Instead, a program called dftables is compiled and run. This outputs
437 the source for new set of tables, created in the default locale of your
438 C runtime system. (This method of replacing the tables does not work if
439 you are cross compiling, because dftables is run on the local host. If
440 you need to create alternative tables when cross compiling, you will
441 have to do so "by hand".)
442
443
444 USING EBCDIC CODE
445
446 PCRE assumes by default that it will run in an environment where the
447 character code is ASCII (or Unicode, which is a superset of ASCII).
448 PCRE can, however, be compiled to run in an EBCDIC environment by
449 adding
450
451 --enable-ebcdic
452
453 to the configure command. This setting implies --enable-rebuild-charta-
454 bles.
455
456
457 SEE ALSO
458
459 pcreapi(3), pcre_config(3).
460
461
462 AUTHOR
463
464 Philip Hazel
465 University Computing Service
466 Cambridge CB2 3QH, England.
467
468
469 REVISION
470
471 Last updated: 20 March 2007
472 Copyright (c) 1997-2007 University of Cambridge.
473 ------------------------------------------------------------------------------
474
475
476 PCREMATCHING(3) PCREMATCHING(3)
477
478
479 NAME
480 PCRE - Perl-compatible regular expressions
481
482
483 PCRE MATCHING ALGORITHMS
484
485 This document describes the two different algorithms that are available
486 in PCRE for matching a compiled regular expression against a given sub-
487 ject string. The "standard" algorithm is the one provided by the
488 pcre_exec() function. This works in the same was as Perl's matching
489 function, and provides a Perl-compatible matching operation.
490
491 An alternative algorithm is provided by the pcre_dfa_exec() function;
492 this operates in a different way, and is not Perl-compatible. It has
493 advantages and disadvantages compared with the standard algorithm, and
494 these are described below.
495
496 When there is only one possible way in which a given subject string can
497 match a pattern, the two algorithms give the same answer. A difference
498 arises, however, when there are multiple possibilities. For example, if
499 the pattern
500
501 ^<.*>
502
503 is matched against the string
504
505 <something> <something else> <something further>
506
507 there are three possible answers. The standard algorithm finds only one
508 of them, whereas the alternative algorithm finds all three.
509
510
511 REGULAR EXPRESSIONS AS TREES
512
513 The set of strings that are matched by a regular expression can be rep-
514 resented as a tree structure. An unlimited repetition in the pattern
515 makes the tree of infinite size, but it is still a tree. Matching the
516 pattern to a given subject string (from a given starting point) can be
517 thought of as a search of the tree. There are two ways to search a
518 tree: depth-first and breadth-first, and these correspond to the two
519 matching algorithms provided by PCRE.
520
521
522 THE STANDARD MATCHING ALGORITHM
523
524 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
525 sions", the standard algorithm is an "NFA algorithm". It conducts a
526 depth-first search of the pattern tree. That is, it proceeds along a
527 single path through the tree, checking that the subject matches what is
528 required. When there is a mismatch, the algorithm tries any alterna-
529 tives at the current point, and if they all fail, it backs up to the
530 previous branch point in the tree, and tries the next alternative
531 branch at that level. This often involves backing up (moving to the
532 left) in the subject string as well. The order in which repetition
533 branches are tried is controlled by the greedy or ungreedy nature of
534 the quantifier.
535
536 If a leaf node is reached, a matching string has been found, and at
537 that point the algorithm stops. Thus, if there is more than one possi-
538 ble match, this algorithm returns the first one that it finds. Whether
539 this is the shortest, the longest, or some intermediate length depends
540 on the way the greedy and ungreedy repetition quantifiers are specified
541 in the pattern.
542
543 Because it ends up with a single path through the tree, it is rela-
544 tively straightforward for this algorithm to keep track of the sub-
545 strings that are matched by portions of the pattern in parentheses.
546 This provides support for capturing parentheses and back references.
547
548
549 THE ALTERNATIVE MATCHING ALGORITHM
550
551 This algorithm conducts a breadth-first search of the tree. Starting
552 from the first matching point in the subject, it scans the subject
553 string from left to right, once, character by character, and as it does
554 this, it remembers all the paths through the tree that represent valid
555 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
556 though it is not implemented as a traditional finite state machine (it
557 keeps multiple states active simultaneously).
558
559 The scan continues until either the end of the subject is reached, or
560 there are no more unterminated paths. At this point, terminated paths
561 represent the different matching possibilities (if there are none, the
562 match has failed). Thus, if there is more than one possible match,
563 this algorithm finds all of them, and in particular, it finds the long-
564 est. In PCRE, there is an option to stop the algorithm after the first
565 match (which is necessarily the shortest) has been found.
566
567 Note that all the matches that are found start at the same point in the
568 subject. If the pattern
569
570 cat(er(pillar)?)
571
572 is matched against the string "the caterpillar catchment", the result
573 will be the three strings "cat", "cater", and "caterpillar" that start
574 at the fourth character of the subject. The algorithm does not automat-
575 ically move on to find matches that start at later positions.
576
577 There are a number of features of PCRE regular expressions that are not
578 supported by the alternative matching algorithm. They are as follows:
579
580 1. Because the algorithm finds all possible matches, the greedy or
581 ungreedy nature of repetition quantifiers is not relevant. Greedy and
582 ungreedy quantifiers are treated in exactly the same way. However, pos-
583 sessive quantifiers can make a difference when what follows could also
584 match what is quantified, for example in a pattern like this:
585
586 ^a++\w!
587
588 This pattern matches "aaab!" but not "aaa!", which would be matched by
589 a non-possessive quantifier. Similarly, if an atomic group is present,
590 it is matched as if it were a standalone pattern at the current point,
591 and the longest match is then "locked in" for the rest of the overall
592 pattern.
593
594 2. When dealing with multiple paths through the tree simultaneously, it
595 is not straightforward to keep track of captured substrings for the
596 different matching possibilities, and PCRE's implementation of this
597 algorithm does not attempt to do this. This means that no captured sub-
598 strings are available.
599
600 3. Because no substrings are captured, back references within the pat-
601 tern are not supported, and cause errors if encountered.
602
603 4. For the same reason, conditional expressions that use a backrefer-
604 ence as the condition or test for a specific group recursion are not
605 supported.
606
607 5. Callouts are supported, but the value of the capture_top field is
608 always 1, and the value of the capture_last field is always -1.
609
610 6. The \C escape sequence, which (in the standard algorithm) matches a
611 single byte, even in UTF-8 mode, is not supported because the alterna-
612 tive algorithm moves through the subject string one character at a
613 time, for all active paths through the tree.
614
615
616 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
617
618 Using the alternative matching algorithm provides the following advan-
619 tages:
620
621 1. All possible matches (at a single point in the subject) are automat-
622 ically found, and in particular, the longest match is found. To find
623 more than one match using the standard algorithm, you have to do kludgy
624 things with callouts.
625
626 2. There is much better support for partial matching. The restrictions
627 on the content of the pattern that apply when using the standard algo-
628 rithm for partial matching do not apply to the alternative algorithm.
629 For non-anchored patterns, the starting position of a partial match is
630 available.
631
632 3. Because the alternative algorithm scans the subject string just
633 once, and never needs to backtrack, it is possible to pass very long
634 subject strings to the matching function in several pieces, checking
635 for partial matching each time.
636
637
638 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
639
640 The alternative algorithm suffers from a number of disadvantages:
641
642 1. It is substantially slower than the standard algorithm. This is
643 partly because it has to search for all possible matches, but is also
644 because it is less susceptible to optimization.
645
646 2. Capturing parentheses and back references are not supported.
647
648 3. Although atomic groups are supported, their use does not provide the
649 performance advantage that it does for the standard algorithm.
650
651
652 AUTHOR
653
654 Philip Hazel
655 University Computing Service
656 Cambridge CB2 3QH, England.
657
658
659 REVISION
660
661 Last updated: 06 March 2007
662 Copyright (c) 1997-2007 University of Cambridge.
663 ------------------------------------------------------------------------------
664
665
666 PCREAPI(3) PCREAPI(3)
667
668
669 NAME
670 PCRE - Perl-compatible regular expressions
671
672
673 PCRE NATIVE API
674
675 #include <pcre.h>
676
677 pcre *pcre_compile(const char *pattern, int options,
678 const char **errptr, int *erroffset,
679 const unsigned char *tableptr);
680
681 pcre *pcre_compile2(const char *pattern, int options,
682 int *errorcodeptr,
683 const char **errptr, int *erroffset,
684 const unsigned char *tableptr);
685
686 pcre_extra *pcre_study(const pcre *code, int options,
687 const char **errptr);
688
689 int pcre_exec(const pcre *code, const pcre_extra *extra,
690 const char *subject, int length, int startoffset,
691 int options, int *ovector, int ovecsize);
692
693 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
694 const char *subject, int length, int startoffset,
695 int options, int *ovector, int ovecsize,
696 int *workspace, int wscount);
697
698 int pcre_copy_named_substring(const pcre *code,
699 const char *subject, int *ovector,
700 int stringcount, const char *stringname,
701 char *buffer, int buffersize);
702
703 int pcre_copy_substring(const char *subject, int *ovector,
704 int stringcount, int stringnumber, char *buffer,
705 int buffersize);
706
707 int pcre_get_named_substring(const pcre *code,
708 const char *subject, int *ovector,
709 int stringcount, const char *stringname,
710 const char **stringptr);
711
712 int pcre_get_stringnumber(const pcre *code,
713 const char *name);
714
715 int pcre_get_stringtable_entries(const pcre *code,
716 const char *name, char **first, char **last);
717
718 int pcre_get_substring(const char *subject, int *ovector,
719 int stringcount, int stringnumber,
720 const char **stringptr);
721
722 int pcre_get_substring_list(const char *subject,
723 int *ovector, int stringcount, const char ***listptr);
724
725 void pcre_free_substring(const char *stringptr);
726
727 void pcre_free_substring_list(const char **stringptr);
728
729 const unsigned char *pcre_maketables(void);
730
731 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
732 int what, void *where);
733
734 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
735
736 int pcre_refcount(pcre *code, int adjust);
737
738 int pcre_config(int what, void *where);
739
740 char *pcre_version(void);
741
742 void *(*pcre_malloc)(size_t);
743
744 void (*pcre_free)(void *);
745
746 void *(*pcre_stack_malloc)(size_t);
747
748 void (*pcre_stack_free)(void *);
749
750 int (*pcre_callout)(pcre_callout_block *);
751
752
753 PCRE API OVERVIEW
754
755 PCRE has its own native API, which is described in this document. There
756 are also some wrapper functions that correspond to the POSIX regular
757 expression API. These are described in the pcreposix documentation.
758 Both of these APIs define a set of C function calls. A C++ wrapper is
759 distributed with PCRE. It is documented in the pcrecpp page.
760
761 The native API C function prototypes are defined in the header file
762 pcre.h, and on Unix systems the library itself is called libpcre. It
763 can normally be accessed by adding -lpcre to the command for linking an
764 application that uses PCRE. The header file defines the macros
765 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
766 bers for the library. Applications can use these to include support
767 for different releases of PCRE.
768
769 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
770 pcre_exec() are used for compiling and matching regular expressions in
771 a Perl-compatible manner. A sample program that demonstrates the sim-
772 plest way of using them is provided in the file called pcredemo.c in
773 the source distribution. The pcresample documentation describes how to
774 run it.
775
776 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
777 ble, is also provided. This uses a different algorithm for the match-
778 ing. The alternative algorithm finds all possible matches (at a given
779 point in the subject), and scans the subject just once. However, this
780 algorithm does not return captured substrings. A description of the two
781 matching algorithms and their advantages and disadvantages is given in
782 the pcrematching documentation.
783
784 In addition to the main compiling and matching functions, there are
785 convenience functions for extracting captured substrings from a subject
786 string that is matched by pcre_exec(). They are:
787
788 pcre_copy_substring()
789 pcre_copy_named_substring()
790 pcre_get_substring()
791 pcre_get_named_substring()
792 pcre_get_substring_list()
793 pcre_get_stringnumber()
794 pcre_get_stringtable_entries()
795
796 pcre_free_substring() and pcre_free_substring_list() are also provided,
797 to free the memory used for extracted strings.
798
799 The function pcre_maketables() is used to build a set of character
800 tables in the current locale for passing to pcre_compile(),
801 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
802 provided for specialist use. Most commonly, no special tables are
803 passed, in which case internal tables that are generated when PCRE is
804 built are used.
805
806 The function pcre_fullinfo() is used to find out information about a
807 compiled pattern; pcre_info() is an obsolete version that returns only
808 some of the available information, but is retained for backwards com-
809 patibility. The function pcre_version() returns a pointer to a string
810 containing the version of PCRE and its date of release.
811
812 The function pcre_refcount() maintains a reference count in a data
813 block containing a compiled pattern. This is provided for the benefit
814 of object-oriented applications.
815
816 The global variables pcre_malloc and pcre_free initially contain the
817 entry points of the standard malloc() and free() functions, respec-
818 tively. PCRE calls the memory management functions via these variables,
819 so a calling program can replace them if it wishes to intercept the
820 calls. This should be done before calling any PCRE functions.
821
822 The global variables pcre_stack_malloc and pcre_stack_free are also
823 indirections to memory management functions. These special functions
824 are used only when PCRE is compiled to use the heap for remembering
825 data, instead of recursive function calls, when running the pcre_exec()
826 function. See the pcrebuild documentation for details of how to do
827 this. It is a non-standard way of building PCRE, for use in environ-
828 ments that have limited stacks. Because of the greater use of memory
829 management, it runs more slowly. Separate functions are provided so
830 that special-purpose external code can be used for this case. When
831 used, these functions are always called in a stack-like manner (last
832 obtained, first freed), and always for memory blocks of the same size.
833 There is a discussion about PCRE's stack usage in the pcrestack docu-
834 mentation.
835
836 The global variable pcre_callout initially contains NULL. It can be set
837 by the caller to a "callout" function, which PCRE will then call at
838 specified points during a matching operation. Details are given in the
839 pcrecallout documentation.
840
841
842 NEWLINES
843
844 PCRE supports four different conventions for indicating line breaks in
845 strings: a single CR (carriage return) character, a single LF (line-
846 feed) character, the two-character sequence CRLF, or any Unicode new-
847 line sequence. The Unicode newline sequences are the three just men-
848 tioned, plus the single characters VT (vertical tab, U+000B), FF (form-
849 feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028),
850 and PS (paragraph separator, U+2029).
851
852 Each of the first three conventions is used by at least one operating
853 system as its standard newline sequence. When PCRE is built, a default
854 can be specified. The default default is LF, which is the Unix stan-
855 dard. When PCRE is run, the default can be overridden, either when a
856 pattern is compiled, or when it is matched.
857
858 In the PCRE documentation the word "newline" is used to mean "the char-
859 acter or pair of characters that indicate a line break". The choice of
860 newline convention affects the handling of the dot, circumflex, and
861 dollar metacharacters, the handling of #-comments in /x mode, and, when
862 CRLF is a recognized line ending sequence, the match position advance-
863 ment for a non-anchored pattern. The choice of newline convention does
864 not affect the interpretation of the \n or \r escape sequences.
865
866
867 MULTITHREADING
868
869 The PCRE functions can be used in multi-threading applications, with
870 the proviso that the memory management functions pointed to by
871 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
872 callout function pointed to by pcre_callout, are shared by all threads.
873
874 The compiled form of a regular expression is not altered during match-
875 ing, so the same compiled pattern can safely be used by several threads
876 at once.
877
878
879 SAVING PRECOMPILED PATTERNS FOR LATER USE
880
881 The compiled form of a regular expression can be saved and re-used at a
882 later time, possibly by a different program, and even on a host other
883 than the one on which it was compiled. Details are given in the
884 pcreprecompile documentation.
885
886
887 CHECKING BUILD-TIME OPTIONS
888
889 int pcre_config(int what, void *where);
890
891 The function pcre_config() makes it possible for a PCRE client to dis-
892 cover which optional features have been compiled into the PCRE library.
893 The pcrebuild documentation has more details about these optional fea-
894 tures.
895
896 The first argument for pcre_config() is an integer, specifying which
897 information is required; the second argument is a pointer to a variable
898 into which the information is placed. The following information is
899 available:
900
901 PCRE_CONFIG_UTF8
902
903 The output is an integer that is set to one if UTF-8 support is avail-
904 able; otherwise it is set to zero.
905
906 PCRE_CONFIG_UNICODE_PROPERTIES
907
908 The output is an integer that is set to one if support for Unicode
909 character properties is available; otherwise it is set to zero.
910
911 PCRE_CONFIG_NEWLINE
912
913 The output is an integer whose value specifies the default character
914 sequence that is recognized as meaning "newline". The four values that
915 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, and -1 for ANY.
916 The default should normally be the standard sequence for your operating
917 system.
918
919 PCRE_CONFIG_LINK_SIZE
920
921 The output is an integer that contains the number of bytes used for
922 internal linkage in compiled regular expressions. The value is 2, 3, or
923 4. Larger values allow larger regular expressions to be compiled, at
924 the expense of slower matching. The default value of 2 is sufficient
925 for all but the most massive patterns, since it allows the compiled
926 pattern to be up to 64K in size.
927
928 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
929
930 The output is an integer that contains the threshold above which the
931 POSIX interface uses malloc() for output vectors. Further details are
932 given in the pcreposix documentation.
933
934 PCRE_CONFIG_MATCH_LIMIT
935
936 The output is an integer that gives the default limit for the number of
937 internal matching function calls in a pcre_exec() execution. Further
938 details are given with pcre_exec() below.
939
940 PCRE_CONFIG_MATCH_LIMIT_RECURSION
941
942 The output is an integer that gives the default limit for the depth of
943 recursion when calling the internal matching function in a pcre_exec()
944 execution. Further details are given with pcre_exec() below.
945
946 PCRE_CONFIG_STACKRECURSE
947
948 The output is an integer that is set to one if internal recursion when
949 running pcre_exec() is implemented by recursive function calls that use
950 the stack to remember their state. This is the usual way that PCRE is
951 compiled. The output is zero if PCRE was compiled to use blocks of data
952 on the heap instead of recursive function calls. In this case,
953 pcre_stack_malloc and pcre_stack_free are called to manage memory
954 blocks on the heap, thus avoiding the use of the stack.
955
956
957 COMPILING A PATTERN
958
959 pcre *pcre_compile(const char *pattern, int options,
960 const char **errptr, int *erroffset,
961 const unsigned char *tableptr);
962
963 pcre *pcre_compile2(const char *pattern, int options,
964 int *errorcodeptr,
965 const char **errptr, int *erroffset,
966 const unsigned char *tableptr);
967
968 Either of the functions pcre_compile() or pcre_compile2() can be called
969 to compile a pattern into an internal form. The only difference between
970 the two interfaces is that pcre_compile2() has an additional argument,
971 errorcodeptr, via which a numerical error code can be returned.
972
973 The pattern is a C string terminated by a binary zero, and is passed in
974 the pattern argument. A pointer to a single block of memory that is
975 obtained via pcre_malloc is returned. This contains the compiled code
976 and related data. The pcre type is defined for the returned block; this
977 is a typedef for a structure whose contents are not externally defined.
978 It is up to the caller to free the memory (via pcre_free) when it is no
979 longer required.
980
981 Although the compiled code of a PCRE regex is relocatable, that is, it
982 does not depend on memory location, the complete pcre data block is not
983 fully relocatable, because it may contain a copy of the tableptr argu-
984 ment, which is an address (see below).
985
986 The options argument contains various bit settings that affect the com-
987 pilation. It should be zero if no options are required. The available
988 options are described below. Some of them, in particular, those that
989 are compatible with Perl, can also be set and unset from within the
990 pattern (see the detailed description in the pcrepattern documenta-
991 tion). For these options, the contents of the options argument speci-
992 fies their initial settings at the start of compilation and execution.
993 The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
994 of matching as well as at compile time.
995
996 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
997 if compilation of a pattern fails, pcre_compile() returns NULL, and
998 sets the variable pointed to by errptr to point to a textual error mes-
999 sage. This is a static string that is part of the library. You must not
1000 try to free it. The offset from the start of the pattern to the charac-
1001 ter where the error was discovered is placed in the variable pointed to
1002 by erroffset, which must not be NULL. If it is, an immediate error is
1003 given.
1004
1005 If pcre_compile2() is used instead of pcre_compile(), and the error-
1006 codeptr argument is not NULL, a non-zero error code number is returned
1007 via this argument in the event of an error. This is in addition to the
1008 textual error message. Error codes and messages are listed below.
1009
1010 If the final argument, tableptr, is NULL, PCRE uses a default set of
1011 character tables that are built when PCRE is compiled, using the
1012 default C locale. Otherwise, tableptr must be an address that is the
1013 result of a call to pcre_maketables(). This value is stored with the
1014 compiled pattern, and used again by pcre_exec(), unless another table
1015 pointer is passed to it. For more discussion, see the section on locale
1016 support below.
1017
1018 This code fragment shows a typical straightforward call to pcre_com-
1019 pile():
1020
1021 pcre *re;
1022 const char *error;
1023 int erroffset;
1024 re = pcre_compile(
1025 "^A.*Z", /* the pattern */
1026 0, /* default options */
1027 &error, /* for error message */
1028 &erroffset, /* for error offset */
1029 NULL); /* use default character tables */
1030
1031 The following names for option bits are defined in the pcre.h header
1032 file:
1033
1034 PCRE_ANCHORED
1035
1036 If this bit is set, the pattern is forced to be "anchored", that is, it
1037 is constrained to match only at the first matching point in the string
1038 that is being searched (the "subject string"). This effect can also be
1039 achieved by appropriate constructs in the pattern itself, which is the
1040 only way to do it in Perl.
1041
1042 PCRE_AUTO_CALLOUT
1043
1044 If this bit is set, pcre_compile() automatically inserts callout items,
1045 all with number 255, before each pattern item. For discussion of the
1046 callout facility, see the pcrecallout documentation.
1047
1048 PCRE_CASELESS
1049
1050 If this bit is set, letters in the pattern match both upper and lower
1051 case letters. It is equivalent to Perl's /i option, and it can be
1052 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1053 always understands the concept of case for characters whose values are
1054 less than 128, so caseless matching is always possible. For characters
1055 with higher values, the concept of case is supported if PCRE is com-
1056 piled with Unicode property support, but not otherwise. If you want to
1057 use caseless matching for characters 128 and above, you must ensure
1058 that PCRE is compiled with Unicode property support as well as with
1059 UTF-8 support.
1060
1061 PCRE_DOLLAR_ENDONLY
1062
1063 If this bit is set, a dollar metacharacter in the pattern matches only
1064 at the end of the subject string. Without this option, a dollar also
1065 matches immediately before a newline at the end of the string (but not
1066 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1067 if PCRE_MULTILINE is set. There is no equivalent to this option in
1068 Perl, and no way to set it within a pattern.
1069
1070 PCRE_DOTALL
1071
1072 If this bit is set, a dot metacharater in the pattern matches all char-
1073 acters, including those that indicate newline. Without it, a dot does
1074 not match when the current position is at a newline. This option is
1075 equivalent to Perl's /s option, and it can be changed within a pattern
1076 by a (?s) option setting. A negative class such as [^a] always matches
1077 newline characters, independent of the setting of this option.
1078
1079 PCRE_DUPNAMES
1080
1081 If this bit is set, names used to identify capturing subpatterns need
1082 not be unique. This can be helpful for certain types of pattern when it
1083 is known that only one instance of the named subpattern can ever be
1084 matched. There are more details of named subpatterns below; see also
1085 the pcrepattern documentation.
1086
1087 PCRE_EXTENDED
1088
1089 If this bit is set, whitespace data characters in the pattern are
1090 totally ignored except when escaped or inside a character class. White-
1091 space does not include the VT character (code 11). In addition, charac-
1092 ters between an unescaped # outside a character class and the next new-
1093 line, inclusive, are also ignored. This is equivalent to Perl's /x
1094 option, and it can be changed within a pattern by a (?x) option set-
1095 ting.
1096
1097 This option makes it possible to include comments inside complicated
1098 patterns. Note, however, that this applies only to data characters.
1099 Whitespace characters may never appear within special character
1100 sequences in a pattern, for example within the sequence (?( which
1101 introduces a conditional subpattern.
1102
1103 PCRE_EXTRA
1104
1105 This option was invented in order to turn on additional functionality
1106 of PCRE that is incompatible with Perl, but it is currently of very
1107 little use. When set, any backslash in a pattern that is followed by a
1108 letter that has no special meaning causes an error, thus reserving
1109 these combinations for future expansion. By default, as in Perl, a
1110 backslash followed by a letter with no special meaning is treated as a
1111 literal. (Perl can, however, be persuaded to give a warning for this.)
1112 There are at present no other features controlled by this option. It
1113 can also be set by a (?X) option setting within a pattern.
1114
1115 PCRE_FIRSTLINE
1116
1117 If this option is set, an unanchored pattern is required to match
1118 before or at the first newline in the subject string, though the
1119 matched text may continue over the newline.
1120
1121 PCRE_MULTILINE
1122
1123 By default, PCRE treats the subject string as consisting of a single
1124 line of characters (even if it actually contains newlines). The "start
1125 of line" metacharacter (^) matches only at the start of the string,
1126 while the "end of line" metacharacter ($) matches only at the end of
1127 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1128 is set). This is the same as Perl.
1129
1130 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1131 constructs match immediately following or immediately before internal
1132 newlines in the subject string, respectively, as well as at the very
1133 start and end. This is equivalent to Perl's /m option, and it can be
1134 changed within a pattern by a (?m) option setting. If there are no new-
1135 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1136 setting PCRE_MULTILINE has no effect.
1137
1138 PCRE_NEWLINE_CR
1139 PCRE_NEWLINE_LF
1140 PCRE_NEWLINE_CRLF
1141 PCRE_NEWLINE_ANY
1142
1143 These options override the default newline definition that was chosen
1144 when PCRE was built. Setting the first or the second specifies that a
1145 newline is indicated by a single character (CR or LF, respectively).
1146 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1147 two-character CRLF sequence. Setting PCRE_NEWLINE_ANY specifies that
1148 any Unicode newline sequence should be recognized. The Unicode newline
1149 sequences are the three just mentioned, plus the single characters VT
1150 (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085),
1151 LS (line separator, U+2028), and PS (paragraph separator, U+2029). The
1152 last two are recognized only in UTF-8 mode.
1153
1154 The newline setting in the options word uses three bits that are
1155 treated as a number, giving eight possibilities. Currently only five
1156 are used (default plus the four values above). This means that if you
1157 set more than one newline option, the combination may or may not be
1158 sensible. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equiva-
1159 lent to PCRE_NEWLINE_CRLF, but other combinations yield unused numbers
1160 and cause an error.
1161
1162 The only time that a line break is specially recognized when compiling
1163 a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1164 character class is encountered. This indicates a comment that lasts
1165 until after the next line break sequence. In other circumstances, line
1166 break sequences are treated as literal data, except that in
1167 PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1168 and are therefore ignored.
1169
1170 The newline option that is set at compile time becomes the default that
1171 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1172
1173 PCRE_NO_AUTO_CAPTURE
1174
1175 If this option is set, it disables the use of numbered capturing paren-
1176 theses in the pattern. Any opening parenthesis that is not followed by
1177 ? behaves as if it were followed by ?: but named parentheses can still
1178 be used for capturing (and they acquire numbers in the usual way).
1179 There is no equivalent of this option in Perl.
1180
1181 PCRE_UNGREEDY
1182
1183 This option inverts the "greediness" of the quantifiers so that they
1184 are not greedy by default, but become greedy if followed by "?". It is
1185 not compatible with Perl. It can also be set by a (?U) option setting
1186 within the pattern.
1187
1188 PCRE_UTF8
1189
1190 This option causes PCRE to regard both the pattern and the subject as
1191 strings of UTF-8 characters instead of single-byte character strings.
1192 However, it is available only when PCRE is built to include UTF-8 sup-
1193 port. If not, the use of this option provokes an error. Details of how
1194 this option changes the behaviour of PCRE are given in the section on
1195 UTF-8 support in the main pcre page.
1196
1197 PCRE_NO_UTF8_CHECK
1198
1199 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1200 automatically checked. If an invalid UTF-8 sequence of bytes is found,
1201 pcre_compile() returns an error. If you already know that your pattern
1202 is valid, and you want to skip this check for performance reasons, you
1203 can set the PCRE_NO_UTF8_CHECK option. When it is set, the effect of
1204 passing an invalid UTF-8 string as a pattern is undefined. It may cause
1205 your program to crash. Note that this option can also be passed to
1206 pcre_exec() and pcre_dfa_exec(), to suppress the UTF-8 validity check-
1207 ing of subject strings.
1208
1209
1210 COMPILATION ERROR CODES
1211
1212 The following table lists the error codes than may be returned by
1213 pcre_compile2(), along with the error messages that may be returned by
1214 both compiling functions. As PCRE has developed, some error codes have
1215 fallen out of use. To avoid confusion, they have not been re-used.
1216
1217 0 no error
1218 1 \ at end of pattern
1219 2 \c at end of pattern
1220 3 unrecognized character follows \
1221 4 numbers out of order in {} quantifier
1222 5 number too big in {} quantifier
1223 6 missing terminating ] for character class
1224 7 invalid escape sequence in character class
1225 8 range out of order in character class
1226 9 nothing to repeat
1227 10 [this code is not in use]
1228 11 internal error: unexpected repeat
1229 12 unrecognized character after (?
1230 13 POSIX named classes are supported only within a class
1231 14 missing )
1232 15 reference to non-existent subpattern
1233 16 erroffset passed as NULL
1234 17 unknown option bit(s) set
1235 18 missing ) after comment
1236 19 [this code is not in use]
1237 20 regular expression too large
1238 21 failed to get memory
1239 22 unmatched parentheses
1240 23 internal error: code overflow
1241 24 unrecognized character after (?<
1242 25 lookbehind assertion is not fixed length
1243 26 malformed number or name after (?(
1244 27 conditional group contains more than two branches
1245 28 assertion expected after (?(
1246 29 (?R or (?digits must be followed by )
1247 30 unknown POSIX class name
1248 31 POSIX collating elements are not supported
1249 32 this version of PCRE is not compiled with PCRE_UTF8 support
1250 33 [this code is not in use]
1251 34 character value in \x{...} sequence is too large
1252 35 invalid condition (?(0)
1253 36 \C not allowed in lookbehind assertion
1254 37 PCRE does not support \L, \l, \N, \U, or \u
1255 38 number after (?C is > 255
1256 39 closing ) for (?C expected
1257 40 recursive call could loop indefinitely
1258 41 unrecognized character after (?P
1259 42 syntax error in subpattern name (missing terminator)
1260 43 two named subpatterns have the same name
1261 44 invalid UTF-8 string
1262 45 support for \P, \p, and \X has not been compiled
1263 46 malformed \P or \p sequence
1264 47 unknown property name after \P or \p
1265 48 subpattern name is too long (maximum 32 characters)
1266 49 too many named subpatterns (maximum 10,000)
1267 50 repeated subpattern is too long
1268 51 octal value is greater than \377 (not in UTF-8 mode)
1269 52 internal error: overran compiling workspace
1270 53 internal error: previously-checked referenced subpattern not
1271 found
1272 54 DEFINE group contains more than one branch
1273 55 repeating a DEFINE group is not allowed
1274 56 inconsistent NEWLINE options"
1275
1276
1277 STUDYING A PATTERN
1278
1279 pcre_extra *pcre_study(const pcre *code, int options
1280 const char **errptr);
1281
1282 If a compiled pattern is going to be used several times, it is worth
1283 spending more time analyzing it in order to speed up the time taken for
1284 matching. The function pcre_study() takes a pointer to a compiled pat-
1285 tern as its first argument. If studying the pattern produces additional
1286 information that will help speed up matching, pcre_study() returns a
1287 pointer to a pcre_extra block, in which the study_data field points to
1288 the results of the study.
1289
1290 The returned value from pcre_study() can be passed directly to
1291 pcre_exec(). However, a pcre_extra block also contains other fields
1292 that can be set by the caller before the block is passed; these are
1293 described below in the section on matching a pattern.
1294
1295 If studying the pattern does not produce any additional information
1296 pcre_study() returns NULL. In that circumstance, if the calling program
1297 wants to pass any of the other fields to pcre_exec(), it must set up
1298 its own pcre_extra block.
1299
1300 The second argument of pcre_study() contains option bits. At present,
1301 no options are defined, and this argument should always be zero.
1302
1303 The third argument for pcre_study() is a pointer for an error message.
1304 If studying succeeds (even if no data is returned), the variable it
1305 points to is set to NULL. Otherwise it is set to point to a textual
1306 error message. This is a static string that is part of the library. You
1307 must not try to free it. You should test the error pointer for NULL
1308 after calling pcre_study(), to be sure that it has run successfully.
1309
1310 This is a typical call to pcre_study():
1311
1312 pcre_extra *pe;
1313 pe = pcre_study(
1314 re, /* result of pcre_compile() */
1315 0, /* no options exist */
1316 &error); /* set to NULL or points to a message */
1317
1318 At present, studying a pattern is useful only for non-anchored patterns
1319 that do not have a single fixed starting character. A bitmap of possi-
1320 ble starting bytes is created.
1321
1322
1323 LOCALE SUPPORT
1324
1325 PCRE handles caseless matching, and determines whether characters are
1326 letters, digits, or whatever, by reference to a set of tables, indexed
1327 by character value. When running in UTF-8 mode, this applies only to
1328 characters with codes less than 128. Higher-valued codes never match
1329 escapes such as \w or \d, but can be tested with \p if PCRE is built
1330 with Unicode character property support. The use of locales with Uni-
1331 code is discouraged. If you are handling characters with codes greater
1332 than 128, you should either use UTF-8 and Unicode, or use locales, but
1333 not try to mix the two.
1334
1335 PCRE contains an internal set of tables that are used when the final
1336 argument of pcre_compile() is NULL. These are sufficient for many
1337 applications. Normally, the internal tables recognize only ASCII char-
1338 acters. However, when PCRE is built, it is possible to cause the inter-
1339 nal tables to be rebuilt in the default "C" locale of the local system,
1340 which may cause them to be different.
1341
1342 The internal tables can always be overridden by tables supplied by the
1343 application that calls PCRE. These may be created in a different locale
1344 from the default. As more and more applications change to using Uni-
1345 code, the need for this locale support is expected to die away.
1346
1347 External tables are built by calling the pcre_maketables() function,
1348 which has no arguments, in the relevant locale. The result can then be
1349 passed to pcre_compile() or pcre_exec() as often as necessary. For
1350 example, to build and use tables that are appropriate for the French
1351 locale (where accented characters with values greater than 128 are
1352 treated as letters), the following code could be used:
1353
1354 setlocale(LC_CTYPE, "fr_FR");
1355 tables = pcre_maketables();
1356 re = pcre_compile(..., tables);
1357
1358 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1359 if you are using Windows, the name for the French locale is "french".
1360
1361 When pcre_maketables() runs, the tables are built in memory that is
1362 obtained via pcre_malloc. It is the caller's responsibility to ensure
1363 that the memory containing the tables remains available for as long as
1364 it is needed.
1365
1366 The pointer that is passed to pcre_compile() is saved with the compiled
1367 pattern, and the same tables are used via this pointer by pcre_study()
1368 and normally also by pcre_exec(). Thus, by default, for any single pat-
1369 tern, compilation, studying and matching all happen in the same locale,
1370 but different patterns can be compiled in different locales.
1371
1372 It is possible to pass a table pointer or NULL (indicating the use of
1373 the internal tables) to pcre_exec(). Although not intended for this
1374 purpose, this facility could be used to match a pattern in a different
1375 locale from the one in which it was compiled. Passing table pointers at
1376 run time is discussed below in the section on matching a pattern.
1377
1378
1379 INFORMATION ABOUT A PATTERN
1380
1381 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1382 int what, void *where);
1383
1384 The pcre_fullinfo() function returns information about a compiled pat-
1385 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1386 less retained for backwards compability (and is documented below).
1387
1388 The first argument for pcre_fullinfo() is a pointer to the compiled
1389 pattern. The second argument is the result of pcre_study(), or NULL if
1390 the pattern was not studied. The third argument specifies which piece
1391 of information is required, and the fourth argument is a pointer to a
1392 variable to receive the data. The yield of the function is zero for
1393 success, or one of the following negative numbers:
1394
1395 PCRE_ERROR_NULL the argument code was NULL
1396 the argument where was NULL
1397 PCRE_ERROR_BADMAGIC the "magic number" was not found
1398 PCRE_ERROR_BADOPTION the value of what was invalid
1399
1400 The "magic number" is placed at the start of each compiled pattern as
1401 an simple check against passing an arbitrary memory pointer. Here is a
1402 typical call of pcre_fullinfo(), to obtain the length of the compiled
1403 pattern:
1404
1405 int rc;
1406 size_t length;
1407 rc = pcre_fullinfo(
1408 re, /* result of pcre_compile() */
1409 pe, /* result of pcre_study(), or NULL */
1410 PCRE_INFO_SIZE, /* what is required */
1411 &length); /* where to put the data */
1412
1413 The possible values for the third argument are defined in pcre.h, and
1414 are as follows:
1415
1416 PCRE_INFO_BACKREFMAX
1417
1418 Return the number of the highest back reference in the pattern. The
1419 fourth argument should point to an int variable. Zero is returned if
1420 there are no back references.
1421
1422 PCRE_INFO_CAPTURECOUNT
1423
1424 Return the number of capturing subpatterns in the pattern. The fourth
1425 argument should point to an int variable.
1426
1427 PCRE_INFO_DEFAULT_TABLES
1428
1429 Return a pointer to the internal default character tables within PCRE.
1430 The fourth argument should point to an unsigned char * variable. This
1431 information call is provided for internal use by the pcre_study() func-
1432 tion. External callers can cause PCRE to use its internal tables by
1433 passing a NULL table pointer.
1434
1435 PCRE_INFO_FIRSTBYTE
1436
1437 Return information about the first byte of any matched string, for a
1438 non-anchored pattern. The fourth argument should point to an int vari-
1439 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1440 is still recognized for backwards compatibility.)
1441
1442 If there is a fixed first byte, for example, from a pattern such as
1443 (cat|cow|coyote), its value is returned. Otherwise, if either
1444
1445 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1446 branch starts with "^", or
1447
1448 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1449 set (if it were set, the pattern would be anchored),
1450
1451 -1 is returned, indicating that the pattern matches only at the start
1452 of a subject string or after any newline within the string. Otherwise
1453 -2 is returned. For anchored patterns, -2 is returned.
1454
1455 PCRE_INFO_FIRSTTABLE
1456
1457 If the pattern was studied, and this resulted in the construction of a
1458 256-bit table indicating a fixed set of bytes for the first byte in any
1459 matching string, a pointer to the table is returned. Otherwise NULL is
1460 returned. The fourth argument should point to an unsigned char * vari-
1461 able.
1462
1463 PCRE_INFO_LASTLITERAL
1464
1465 Return the value of the rightmost literal byte that must exist in any
1466 matched string, other than at its start, if such a byte has been
1467 recorded. The fourth argument should point to an int variable. If there
1468 is no such byte, -1 is returned. For anchored patterns, a last literal
1469 byte is recorded only if it follows something of variable length. For
1470 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1471 /^a\dz\d/ the returned value is -1.
1472
1473 PCRE_INFO_NAMECOUNT
1474 PCRE_INFO_NAMEENTRYSIZE
1475 PCRE_INFO_NAMETABLE
1476
1477 PCRE supports the use of named as well as numbered capturing parenthe-
1478 ses. The names are just an additional way of identifying the parenthe-
1479 ses, which still acquire numbers. Several convenience functions such as
1480 pcre_get_named_substring() are provided for extracting captured sub-
1481 strings by name. It is also possible to extract the data directly, by
1482 first converting the name to a number in order to access the correct
1483 pointers in the output vector (described with pcre_exec() below). To do
1484 the conversion, you need to use the name-to-number map, which is
1485 described by these three values.
1486
1487 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1488 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1489 of each entry; both of these return an int value. The entry size
1490 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1491 a pointer to the first entry of the table (a pointer to char). The
1492 first two bytes of each entry are the number of the capturing parenthe-
1493 sis, most significant byte first. The rest of the entry is the corre-
1494 sponding name, zero terminated. The names are in alphabetical order.
1495 When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1496 theses numbers. For example, consider the following pattern (assume
1497 PCRE_EXTENDED is set, so white space - including newlines - is
1498 ignored):
1499
1500 (?<date> (?<year>(\d\d)?\d\d) -
1501 (?<month>\d\d) - (?<day>\d\d) )
1502
1503 There are four named subpatterns, so the table has four entries, and
1504 each entry in the table is eight bytes long. The table is as follows,
1505 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1506 as ??:
1507
1508 00 01 d a t e 00 ??
1509 00 05 d a y 00 ?? ??
1510 00 04 m o n t h 00
1511 00 02 y e a r 00 ??
1512
1513 When writing code to extract data from named subpatterns using the
1514 name-to-number map, remember that the length of the entries is likely
1515 to be different for each compiled pattern.
1516
1517 PCRE_INFO_OPTIONS
1518
1519 Return a copy of the options with which the pattern was compiled. The
1520 fourth argument should point to an unsigned long int variable. These
1521 option bits are those specified in the call to pcre_compile(), modified
1522 by any top-level option settings within the pattern itself.
1523
1524 A pattern is automatically anchored by PCRE if all of its top-level
1525 alternatives begin with one of the following:
1526
1527 ^ unless PCRE_MULTILINE is set
1528 \A always
1529 \G always
1530 .* if PCRE_DOTALL is set and there are no back
1531 references to the subpattern in which .* appears
1532
1533 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1534 by pcre_fullinfo().
1535
1536 PCRE_INFO_SIZE
1537
1538 Return the size of the compiled pattern, that is, the value that was
1539 passed as the argument to pcre_malloc() when PCRE was getting memory in
1540 which to place the compiled data. The fourth argument should point to a
1541 size_t variable.
1542
1543 PCRE_INFO_STUDYSIZE
1544
1545 Return the size of the data block pointed to by the study_data field in
1546 a pcre_extra block. That is, it is the value that was passed to
1547 pcre_malloc() when PCRE was getting memory into which to place the data
1548 created by pcre_study(). The fourth argument should point to a size_t
1549 variable.
1550
1551
1552 OBSOLETE INFO FUNCTION
1553
1554 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1555
1556 The pcre_info() function is now obsolete because its interface is too
1557 restrictive to return all the available data about a compiled pattern.
1558 New programs should use pcre_fullinfo() instead. The yield of
1559 pcre_info() is the number of capturing subpatterns, or one of the fol-
1560 lowing negative numbers:
1561
1562 PCRE_ERROR_NULL the argument code was NULL
1563 PCRE_ERROR_BADMAGIC the "magic number" was not found
1564
1565 If the optptr argument is not NULL, a copy of the options with which
1566 the pattern was compiled is placed in the integer it points to (see
1567 PCRE_INFO_OPTIONS above).
1568
1569 If the pattern is not anchored and the firstcharptr argument is not
1570 NULL, it is used to pass back information about the first character of
1571 any matched string (see PCRE_INFO_FIRSTBYTE above).
1572
1573
1574 REFERENCE COUNTS
1575
1576 int pcre_refcount(pcre *code, int adjust);
1577
1578 The pcre_refcount() function is used to maintain a reference count in
1579 the data block that contains a compiled pattern. It is provided for the
1580 benefit of applications that operate in an object-oriented manner,
1581 where different parts of the application may be using the same compiled
1582 pattern, but you want to free the block when they are all done.
1583
1584 When a pattern is compiled, the reference count field is initialized to
1585 zero. It is changed only by calling this function, whose action is to
1586 add the adjust value (which may be positive or negative) to it. The
1587 yield of the function is the new value. However, the value of the count
1588 is constrained to lie between 0 and 65535, inclusive. If the new value
1589 is outside these limits, it is forced to the appropriate limit value.
1590
1591 Except when it is zero, the reference count is not correctly preserved
1592 if a pattern is compiled on one host and then transferred to a host
1593 whose byte-order is different. (This seems a highly unlikely scenario.)
1594
1595
1596 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1597
1598 int pcre_exec(const pcre *code, const pcre_extra *extra,
1599 const char *subject, int length, int startoffset,
1600 int options, int *ovector, int ovecsize);
1601
1602 The function pcre_exec() is called to match a subject string against a
1603 compiled pattern, which is passed in the code argument. If the pattern
1604 has been studied, the result of the study should be passed in the extra
1605 argument. This function is the main matching facility of the library,
1606 and it operates in a Perl-like manner. For specialist use there is also
1607 an alternative matching function, which is described below in the sec-
1608 tion about the pcre_dfa_exec() function.
1609
1610 In most applications, the pattern will have been compiled (and option-
1611 ally studied) in the same process that calls pcre_exec(). However, it
1612 is possible to save compiled patterns and study data, and then use them
1613 later in different processes, possibly even on different hosts. For a
1614 discussion about this, see the pcreprecompile documentation.
1615
1616 Here is an example of a simple call to pcre_exec():
1617
1618 int rc;
1619 int ovector[30];
1620 rc = pcre_exec(
1621 re, /* result of pcre_compile() */
1622 NULL, /* we didn't study the pattern */
1623 "some string", /* the subject string */
1624 11, /* the length of the subject string */
1625 0, /* start at offset 0 in the subject */
1626 0, /* default options */
1627 ovector, /* vector of integers for substring information */
1628 30); /* number of elements (NOT size in bytes) */
1629
1630 Extra data for pcre_exec()
1631
1632 If the extra argument is not NULL, it must point to a pcre_extra data
1633 block. The pcre_study() function returns such a block (when it doesn't
1634 return NULL), but you can also create one for yourself, and pass addi-
1635 tional information in it. The pcre_extra block contains the following
1636 fields (not necessarily in this order):
1637
1638 unsigned long int flags;
1639 void *study_data;
1640 unsigned long int match_limit;
1641 unsigned long int match_limit_recursion;
1642 void *callout_data;
1643 const unsigned char *tables;
1644
1645 The flags field is a bitmap that specifies which of the other fields
1646 are set. The flag bits are:
1647
1648 PCRE_EXTRA_STUDY_DATA
1649 PCRE_EXTRA_MATCH_LIMIT
1650 PCRE_EXTRA_MATCH_LIMIT_RECURSION
1651 PCRE_EXTRA_CALLOUT_DATA
1652 PCRE_EXTRA_TABLES
1653
1654 Other flag bits should be set to zero. The study_data field is set in
1655 the pcre_extra block that is returned by pcre_study(), together with
1656 the appropriate flag bit. You should not set this yourself, but you may
1657 add to the block by setting the other fields and their corresponding
1658 flag bits.
1659
1660 The match_limit field provides a means of preventing PCRE from using up
1661 a vast amount of resources when running patterns that are not going to
1662 match, but which have a very large number of possibilities in their
1663 search trees. The classic example is the use of nested unlimited
1664 repeats.
1665
1666 Internally, PCRE uses a function called match() which it calls repeat-
1667 edly (sometimes recursively). The limit set by match_limit is imposed
1668 on the number of times this function is called during a match, which
1669 has the effect of limiting the amount of backtracking that can take
1670 place. For patterns that are not anchored, the count restarts from zero
1671 for each position in the subject string.
1672
1673 The default value for the limit can be set when PCRE is built; the
1674 default default is 10 million, which handles all but the most extreme
1675 cases. You can override the default by suppling pcre_exec() with a
1676 pcre_extra block in which match_limit is set, and
1677 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1678 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1679
1680 The match_limit_recursion field is similar to match_limit, but instead
1681 of limiting the total number of times that match() is called, it limits
1682 the depth of recursion. The recursion depth is a smaller number than
1683 the total number of calls, because not all calls to match() are recur-
1684 sive. This limit is of use only if it is set smaller than match_limit.
1685
1686 Limiting the recursion depth limits the amount of stack that can be
1687 used, or, when PCRE has been compiled to use memory on the heap instead
1688 of the stack, the amount of heap memory that can be used.
1689
1690 The default value for match_limit_recursion can be set when PCRE is
1691 built; the default default is the same value as the default for
1692 match_limit. You can override the default by suppling pcre_exec() with
1693 a pcre_extra block in which match_limit_recursion is set, and
1694 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1695 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1696
1697 The pcre_callout field is used in conjunction with the "callout" fea-
1698 ture, which is described in the pcrecallout documentation.
1699
1700 The tables field is used to pass a character tables pointer to
1701 pcre_exec(); this overrides the value that is stored with the compiled
1702 pattern. A non-NULL value is stored with the compiled pattern only if
1703 custom tables were supplied to pcre_compile() via its tableptr argu-
1704 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1705 PCRE's internal tables to be used. This facility is helpful when re-
1706 using patterns that have been saved after compiling with an external
1707 set of tables, because the external tables might be at a different
1708 address when pcre_exec() is called. See the pcreprecompile documenta-
1709 tion for a discussion of saving compiled patterns for later use.
1710
1711 Option bits for pcre_exec()
1712
1713 The unused bits of the options argument for pcre_exec() must be zero.
1714 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1715 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
1716 PCRE_PARTIAL.
1717
1718 PCRE_ANCHORED
1719
1720 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
1721 matching position. If a pattern was compiled with PCRE_ANCHORED, or
1722 turned out to be anchored by virtue of its contents, it cannot be made
1723 unachored at matching time.
1724
1725 PCRE_NEWLINE_CR
1726 PCRE_NEWLINE_LF
1727 PCRE_NEWLINE_CRLF
1728 PCRE_NEWLINE_ANY
1729
1730 These options override the newline definition that was chosen or
1731 defaulted when the pattern was compiled. For details, see the descrip-
1732 tion of pcre_compile() above. During matching, the newline choice
1733 affects the behaviour of the dot, circumflex, and dollar metacharac-
1734 ters. It may also alter the way the match position is advanced after a
1735 match failure for an unanchored pattern. When PCRE_NEWLINE_CRLF or
1736 PCRE_NEWLINE_ANY is set, and a match attempt fails when the current
1737 position is at a CRLF sequence, the match position is advanced by two
1738 characters instead of one, in other words, to after the CRLF.
1739
1740 PCRE_NOTBOL
1741
1742 This option specifies that first character of the subject string is not
1743 the beginning of a line, so the circumflex metacharacter should not
1744 match before it. Setting this without PCRE_MULTILINE (at compile time)
1745 causes circumflex never to match. This option affects only the behav-
1746 iour of the circumflex metacharacter. It does not affect \A.
1747
1748 PCRE_NOTEOL
1749
1750 This option specifies that the end of the subject string is not the end
1751 of a line, so the dollar metacharacter should not match it nor (except
1752 in multiline mode) a newline immediately before it. Setting this with-
1753 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1754 option affects only the behaviour of the dollar metacharacter. It does
1755 not affect \Z or \z.
1756
1757 PCRE_NOTEMPTY
1758
1759 An empty string is not considered to be a valid match if this option is
1760 set. If there are alternatives in the pattern, they are tried. If all
1761 the alternatives match the empty string, the entire match fails. For
1762 example, if the pattern
1763
1764 a?b?
1765
1766 is applied to a string not beginning with "a" or "b", it matches the
1767 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
1768 match is not valid, so PCRE searches further into the string for occur-
1769 rences of "a" or "b".
1770
1771 Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1772 cial case of a pattern match of the empty string within its split()
1773 function, and when using the /g modifier. It is possible to emulate
1774 Perl's behaviour after matching a null string by first trying the match
1775 again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1776 if that fails by advancing the starting offset (see below) and trying
1777 an ordinary match again. There is some code that demonstrates how to do
1778 this in the pcredemo.c sample program.
1779
1780 PCRE_NO_UTF8_CHECK
1781
1782 When PCRE_UTF8 is set at compile time, the validity of the subject as a
1783 UTF-8 string is automatically checked when pcre_exec() is subsequently
1784 called. The value of startoffset is also checked to ensure that it
1785 points to the start of a UTF-8 character. If an invalid UTF-8 sequence
1786 of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1787 startoffset contains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is
1788 returned.
1789
1790 If you already know that your subject is valid, and you want to skip
1791 these checks for performance reasons, you can set the
1792 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
1793 do this for the second and subsequent calls to pcre_exec() if you are
1794 making repeated calls to find all the matches in a single subject
1795 string. However, you should be sure that the value of startoffset
1796 points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1797 set, the effect of passing an invalid UTF-8 string as a subject, or a
1798 value of startoffset that does not point to the start of a UTF-8 char-
1799 acter, is undefined. Your program may crash.
1800
1801 PCRE_PARTIAL
1802
1803 This option turns on the partial matching feature. If the subject
1804 string fails to match the pattern, but at some point during the match-
1805 ing process the end of the subject was reached (that is, the subject
1806 partially matches the pattern and the failure to match occurred only
1807 because there were not enough subject characters), pcre_exec() returns
1808 PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1809 used, there are restrictions on what may appear in the pattern. These
1810 are discussed in the pcrepartial documentation.
1811
1812 The string to be matched by pcre_exec()
1813
1814 The subject string is passed to pcre_exec() as a pointer in subject, a
1815 length in length, and a starting byte offset in startoffset. In UTF-8
1816 mode, the byte offset must point to the start of a UTF-8 character.
1817 Unlike the pattern string, the subject may contain binary zero bytes.
1818 When the starting offset is zero, the search for a match starts at the
1819 beginning of the subject, and this is by far the most common case.
1820
1821 A non-zero starting offset is useful when searching for another match
1822 in the same subject by calling pcre_exec() again after a previous suc-
1823 cess. Setting startoffset differs from just passing over a shortened
1824 string and setting PCRE_NOTBOL in the case of a pattern that begins
1825 with any kind of lookbehind. For example, consider the pattern
1826
1827 \Biss\B
1828
1829 which finds occurrences of "iss" in the middle of words. (\B matches
1830 only if the current position in the subject is not a word boundary.)
1831 When applied to the string "Mississipi" the first call to pcre_exec()
1832 finds the first occurrence. If pcre_exec() is called again with just
1833 the remainder of the subject, namely "issipi", it does not match,
1834 because \B is always false at the start of the subject, which is deemed
1835 to be a word boundary. However, if pcre_exec() is passed the entire
1836 string again, but with startoffset set to 4, it finds the second occur-
1837 rence of "iss" because it is able to look behind the starting point to
1838 discover that it is preceded by a letter.
1839
1840 If a non-zero starting offset is passed when the pattern is anchored,
1841 one attempt to match at the given offset is made. This can only succeed
1842 if the pattern does not require the match to be at the start of the
1843 subject.
1844
1845 How pcre_exec() returns captured substrings
1846
1847 In general, a pattern matches a certain portion of the subject, and in
1848 addition, further substrings from the subject may be picked out by
1849 parts of the pattern. Following the usage in Jeffrey Friedl's book,
1850 this is called "capturing" in what follows, and the phrase "capturing
1851 subpattern" is used for a fragment of a pattern that picks out a sub-
1852 string. PCRE supports several other kinds of parenthesized subpattern
1853 that do not cause substrings to be captured.
1854
1855 Captured substrings are returned to the caller via a vector of integer
1856 offsets whose address is passed in ovector. The number of elements in
1857 the vector is passed in ovecsize, which must be a non-negative number.
1858 Note: this argument is NOT the size of ovector in bytes.
1859
1860 The first two-thirds of the vector is used to pass back captured sub-
1861 strings, each substring using a pair of integers. The remaining third
1862 of the vector is used as workspace by pcre_exec() while matching cap-
1863 turing subpatterns, and is not available for passing back information.
1864 The length passed in ovecsize should always be a multiple of three. If
1865 it is not, it is rounded down.
1866
1867 When a match is successful, information about captured substrings is
1868 returned in pairs of integers, starting at the beginning of ovector,
1869 and continuing up to two-thirds of its length at the most. The first
1870 element of a pair is set to the offset of the first character in a sub-
1871 string, and the second is set to the offset of the first character
1872 after the end of a substring. The first pair, ovector[0] and ovec-
1873 tor[1], identify the portion of the subject string matched by the
1874 entire pattern. The next pair is used for the first capturing subpat-
1875 tern, and so on. The value returned by pcre_exec() is one more than the
1876 highest numbered pair that has been set. For example, if two substrings
1877 have been captured, the returned value is 3. If there are no capturing
1878 subpatterns, the return value from a successful match is 1, indicating
1879 that just the first pair of offsets has been set.
1880
1881 If a capturing subpattern is matched repeatedly, it is the last portion
1882 of the string that it matched that is returned.
1883
1884 If the vector is too small to hold all the captured substring offsets,
1885 it is used as far as possible (up to two-thirds of its length), and the
1886 function returns a value of zero. In particular, if the substring off-
1887 sets are not of interest, pcre_exec() may be called with ovector passed
1888 as NULL and ovecsize as zero. However, if the pattern contains back
1889 references and the ovector is not big enough to remember the related
1890 substrings, PCRE has to get additional memory for use during matching.
1891 Thus it is usually advisable to supply an ovector.
1892
1893 The pcre_info() function can be used to find out how many capturing
1894 subpatterns there are in a compiled pattern. The smallest size for
1895 ovector that will allow for n captured substrings, in addition to the
1896 offsets of the substring matched by the whole pattern, is (n+1)*3.
1897
1898 It is possible for capturing subpattern number n+1 to match some part
1899 of the subject when subpattern n has not been used at all. For example,
1900 if the string "abc" is matched against the pattern (a|(z))(bc) the
1901 return from the function is 4, and subpatterns 1 and 3 are matched, but
1902 2 is not. When this happens, both values in the offset pairs corre-
1903 sponding to unused subpatterns are set to -1.
1904
1905 Offset values that correspond to unused subpatterns at the end of the
1906 expression are also set to -1. For example, if the string "abc" is
1907 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
1908 matched. The return from the function is 2, because the highest used
1909 capturing subpattern number is 1. However, you can refer to the offsets
1910 for the second and third capturing subpatterns if you wish (assuming
1911 the vector is large enough, of course).
1912
1913 Some convenience functions are provided for extracting the captured
1914 substrings as separate strings. These are described below.
1915
1916 Error return values from pcre_exec()
1917
1918 If pcre_exec() fails, it returns a negative number. The following are
1919 defined in the header file:
1920
1921 PCRE_ERROR_NOMATCH (-1)
1922
1923 The subject string did not match the pattern.
1924
1925 PCRE_ERROR_NULL (-2)
1926
1927 Either code or subject was passed as NULL, or ovector was NULL and
1928 ovecsize was not zero.
1929
1930 PCRE_ERROR_BADOPTION (-3)
1931
1932 An unrecognized bit was set in the options argument.
1933
1934 PCRE_ERROR_BADMAGIC (-4)
1935
1936 PCRE stores a 4-byte "magic number" at the start of the compiled code,
1937 to catch the case when it is passed a junk pointer and to detect when a
1938 pattern that was compiled in an environment of one endianness is run in
1939 an environment with the other endianness. This is the error that PCRE
1940 gives when the magic number is not present.
1941
1942 PCRE_ERROR_UNKNOWN_OPCODE (-5)
1943
1944 While running the pattern match, an unknown item was encountered in the
1945 compiled pattern. This error could be caused by a bug in PCRE or by
1946 overwriting of the compiled pattern.
1947
1948 PCRE_ERROR_NOMEMORY (-6)
1949
1950 If a pattern contains back references, but the ovector that is passed
1951 to pcre_exec() is not big enough to remember the referenced substrings,
1952 PCRE gets a block of memory at the start of matching to use for this
1953 purpose. If the call via pcre_malloc() fails, this error is given. The
1954 memory is automatically freed at the end of matching.
1955
1956 PCRE_ERROR_NOSUBSTRING (-7)
1957
1958 This error is used by the pcre_copy_substring(), pcre_get_substring(),
1959 and pcre_get_substring_list() functions (see below). It is never
1960 returned by pcre_exec().
1961
1962 PCRE_ERROR_MATCHLIMIT (-8)
1963
1964 The backtracking limit, as specified by the match_limit field in a
1965 pcre_extra structure (or defaulted) was reached. See the description
1966 above.
1967
1968 PCRE_ERROR_CALLOUT (-9)
1969
1970 This error is never generated by pcre_exec() itself. It is provided for
1971 use by callout functions that want to yield a distinctive error code.
1972 See the pcrecallout documentation for details.
1973
1974 PCRE_ERROR_BADUTF8 (-10)
1975
1976 A string that contains an invalid UTF-8 byte sequence was passed as a
1977 subject.
1978
1979 PCRE_ERROR_BADUTF8_OFFSET (-11)
1980
1981 The UTF-8 byte sequence that was passed as a subject was valid, but the
1982 value of startoffset did not point to the beginning of a UTF-8 charac-
1983 ter.
1984
1985 PCRE_ERROR_PARTIAL (-12)
1986
1987 The subject string did not match, but it did match partially. See the
1988 pcrepartial documentation for details of partial matching.
1989
1990 PCRE_ERROR_BADPARTIAL (-13)
1991
1992 The PCRE_PARTIAL option was used with a compiled pattern containing
1993 items that are not supported for partial matching. See the pcrepartial
1994 documentation for details of partial matching.
1995
1996 PCRE_ERROR_INTERNAL (-14)
1997
1998 An unexpected internal error has occurred. This error could be caused
1999 by a bug in PCRE or by overwriting of the compiled pattern.
2000
2001 PCRE_ERROR_BADCOUNT (-15)
2002
2003 This error is given if the value of the ovecsize argument is negative.
2004
2005 PCRE_ERROR_RECURSIONLIMIT (-21)
2006
2007 The internal recursion limit, as specified by the match_limit_recursion
2008 field in a pcre_extra structure (or defaulted) was reached. See the
2009 description above.
2010
2011 PCRE_ERROR_NULLWSLIMIT (-22)
2012
2013 When a group that can match an empty substring is repeated with an
2014 unbounded upper limit, the subject position at the start of the group
2015 must be remembered, so that a test for an empty string can be made when
2016 the end of the group is reached. Some workspace is required for this;
2017 if it runs out, this error is given.
2018
2019 PCRE_ERROR_BADNEWLINE (-23)
2020
2021 An invalid combination of PCRE_NEWLINE_xxx options was given.
2022
2023 Error numbers -16 to -20 are not used by pcre_exec().
2024
2025
2026 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2027
2028 int pcre_copy_substring(const char *subject, int *ovector,
2029 int stringcount, int stringnumber, char *buffer,
2030 int buffersize);
2031
2032 int pcre_get_substring(const char *subject, int *ovector,
2033 int stringcount, int stringnumber,
2034 const char **stringptr);
2035
2036 int pcre_get_substring_list(const char *subject,
2037 int *ovector, int stringcount, const char ***listptr);
2038
2039 Captured substrings can be accessed directly by using the offsets
2040 returned by pcre_exec() in ovector. For convenience, the functions
2041 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2042 string_list() are provided for extracting captured substrings as new,
2043 separate, zero-terminated strings. These functions identify substrings
2044 by number. The next section describes functions for extracting named
2045 substrings.
2046
2047 A substring that contains a binary zero is correctly extracted and has
2048 a further zero added on the end, but the result is not, of course, a C
2049 string. However, you can process such a string by referring to the
2050 length that is returned by pcre_copy_substring() and pcre_get_sub-
2051 string(). Unfortunately, the interface to pcre_get_substring_list() is
2052 not adequate for handling strings containing binary zeros, because the
2053 end of the final string is not independently indicated.
2054
2055 The first three arguments are the same for all three of these func-
2056 tions: subject is the subject string that has just been successfully
2057 matched, ovector is a pointer to the vector of integer offsets that was
2058 passed to pcre_exec(), and stringcount is the number of substrings that
2059 were captured by the match, including the substring that matched the
2060 entire regular expression. This is the value returned by pcre_exec() if
2061 it is greater than zero. If pcre_exec() returned zero, indicating that
2062 it ran out of space in ovector, the value passed as stringcount should
2063 be the number of elements in the vector divided by three.
2064
2065 The functions pcre_copy_substring() and pcre_get_substring() extract a
2066 single substring, whose number is given as stringnumber. A value of
2067 zero extracts the substring that matched the entire pattern, whereas
2068 higher values extract the captured substrings. For pcre_copy_sub-
2069 string(), the string is placed in buffer, whose length is given by
2070 buffersize, while for pcre_get_substring() a new block of memory is
2071 obtained via pcre_malloc, and its address is returned via stringptr.
2072 The yield of the function is the length of the string, not including
2073 the terminating zero, or one of these error codes:
2074
2075 PCRE_ERROR_NOMEMORY (-6)
2076
2077 The buffer was too small for pcre_copy_substring(), or the attempt to
2078 get memory failed for pcre_get_substring().
2079
2080 PCRE_ERROR_NOSUBSTRING (-7)
2081
2082 There is no substring whose number is stringnumber.
2083
2084 The pcre_get_substring_list() function extracts all available sub-
2085 strings and builds a list of pointers to them. All this is done in a
2086 single block of memory that is obtained via pcre_malloc. The address of
2087 the memory block is returned via listptr, which is also the start of
2088 the list of string pointers. The end of the list is marked by a NULL
2089 pointer. The yield of the function is zero if all went well, or the
2090 error code
2091
2092 PCRE_ERROR_NOMEMORY (-6)
2093
2094 if the attempt to get the memory block failed.
2095
2096 When any of these functions encounter a substring that is unset, which
2097 can happen when capturing subpattern number n+1 matches some part of
2098 the subject, but subpattern n has not been used at all, they return an
2099 empty string. This can be distinguished from a genuine zero-length sub-
2100 string by inspecting the appropriate offset in ovector, which is nega-
2101 tive for unset substrings.
2102
2103 The two convenience functions pcre_free_substring() and pcre_free_sub-
2104 string_list() can be used to free the memory returned by a previous
2105 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2106 tively. They do nothing more than call the function pointed to by
2107 pcre_free, which of course could be called directly from a C program.
2108 However, PCRE is used in some situations where it is linked via a spe-
2109 cial interface to another programming language that cannot use
2110 pcre_free directly; it is for these cases that the functions are pro-
2111 vided.
2112
2113
2114 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2115
2116 int pcre_get_stringnumber(const pcre *code,
2117 const char *name);
2118
2119 int pcre_copy_named_substring(const pcre *code,
2120 const char *subject, int *ovector,
2121 int stringcount, const char *stringname,
2122 char *buffer, int buffersize);
2123
2124 int pcre_get_named_substring(const pcre *code,
2125 const char *subject, int *ovector,
2126 int stringcount, const char *stringname,
2127 const char **stringptr);
2128
2129 To extract a substring by name, you first have to find associated num-
2130 ber. For example, for this pattern
2131
2132 (a+)b(?<xxx>\d+)...
2133
2134 the number of the subpattern called "xxx" is 2. If the name is known to
2135 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2136 name by calling pcre_get_stringnumber(). The first argument is the com-
2137 piled pattern, and the second is the name. The yield of the function is
2138 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2139 subpattern of that name.
2140
2141 Given the number, you can extract the substring directly, or use one of
2142 the functions described in the previous section. For convenience, there
2143 are also two functions that do the whole job.
2144
2145 Most of the arguments of pcre_copy_named_substring() and
2146 pcre_get_named_substring() are the same as those for the similarly
2147 named functions that extract by number. As these are described in the
2148 previous section, they are not re-described here. There are just two
2149 differences:
2150
2151 First, instead of a substring number, a substring name is given. Sec-
2152 ond, there is an extra argument, given at the start, which is a pointer
2153 to the compiled pattern. This is needed in order to gain access to the
2154 name-to-number translation table.
2155
2156 These functions call pcre_get_stringnumber(), and if it succeeds, they
2157 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2158 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2159 behaviour may not be what you want (see the next section).
2160
2161
2162 DUPLICATE SUBPATTERN NAMES
2163
2164 int pcre_get_stringtable_entries(const pcre *code,
2165 const char *name, char **first, char **last);
2166
2167 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2168 subpatterns are not required to be unique. Normally, patterns with
2169 duplicate names are such that in any one match, only one of the named
2170 subpatterns participates. An example is shown in the pcrepattern docu-
2171 mentation. When duplicates are present, pcre_copy_named_substring() and
2172 pcre_get_named_substring() return the first substring corresponding to
2173 the given name that is set. If none are set, an empty string is
2174 returned. The pcre_get_stringnumber() function returns one of the num-
2175 bers that are associated with the name, but it is not defined which it
2176 is.
2177
2178 If you want to get full details of all captured substrings for a given
2179 name, you must use the pcre_get_stringtable_entries() function. The
2180 first argument is the compiled pattern, and the second is the name. The
2181 third and fourth are pointers to variables which are updated by the
2182 function. After it has run, they point to the first and last entries in
2183 the name-to-number table for the given name. The function itself
2184 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2185 there are none. The format of the table is described above in the sec-
2186 tion entitled Information about a pattern. Given all the relevant
2187 entries for the name, you can extract each of their numbers, and hence
2188 the captured data, if any.
2189
2190
2191 FINDING ALL POSSIBLE MATCHES
2192
2193 The traditional matching function uses a similar algorithm to Perl,
2194 which stops when it finds the first match, starting at a given point in
2195 the subject. If you want to find all possible matches, or the longest
2196 possible match, consider using the alternative matching function (see
2197 below) instead. If you cannot use the alternative function, but still
2198 need to find all possible matches, you can kludge it up by making use
2199 of the callout facility, which is described in the pcrecallout documen-
2200 tation.
2201
2202 What you have to do is to insert a callout right at the end of the pat-
2203 tern. When your callout function is called, extract and save the cur-
2204 rent matched substring. Then return 1, which forces pcre_exec() to
2205 backtrack and try other alternatives. Ultimately, when it runs out of
2206 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2207
2208
2209 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2210
2211 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2212 const char *subject, int length, int startoffset,
2213 int options, int *ovector, int ovecsize,
2214 int *workspace, int wscount);
2215
2216 The function pcre_dfa_exec() is called to match a subject string
2217 against a compiled pattern, using a matching algorithm that scans the
2218 subject string just once, and does not backtrack. This has different
2219 characteristics to the normal algorithm, and is not compatible with
2220 Perl. Some of the features of PCRE patterns are not supported. Never-
2221 theless, there are times when this kind of matching can be useful. For
2222 a discussion of the two matching algorithms, see the pcrematching docu-
2223 mentation.
2224
2225 The arguments for the pcre_dfa_exec() function are the same as for
2226 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2227 ent way, and this is described below. The other common arguments are
2228 used in the same way as for pcre_exec(), so their description is not
2229 repeated here.
2230
2231 The two additional arguments provide workspace for the function. The
2232 workspace vector should contain at least 20 elements. It is used for
2233 keeping track of multiple paths through the pattern tree. More
2234 workspace will be needed for patterns and subjects where there are a
2235 lot of potential matches.
2236
2237 Here is an example of a simple call to pcre_dfa_exec():
2238
2239 int rc;
2240 int ovector[10];
2241 int wspace[20];
2242 rc = pcre_dfa_exec(
2243 re, /* result of pcre_compile() */
2244 NULL, /* we didn't study the pattern */
2245 "some string", /* the subject string */
2246 11, /* the length of the subject string */
2247 0, /* start at offset 0 in the subject */
2248 0, /* default options */
2249 ovector, /* vector of integers for substring information */
2250 10, /* number of elements (NOT size in bytes) */
2251 wspace, /* working space vector */
2252 20); /* number of elements (NOT size in bytes) */
2253
2254 Option bits for pcre_dfa_exec()
2255
2256 The unused bits of the options argument for pcre_dfa_exec() must be
2257 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2258 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2259 PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2260 three of these are the same as for pcre_exec(), so their description is
2261 not repeated here.
2262
2263 PCRE_PARTIAL
2264
2265 This has the same general effect as it does for pcre_exec(), but the
2266 details are slightly different. When PCRE_PARTIAL is set for
2267 pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
2268 PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
2269 been no complete matches, but there is still at least one matching pos-
2270 sibility. The portion of the string that provided the partial match is
2271 set as the first matching string.
2272
2273 PCRE_DFA_SHORTEST
2274
2275 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2276 stop as soon as it has found one match. Because of the way the alterna-
2277 tive algorithm works, this is necessarily the shortest possible match
2278 at the first possible matching point in the subject string.
2279
2280 PCRE_DFA_RESTART
2281
2282 When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
2283 returns a partial match, it is possible to call it again, with addi-
2284 tional subject characters, and have it continue with the same match.
2285 The PCRE_DFA_RESTART option requests this action; when it is set, the
2286 workspace and wscount options must reference the same vector as before
2287 because data about the match so far is left in them after a partial
2288 match. There is more discussion of this facility in the pcrepartial
2289 documentation.
2290
2291 Successful returns from pcre_dfa_exec()
2292
2293 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2294 string in the subject. Note, however, that all the matches from one run
2295 of the function start at the same point in the subject. The shorter
2296 matches are all initial substrings of the longer matches. For example,
2297 if the pattern
2298
2299 <.*>
2300
2301 is matched against the string
2302
2303 This is <something> <something else> <something further> no more
2304
2305 the three matched strings are
2306
2307 <something>
2308 <something> <something else>
2309 <something> <something else> <something further>
2310
2311 On success, the yield of the function is a number greater than zero,
2312 which is the number of matched substrings. The substrings themselves
2313 are returned in ovector. Each string uses two elements; the first is
2314 the offset to the start, and the second is the offset to the end. In
2315 fact, all the strings have the same start offset. (Space could have
2316 been saved by giving this only once, but it was decided to retain some
2317 compatibility with the way pcre_exec() returns data, even though the
2318 meaning of the strings is different.)
2319
2320 The strings are returned in reverse order of length; that is, the long-
2321 est matching string is given first. If there were too many matches to
2322 fit into ovector, the yield of the function is zero, and the vector is
2323 filled with the longest matches.
2324
2325 Error returns from pcre_dfa_exec()
2326
2327 The pcre_dfa_exec() function returns a negative number when it fails.
2328 Many of the errors are the same as for pcre_exec(), and these are
2329 described above. There are in addition the following errors that are
2330 specific to pcre_dfa_exec():
2331
2332 PCRE_ERROR_DFA_UITEM (-16)
2333
2334 This return is given if pcre_dfa_exec() encounters an item in the pat-
2335 tern that it does not support, for instance, the use of \C or a back
2336 reference.
2337
2338 PCRE_ERROR_DFA_UCOND (-17)
2339
2340 This return is given if pcre_dfa_exec() encounters a condition item
2341 that uses a back reference for the condition, or a test for recursion
2342 in a specific group. These are not supported.
2343
2344 PCRE_ERROR_DFA_UMLIMIT (-18)
2345
2346 This return is given if pcre_dfa_exec() is called with an extra block
2347 that contains a setting of the match_limit field. This is not supported
2348 (it is meaningless).
2349
2350 PCRE_ERROR_DFA_WSSIZE (-19)
2351
2352 This return is given if pcre_dfa_exec() runs out of space in the
2353 workspace vector.
2354
2355 PCRE_ERROR_DFA_RECURSE (-20)
2356
2357 When a recursive subpattern is processed, the matching function calls
2358 itself recursively, using private vectors for ovector and workspace.
2359 This error is given if the output vector is not large enough. This
2360 should be extremely rare, as a vector of size 1000 is used.
2361
2362
2363 SEE ALSO
2364
2365 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2366 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2367
2368
2369 AUTHOR
2370
2371 Philip Hazel
2372 University Computing Service
2373 Cambridge CB2 3QH, England.
2374
2375
2376 REVISION
2377
2378 Last updated: 06 March 2007
2379 Copyright (c) 1997-2007 University of Cambridge.
2380 ------------------------------------------------------------------------------
2381
2382
2383 PCRECALLOUT(3) PCRECALLOUT(3)
2384
2385
2386 NAME
2387 PCRE - Perl-compatible regular expressions
2388
2389
2390 PCRE CALLOUTS
2391
2392 int (*pcre_callout)(pcre_callout_block *);
2393
2394 PCRE provides a feature called "callout", which is a means of temporar-
2395 ily passing control to the caller of PCRE in the middle of pattern
2396 matching. The caller of PCRE provides an external function by putting
2397 its entry point in the global variable pcre_callout. By default, this
2398 variable contains NULL, which disables all calling out.
2399
2400 Within a regular expression, (?C) indicates the points at which the
2401 external function is to be called. Different callout points can be
2402 identified by putting a number less than 256 after the letter C. The
2403 default value is zero. For example, this pattern has two callout
2404 points:
2405
2406 (?C1)eabc(?C2)def
2407
2408 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
2409 called, PCRE automatically inserts callouts, all with number 255,
2410 before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2411 used with the pattern
2412
2413 A(\d{2}|--)
2414
2415 it is processed as if it were
2416
2417 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2418
2419 Notice that there is a callout before and after each parenthesis and
2420 alternation bar. Automatic callouts can be used for tracking the
2421 progress of pattern matching. The pcretest command has an option that
2422 sets automatic callouts; when it is used, the output indicates how the
2423 pattern is matched. This is useful information when you are trying to
2424 optimize the performance of a particular pattern.
2425
2426
2427 MISSING CALLOUTS
2428
2429 You should be aware that, because of optimizations in the way PCRE
2430 matches patterns, callouts sometimes do not happen. For example, if the
2431 pattern is
2432
2433 ab(?C4)cd
2434
2435 PCRE knows that any matching string must contain the letter "d". If the
2436 subject string is "abyz", the lack of "d" means that matching doesn't
2437 ever start, and the callout is never reached. However, with "abyd",
2438 though the result is still no match, the callout is obeyed.
2439
2440
2441 THE CALLOUT INTERFACE
2442
2443 During matching, when PCRE reaches a callout point, the external func-
2444 tion defined by pcre_callout is called (if it is set). This applies to
2445 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2446 only argument to the callout function is a pointer to a pcre_callout
2447 block. This structure contains the following fields:
2448
2449 int version;
2450 int callout_number;
2451 int *offset_vector;
2452 const char *subject;
2453 int subject_length;
2454 int start_match;
2455 int current_position;
2456 int capture_top;
2457 int capture_last;
2458 void *callout_data;
2459 int pattern_position;
2460 int next_item_length;
2461
2462 The version field is an integer containing the version number of the
2463 block format. The initial version was 0; the current version is 1. The
2464 version number will change again in future if additional fields are
2465 added, but the intention is never to remove any of the existing fields.
2466
2467 The callout_number field contains the number of the callout, as com-
2468 piled into the pattern (that is, the number after ?C for manual call-
2469 outs, and 255 for automatically generated callouts).
2470
2471 The offset_vector field is a pointer to the vector of offsets that was
2472 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2473 pcre_exec() is used, the contents can be inspected in order to extract
2474 substrings that have been matched so far, in the same way as for
2475 extracting substrings after a match has completed. For pcre_dfa_exec()
2476 this field is not useful.
2477
2478 The subject and subject_length fields contain copies of the values that
2479 were passed to pcre_exec().
2480
2481 The start_match field contains the offset within the subject at which
2482 the current match attempt started. If the pattern is not anchored, the
2483 callout function may be called several times from the same point in the
2484 pattern for different starting points in the subject.
2485
2486 The current_position field contains the offset within the subject of
2487 the current match pointer.
2488
2489 When the pcre_exec() function is used, the capture_top field contains
2490 one more than the number of the highest numbered captured substring so
2491 far. If no substrings have been captured, the value of capture_top is
2492 one. This is always the case when pcre_dfa_exec() is used, because it
2493 does not support captured substrings.
2494
2495 The capture_last field contains the number of the most recently cap-
2496 tured substring. If no substrings have been captured, its value is -1.
2497 This is always the case when pcre_dfa_exec() is used.
2498
2499 The callout_data field contains a value that is passed to pcre_exec()
2500 or pcre_dfa_exec() specifically so that it can be passed back in call-
2501 outs. It is passed in the pcre_callout field of the pcre_extra data
2502 structure. If no such data was passed, the value of callout_data in a
2503 pcre_callout block is NULL. There is a description of the pcre_extra
2504 structure in the pcreapi documentation.
2505
2506 The pattern_position field is present from version 1 of the pcre_call-
2507 out structure. It contains the offset to the next item to be matched in
2508 the pattern string.
2509
2510 The next_item_length field is present from version 1 of the pcre_call-
2511 out structure. It contains the length of the next item to be matched in
2512 the pattern string. When the callout immediately precedes an alterna-
2513 tion bar, a closing parenthesis, or the end of the pattern, the length
2514 is zero. When the callout precedes an opening parenthesis, the length
2515 is that of the entire subpattern.
2516
2517 The pattern_position and next_item_length fields are intended to help
2518 in distinguishing between different automatic callouts, which all have
2519 the same callout number. However, they are set for all callouts.
2520
2521
2522 RETURN VALUES
2523
2524 The external callout function returns an integer to PCRE. If the value
2525 is zero, matching proceeds as normal. If the value is greater than
2526 zero, matching fails at the current point, but the testing of other
2527 matching possibilities goes ahead, just as if a lookahead assertion had
2528 failed. If the value is less than zero, the match is abandoned, and
2529 pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2530
2531 Negative values should normally be chosen from the set of
2532 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2533 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2534 reserved for use by callout functions; it will never be used by PCRE
2535 itself.
2536
2537
2538 AUTHOR
2539
2540 Philip Hazel
2541 University Computing Service
2542 Cambridge CB2 3QH, England.
2543
2544
2545 REVISION
2546
2547 Last updated: 06 March 2007
2548 Copyright (c) 1997-2007 University of Cambridge.
2549 ------------------------------------------------------------------------------
2550
2551
2552 PCRECOMPAT(3) PCRECOMPAT(3)
2553
2554
2555 NAME
2556 PCRE - Perl-compatible regular expressions
2557
2558
2559 DIFFERENCES BETWEEN PCRE AND PERL
2560
2561 This document describes the differences in the ways that PCRE and Perl
2562 handle regular expressions. The differences described here are mainly
2563 with respect to Perl 5.8, though PCRE version 7.0 contains some fea-
2564 tures that are expected to be in the forthcoming Perl 5.10.
2565
2566 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2567 of what it does have are given in the section on UTF-8 support in the
2568 main pcre page.
2569
2570 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2571 permits them, but they do not mean what you might think. For example,
2572 (?!a){3} does not assert that the next three characters are not "a". It
2573 just asserts that the next character is not "a" three times.
2574
2575 3. Capturing subpatterns that occur inside negative lookahead asser-
2576 tions are counted, but their entries in the offsets vector are never
2577 set. Perl sets its numerical variables from any such patterns that are
2578 matched before the assertion fails to match something (thereby succeed-
2579 ing), but only if the negative lookahead assertion contains just one
2580 branch.
2581
2582 4. Though binary zero characters are supported in the subject string,
2583 they are not allowed in a pattern string because it is passed as a nor-
2584 mal C string, terminated by zero. The escape sequence \0 can be used in
2585 the pattern to represent a binary zero.
2586
2587 5. The following Perl escape sequences are not supported: \l, \u, \L,
2588 \U, and \N. In fact these are implemented by Perl's general string-han-
2589 dling and are not part of its pattern matching engine. If any of these
2590 are encountered by PCRE, an error is generated.
2591
2592 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2593 is built with Unicode character property support. The properties that
2594 can be tested with \p and \P are limited to the general category prop-
2595 erties such as Lu and Nd, script names such as Greek or Han, and the
2596 derived properties Any and L&.
2597
2598 7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2599 ters in between are treated as literals. This is slightly different
2600 from Perl in that $ and @ are also handled as literals inside the
2601 quotes. In Perl, they cause variable interpolation (but of course PCRE
2602 does not have variables). Note the following examples:
2603
2604 Pattern PCRE matches Perl matches
2605
2606 \Qabc$xyz\E abc$xyz abc followed by the
2607 contents of $xyz
2608 \Qabc\$xyz\E abc\$xyz abc\$xyz
2609 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2610
2611 The \Q...\E sequence is recognized both inside and outside character
2612 classes.
2613
2614 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2615 constructions. However, there is support for recursive patterns. This
2616 is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2617 "callout" feature allows an external function to be called during pat-
2618 tern matching. See the pcrecallout documentation for details.
2619
2620 9. Subpatterns that are called recursively or as "subroutines" are
2621 always treated as atomic groups in PCRE. This is like Python, but
2622 unlike Perl.
2623
2624 10. There are some differences that are concerned with the settings of
2625 captured strings when part of a pattern is repeated. For example,
2626 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
2627 unset, but in PCRE it is set to "b".
2628
2629 11. PCRE provides some extensions to the Perl regular expression facil-
2630 ities. Perl 5.10 will include new features that are not in earlier
2631 versions, some of which (such as named parentheses) have been in PCRE
2632 for some time. This list is with respect to Perl 5.10:
2633
2634 (a) Although lookbehind assertions must match fixed length strings,
2635 each alternative branch of a lookbehind assertion can match a different
2636 length of string. Perl requires them all to have the same length.
2637
2638 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2639 meta-character matches only at the very end of the string.
2640
2641 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2642 cial meaning is faulted. Otherwise, like Perl, the backslash is
2643 ignored. (Perl can be made to issue a warning.)
2644
2645 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
2646 fiers is inverted, that is, by default they are not greedy, but if fol-
2647 lowed by a question mark they are.
2648
2649 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2650 tried only at the first matching position in the subject string.
2651
2652 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2653 TURE options for pcre_exec() have no Perl equivalents.
2654
2655 (g) The callout facility is PCRE-specific.
2656
2657 (h) The partial matching facility is PCRE-specific.
2658
2659 (i) Patterns compiled by PCRE can be saved and re-used at a later time,
2660 even on different hosts that have the other endianness.
2661
2662 (j) The alternative matching function (pcre_dfa_exec()) matches in a
2663 different way and is not Perl-compatible.
2664
2665
2666 AUTHOR
2667
2668 Philip Hazel
2669 University Computing Service
2670 Cambridge CB2 3QH, England.
2671
2672
2673 REVISION
2674
2675 Last updated: 06 March 2007
2676 Copyright (c) 1997-2007 University of Cambridge.
2677 ------------------------------------------------------------------------------
2678
2679
2680 PCREPATTERN(3) PCREPATTERN(3)
2681
2682
2683 NAME
2684 PCRE - Perl-compatible regular expressions
2685
2686
2687 PCRE REGULAR EXPRESSION DETAILS
2688
2689 The syntax and semantics of the regular expressions supported by PCRE
2690 are described below. Regular expressions are also described in the Perl
2691 documentation and in a number of books, some of which have copious
2692 examples. Jeffrey Friedl's "Mastering Regular Expressions", published
2693 by O'Reilly, covers regular expressions in great detail. This descrip-
2694 tion of PCRE's regular expressions is intended as reference material.
2695
2696 The original operation of PCRE was on strings of one-byte characters.
2697 However, there is now also support for UTF-8 character strings. To use
2698 this, you must build PCRE to include UTF-8 support, and then call
2699 pcre_compile() with the PCRE_UTF8 option. How this affects pattern
2700 matching is mentioned in several places below. There is also a summary
2701 of UTF-8 features in the section on UTF-8 support in the main pcre
2702 page.
2703
2704 The remainder of this document discusses the patterns that are sup-
2705 ported by PCRE when its main matching function, pcre_exec(), is used.
2706 From release 6.0, PCRE offers a second matching function,
2707 pcre_dfa_exec(), which matches using a different algorithm that is not
2708 Perl-compatible. The advantages and disadvantages of the alternative
2709 function, and how it differs from the normal function, are discussed in
2710 the pcrematching page.
2711
2712
2713 CHARACTERS AND METACHARACTERS
2714
2715 A regular expression is a pattern that is matched against a subject
2716 string from left to right. Most characters stand for themselves in a
2717 pattern, and match the corresponding characters in the subject. As a
2718 trivial example, the pattern
2719
2720 The quick brown fox
2721
2722 matches a portion of a subject string that is identical to itself. When
2723 caseless matching is specified (the PCRE_CASELESS option), letters are
2724 matched independently of case. In UTF-8 mode, PCRE always understands
2725 the concept of case for characters whose values are less than 128, so
2726 caseless matching is always possible. For characters with higher val-
2727 ues, the concept of case is supported if PCRE is compiled with Unicode
2728 property support, but not otherwise. If you want to use caseless
2729 matching for characters 128 and above, you must ensure that PCRE is
2730 compiled with Unicode property support as well as with UTF-8 support.
2731
2732 The power of regular expressions comes from the ability to include
2733 alternatives and repetitions in the pattern. These are encoded in the
2734 pattern by the use of metacharacters, which do not stand for themselves
2735 but instead are interpreted in some special way.
2736
2737 There are two different sets of metacharacters: those that are recog-
2738 nized anywhere in the pattern except within square brackets, and those
2739 that are recognized within square brackets. Outside square brackets,
2740 the metacharacters are as follows:
2741
2742 \ general escape character with several uses
2743 ^ assert start of string (or line, in multiline mode)
2744 $ assert end of string (or line, in multiline mode)
2745 . match any character except newline (by default)
2746 [ start character class definition
2747 | start of alternative branch
2748 ( start subpattern
2749 ) end subpattern
2750 ? extends the meaning of (
2751 also 0 or 1 quantifier
2752 also quantifier minimizer
2753 * 0 or more quantifier
2754 + 1 or more quantifier
2755 also "possessive quantifier"
2756 { start min/max quantifier
2757
2758 Part of a pattern that is in square brackets is called a "character
2759 class". In a character class the only metacharacters are:
2760
2761 \ general escape character
2762 ^ negate the class, but only if the first character
2763 - indicates character range
2764 [ POSIX character class (only if followed by POSIX
2765 syntax)
2766 ] terminates the character class
2767
2768 The following sections describe the use of each of the metacharacters.
2769
2770
2771 BACKSLASH
2772
2773 The backslash character has several uses. Firstly, if it is followed by
2774 a non-alphanumeric character, it takes away any special meaning that
2775 character may have. This use of backslash as an escape character
2776 applies both inside and outside character classes.
2777
2778 For example, if you want to match a * character, you write \* in the
2779 pattern. This escaping action applies whether or not the following
2780 character would otherwise be interpreted as a metacharacter, so it is
2781 always safe to precede a non-alphanumeric with backslash to specify
2782 that it stands for itself. In particular, if you want to match a back-
2783 slash, you write \\.
2784
2785 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
2786 the pattern (other than in a character class) and characters between a
2787 # outside a character class and the next newline are ignored. An escap-
2788 ing backslash can be used to include a whitespace or # character as
2789 part of the pattern.
2790
2791 If you want to remove the special meaning from a sequence of charac-
2792 ters, you can do so by putting them between \Q and \E. This is differ-
2793 ent from Perl in that $ and @ are handled as literals in \Q...\E
2794 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
2795 tion. Note the following examples:
2796
2797 Pattern PCRE matches Perl matches
2798
2799 \Qabc$xyz\E abc$xyz abc followed by the
2800 contents of $xyz
2801 \Qabc\$xyz\E abc\$xyz abc\$xyz
2802 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2803
2804 The \Q...\E sequence is recognized both inside and outside character
2805 classes.
2806
2807 Non-printing characters
2808
2809 A second use of backslash provides a way of encoding non-printing char-
2810 acters in patterns in a visible manner. There is no restriction on the
2811 appearance of non-printing characters, apart from the binary zero that
2812 terminates a pattern, but when a pattern is being prepared by text
2813 editing, it is usually easier to use one of the following escape
2814 sequences than the binary character it represents:
2815
2816 \a alarm, that is, the BEL character (hex 07)
2817 \cx "control-x", where x is any character
2818 \e escape (hex 1B)
2819 \f formfeed (hex 0C)
2820 \n newline (hex 0A)
2821 \r carriage return (hex 0D)
2822 \t tab (hex 09)
2823 \ddd character with octal code ddd, or backreference
2824 \xhh character with hex code hh
2825 \x{hhh..} character with hex code hhh..
2826
2827 The precise effect of \cx is as follows: if x is a lower case letter,
2828 it is converted to upper case. Then bit 6 of the character (hex 40) is
2829 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
2830 becomes hex 7B.
2831
2832 After \x, from zero to two hexadecimal digits are read (letters can be
2833 in upper or lower case). Any number of hexadecimal digits may appear
2834 between \x{ and }, but the value of the character code must be less
2835 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
2836 the maximum hexadecimal value is 7FFFFFFF). If characters other than
2837 hexadecimal digits appear between \x{ and }, or if there is no termi-
2838 nating }, this form of escape is not recognized. Instead, the initial
2839 \x will be interpreted as a basic hexadecimal escape, with no following
2840 digits, giving a character whose value is zero.
2841
2842 Characters whose value is less than 256 can be defined by either of the
2843 two syntaxes for \x. There is no difference in the way they are han-
2844 dled. For example, \xdc is exactly the same as \x{dc}.
2845
2846 After \0 up to two further octal digits are read. If there are fewer
2847 than two digits, just those that are present are used. Thus the
2848 sequence \0\x\07 specifies two binary zeros followed by a BEL character
2849 (code value 7). Make sure you supply two digits after the initial zero
2850 if the pattern character that follows is itself an octal digit.
2851
2852 The handling of a backslash followed by a digit other than 0 is compli-
2853 cated. Outside a character class, PCRE reads it and any following dig-
2854 its as a decimal number. If the number is less than 10, or if there
2855 have been at least that many previous capturing left parentheses in the
2856 expression, the entire sequence is taken as a back reference. A
2857 description of how this works is given later, following the discussion
2858 of parenthesized subpatterns.
2859
2860 Inside a character class, or if the decimal number is greater than 9
2861 and there have not been that many capturing subpatterns, PCRE re-reads
2862 up to three octal digits following the backslash, and uses them to gen-
2863 erate a data character. Any subsequent digits stand for themselves. In
2864 non-UTF-8 mode, the value of a character specified in octal must be
2865 less than \400. In UTF-8 mode, values up to \777 are permitted. For
2866 example:
2867
2868 \040 is another way of writing a space
2869 \40 is the same, provided there are fewer than 40
2870 previous capturing subpatterns
2871 \7 is always a back reference
2872 \11 might be a back reference, or another way of
2873 writing a tab
2874 \011 is always a tab
2875 \0113 is a tab followed by the character "3"
2876 \113 might be a back reference, otherwise the
2877 character with octal code 113
2878 \377 might be a back reference, otherwise
2879 the byte consisting entirely of 1 bits
2880 \81 is either a back reference, or a binary zero
2881 followed by the two characters "8" and "1"
2882
2883 Note that octal values of 100 or greater must not be introduced by a
2884 leading zero, because no more than three octal digits are ever read.
2885
2886 All the sequences that define a single character value can be used both
2887 inside and outside character classes. In addition, inside a character
2888 class, the sequence \b is interpreted as the backspace character (hex
2889 08), and the sequences \R and \X are interpreted as the characters "R"
2890 and "X", respectively. Outside a character class, these sequences have
2891 different meanings (see below).
2892
2893 Absolute and relative back references
2894
2895 The sequence \g followed by a positive or negative number, optionally
2896 enclosed in braces, is an absolute or relative back reference. Back
2897 references are discussed later, following the discussion of parenthe-
2898 sized subpatterns.
2899
2900 Generic character types
2901
2902 Another use of backslash is for specifying generic character types. The
2903 following are always recognized:
2904
2905 \d any decimal digit
2906 \D any character that is not a decimal digit
2907 \s any whitespace character
2908 \S any character that is not a whitespace character
2909 \w any "word" character
2910 \W any "non-word" character
2911
2912 Each pair of escape sequences partitions the complete set of characters
2913 into two disjoint sets. Any given character matches one, and only one,
2914 of each pair.
2915
2916 These character type sequences can appear both inside and outside char-
2917 acter classes. They each match one character of the appropriate type.
2918 If the current matching point is at the end of the subject string, all
2919 of them fail, since there is no character to match.
2920
2921 For compatibility with Perl, \s does not match the VT character (code
2922 11). This makes it different from the the POSIX "space" class. The \s
2923 characters are HT (9), LF (10), FF (12), CR (13), and space (32). (If
2924 "use locale;" is included in a Perl script, \s may match the VT charac-
2925 ter. In PCRE, it never does.)
2926
2927 A "word" character is an underscore or any character less than 256 that
2928 is a letter or digit. The definition of letters and digits is con-
2929 trolled by PCRE's low-valued character tables, and may vary if locale-
2930 specific matching is taking place (see "Locale support" in the pcreapi
2931 page). For example, in a French locale such as "fr_FR" in Unix-like
2932 systems, or "french" in Windows, some character codes greater than 128
2933 are used for accented letters, and these are matched by \w.
2934
2935 In UTF-8 mode, characters with values greater than 128 never match \d,
2936 \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
2937 code character property support is available. The use of locales with
2938 Unicode is discouraged.
2939
2940 Newline sequences
2941
2942 Outside a character class, the escape sequence \R matches any Unicode
2943 newline sequence. This is an extension to Perl. In non-UTF-8 mode \R is
2944 equivalent to the following:
2945
2946 (?>\r\n|\n|\x0b|\f|\r|\x85)
2947
2948 This is an example of an "atomic group", details of which are given
2949 below. This particular group matches either the two-character sequence
2950 CR followed by LF, or one of the single characters LF (linefeed,
2951 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
2952 return, U+000D), or NEL (next line, U+0085). The two-character sequence
2953 is treated as a single unit that cannot be split.
2954
2955 In UTF-8 mode, two additional characters whose codepoints are greater
2956 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
2957 rator, U+2029). Unicode character property support is not needed for
2958 these characters to be recognized.
2959
2960 Inside a character class, \R matches the letter "R".
2961
2962 Unicode character properties
2963
2964 When PCRE is built with Unicode character property support, three addi-
2965 tional escape sequences to match character properties are available
2966 when UTF-8 mode is selected. They are:
2967
2968 \p{xx} a character with the xx property
2969 \P{xx} a character without the xx property
2970 \X an extended Unicode sequence
2971
2972 The property names represented by xx above are limited to the Unicode
2973 script names, the general category properties, and "Any", which matches
2974 any character (including newline). Other properties such as "InMusical-
2975 Symbols" are not currently supported by PCRE. Note that \P{Any} does
2976 not match any characters, so always causes a match failure.
2977
2978 Sets of Unicode characters are defined as belonging to certain scripts.
2979 A character from one of these sets can be matched using a script name.
2980 For example:
2981
2982 \p{Greek}
2983 \P{Han}
2984
2985 Those that are not part of an identified script are lumped together as
2986 "Common". The current list of scripts is:
2987
2988 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
2989 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
2990 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
2991 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
2992 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
2993 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
2994 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
2995 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
2996 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
2997
2998 Each character has exactly one general category property, specified by
2999 a two-letter abbreviation. For compatibility with Perl, negation can be
3000 specified by including a circumflex between the opening brace and the
3001 property name. For example, \p{^Lu} is the same as \P{Lu}.
3002
3003 If only one letter is specified with \p or \P, it includes all the gen-
3004 eral category properties that start with that letter. In this case, in
3005 the absence of negation, the curly brackets in the escape sequence are
3006 optional; these two examples have the same effect:
3007
3008 \p{L}
3009 \pL
3010
3011 The following general category property codes are supported:
3012
3013 C Other
3014 Cc Control
3015 Cf Format
3016 Cn Unassigned
3017 Co Private use
3018 Cs Surrogate
3019
3020 L Letter
3021 Ll Lower case letter
3022 Lm Modifier letter
3023 Lo Other letter
3024 Lt Title case letter
3025 Lu Upper case letter
3026
3027 M Mark
3028 Mc Spacing mark
3029 Me Enclosing mark
3030 Mn Non-spacing mark
3031
3032 N Number
3033 Nd Decimal number
3034 Nl Letter number
3035 No Other number
3036
3037 P Punctuation
3038 Pc Connector punctuation
3039 Pd Dash punctuation
3040 Pe Close punctuation
3041 Pf Final punctuation
3042 Pi Initial punctuation
3043 Po Other punctuation
3044 Ps Open punctuation
3045
3046 S Symbol
3047 Sc Currency symbol
3048 Sk Modifier symbol
3049 Sm Mathematical symbol
3050 So Other symbol
3051
3052 Z Separator
3053 Zl Line separator
3054 Zp Paragraph separator
3055 Zs Space separator
3056
3057 The special property L& is also supported: it matches a character that
3058 has the Lu, Ll, or Lt property, in other words, a letter that is not
3059 classified as a modifier or "other".
3060
3061 The long synonyms for these properties that Perl supports (such as
3062 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3063 any of these properties with "Is".
3064
3065 No character that is in the Unicode table has the Cn (unassigned) prop-
3066 erty. Instead, this property is assumed for any code point that is not
3067 in the Unicode table.
3068
3069 Specifying caseless matching does not affect these escape sequences.
3070 For example, \p{Lu} always matches only upper case letters.
3071
3072 The \X escape matches any number of Unicode characters that form an
3073 extended Unicode sequence. \X is equivalent to
3074
3075 (?>\PM\pM*)
3076
3077 That is, it matches a character without the "mark" property, followed
3078 by zero or more characters with the "mark" property, and treats the
3079 sequence as an atomic group (see below). Characters with the "mark"
3080 property are typically accents that affect the preceding character.
3081
3082 Matching characters by Unicode property is not fast, because PCRE has
3083 to search a structure that contains data for over fifteen thousand
3084 characters. That is why the traditional escape sequences such as \d and
3085 \w do not use Unicode properties in PCRE.
3086
3087 Simple assertions
3088
3089 The final use of backslash is for certain simple assertions. An asser-
3090 tion specifies a condition that has to be met at a particular point in
3091 a match, without consuming any characters from the subject string. The
3092 use of subpatterns for more complicated assertions is described below.
3093 The backslashed assertions are:
3094
3095 \b matches at a word boundary
3096 \B matches when not at a word boundary
3097 \A matches at the start of the subject
3098 \Z matches at the end of the subject
3099 also matches before a newline at the end of the subject
3100 \z matches only at the end of the subject
3101 \G matches at the first matching position in the subject
3102
3103 These assertions may not appear in character classes (but note that \b
3104 has a different meaning, namely the backspace character, inside a char-
3105 acter class).
3106
3107 A word boundary is a position in the subject string where the current
3108 character and the previous character do not both match \w or \W (i.e.
3109 one matches \w and the other matches \W), or the start or end of the
3110 string if the first or last character matches \w, respectively.
3111
3112 The \A, \Z, and \z assertions differ from the traditional circumflex
3113 and dollar (described in the next section) in that they only ever match
3114 at the very start and end of the subject string, whatever options are
3115 set. Thus, they are independent of multiline mode. These three asser-
3116 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3117 affect only the behaviour of the circumflex and dollar metacharacters.
3118 However, if the startoffset argument of pcre_exec() is non-zero, indi-
3119 cating that matching is to start at a point other than the beginning of
3120 the subject, \A can never match. The difference between \Z and \z is
3121 that \Z matches before a newline at the end of the string as well as at
3122 the very end, whereas \z matches only at the end.
3123
3124 The \G assertion is true only when the current matching position is at
3125 the start point of the match, as specified by the startoffset argument
3126 of pcre_exec(). It differs from \A when the value of startoffset is
3127 non-zero. By calling pcre_exec() multiple times with appropriate argu-
3128 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3129 mentation where \G can be useful.
3130
3131 Note, however, that PCRE's interpretation of \G, as the start of the
3132 current match, is subtly different from Perl's, which defines it as the
3133 end of the previous match. In Perl, these can be different when the
3134 previously matched string was empty. Because PCRE does just one match
3135 at a time, it cannot reproduce this behaviour.
3136
3137 If all the alternatives of a pattern begin with \G, the expression is
3138 anchored to the starting match position, and the "anchored" flag is set
3139 in the compiled regular expression.
3140
3141
3142 CIRCUMFLEX AND DOLLAR
3143
3144 Outside a character class, in the default matching mode, the circumflex
3145 character is an assertion that is true only if the current matching
3146 point is at the start of the subject string. If the startoffset argu-
3147 ment of pcre_exec() is non-zero, circumflex can never match if the
3148 PCRE_MULTILINE option is unset. Inside a character class, circumflex
3149 has an entirely different meaning (see below).
3150
3151 Circumflex need not be the first character of the pattern if a number
3152 of alternatives are involved, but it should be the first thing in each
3153 alternative in which it appears if the pattern is ever to match that
3154 branch. If all possible alternatives start with a circumflex, that is,
3155 if the pattern is constrained to match only at the start of the sub-
3156 ject, it is said to be an "anchored" pattern. (There are also other
3157 constructs that can cause a pattern to be anchored.)
3158
3159 A dollar character is an assertion that is true only if the current
3160 matching point is at the end of the subject string, or immediately
3161 before a newline at the end of the string (by default). Dollar need not
3162 be the last character of the pattern if a number of alternatives are
3163 involved, but it should be the last item in any branch in which it
3164 appears. Dollar has no special meaning in a character class.
3165
3166 The meaning of dollar can be changed so that it matches only at the
3167 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3168 compile time. This does not affect the \Z assertion.
3169
3170 The meanings of the circumflex and dollar characters are changed if the
3171 PCRE_MULTILINE option is set. When this is the case, a circumflex
3172 matches immediately after internal newlines as well as at the start of
3173 the subject string. It does not match after a newline that ends the
3174 string. A dollar matches before any newlines in the string, as well as
3175 at the very end, when PCRE_MULTILINE is set. When newline is specified
3176 as the two-character sequence CRLF, isolated CR and LF characters do
3177 not indicate newlines.
3178
3179 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3180 (where \n represents a newline) in multiline mode, but not otherwise.
3181 Consequently, patterns that are anchored in single line mode because
3182 all branches start with ^ are not anchored in multiline mode, and a
3183 match for circumflex is possible when the startoffset argument of
3184 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3185 PCRE_MULTILINE is set.
3186
3187 Note that the sequences \A, \Z, and \z can be used to match the start
3188 and end of the subject in both modes, and if all branches of a pattern
3189 start with \A it is always anchored, whether or not PCRE_MULTILINE is
3190 set.
3191
3192
3193 FULL STOP (PERIOD, DOT)
3194
3195 Outside a character class, a dot in the pattern matches any one charac-
3196 ter in the subject string except (by default) a character that signi-
3197 fies the end of a line. In UTF-8 mode, the matched character may be
3198 more than one byte long.
3199
3200 When a line ending is defined as a single character, dot never matches
3201 that character; when the two-character sequence CRLF is used, dot does
3202 not match CR if it is immediately followed by LF, but otherwise it
3203 matches all characters (including isolated CRs and LFs). When any Uni-
3204 code line endings are being recognized, dot does not match CR or LF or
3205 any of the other line ending characters.
3206
3207 The behaviour of dot with regard to newlines can be changed. If the
3208 PCRE_DOTALL option is set, a dot matches any one character, without
3209 exception. If the two-character sequence CRLF is present in the subject
3210 string, it takes two dots to match it.
3211
3212 The handling of dot is entirely independent of the handling of circum-
3213 flex and dollar, the only relationship being that they both involve
3214 newlines. Dot has no special meaning in a character class.
3215
3216
3217 MATCHING A SINGLE BYTE
3218
3219 Outside a character class, the escape sequence \C matches any one byte,
3220 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3221 line-ending characters. The feature is provided in Perl in order to
3222 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3223 acters into individual bytes, what remains in the string may be a mal-
3224 formed UTF-8 string. For this reason, the \C escape sequence is best
3225 avoided.
3226
3227 PCRE does not allow \C to appear in lookbehind assertions (described
3228 below), because in UTF-8 mode this would make it impossible to calcu-
3229 late the length of the lookbehind.
3230
3231
3232 SQUARE BRACKETS AND CHARACTER CLASSES
3233
3234 An opening square bracket introduces a character class, terminated by a
3235 closing square bracket. A closing square bracket on its own is not spe-
3236 cial. If a closing square bracket is required as a member of the class,
3237 it should be the first data character in the class (after an initial
3238 circumflex, if present) or escaped with a backslash.
3239
3240 A character class matches a single character in the subject. In UTF-8
3241 mode, the character may occupy more than one byte. A matched character
3242 must be in the set of characters defined by the class, unless the first
3243 character in the class definition is a circumflex, in which case the
3244 subject character must not be in the set defined by the class. If a
3245 circumflex is actually required as a member of the class, ensure it is
3246 not the first character, or escape it with a backslash.
3247
3248 For example, the character class [aeiou] matches any lower case vowel,
3249 while [^aeiou] matches any character that is not a lower case vowel.
3250 Note that a circumflex is just a convenient notation for specifying the
3251 characters that are in the class by enumerating those that are not. A
3252 class that starts with a circumflex is not an assertion: it still con-
3253 sumes a character from the subject string, and therefore it fails if
3254 the current pointer is at the end of the string.
3255
3256 In UTF-8 mode, characters with values greater than 255 can be included
3257 in a class as a literal string of bytes, or by using the \x{ escaping
3258 mechanism.
3259
3260 When caseless matching is set, any letters in a class represent both
3261 their upper case and lower case versions, so for example, a caseless
3262 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3263 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3264 understands the concept of case for characters whose values are less
3265 than 128, so caseless matching is always possible. For characters with
3266 higher values, the concept of case is supported if PCRE is compiled
3267 with Unicode property support, but not otherwise. If you want to use
3268 caseless matching for characters 128 and above, you must ensure that
3269 PCRE is compiled with Unicode property support as well as with UTF-8
3270 support.
3271
3272 Characters that might indicate line breaks are never treated in any
3273 special way when matching character classes, whatever line-ending
3274 sequence is in use, and whatever setting of the PCRE_DOTALL and
3275 PCRE_MULTILINE options is used. A class such as [^a] always matches one
3276 of these characters.
3277
3278 The minus (hyphen) character can be used to specify a range of charac-
3279 ters in a character class. For example, [d-m] matches any letter
3280 between d and m, inclusive. If a minus character is required in a
3281 class, it must be escaped with a backslash or appear in a position
3282 where it cannot be interpreted as indicating a range, typically as the
3283 first or last character in the class.
3284
3285 It is not possible to have the literal character "]" as the end charac-
3286 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3287 two characters ("W" and "-") followed by a literal string "46]", so it
3288 would match "W46]" or "-46]". However, if the "]" is escaped with a
3289 backslash it is interpreted as the end of range, so [W-\]46] is inter-
3290 preted as a class containing a range followed by two other characters.
3291 The octal or hexadecimal representation of "]" can also be used to end
3292 a range.
3293
3294 Ranges operate in the collating sequence of character values. They can
3295 also be used for characters specified numerically, for example
3296 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3297 are greater than 255, for example [\x{100}-\x{2ff}].
3298
3299 If a range that includes letters is used when caseless matching is set,
3300 it matches the letters in either case. For example, [W-c] is equivalent
3301 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3302 character tables for a French locale are in use, [\xc8-\xcb] matches
3303 accented E characters in both cases. In UTF-8 mode, PCRE supports the
3304 concept of case for characters with values greater than 128 only when
3305 it is compiled with Unicode property support.
3306
3307 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3308 in a character class, and add the characters that they match to the
3309 class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3310 flex can conveniently be used with the upper case character types to
3311 specify a more restricted set of characters than the matching lower
3312 case type. For example, the class [^\W_] matches any letter or digit,
3313 but not underscore.
3314
3315 The only metacharacters that are recognized in character classes are
3316 backslash, hyphen (only where it can be interpreted as specifying a
3317 range), circumflex (only at the start), opening square bracket (only
3318 when it can be interpreted as introducing a POSIX class name - see the
3319 next section), and the terminating closing square bracket. However,
3320 escaping other non-alphanumeric characters does no harm.
3321
3322
3323 POSIX CHARACTER CLASSES
3324
3325 Perl supports the POSIX notation for character classes. This uses names
3326 enclosed by [: and :] within the enclosing square brackets. PCRE also
3327 supports this notation. For example,
3328
3329 [01[:alpha:]%]
3330
3331 matches "0", "1", any alphabetic character, or "%". The supported class
3332 names are
3333
3334 alnum letters and digits
3335 alpha letters
3336 ascii character codes 0 - 127
3337 blank space or tab only
3338 cntrl control characters
3339 digit decimal digits (same as \d)
3340 graph printing characters, excluding space
3341 lower lower case letters
3342 print printing characters, including space
3343 punct printing characters, excluding letters and digits
3344 space white space (not quite the same as \s)
3345 upper upper case letters
3346 word "word" characters (same as \w)
3347 xdigit hexadecimal digits
3348
3349 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3350 and space (32). Notice that this list includes the VT character (code
3351 11). This makes "space" different to \s, which does not include VT (for
3352 Perl compatibility).
3353
3354 The name "word" is a Perl extension, and "blank" is a GNU extension
3355 from Perl 5.8. Another Perl extension is negation, which is indicated
3356 by a ^ character after the colon. For example,
3357
3358 [12[:^digit:]]
3359
3360 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3361 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3362 these are not supported, and an error is given if they are encountered.
3363
3364 In UTF-8 mode, characters with values greater than 128 do not match any
3365 of the POSIX character classes.
3366
3367
3368 VERTICAL BAR
3369
3370 Vertical bar characters are used to separate alternative patterns. For
3371 example, the pattern
3372
3373 gilbert|sullivan
3374
3375 matches either "gilbert" or "sullivan". Any number of alternatives may
3376 appear, and an empty alternative is permitted (matching the empty
3377 string). The matching process tries each alternative in turn, from left
3378 to right, and the first one that succeeds is used. If the alternatives
3379 are within a subpattern (defined below), "succeeds" means matching the
3380 rest of the main pattern as well as the alternative in the subpattern.
3381
3382
3383 INTERNAL OPTION SETTING
3384
3385 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3386 PCRE_EXTENDED options can be changed from within the pattern by a
3387 sequence of Perl option letters enclosed between "(?" and ")". The
3388 option letters are
3389
3390 i for PCRE_CASELESS
3391 m for PCRE_MULTILINE
3392 s for PCRE_DOTALL
3393 x for PCRE_EXTENDED
3394
3395 For example, (?im) sets caseless, multiline matching. It is also possi-
3396 ble to unset these options by preceding the letter with a hyphen, and a
3397 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3398 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3399 is also permitted. If a letter appears both before and after the
3400 hyphen, the option is unset.
3401
3402 When an option change occurs at top level (that is, not inside subpat-
3403 tern parentheses), the change applies to the remainder of the pattern
3404 that follows. If the change is placed right at the start of a pattern,
3405 PCRE extracts it into the global options (and it will therefore show up
3406 in data extracted by the pcre_fullinfo() function).
3407
3408 An option change within a subpattern (see below for a description of
3409 subpatterns) affects only that part of the current pattern that follows
3410 it, so
3411
3412 (a(?i)b)c
3413
3414 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3415 used). By this means, options can be made to have different settings
3416 in different parts of the pattern. Any changes made in one alternative
3417 do carry on into subsequent branches within the same subpattern. For
3418 example,
3419
3420 (a(?i)b|c)
3421
3422 matches "ab", "aB", "c", and "C", even though when matching "C" the
3423 first branch is abandoned before the option setting. This is because
3424 the effects of option settings happen at compile time. There would be
3425 some very weird behaviour otherwise.
3426
3427 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3428 can be changed in the same way as the Perl-compatible options by using
3429 the characters J, U and X respectively.
3430
3431
3432 SUBPATTERNS
3433
3434 Subpatterns are delimited by parentheses (round brackets), which can be
3435 nested. Turning part of a pattern into a subpattern does two things:
3436
3437 1. It localizes a set of alternatives. For example, the pattern
3438
3439 cat(aract|erpillar|)
3440
3441 matches one of the words "cat", "cataract", or "caterpillar". Without
3442 the parentheses, it would match "cataract", "erpillar" or an empty
3443 string.
3444
3445 2. It sets up the subpattern as a capturing subpattern. This means
3446 that, when the whole pattern matches, that portion of the subject
3447 string that matched the subpattern is passed back to the caller via the
3448 ovector argument of pcre_exec(). Opening parentheses are counted from
3449 left to right (starting from 1) to obtain numbers for the capturing
3450 subpatterns.
3451
3452 For example, if the string "the red king" is matched against the pat-
3453 tern
3454
3455 the ((red|white) (king|queen))
3456
3457 the captured substrings are "red king", "red", and "king", and are num-
3458 bered 1, 2, and 3, respectively.
3459
3460 The fact that plain parentheses fulfil two functions is not always
3461 helpful. There are often times when a grouping subpattern is required
3462 without a capturing requirement. If an opening parenthesis is followed
3463 by a question mark and a colon, the subpattern does not do any captur-
3464 ing, and is not counted when computing the number of any subsequent
3465 capturing subpatterns. For example, if the string "the white queen" is
3466 matched against the pattern
3467
3468 the ((?:red|white) (king|queen))
3469
3470 the captured substrings are "white queen" and "queen", and are numbered
3471 1 and 2. The maximum number of capturing subpatterns is 65535.
3472
3473 As a convenient shorthand, if any option settings are required at the
3474 start of a non-capturing subpattern, the option letters may appear
3475 between the "?" and the ":". Thus the two patterns
3476
3477 (?i:saturday|sunday)
3478 (?:(?i)saturday|sunday)
3479
3480 match exactly the same set of strings. Because alternative branches are
3481 tried from left to right, and options are not reset until the end of
3482 the subpattern is reached, an option setting in one branch does affect
3483 subsequent branches, so the above patterns match "SUNDAY" as well as
3484 "Saturday".
3485
3486
3487 NAMED SUBPATTERNS
3488
3489 Identifying capturing parentheses by number is simple, but it can be
3490 very hard to keep track of the numbers in complicated regular expres-
3491 sions. Furthermore, if an expression is modified, the numbers may
3492 change. To help with this difficulty, PCRE supports the naming of sub-
3493 patterns. This feature was not added to Perl until release 5.10. Python
3494 had the feature earlier, and PCRE introduced it at release 4.0, using
3495 the Python syntax. PCRE now supports both the Perl and the Python syn-
3496 tax.
3497
3498 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
3499 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
3500 to capturing parentheses from other parts of the pattern, such as back-
3501 references, recursion, and conditions, can be made by name as well as
3502 by number.
3503
3504 Names consist of up to 32 alphanumeric characters and underscores.
3505 Named capturing parentheses are still allocated numbers as well as
3506 names, exactly as if the names were not present. The PCRE API provides
3507 function calls for extracting the name-to-number translation table from
3508 a compiled pattern. There is also a convenience function for extracting
3509 a captured substring by name.
3510
3511 By default, a name must be unique within a pattern, but it is possible
3512 to relax this constraint by setting the PCRE_DUPNAMES option at compile
3513 time. This can be useful for patterns where only one instance of the
3514 named parentheses can match. Suppose you want to match the name of a
3515 weekday, either as a 3-letter abbreviation or as the full name, and in
3516 both cases you want to extract the abbreviation. This pattern (ignoring
3517 the line breaks) does the job:
3518
3519 (?<DN>Mon|Fri|Sun)(?:day)?|
3520 (?<DN>Tue)(?:sday)?|
3521 (?<DN>Wed)(?:nesday)?|
3522 (?<DN>Thu)(?:rsday)?|
3523 (?<DN>Sat)(?:urday)?
3524
3525 There are five capturing substrings, but only one is ever set after a
3526 match. The convenience function for extracting the data by name
3527 returns the substring for the first (and in this example, the only)
3528 subpattern of that name that matched. This saves searching to find
3529 which numbered subpattern it was. If you make a reference to a non-
3530 unique named subpattern from elsewhere in the pattern, the one that
3531 corresponds to the lowest number is used. For further details of the
3532 interfaces for handling named subpatterns, see the pcreapi documenta-
3533 tion.
3534
3535
3536 REPETITION
3537
3538 Repetition is specified by quantifiers, which can follow any of the
3539 following items:
3540
3541 a literal data character
3542 the dot metacharacter
3543 the \C escape sequence
3544 the \X escape sequence (in UTF-8 mode with Unicode properties)
3545 the \R escape sequence
3546 an escape such as \d that matches a single character
3547 a character class
3548 a back reference (see next section)
3549 a parenthesized subpattern (unless it is an assertion)
3550
3551 The general repetition quantifier specifies a minimum and maximum num-
3552 ber of permitted matches, by giving the two numbers in curly brackets
3553 (braces), separated by a comma. The numbers must be less than 65536,
3554 and the first must be less than or equal to the second. For example:
3555
3556 z{2,4}
3557
3558 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
3559 special character. If the second number is omitted, but the comma is
3560 present, there is no upper limit; if the second number and the comma
3561 are both omitted, the quantifier specifies an exact number of required
3562 matches. Thus
3563
3564 [aeiou]{3,}
3565
3566 matches at least 3 successive vowels, but may match many more, while
3567
3568 \d{8}
3569
3570 matches exactly 8 digits. An opening curly bracket that appears in a
3571 position where a quantifier is not allowed, or one that does not match
3572 the syntax of a quantifier, is taken as a literal character. For exam-
3573 ple, {,6} is not a quantifier, but a literal string of four characters.
3574
3575 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
3576 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
3577 acters, each of which is represented by a two-byte sequence. Similarly,
3578 when Unicode property support is available, \X{3} matches three Unicode
3579 extended sequences, each of which may be several bytes long (and they
3580 may be of different lengths).
3581
3582 The quantifier {0} is permitted, causing the expression to behave as if
3583 the previous item and the quantifier were not present.
3584
3585 For convenience, the three most common quantifiers have single-charac-
3586 ter abbreviations:
3587
3588 * is equivalent to {0,}
3589 + is equivalent to {1,}
3590 ? is equivalent to {0,1}
3591
3592 It is possible to construct infinite loops by following a subpattern
3593 that can match no characters with a quantifier that has no upper limit,
3594 for example:
3595
3596 (a?)*
3597
3598 Earlier versions of Perl and PCRE used to give an error at compile time
3599 for such patterns. However, because there are cases where this can be
3600 useful, such patterns are now accepted, but if any repetition of the
3601 subpattern does in fact match no characters, the loop is forcibly bro-
3602 ken.
3603
3604 By default, the quantifiers are "greedy", that is, they match as much
3605 as possible (up to the maximum number of permitted times), without
3606 causing the rest of the pattern to fail. The classic example of where
3607 this gives problems is in trying to match comments in C programs. These
3608 appear between /* and */ and within the comment, individual * and /
3609 characters may appear. An attempt to match C comments by applying the
3610 pattern
3611
3612 /\*.*\*/
3613
3614 to the string
3615
3616 /* first comment */ not comment /* second comment */
3617
3618 fails, because it matches the entire string owing to the greediness of
3619 the .* item.
3620
3621 However, if a quantifier is followed by a question mark, it ceases to
3622 be greedy, and instead matches the minimum number of times possible, so
3623 the pattern
3624
3625 /\*.*?\*/
3626
3627 does the right thing with the C comments. The meaning of the various
3628 quantifiers is not otherwise changed, just the preferred number of
3629 matches. Do not confuse this use of question mark with its use as a
3630 quantifier in its own right. Because it has two uses, it can sometimes
3631 appear doubled, as in
3632
3633 \d??\d
3634
3635 which matches one digit by preference, but can match two if that is the
3636 only way the rest of the pattern matches.
3637
3638 If the PCRE_UNGREEDY option is set (an option that is not available in
3639 Perl), the quantifiers are not greedy by default, but individual ones
3640 can be made greedy by following them with a question mark. In other
3641 words, it inverts the default behaviour.
3642
3643 When a parenthesized subpattern is quantified with a minimum repeat
3644 count that is greater than 1 or with a limited maximum, more memory is
3645 required for the compiled pattern, in proportion to the size of the
3646 minimum or maximum.
3647
3648 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
3649 alent to Perl's /s) is set, thus allowing the dot to match newlines,
3650 the pattern is implicitly anchored, because whatever follows will be
3651 tried against every character position in the subject string, so there
3652 is no point in retrying the overall match at any position after the
3653 first. PCRE normally treats such a pattern as though it were preceded
3654 by \A.
3655
3656 In cases where it is known that the subject string contains no new-
3657 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
3658 mization, or alternatively using ^ to indicate anchoring explicitly.
3659
3660 However, there is one situation where the optimization cannot be used.
3661 When .* is inside capturing parentheses that are the subject of a
3662 backreference elsewhere in the pattern, a match at the start may fail
3663 where a later one succeeds. Consider, for example:
3664
3665 (.*)abc\1
3666
3667 If the subject is "xyz123abc123" the match point is the fourth charac-
3668 ter. For this reason, such a pattern is not implicitly anchored.
3669
3670 When a capturing subpattern is repeated, the value captured is the sub-
3671 string that matched the final iteration. For example, after
3672
3673 (tweedle[dume]{3}\s*)+
3674
3675 has matched "tweedledum tweedledee" the value of the captured substring
3676 is "tweedledee". However, if there are nested capturing subpatterns,
3677 the corresponding captured values may have been set in previous itera-
3678 tions. For example, after
3679
3680 /(a|(b))+/
3681
3682 matches "aba" the value of the second captured substring is "b".
3683
3684
3685 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
3686
3687 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
3688 repetition, failure of what follows normally causes the repeated item
3689 to be re-evaluated to see if a different number of repeats allows the
3690 rest of the pattern to match. Sometimes it is useful to prevent this,
3691 either to change the nature of the match, or to cause it fail earlier
3692 than it otherwise might, when the author of the pattern knows there is
3693 no point in carrying on.
3694
3695 Consider, for example, the pattern \d+foo when applied to the subject
3696 line
3697
3698 123456bar
3699
3700 After matching all 6 digits and then failing to match "foo", the normal
3701 action of the matcher is to try again with only 5 digits matching the
3702 \d+ item, and then with 4, and so on, before ultimately failing.
3703 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
3704 the means for specifying that once a subpattern has matched, it is not
3705 to be re-evaluated in this way.
3706
3707 If we use atomic grouping for the previous example, the matcher gives
3708 up immediately on failing to match "foo" the first time. The notation
3709 is a kind of special parenthesis, starting with (?> as in this example:
3710
3711 (?>\d+)foo
3712
3713 This kind of parenthesis "locks up" the part of the pattern it con-
3714 tains once it has matched, and a failure further into the pattern is
3715 prevented from backtracking into it. Backtracking past it to previous
3716 items, however, works as normal.
3717
3718 An alternative description is that a subpattern of this type matches
3719 the string of characters that an identical standalone pattern would
3720 match, if anchored at the current point in the subject string.
3721
3722 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
3723 such as the above example can be thought of as a maximizing repeat that
3724 must swallow everything it can. So, while both \d+ and \d+? are pre-
3725 pared to adjust the number of digits they match in order to make the
3726 rest of the pattern match, (?>\d+) can only match an entire sequence of
3727 digits.
3728
3729 Atomic groups in general can of course contain arbitrarily complicated
3730 subpatterns, and can be nested. However, when the subpattern for an
3731 atomic group is just a single repeated item, as in the example above, a
3732 simpler notation, called a "possessive quantifier" can be used. This
3733 consists of an additional + character following a quantifier. Using
3734 this notation, the previous example can be rewritten as
3735
3736 \d++foo
3737
3738 Possessive quantifiers are always greedy; the setting of the
3739 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
3740 simpler forms of atomic group. However, there is no difference in the
3741 meaning of a possessive quantifier and the equivalent atomic group,
3742 though there may be a performance difference; possessive quantifiers
3743 should be slightly faster.
3744
3745 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
3746 tax. Jeffrey Friedl originated the idea (and the name) in the first
3747 edition of his book. Mike McCloskey liked it, so implemented it when he
3748 built Sun's Java package, and PCRE copied it from there. It ultimately
3749 found its way into Perl at release 5.10.
3750
3751 PCRE has an optimization that automatically "possessifies" certain sim-
3752 ple pattern constructs. For example, the sequence A+B is treated as
3753 A++B because there is no point in backtracking into a sequence of A's
3754 when B must follow.
3755
3756 When a pattern contains an unlimited repeat inside a subpattern that
3757 can itself be repeated an unlimited number of times, the use of an
3758 atomic group is the only way to avoid some failing matches taking a
3759 very long time indeed. The pattern
3760
3761 (\D+|<\d+>)*[!?]
3762
3763 matches an unlimited number of substrings that either consist of non-
3764 digits, or digits enclosed in <>, followed by either ! or ?. When it
3765 matches, it runs quickly. However, if it is applied to
3766
3767 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
3768
3769 it takes a long time before reporting failure. This is because the
3770 string can be divided between the internal \D+ repeat and the external
3771 * repeat in a large number of ways, and all have to be tried. (The
3772 example uses [!?] rather than a single character at the end, because
3773 both PCRE and Perl have an optimization that allows for fast failure
3774 when a single character is used. They remember the last single charac-
3775 ter that is required for a match, and fail early if it is not present
3776 in the string.) If the pattern is changed so that it uses an atomic
3777 group, like this:
3778
3779 ((?>\D+)|<\d+>)*[!?]
3780
3781 sequences of non-digits cannot be broken, and failure happens quickly.
3782
3783
3784 BACK REFERENCES
3785
3786 Outside a character class, a backslash followed by a digit greater than
3787 0 (and possibly further digits) is a back reference to a capturing sub-
3788 pattern earlier (that is, to its left) in the pattern, provided there
3789 have been that many previous capturing left parentheses.
3790
3791 However, if the decimal number following the backslash is less than 10,
3792 it is always taken as a back reference, and causes an error only if
3793 there are not that many capturing left parentheses in the entire pat-
3794 tern. In other words, the parentheses that are referenced need not be
3795 to the left of the reference for numbers less than 10. A "forward back
3796 reference" of this type can make sense when a repetition is involved
3797 and the subpattern to the right has participated in an earlier itera-
3798 tion.
3799
3800 It is not possible to have a numerical "forward back reference" to a
3801 subpattern whose number is 10 or more using this syntax because a
3802 sequence such as \50 is interpreted as a character defined in octal.
3803 See the subsection entitled "Non-printing characters" above for further
3804 details of the handling of digits following a backslash. There is no
3805 such problem when named parentheses are used. A back reference to any
3806 subpattern is possible using named parentheses (see below).
3807
3808 Another way of avoiding the ambiguity inherent in the use of digits
3809 following a backslash is to use the \g escape sequence, which is a fea-
3810 ture introduced in Perl 5.10. This escape must be followed by a posi-
3811 tive or a negative number, optionally enclosed in braces. These exam-
3812 ples are all identical:
3813
3814 (ring), \1
3815 (ring), \g1
3816 (ring), \g{1}
3817
3818 A positive number specifies an absolute reference without the ambiguity
3819 that is present in the older syntax. It is also useful when literal
3820 digits follow the reference. A negative number is a relative reference.
3821 Consider this example:
3822
3823 (abc(def)ghi)\g{-1}
3824
3825 The sequence \g{-1} is a reference to the most recently started captur-
3826 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
3827 \g{-2} would be equivalent to \1. The use of relative references can be
3828 helpful in long patterns, and also in patterns that are created by
3829 joining together fragments that contain references within themselves.
3830
3831 A back reference matches whatever actually matched the capturing sub-
3832 pattern in the current subject string, rather than anything matching
3833 the subpattern itself (see "Subpatterns as subroutines" below for a way
3834 of doing that). So the pattern
3835
3836 (sens|respons)e and \1ibility
3837
3838 matches "sense and sensibility" and "response and responsibility", but
3839 not "sense and responsibility". If caseful matching is in force at the
3840 time of the back reference, the case of letters is relevant. For exam-
3841 ple,
3842
3843 ((?i)rah)\s+\1
3844
3845 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
3846 original capturing subpattern is matched caselessly.
3847
3848 Back references to named subpatterns use the Perl syntax \k<name> or
3849 \k'name' or the Python syntax (?P=name). We could rewrite the above
3850 example in either of the following ways:
3851
3852 (?<p1>(?i)rah)\s+\k<p1>
3853 (?P<p1>(?i)rah)\s+(?P=p1)
3854
3855 A subpattern that is referenced by name may appear in the pattern
3856 before or after the reference.
3857
3858 There may be more than one back reference to the same subpattern. If a
3859 subpattern has not actually been used in a particular match, any back
3860 references to it always fail. For example, the pattern
3861
3862 (a|(bc))\2
3863
3864 always fails if it starts to match "a" rather than "bc". Because there
3865 may be many capturing parentheses in a pattern, all digits following
3866 the backslash are taken as part of a potential back reference number.
3867 If the pattern continues with a digit character, some delimiter must be
3868 used to terminate the back reference. If the PCRE_EXTENDED option is
3869 set, this can be whitespace. Otherwise an empty comment (see "Com-
3870 ments" below) can be used.
3871
3872 A back reference that occurs inside the parentheses to which it refers
3873 fails when the subpattern is first used, so, for example, (a\1) never
3874 matches. However, such references can be useful inside repeated sub-
3875 patterns. For example, the pattern
3876
3877 (a|b\1)+
3878
3879 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
3880 ation of the subpattern, the back reference matches the character
3881 string corresponding to the previous iteration. In order for this to
3882 work, the pattern must be such that the first iteration does not need
3883 to match the back reference. This can be done using alternation, as in
3884 the example above, or by a quantifier with a minimum of zero.
3885
3886
3887 ASSERTIONS
3888
3889 An assertion is a test on the characters following or preceding the
3890 current matching point that does not actually consume any characters.
3891 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
3892 described above.
3893
3894 More complicated assertions are coded as subpatterns. There are two
3895 kinds: those that look ahead of the current position in the subject
3896 string, and those that look behind it. An assertion subpattern is
3897 matched in the normal way, except that it does not cause the current
3898 matching position to be changed.
3899
3900 Assertion subpatterns are not capturing subpatterns, and may not be
3901 repeated, because it makes no sense to assert the same thing several
3902 times. If any kind of assertion contains capturing subpatterns within
3903 it, these are counted for the purposes of numbering the capturing sub-
3904 patterns in the whole pattern. However, substring capturing is carried
3905 out only for positive assertions, because it does not make sense for
3906 negative assertions.
3907
3908 Lookahead assertions
3909
3910 Lookahead assertions start with (?= for positive assertions and (?! for
3911 negative assertions. For example,
3912
3913 \w+(?=;)
3914
3915 matches a word followed by a semicolon, but does not include the semi-
3916 colon in the match, and
3917
3918 foo(?!bar)
3919
3920 matches any occurrence of "foo" that is not followed by "bar". Note
3921 that the apparently similar pattern
3922
3923 (?!foo)bar
3924
3925 does not find an occurrence of "bar" that is preceded by something
3926 other than "foo"; it finds any occurrence of "bar" whatsoever, because
3927 the assertion (?!foo) is always true when the next three characters are
3928 "bar". A lookbehind assertion is needed to achieve the other effect.
3929
3930 If you want to force a matching failure at some point in a pattern, the
3931 most convenient way to do it is with (?!) because an empty string
3932 always matches, so an assertion that requires there not to be an empty
3933 string must always fail.
3934
3935 Lookbehind assertions
3936
3937 Lookbehind assertions start with (?<= for positive assertions and (?<!
3938 for negative assertions. For example,
3939
3940 (?<!foo)bar
3941
3942 does find an occurrence of "bar" that is not preceded by "foo". The
3943 contents of a lookbehind assertion are restricted such that all the
3944 strings it matches must have a fixed length. However, if there are sev-
3945 eral top-level alternatives, they do not all have to have the same
3946 fixed length. Thus
3947
3948 (?<=bullock|donkey)
3949
3950 is permitted, but
3951
3952 (?<!dogs?|cats?)
3953
3954 causes an error at compile time. Branches that match different length
3955 strings are permitted only at the top level of a lookbehind assertion.
3956 This is an extension compared with Perl (at least for 5.8), which
3957 requires all branches to match the same length of string. An assertion
3958 such as
3959
3960 (?<=ab(c|de))
3961
3962 is not permitted, because its single top-level branch can match two
3963 different lengths, but it is acceptable if rewritten to use two top-
3964 level branches:
3965
3966 (?<=abc|abde)
3967
3968 The implementation of lookbehind assertions is, for each alternative,
3969 to temporarily move the current position back by the fixed length and
3970 then try to match. If there are insufficient characters before the cur-
3971 rent position, the assertion fails.
3972
3973 PCRE does not allow the \C escape (which matches a single byte in UTF-8
3974 mode) to appear in lookbehind assertions, because it makes it impossi-
3975 ble to calculate the length of the lookbehind. The \X and \R escapes,
3976 which can match different numbers of bytes, are also not permitted.
3977
3978 Possessive quantifiers can be used in conjunction with lookbehind
3979 assertions to specify efficient matching at the end of the subject
3980 string. Consider a simple pattern such as
3981
3982 abcd$
3983
3984 when applied to a long string that does not match. Because matching
3985 proceeds from left to right, PCRE will look for each "a" in the subject
3986 and then see if what follows matches the rest of the pattern. If the
3987 pattern is specified as
3988
3989 ^.*abcd$
3990
3991 the initial .* matches the entire string at first, but when this fails
3992 (because there is no following "a"), it backtracks to match all but the
3993 last character, then all but the last two characters, and so on. Once
3994 again the search for "a" covers the entire string, from right to left,
3995 so we are no better off. However, if the pattern is written as
3996
3997 ^.*+(?<=abcd)
3998
3999 there can be no backtracking for the .*+ item; it can match only the
4000 entire string. The subsequent lookbehind assertion does a single test
4001 on the last four characters. If it fails, the match fails immediately.
4002 For long strings, this approach makes a significant difference to the
4003 processing time.
4004
4005 Using multiple assertions
4006
4007 Several assertions (of any sort) may occur in succession. For example,
4008
4009 (?<=\d{3})(?<!999)foo
4010
4011 matches "foo" preceded by three digits that are not "999". Notice that
4012 each of the assertions is applied independently at the same point in
4013 the subject string. First there is a check that the previous three
4014 characters are all digits, and then there is a check that the same
4015 three characters are not "999". This pattern does not match "foo" pre-
4016 ceded by six characters, the first of which are digits and the last
4017 three of which are not "999". For example, it doesn't match "123abc-
4018 foo". A pattern to do that is
4019
4020 (?<=\d{3}...)(?<!999)foo
4021
4022 This time the first assertion looks at the preceding six characters,
4023 checking that the first three are digits, and then the second assertion
4024 checks that the preceding three characters are not "999".
4025
4026 Assertions can be nested in any combination. For example,
4027
4028 (?<=(?<!foo)bar)baz
4029
4030 matches an occurrence of "baz" that is preceded by "bar" which in turn
4031 is not preceded by "foo", while
4032
4033 (?<=\d{3}(?!999)...)foo
4034
4035 is another pattern that matches "foo" preceded by three digits and any
4036 three characters that are not "999".
4037
4038
4039 CONDITIONAL SUBPATTERNS
4040
4041 It is possible to cause the matching process to obey a subpattern con-
4042 ditionally or to choose between two alternative subpatterns, depending
4043 on the result of an assertion, or whether a previous capturing subpat-
4044 tern matched or not. The two possible forms of conditional subpattern
4045 are
4046
4047 (?(condition)yes-pattern)
4048 (?(condition)yes-pattern|no-pattern)
4049
4050 If the condition is satisfied, the yes-pattern is used; otherwise the
4051 no-pattern (if present) is used. If there are more than two alterna-
4052 tives in the subpattern, a compile-time error occurs.
4053
4054 There are four kinds of condition: references to subpatterns, refer-
4055 ences to recursion, a pseudo-condition called DEFINE, and assertions.
4056
4057 Checking for a used subpattern by number
4058
4059 If the text between the parentheses consists of a sequence of digits,
4060 the condition is true if the capturing subpattern of that number has
4061 previously matched.
4062
4063 Consider the following pattern, which contains non-significant white
4064 space to make it more readable (assume the PCRE_EXTENDED option) and to
4065 divide it into three parts for ease of discussion:
4066
4067 ( \( )? [^()]+ (?(1) \) )
4068
4069 The first part matches an optional opening parenthesis, and if that
4070 character is present, sets it as the first captured substring. The sec-
4071 ond part matches one or more characters that are not parentheses. The
4072 third part is a conditional subpattern that tests whether the first set
4073 of parentheses matched or not. If they did, that is, if subject started
4074 with an opening parenthesis, the condition is true, and so the yes-pat-
4075 tern is executed and a closing parenthesis is required. Otherwise,
4076 since no-pattern is not present, the subpattern matches nothing. In
4077 other words, this pattern matches a sequence of non-parentheses,
4078 optionally enclosed in parentheses.
4079
4080 Checking for a used subpattern by name
4081
4082 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
4083 used subpattern by name. For compatibility with earlier versions of
4084 PCRE, which had this facility before Perl, the syntax (?(name)...) is
4085 also recognized. However, there is a possible ambiguity with this syn-
4086 tax, because subpattern names may consist entirely of digits. PCRE
4087 looks first for a named subpattern; if it cannot find one and the name
4088 consists entirely of digits, PCRE looks for a subpattern of that num-
4089 ber, which must be greater than zero. Using subpattern names that con-
4090 sist entirely of digits is not recommended.
4091
4092 Rewriting the above example to use a named subpattern gives this:
4093
4094 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
4095
4096
4097 Checking for pattern recursion
4098
4099 If the condition is the string (R), and there is no subpattern with the
4100 name R, the condition is true if a recursive call to the whole pattern
4101 or any subpattern has been made. If digits or a name preceded by amper-
4102 sand follow the letter R, for example:
4103
4104 (?(R3)...) or (?(R&name)...)
4105
4106 the condition is true if the most recent recursion is into the subpat-
4107 tern whose number or name is given. This condition does not check the
4108 entire recursion stack.
4109
4110 At "top level", all these recursion test conditions are false. Recur-
4111 sive patterns are described below.
4112
4113 Defining subpatterns for use by reference only
4114
4115 If the condition is the string (DEFINE), and there is no subpattern
4116 with the name DEFINE, the condition is always false. In this case,
4117 there may be only one alternative in the subpattern. It is always
4118 skipped if control reaches this point in the pattern; the idea of
4119 DEFINE is that it can be used to define "subroutines" that can be ref-
4120 erenced from elsewhere. (The use of "subroutines" is described below.)
4121 For example, a pattern to match an IPv4 address could be written like
4122 this (ignore whitespace and line breaks):
4123
4124 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4125 \b (?&byte) (\.(?&byte)){3} \b
4126
4127 The first part of the pattern is a DEFINE group inside which a another
4128 group named "byte" is defined. This matches an individual component of
4129 an IPv4 address (a number less than 256). When matching takes place,
4130 this part of the pattern is skipped because DEFINE acts like a false
4131 condition.
4132
4133 The rest of the pattern uses references to the named group to match the
4134 four dot-separated components of an IPv4 address, insisting on a word
4135 boundary at each end.
4136
4137 Assertion conditions
4138
4139 If the condition is not in any of the above formats, it must be an
4140 assertion. This may be a positive or negative lookahead or lookbehind
4141 assertion. Consider this pattern, again containing non-significant
4142 white space, and with the two alternatives on the second line:
4143
4144 (?(?=[^a-z]*[a-z])
4145 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
4146
4147 The condition is a positive lookahead assertion that matches an
4148 optional sequence of non-letters followed by a letter. In other words,
4149 it tests for the presence of at least one letter in the subject. If a
4150 letter is found, the subject is matched against the first alternative;
4151 otherwise it is matched against the second. This pattern matches
4152 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4153 letters and dd are digits.
4154
4155
4156 COMMENTS
4157
4158 The sequence (?# marks the start of a comment that continues up to the
4159 next closing parenthesis. Nested parentheses are not permitted. The
4160 characters that make up a comment play no part in the pattern matching
4161 at all.
4162
4163 If the PCRE_EXTENDED option is set, an unescaped # character outside a
4164 character class introduces a comment that continues to immediately
4165 after the next newline in the pattern.
4166
4167
4168 RECURSIVE PATTERNS
4169
4170 Consider the problem of matching a string in parentheses, allowing for
4171 unlimited nested parentheses. Without the use of recursion, the best
4172 that can be done is to use a pattern that matches up to some fixed
4173 depth of nesting. It is not possible to handle an arbitrary nesting
4174 depth.
4175
4176 For some time, Perl has provided a facility that allows regular expres-
4177 sions to recurse (amongst other things). It does this by interpolating
4178 Perl code in the expression at run time, and the code can refer to the
4179 expression itself. A Perl pattern using code interpolation to solve the
4180 parentheses problem can be created like this:
4181
4182 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4183
4184 The (?p{...}) item interpolates Perl code at run time, and in this case
4185 refers recursively to the pattern in which it appears.
4186
4187 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4188 it supports special syntax for recursion of the entire pattern, and
4189 also for individual subpattern recursion. After its introduction in
4190 PCRE and Python, this kind of recursion was introduced into Perl at
4191 release 5.10.
4192
4193 A special item that consists of (? followed by a number greater than
4194 zero and a closing parenthesis is a recursive call of the subpattern of
4195 the given number, provided that it occurs inside that subpattern. (If
4196 not, it is a "subroutine" call, which is described in the next sec-
4197 tion.) The special item (?R) or (?0) is a recursive call of the entire
4198 regular expression.
4199
4200 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
4201 always treated as an atomic group. That is, once it has matched some of
4202 the subject string, it is never re-entered, even if it contains untried
4203 alternatives and there is a subsequent matching failure.
4204
4205 This PCRE pattern solves the nested parentheses problem (assume the
4206 PCRE_EXTENDED option is set so that white space is ignored):
4207
4208 \( ( (?>[^()]+) | (?R) )* \)
4209
4210 First it matches an opening parenthesis. Then it matches any number of
4211 substrings which can either be a sequence of non-parentheses, or a
4212 recursive match of the pattern itself (that is, a correctly parenthe-
4213 sized substring). Finally there is a closing parenthesis.
4214
4215 If this were part of a larger pattern, you would not want to recurse
4216 the entire pattern, so instead you could use this:
4217
4218 ( \( ( (?>[^()]+) | (?1) )* \) )
4219
4220 We have put the pattern into parentheses, and caused the recursion to
4221 refer to them instead of the whole pattern. In a larger pattern, keep-
4222 ing track of parenthesis numbers can be tricky. It may be more conve-
4223 nient to use named parentheses instead. The Perl syntax for this is
4224 (?&name); PCRE's earlier syntax (?P>name) is also supported. We could
4225 rewrite the above example as follows:
4226
4227 (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4228
4229 If there is more than one subpattern with the same name, the earliest
4230 one is used. This particular example pattern contains nested unlimited
4231 repeats, and so the use of atomic grouping for matching strings of non-
4232 parentheses is important when applying the pattern to strings that do
4233 not match. For example, when this pattern is applied to
4234
4235 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4236
4237 it yields "no match" quickly. However, if atomic grouping is not used,
4238 the match runs for a very long time indeed because there are so many
4239 different ways the + and * repeats can carve up the subject, and all
4240 have to be tested before failure can be reported.
4241
4242 At the end of a match, the values set for any capturing subpatterns are
4243 those from the outermost level of the recursion at which the subpattern
4244 value is set. If you want to obtain intermediate values, a callout
4245 function can be used (see below and the pcrecallout documentation). If
4246 the pattern above is matched against
4247
4248 (ab(cd)ef)
4249
4250 the value for the capturing parentheses is "ef", which is the last
4251 value taken on at the top level. If additional parentheses are added,
4252 giving
4253
4254 \( ( ( (?>[^()]+) | (?R) )* ) \)
4255 ^ ^
4256 ^ ^
4257
4258 the string they capture is "ab(cd)ef", the contents of the top level
4259 parentheses. If there are more than 15 capturing parentheses in a pat-
4260 tern, PCRE has to obtain extra memory to store data during a recursion,
4261 which it does by using pcre_malloc, freeing it via pcre_free after-
4262 wards. If no memory can be obtained, the match fails with the
4263 PCRE_ERROR_NOMEMORY error.
4264
4265 Do not confuse the (?R) item with the condition (R), which tests for
4266 recursion. Consider this pattern, which matches text in angle brack-
4267 ets, allowing for arbitrary nesting. Only digits are allowed in nested
4268 brackets (that is, when recursing), whereas any characters are permit-
4269 ted at the outer level.
4270
4271 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
4272
4273 In this pattern, (?(R) is the start of a conditional subpattern, with
4274 two different alternatives for the recursive and non-recursive cases.
4275 The (?R) item is the actual recursive call.
4276
4277
4278 SUBPATTERNS AS SUBROUTINES
4279
4280 If the syntax for a recursive subpattern reference (either by number or
4281 by name) is used outside the parentheses to which it refers, it oper-
4282 ates like a subroutine in a programming language. The "called" subpat-
4283 tern may be defined before or after the reference. An earlier example
4284 pointed out that the pattern
4285
4286 (sens|respons)e and \1ibility
4287
4288 matches "sense and sensibility" and "response and responsibility", but
4289 not "sense and responsibility". If instead the pattern
4290
4291 (sens|respons)e and (?1)ibility
4292
4293 is used, it does match "sense and responsibility" as well as the other
4294 two strings. Another example is given in the discussion of DEFINE
4295 above.
4296
4297 Like recursive subpatterns, a "subroutine" call is always treated as an
4298 atomic group. That is, once it has matched some of the subject string,
4299 it is never re-entered, even if it contains untried alternatives and
4300 there is a subsequent matching failure.
4301
4302 When a subpattern is used as a subroutine, processing options such as
4303 case-independence are fixed when the subpattern is defined. They cannot
4304 be changed for different calls. For example, consider this pattern:
4305
4306 (abc)(?i:(?1))
4307
4308 It matches "abcabc". It does not match "abcABC" because the change of
4309 processing option does not affect the called subpattern.
4310
4311
4312 CALLOUTS
4313
4314 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4315 Perl code to be obeyed in the middle of matching a regular expression.
4316 This makes it possible, amongst other things, to extract different sub-
4317 strings that match the same pair of parentheses when there is a repeti-
4318 tion.
4319
4320 PCRE provides a similar feature, but of course it cannot obey arbitrary
4321 Perl code. The feature is called "callout". The caller of PCRE provides
4322 an external function by putting its entry point in the global variable
4323 pcre_callout. By default, this variable contains NULL, which disables
4324 all calling out.
4325
4326 Within a regular expression, (?C) indicates the points at which the
4327 external function is to be called. If you want to identify different
4328 callout points, you can put a number less than 256 after the letter C.
4329 The default value is zero. For example, this pattern has two callout
4330 points:
4331
4332 (?C1)abc(?C2)def
4333
4334 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4335 automatically installed before each item in the pattern. They are all
4336 numbered 255.
4337
4338 During matching, when PCRE reaches a callout point (and pcre_callout is
4339 set), the external function is called. It is provided with the number
4340 of the callout, the position in the pattern, and, optionally, one item
4341 of data originally supplied by the caller of pcre_exec(). The callout
4342 function may cause matching to proceed, to backtrack, or to fail alto-
4343 gether. A complete description of the interface to the callout function
4344 is given in the pcrecallout documentation.
4345
4346
4347 SEE ALSO
4348
4349 pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
4350
4351
4352 AUTHOR
4353
4354 Philip Hazel
4355 University Computing Service
4356 Cambridge CB2 3QH, England.
4357
4358
4359 REVISION
4360
4361 Last updated: 06 March 2007
4362 Copyright (c) 1997-2007 University of Cambridge.
4363 ------------------------------------------------------------------------------
4364
4365
4366 PCREPARTIAL(3) PCREPARTIAL(3)
4367
4368
4369 NAME
4370 PCRE - Perl-compatible regular expressions
4371
4372
4373 PARTIAL MATCHING IN PCRE
4374
4375 In normal use of PCRE, if the subject string that is passed to
4376 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
4377 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
4378 There are circumstances where it might be helpful to distinguish this
4379 case from other cases in which there is no match.
4380
4381 Consider, for example, an application where a human is required to type
4382 in data for a field with specific formatting requirements. An example
4383 might be a date in the form ddmmmyy, defined by this pattern:
4384
4385 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
4386
4387 If the application sees the user's keystrokes one by one, and can check
4388 that what has been typed so far is potentially valid, it is able to
4389 raise an error as soon as a mistake is made, possibly beeping and not
4390 reflecting the character that has been typed. This immediate feedback
4391 is likely to be a better user interface than a check that is delayed
4392 until the entire string has been entered.
4393
4394 PCRE supports the concept of partial matching by means of the PCRE_PAR-
4395 TIAL option, which can be set when calling pcre_exec() or
4396 pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
4397 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
4398 during the matching process the last part of the subject string matched
4399 part of the pattern. Unfortunately, for non-anchored matching, it is
4400 not possible to obtain the position of the start of the partial match.
4401 No captured data is set when PCRE_ERROR_PARTIAL is returned.
4402
4403 When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code
4404 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of
4405 the subject is reached, there have been no complete matches, but there
4406 is still at least one matching possibility. The portion of the string
4407 that provided the partial match is set as the first matching string.
4408
4409 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
4410 the last literal byte in a pattern, and abandons matching immediately
4411 if such a byte is not present in the subject string. This optimization
4412 cannot be used for a subject string that might match only partially.
4413
4414
4415 RESTRICTED PATTERNS FOR PCRE_PARTIAL
4416
4417 Because of the way certain internal optimizations are implemented in
4418 the pcre_exec() function, the PCRE_PARTIAL option cannot be used with
4419 all patterns. These restrictions do not apply when pcre_dfa_exec() is
4420 used. For pcre_exec(), repeated single characters such as
4421
4422 a{2,4}
4423
4424 and repeated single metasequences such as
4425
4426 \d+
4427
4428 are not permitted if the maximum number of occurrences is greater than
4429 one. Optional items such as \d? (where the maximum is one) are permit-
4430 ted. Quantifiers with any values are permitted after parentheses, so
4431 the invalid examples above can be coded thus:
4432
4433 (a){2,4}
4434 (\d)+
4435
4436 These constructions run more slowly, but for the kinds of application
4437 that are envisaged for this facility, this is not felt to be a major
4438 restriction.
4439
4440 If PCRE_PARTIAL is set for a pattern that does not conform to the
4441 restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
4442 (-13).
4443
4444
4445 EXAMPLE OF PARTIAL MATCHING USING PCRETEST
4446
4447 If the escape sequence \P is present in a pcretest data line, the
4448 PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
4449 uses the date example quoted above:
4450
4451 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
4452 data> 25jun04\P
4453 0: 25jun04
4454 1: jun
4455 data> 25dec3\P
4456 Partial match
4457 data> 3ju\P
4458 Partial match
4459 data> 3juj\P
4460 No match
4461 data> j\P
4462 No match
4463
4464 The first data string is matched completely, so pcretest shows the
4465 matched substrings. The remaining four strings do not match the com-
4466 plete pattern, but the first two are partial matches. The same test,
4467 using pcre_dfa_exec() matching (by means of the \D escape sequence),
4468 produces the following output:
4469
4470 re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/
4471 data> 25jun04\P\D
4472 0: 25jun04
4473 data> 23dec3\P\D
4474 Partial match: 23dec3
4475 data> 3ju\P\D
4476 Partial match: 3ju
4477 data> 3juj\P\D
4478 No match
4479 data> j\P\D
4480 No match
4481
4482 Notice that in this case the portion of the string that was matched is
4483 made available.
4484
4485
4486 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
4487
4488 When a partial match has been found using pcre_dfa_exec(), it is possi-
4489 ble to continue the match by providing additional subject data and
4490 calling pcre_dfa_exec() again with the same compiled regular expres-
4491 sion, this time setting the PCRE_DFA_RESTART option. You must also pass
4492 the same working space as before, because this is where details of the
4493 previous partial match are stored. Here is an example using pcretest,
4494 using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
4495 \D are as above):
4496
4497 re> /^?(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)$/
4498 data> 23ja\P\D
4499 Partial match: 23ja
4500 data> n05\R\D
4501 0: n05
4502
4503 The first call has "23ja" as the subject, and requests partial match-
4504 ing; the second call has "n05" as the subject for the continued
4505 (restarted) match. Notice that when the match is complete, only the
4506 last part is shown; PCRE does not retain the previously partially-
4507 matched string. It is up to the calling program to do that if it needs
4508 to.
4509
4510 You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial
4511 matching over multiple segments. This facility can be used to pass very
4512 long subject strings to pcre_dfa_exec(). However, some care is needed
4513 for certain types of pattern.
4514
4515 1. If the pattern contains tests for the beginning or end of a line,
4516 you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
4517 ate, when the subject string for any call does not contain the begin-
4518 ning or end of a line.
4519
4520 2. If the pattern contains backward assertions (including \b or \B),
4521 you need to arrange for some overlap in the subject strings to allow
4522 for this. For example, you could pass the subject in chunks that are
4523 500 bytes long, but in a buffer of 700 bytes, with the starting offset
4524 set to 200 and the previous 200 bytes at the start of the buffer.
4525
4526 3. Matching a subject string that is split into multiple segments does
4527 not always produce exactly the same result as matching over one single
4528 long string. The difference arises when there are multiple matching
4529 possibilities, because a partial match result is given only when there
4530 are no completed matches in a call to pcre_dfa_exec(). This means that
4531 as soon as the shortest match has been found, continuation to a new
4532 subject segment is no longer possible. Consider this pcretest example:
4533
4534 re> /dog(sbody)?/
4535 data> do\P\D
4536 Partial match: do
4537 data> gsb\R\P\D
4538 0: g
4539 data> dogsbody\D
4540 0: dogsbody
4541 1: dog
4542
4543 The pattern matches the words "dog" or "dogsbody". When the subject is
4544 presented in several parts ("do" and "gsb" being the first two) the
4545 match stops when "dog" has been found, and it is not possible to con-
4546 tinue. On the other hand, if "dogsbody" is presented as a single
4547 string, both matches are found.
4548
4549 Because of this phenomenon, it does not usually make sense to end a
4550 pattern that is going to be matched in this way with a variable repeat.
4551
4552 4. Patterns that contain alternatives at the top level which do not all
4553 start with the same pattern item may not work as expected. For example,
4554 consider this pattern:
4555
4556 1234|3789
4557
4558 If the first part of the subject is "ABC123", a partial match of the
4559 first alternative is found at offset 3. There is no partial match for
4560 the second alternative, because such a match does not start at the same
4561 point in the subject string. Attempting to continue with the string
4562 "789" does not yield a match because only those alternatives that match
4563 at one point in the subject are remembered. The problem arises because
4564 the start of the second alternative matches within the first alterna-
4565 tive. There is no problem with anchored patterns or patterns such as:
4566
4567 1234|ABCD
4568
4569 where no string can be a partial match for both alternatives.
4570
4571
4572 AUTHOR
4573
4574 Philip Hazel
4575 University Computing Service
4576 Cambridge CB2 3QH, England.
4577
4578
4579 REVISION
4580
4581 Last updated: 06 March 2007
4582 Copyright (c) 1997-2007 University of Cambridge.
4583 ------------------------------------------------------------------------------
4584
4585
4586 PCREPRECOMPILE(3) PCREPRECOMPILE(3)
4587
4588
4589 NAME
4590 PCRE - Perl-compatible regular expressions
4591
4592
4593 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
4594
4595 If you are running an application that uses a large number of regular
4596 expression patterns, it may be useful to store them in a precompiled
4597 form instead of having to compile them every time the application is
4598 run. If you are not using any private character tables (see the
4599 pcre_maketables() documentation), this is relatively straightforward.
4600 If you are using private tables, it is a little bit more complicated.
4601
4602 If you save compiled patterns to a file, you can copy them to a differ-
4603 ent host and run them there. This works even if the new host has the
4604 opposite endianness to the one on which the patterns were compiled.
4605 There may be a small performance penalty, but it should be insignifi-
4606 cant.
4607
4608
4609 SAVING A COMPILED PATTERN
4610 The value returned by pcre_compile() points to a single block of memory
4611 that holds the compiled pattern and associated data. You can find the
4612 length of this block in bytes by calling pcre_fullinfo() with an argu-
4613 ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
4614 manner. Here is sample code that compiles a pattern and writes it to a
4615 file. It assumes that the variable fd refers to a file that is open for
4616 output:
4617
4618 int erroroffset, rc, size;
4619 char *error;
4620 pcre *re;
4621
4622 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
4623 if (re == NULL) { ... handle errors ... }
4624 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
4625 if (rc < 0) { ... handle errors ... }
4626 rc = fwrite(re, 1, size, fd);
4627 if (rc != size) { ... handle errors ... }
4628
4629 In this example, the bytes that comprise the compiled pattern are
4630 copied exactly. Note that this is binary data that may contain any of
4631 the 256 possible byte values. On systems that make a distinction
4632 between binary and non-binary data, be sure that the file is opened for
4633 binary output.
4634
4635 If you want to write more than one pattern to a file, you will have to
4636 devise a way of separating them. For binary data, preceding each pat-
4637 tern with its length is probably the most straightforward approach.
4638 Another possibility is to write out the data in hexadecimal instead of
4639 binary, one pattern to a line.
4640
4641 Saving compiled patterns in a file is only one possible way of storing
4642 them for later use. They could equally well be saved in a database, or
4643 in the memory of some daemon process that passes them via sockets to
4644 the processes that want them.
4645
4646 If the pattern has been studied, it is also possible to save the study
4647 data in a similar way to the compiled pattern itself. When studying
4648 generates additional information, pcre_study() returns a pointer to a
4649 pcre_extra data block. Its format is defined in the section on matching
4650 a pattern in the pcreapi documentation. The study_data field points to
4651 the binary study data, and this is what you must save (not the
4652 pcre_extra block itself). The length of the study data can be obtained
4653 by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
4654 Remember to check that pcre_study() did return a non-NULL value before
4655 trying to save the study data.
4656
4657
4658 RE-USING A PRECOMPILED PATTERN
4659
4660 Re-using a precompiled pattern is straightforward. Having reloaded it
4661 into main memory, you pass its pointer to pcre_exec() or
4662 pcre_dfa_exec() in the usual way. This should work even on another
4663 host, and even if that host has the opposite endianness to the one
4664 where the pattern was compiled.
4665
4666 However, if you passed a pointer to custom character tables when the
4667 pattern was compiled (the tableptr argument of pcre_compile()), you
4668 must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
4669 because the value saved with the compiled pattern will obviously be
4670 nonsense. A field in a pcre_extra() block is used to pass this data, as
4671 described in the section on matching a pattern in the pcreapi documen-
4672 tation.
4673
4674 If you did not provide custom character tables when the pattern was
4675 compiled, the pointer in the compiled pattern is NULL, which causes
4676 pcre_exec() to use PCRE's internal tables. Thus, you do not need to
4677 take any special action at run time in this case.
4678
4679 If you saved study data with the compiled pattern, you need to create
4680 your own pcre_extra data block and set the study_data field to point to
4681 the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
4682 bit in the flags field to indicate that study data is present. Then
4683 pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
4684 usual way.
4685
4686
4687 COMPATIBILITY WITH DIFFERENT PCRE RELEASES
4688
4689 The layout of the control block that is at the start of the data that
4690 makes up a compiled pattern was changed for release 5.0. If you have
4691 any saved patterns that were compiled with previous releases (not a
4692 facility that was previously advertised), you will have to recompile
4693 them for release 5.0 and above.
4694
4695 If you have any saved patterns in UTF-8 mode that use \p or \P that
4696 were compiled with any release up to and including 6.4, you will have
4697 to recompile them for release 6.5 and above.
4698
4699 All saved patterns from earlier releases must be recompiled for release
4700 7.0 or higher, because there was an internal reorganization at that
4701 release.
4702
4703
4704 AUTHOR
4705
4706 Philip Hazel
4707 University Computing Service
4708 Cambridge CB2 3QH, England.
4709
4710
4711 REVISION
4712
4713 Last updated: 06 March 2007
4714 Copyright (c) 1997-2007 University of Cambridge.
4715 ------------------------------------------------------------------------------
4716
4717
4718 PCREPERFORM(3) PCREPERFORM(3)
4719
4720
4721 NAME
4722 PCRE - Perl-compatible regular expressions
4723
4724
4725 PCRE PERFORMANCE
4726
4727 Two aspects of performance are discussed below: memory usage and pro-
4728 cessing time. The way you express your pattern as a regular expression
4729 can affect both of them.
4730
4731
4732 MEMORY USAGE
4733
4734 Patterns are compiled by PCRE into a reasonably efficient byte code, so
4735 that most simple patterns do not use much memory. However, there is one
4736 case where memory usage can be unexpectedly large. When a parenthesized
4737 subpattern has a quantifier with a minimum greater than 1 and/or a lim-
4738 ited maximum, the whole subpattern is repeated in the compiled code.
4739 For example, the pattern
4740
4741 (abc|def){2,4}
4742
4743 is compiled as if it were
4744
4745 (abc|def)(abc|def)((abc|def)(abc|def)?)?
4746
4747 (Technical aside: It is done this way so that backtrack points within
4748 each of the repetitions can be independently maintained.)
4749
4750 For regular expressions whose quantifiers use only small numbers, this
4751 is not usually a problem. However, if the numbers are large, and par-
4752 ticularly if such repetitions are nested, the memory usage can become
4753 an embarrassment. For example, the very simple pattern
4754
4755 ((ab){1,1000}c){1,3}
4756
4757 uses 51K bytes when compiled. When PCRE is compiled with its default
4758 internal pointer size of two bytes, the size limit on a compiled pat-
4759 tern is 64K, and this is reached with the above pattern if the outer
4760 repetition is increased from 3 to 4. PCRE can be compiled to use larger
4761 internal pointers and thus handle larger compiled patterns, but it is
4762 better to try to rewrite your pattern to use less memory if you can.
4763
4764 One way of reducing the memory usage for such patterns is to make use
4765 of PCRE's "subroutine" facility. Re-writing the above pattern as
4766
4767 ((ab)(?2){0,999}c)(?1){0,2}
4768
4769 reduces the memory requirements to 18K, and indeed it remains under 20K
4770 even with the outer repetition increased to 100. However, this pattern
4771 is not exactly equivalent, because the "subroutine" calls are treated
4772 as atomic groups into which there can be no backtracking if there is a
4773 subsequent matching failure. Therefore, PCRE cannot do this kind of
4774 rewriting automatically. Furthermore, there is a noticeable loss of
4775 speed when executing the modified pattern. Nevertheless, if the atomic
4776 grouping is not a problem and the loss of speed is acceptable, this
4777 kind of rewriting will allow you to process patterns that PCRE cannot
4778 otherwise handle.
4779
4780
4781 PROCESSING TIME
4782
4783 Certain items in regular expression patterns are processed more effi-
4784 ciently than others. It is more efficient to use a character class like
4785 [aeiou] than a set of single-character alternatives such as
4786 (a|e|i|o|u). In general, the simplest construction that provides the
4787 required behaviour is usually the most efficient. Jeffrey Friedl's book
4788 contains a lot of useful general discussion about optimizing regular
4789 expressions for efficient performance. This document contains a few
4790 observations about PCRE.
4791
4792 Using Unicode character properties (the \p, \P, and \X escapes) is
4793 slow, because PCRE has to scan a structure that contains data for over
4794 fifteen thousand characters whenever it needs a character's property.
4795 If you can find an alternative pattern that does not use character
4796 properties, it will probably be faster.
4797
4798 When a pattern begins with .* not in parentheses, or in parentheses
4799 that are not the subject of a backreference, and the PCRE_DOTALL option
4800 is set, the pattern is implicitly anchored by PCRE, since it can match
4801 only at the start of a subject string. However, if PCRE_DOTALL is not
4802 set, PCRE cannot make this optimization, because the . metacharacter
4803 does not then match a newline, and if the subject string contains new-
4804 lines, the pattern may match from the character immediately following
4805 one of them instead of from the very start. For example, the pattern
4806
4807 .*second
4808
4809 matches the subject "first\nand second" (where \n stands for a newline
4810 character), with the match starting at the seventh character. In order
4811 to do this, PCRE has to retry the match starting after every newline in
4812 the subject.
4813
4814 If you are using such a pattern with subject strings that do not con-
4815 tain newlines, the best performance is obtained by setting PCRE_DOTALL,
4816 or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
4817 ing. That saves PCRE from having to scan along the subject looking for
4818 a newline to restart at.
4819
4820 Beware of patterns that contain nested indefinite repeats. These can
4821 take a long time to run when applied to a string that does not match.
4822 Consider the pattern fragment
4823
4824 ^(a+)*
4825
4826 This can match "aaaa" in 16 different ways, and this number increases
4827 very rapidly as the string gets longer. (The * repeat can match 0, 1,
4828 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
4829 repeats can match different numbers of times.) When the remainder of
4830 the pattern is such that the entire match is going to fail, PCRE has in
4831 principle to try every possible variation, and this can take an
4832 extremely long time, even for relatively short strings.
4833
4834 An optimization catches some of the more simple cases such as
4835
4836 (a+)*b
4837
4838 where a literal character follows. Before embarking on the standard
4839 matching procedure, PCRE checks that there is a "b" later in the sub-
4840 ject string, and if there is not, it fails the match immediately. How-
4841 ever, when there is no following literal this optimization cannot be
4842 used. You can see the difference by comparing the behaviour of
4843
4844 (a+)*\d
4845
4846 with the pattern above. The former gives a failure almost instantly
4847 when applied to a whole line of "a" characters, whereas the latter
4848 takes an appreciable time with strings longer than about 20 characters.
4849
4850 In many cases, the solution to this kind of performance issue is to use
4851 an atomic group or a possessive quantifier.
4852
4853
4854 AUTHOR
4855
4856 Philip Hazel
4857 University Computing Service
4858 Cambridge CB2 3QH, England.
4859
4860
4861 REVISION
4862
4863 Last updated: 06 March 2007
4864 Copyright (c) 1997-2007 University of Cambridge.
4865 ------------------------------------------------------------------------------
4866
4867
4868 PCREPOSIX(3) PCREPOSIX(3)
4869
4870
4871 NAME
4872 PCRE - Perl-compatible regular expressions.
4873
4874
4875 SYNOPSIS OF POSIX API
4876
4877 #include <pcreposix.h>
4878
4879 int regcomp(regex_t *preg, const char *pattern,
4880 int cflags);
4881
4882 int regexec(regex_t *preg, const char *string,
4883 size_t nmatch, regmatch_t pmatch[], int eflags);
4884
4885 size_t regerror(int errcode, const regex_t *preg,
4886 char *errbuf, size_t errbuf_size);
4887
4888 void regfree(regex_t *preg);
4889
4890
4891 DESCRIPTION
4892
4893 This set of functions provides a POSIX-style API to the PCRE regular
4894 expression package. See the pcreapi documentation for a description of
4895 PCRE's native API, which contains much additional functionality.
4896
4897 The functions described here are just wrapper functions that ultimately
4898 call the PCRE native API. Their prototypes are defined in the
4899 pcreposix.h header file, and on Unix systems the library itself is
4900 called pcreposix.a, so can be accessed by adding -lpcreposix to the
4901 command for linking an application that uses them. Because the POSIX
4902 functions call the native ones, it is also necessary to add -lpcre.
4903
4904 I have implemented only those option bits that can be reasonably mapped
4905 to PCRE native options. In addition, the option REG_EXTENDED is defined
4906 with the value zero. This has no effect, but since programs that are
4907 written to the POSIX interface often use it, this makes it easier to
4908 slot in PCRE as a replacement library. Other POSIX options are not even
4909 defined.
4910
4911 When PCRE is called via these functions, it is only the API that is
4912 POSIX-like in style. The syntax and semantics of the regular expres-
4913 sions themselves are still those of Perl, subject to the setting of
4914 various PCRE options, as described below. "POSIX-like in style" means
4915 that the API approximates to the POSIX definition; it is not fully
4916 POSIX-compatible, and in multi-byte encoding domains it is probably
4917 even less compatible.
4918
4919 The header for these functions is supplied as pcreposix.h to avoid any
4920 potential clash with other POSIX libraries. It can, of course, be
4921 renamed or aliased as regex.h, which is the "correct" name. It provides
4922 two structure types, regex_t for compiled internal forms, and reg-
4923 match_t for returning captured substrings. It also defines some con-
4924 stants whose names start with "REG_"; these are used for setting
4925 options and identifying error codes.
4926
4927
4928 COMPILING A PATTERN
4929
4930 The function regcomp() is called to compile a pattern into an internal
4931 form. The pattern is a C string terminated by a binary zero, and is
4932 passed in the argument pattern. The preg argument is a pointer to a
4933 regex_t structure that is used as a base for storing information about
4934 the compiled regular expression.
4935
4936 The argument cflags is either zero, or contains one or more of the bits
4937 defined by the following macros:
4938
4939 REG_DOTALL
4940
4941 The PCRE_DOTALL option is set when the regular expression is passed for
4942 compilation to the native function. Note that REG_DOTALL is not part of
4943 the POSIX standard.
4944
4945 REG_ICASE
4946
4947 The PCRE_CASELESS option is set when the regular expression is passed
4948 for compilation to the native function.
4949
4950 REG_NEWLINE
4951
4952 The PCRE_MULTILINE option is set when the regular expression is passed
4953 for compilation to the native function. Note that this does not mimic
4954 the defined POSIX behaviour for REG_NEWLINE (see the following sec-
4955 tion).
4956
4957 REG_NOSUB
4958
4959 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
4960 passed for compilation to the native function. In addition, when a pat-
4961 tern that is compiled with this flag is passed to regexec() for match-
4962 ing, the nmatch and pmatch arguments are ignored, and no captured
4963 strings are returned.
4964
4965 REG_UTF8
4966
4967 The PCRE_UTF8 option is set when the regular expression is passed for
4968 compilation to the native function. This causes the pattern itself and
4969 all data strings used for matching it to be treated as UTF-8 strings.
4970 Note that REG_UTF8 is not part of the POSIX standard.
4971
4972 In the absence of these flags, no options are passed to the native
4973 function. This means the the regex is compiled with PCRE default
4974 semantics. In particular, the way it handles newline characters in the
4975 subject string is the Perl way, not the POSIX way. Note that setting
4976 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
4977 It does not affect the way newlines are matched by . (they aren't) or
4978 by a negative class such as [^a] (they are).
4979
4980 The yield of regcomp() is zero on success, and non-zero otherwise. The
4981 preg structure is filled in on success, and one member of the structure
4982 is public: re_nsub contains the number of capturing subpatterns in the
4983 regular expression. Various error codes are defined in the header file.
4984
4985
4986 MATCHING NEWLINE CHARACTERS
4987
4988 This area is not simple, because POSIX and Perl take different views of
4989 things. It is not possible to get PCRE to obey POSIX semantics, but
4990 then PCRE was never intended to be a POSIX engine. The following table
4991 lists the different possibilities for matching newline characters in
4992 PCRE:
4993
4994 Default Change with
4995
4996 . matches newline no PCRE_DOTALL
4997 newline matches [^a] yes not changeable
4998 $ matches \n at end yes PCRE_DOLLARENDONLY
4999 $ matches \n in middle no PCRE_MULTILINE
5000 ^ matches \n in middle no PCRE_MULTILINE
5001
5002 This is the equivalent table for POSIX:
5003
5004 Default Change with
5005
5006 . matches newline yes REG_NEWLINE
5007 newline matches [^a] yes REG_NEWLINE
5008 $ matches \n at end no REG_NEWLINE
5009 $ matches \n in middle no REG_NEWLINE
5010 ^ matches \n in middle no REG_NEWLINE
5011
5012 PCRE's behaviour is the same as Perl's, except that there is no equiva-
5013 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
5014 no way to stop newline from matching [^a].
5015
5016 The default POSIX newline handling can be obtained by setting
5017 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
5018 behave exactly as for the REG_NEWLINE action.
5019
5020
5021 MATCHING A PATTERN
5022
5023 The function regexec() is called to match a compiled pattern preg
5024 against a given string, which is terminated by a zero byte, subject to
5025 the options in eflags. These can be:
5026
5027 REG_NOTBOL
5028
5029 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
5030 function.
5031
5032 REG_NOTEOL
5033
5034 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
5035 function.
5036
5037 If the pattern was compiled with the REG_NOSUB flag, no data about any
5038 matched strings is returned. The nmatch and pmatch arguments of
5039 regexec() are ignored.
5040
5041 Otherwise,the portion of the string that was matched, and also any cap-
5042 tured substrings, are returned via the pmatch argument, which points to
5043 an array of nmatch structures of type regmatch_t, containing the mem-
5044 bers rm_so and rm_eo. These contain the offset to the first character
5045 of each substring and the offset to the first character after the end
5046 of each substring, respectively. The 0th element of the vector relates
5047 to the entire portion of string that was matched; subsequent elements
5048 relate to the capturing subpatterns of the regular expression. Unused
5049 entries in the array have both structure members set to -1.
5050
5051 A successful match yields a zero return; various error codes are
5052 defined in the header file, of which REG_NOMATCH is the "expected"
5053 failure code.
5054
5055
5056 ERROR MESSAGES
5057
5058 The regerror() function maps a non-zero errorcode from either regcomp()
5059 or regexec() to a printable message. If preg is not NULL, the error
5060 should have arisen from the use of that structure. A message terminated
5061 by a binary zero is placed in errbuf. The length of the message,
5062 including the zero, is limited to errbuf_size. The yield of the func-
5063 tion is the size of buffer needed to hold the whole message.
5064
5065
5066 MEMORY USAGE
5067
5068 Compiling a regular expression causes memory to be allocated and asso-
5069 ciated with the preg structure. The function regfree() frees all such
5070 memory, after which preg may no longer be used as a compiled expres-
5071 sion.
5072
5073
5074 AUTHOR
5075
5076 Philip Hazel
5077 University Computing Service
5078 Cambridge CB2 3QH, England.
5079
5080
5081 REVISION
5082
5083 Last updated: 06 March 2007
5084 Copyright (c) 1997-2007 University of Cambridge.
5085 ------------------------------------------------------------------------------
5086
5087
5088 PCRECPP(3) PCRECPP(3)
5089
5090
5091 NAME
5092 PCRE - Perl-compatible regular expressions.
5093
5094
5095 SYNOPSIS OF C++ WRAPPER
5096
5097 #include <pcrecpp.h>
5098
5099
5100 DESCRIPTION
5101
5102 The C++ wrapper for PCRE was provided by Google Inc. Some additional
5103 functionality was added by Giuseppe Maxia. This brief man page was con-
5104 structed from the notes in the pcrecpp.h file, which should be con-
5105 sulted for further details.
5106
5107
5108 MATCHING INTERFACE
5109
5110 The "FullMatch" operation checks that supplied text matches a supplied
5111 pattern exactly. If pointer arguments are supplied, it copies matched
5112 sub-strings that match sub-patterns into them.
5113
5114 Example: successful match
5115 pcrecpp::RE re("h.*o");
5116 re.FullMatch("hello");
5117
5118 Example: unsuccessful match (requires full match):
5119 pcrecpp::RE re("e");
5120 !re.FullMatch("hello");
5121
5122 Example: creating a temporary RE object:
5123 pcrecpp::RE("h.*o").FullMatch("hello");
5124
5125 You can pass in a "const char*" or a "string" for "text". The examples
5126 below tend to use a const char*. You can, as in the different examples
5127 above, store the RE object explicitly in a variable or use a temporary
5128 RE object. The examples below use one mode or the other arbitrarily.
5129 Either could correctly be used for any of these examples.
5130
5131 You must supply extra pointer arguments to extract matched subpieces.
5132
5133 Example: extracts "ruby" into "s" and 1234 into "i"
5134 int i;
5135 string s;
5136 pcrecpp::RE re("(\\w+):(\\d+)");
5137 re.FullMatch("ruby:1234", &s, &i);
5138
5139 Example: does not try to extract any extra sub-patterns
5140 re.FullMatch("ruby:1234", &s);
5141
5142 Example: does not try to extract into NULL
5143 re.FullMatch("ruby:1234", NULL, &i);
5144
5145 Example: integer overflow causes failure
5146 !re.FullMatch("ruby:1234567891234", NULL, &i);
5147
5148 Example: fails because there aren't enough sub-patterns:
5149 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
5150
5151 Example: fails because string cannot be stored in integer
5152 !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
5153
5154 The provided pointer arguments can be pointers to any scalar numeric
5155 type, or one of:
5156
5157 string (matched piece is copied to string)
5158 StringPiece (StringPiece is mutated to point to matched piece)
5159 T (where "bool T::ParseFrom(const char*, int)" exists)
5160 NULL (the corresponding matched sub-pattern is not copied)
5161
5162 The function returns true iff all of the following conditions are sat-
5163 isfied:
5164
5165 a. "text" matches "pattern" exactly;
5166
5167 b. The number of matched sub-patterns is >= number of supplied
5168 pointers;
5169
5170 c. The "i"th argument has a suitable type for holding the
5171 string captured as the "i"th sub-pattern. If you pass in
5172 NULL for the "i"th argument, or pass fewer arguments than
5173 number of sub-patterns, "i"th captured sub-pattern is
5174 ignored.
5175
5176 CAVEAT: An optional sub-pattern that does not exist in the matched
5177 string is assigned the empty string. Therefore, the following will
5178 return false (because the empty string is not a valid number):
5179
5180 int number;
5181 pcrecpp::RE::FullMatch("abc", "[a-z]+(\d+)?", &number);
5182
5183 The matching interface supports at most 16 arguments per call. If you
5184 need more, consider using the more general interface
5185 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
5186
5187
5188 QUOTING METACHARACTERS
5189
5190 You can use the "QuoteMeta" operation to insert backslashes before all
5191 potentially meaningful characters in a string. The returned string,
5192 used as a regular expression, will exactly match the original string.
5193
5194 Example:
5195 string quoted = RE::QuoteMeta(unquoted);
5196
5197 Note that it's legal to escape a character even if it has no special
5198 meaning in a regular expression -- so this function does that. (This
5199 also makes it identical to the perl function of the same name; see
5200 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
5201 "1\.5\-2\.0\?".
5202
5203
5204 PARTIAL MATCHES
5205
5206 You can use the "PartialMatch" operation when you want the pattern to
5207 match any substring of the text.
5208
5209 Example: simple search for a string:
5210 pcrecpp::RE("ell").PartialMatch("hello");
5211
5212 Example: find first number in a string:
5213 int number;
5214 pcrecpp::RE re("(\\d+)");
5215 re.PartialMatch("x*100 + 20", &number);
5216 assert(number == 100);
5217
5218
5219 UTF-8 AND THE MATCHING INTERFACE
5220
5221 By default, pattern and text are plain text, one byte per character.
5222 The UTF8 flag, passed to the constructor, causes both pattern and
5223 string to be treated as UTF-8 text, still a byte stream but potentially
5224 multiple bytes per character. In practice, the text is likelier to be
5225 UTF-8 than the pattern, but the match returned may depend on the UTF8
5226 flag, so always use it when matching UTF8 text. For example, "." will
5227 match one byte normally but with UTF8 set may match up to three bytes
5228 of a multi-byte character.
5229
5230 Example:
5231 pcrecpp::RE_Options options;
5232 options.set_utf8();
5233 pcrecpp::RE re(utf8_pattern, options);
5234 re.FullMatch(utf8_string);
5235
5236 Example: using the convenience function UTF8():
5237 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
5238 re.FullMatch(utf8_string);
5239
5240 NOTE: The UTF8 flag is ignored if pcre was not configured with the
5241 --enable-utf8 flag.
5242
5243
5244 PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
5245
5246 PCRE defines some modifiers to change the behavior of the regular
5247 expression engine. The C++ wrapper defines an auxiliary class,
5248 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
5249 rently, the following modifiers are supported:
5250
5251 modifier description Perl corresponding
5252
5253 PCRE_CASELESS case insensitive match /i
5254 PCRE_MULTILINE multiple lines match /m
5255 PCRE_DOTALL dot matches newlines /s
5256 PCRE_DOLLAR_ENDONLY $ matches only at end N/A
5257 PCRE_EXTRA strict escape parsing N/A
5258 PCRE_EXTENDED ignore whitespaces /x
5259 PCRE_UTF8 handles UTF8 chars built-in
5260 PCRE_UNGREEDY reverses * and *? N/A
5261 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
5262
5263 (*) Both Perl and PCRE allow non capturing parentheses by means of the
5264 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
5265 ture, while (ab|cd) does.
5266
5267 For a full account on how each modifier works, please check the PCRE
5268 API reference page.
5269
5270 For each modifier, there are two member functions whose name is made
5271 out of the modifier in lowercase, without the "PCRE_" prefix. For
5272 instance, PCRE_CASELESS is handled by
5273
5274 bool caseless()
5275
5276 which returns true if the modifier is set, and
5277
5278 RE_Options & set_caseless(bool)
5279
5280 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
5281 be accessed through the set_match_limit() and match_limit() member
5282 functions. Setting match_limit to a non-zero value will limit the exe-
5283 cution of pcre to keep it from doing bad things like blowing the stack
5284 or taking an eternity to return a result. A value of 5000 is good
5285 enough to stop stack blowup in a 2MB thread stack. Setting match_limit
5286 to zero disables match limiting. Alternatively, you can call
5287 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
5288 limit how much PCRE recurses. match_limit() limits the number of
5289 matches PCRE does; match_limit_recursion() limits the depth of internal
5290 recursion, and therefore the amount of stack that is used.
5291
5292 Normally, to pass one or more modifiers to a RE class, you declare a
5293 RE_Options object, set the appropriate options, and pass this object to
5294 a RE constructor. Example:
5295
5296 RE_options opt;
5297 opt.set_caseless(true);
5298 if (RE("HELLO", opt).PartialMatch("hello world")) ...
5299
5300 RE_options has two constructors. The default constructor takes no argu-
5301 ments and creates a set of flags that are off by default. The optional
5302 parameter option_flags is to facilitate transfer of legacy code from C
5303 programs. This lets you do
5304
5305 RE(pattern,
5306 RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);
5307
5308 However, new code is better off doing
5309
5310 RE(pattern,
5311 RE_Options().set_caseless(true).set_multiline(true))
5312 .PartialMatch(str);
5313
5314 If you are going to pass one of the most used modifiers, there are some
5315 convenience functions that return a RE_Options class with the appropri-
5316 ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
5317 and EXTENDED().
5318
5319 If you need to set several options at once, and you don't want to go
5320 through the pains of declaring a RE_Options object and setting several
5321 options, there is a parallel method that give you such ability on the
5322 fly. You can concatenate several set_xxxxx() member functions, since
5323 each of them returns a reference to its class object. For example, to
5324 pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
5325 statement, you may write:
5326
5327 RE(" ^ xyz \\s+ .* blah$",
5328 RE_Options()
5329 .set_caseless(true)
5330 .set_extended(true)
5331 .set_multiline(true)).PartialMatch(sometext);
5332
5333
5334 SCANNING TEXT INCREMENTALLY
5335
5336 The "Consume" operation may be useful if you want to repeatedly match
5337 regular expressions at the front of a string and skip over them as they
5338 match. This requires use of the "StringPiece" type, which represents a
5339 sub-range of a real string. Like RE, StringPiece is defined in the
5340 pcrecpp namespace.
5341
5342 Example: read lines of the form "var = value" from a string.
5343 string contents = ...; // Fill string somehow
5344 pcrecpp::StringPiece input(contents); // Wrap in a StringPiece
5345
5346 string var;
5347 int value;
5348 pcrecpp::RE re("(\\w+) = (\\d+)\n");
5349 while (re.Consume(&input, &var, &value)) {
5350 ...;
5351 }
5352
5353 Each successful call to "Consume" will set "var/value", and also
5354 advance "input" so it points past the matched text.
5355
5356 The "FindAndConsume" operation is similar to "Consume" but does not
5357 anchor your match at the beginning of the string. For example, you
5358 could extract all words from a string by repeatedly calling
5359
5360 pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)
5361
5362
5363 PARSING HEX/OCTAL/C-RADIX NUMBERS
5364
5365 By default, if you pass a pointer to a numeric value, the corresponding
5366 text is interpreted as a base-10 number. You can instead wrap the
5367 pointer with a call to one of the operators Hex(), Octal(), or CRadix()
5368 to interpret the text in another base. The CRadix operator interprets
5369 C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to
5370 base-10.
5371
5372 Example:
5373 int a, b, c, d;
5374 pcrecpp::RE re("(.*) (.*) (.*) (.*)");
5375 re.FullMatch("100 40 0100 0x40",
5376 pcrecpp::Octal(&a), pcrecpp::Hex(&b),
5377 pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));
5378
5379 will leave 64 in a, b, c, and d.
5380
5381
5382 REPLACING PARTS OF STRINGS
5383
5384 You can replace the first match of "pattern" in "str" with "rewrite".
5385 Within "rewrite", backslash-escaped digits (\1 to \9) can be used to
5386 insert text matching corresponding parenthesized group from the pat-
5387 tern. \0 in "rewrite" refers to the entire matching text. For example:
5388
5389 string s = "yabba dabba doo";
5390 pcrecpp::RE("b+").Replace("d", &s);
5391
5392 will leave "s" containing "yada dabba doo". The result is true if the
5393 pattern matches and a replacement occurs, false otherwise.
5394
5395 GlobalReplace is like Replace except that it replaces all occurrences
5396 of the pattern in the string with the rewrite. Replacements are not
5397 subject to re-matching. For example:
5398
5399 string s = "yabba dabba doo";
5400 pcrecpp::RE("b+").GlobalReplace("d", &s);
5401
5402 will leave "s" containing "yada dada doo". It returns the number of
5403 replacements made.
5404
5405 Extract is like Replace, except that if the pattern matches, "rewrite"
5406 is copied into "out" (an additional argument) with substitutions. The
5407 non-matching portions of "text" are ignored. Returns true iff a match
5408 occurred and the extraction happened successfully; if no match occurs,
5409 the string is left unaffected.
5410
5411
5412 AUTHOR
5413
5414 The C++ wrapper was contributed by Google Inc.
5415 Copyright (c) 2007 Google Inc.
5416
5417
5418 REVISION
5419
5420 Last updated: 06 March 2007
5421 ------------------------------------------------------------------------------
5422
5423
5424 PCRESAMPLE(3) PCRESAMPLE(3)
5425
5426
5427 NAME
5428 PCRE - Perl-compatible regular expressions
5429
5430
5431 PCRE SAMPLE PROGRAM
5432
5433 A simple, complete demonstration program, to get you started with using
5434 PCRE, is supplied in the file pcredemo.c in the PCRE distribution.
5435
5436 The program compiles the regular expression that is its first argument,
5437 and matches it against the subject string in its second argument. No
5438 PCRE options are set, and default character tables are used. If match-
5439 ing succeeds, the program outputs the portion of the subject that
5440 matched, together with the contents of any captured substrings.
5441
5442 If the -g option is given on the command line, the program then goes on
5443 to check for further matches of the same regular expression in the same
5444 subject string. The logic is a little bit tricky because of the possi-
5445 bility of matching an empty string. Comments in the code explain what
5446 is going on.
5447
5448 If PCRE is installed in the standard include and library directories
5449 for your system, you should be able to compile the demonstration pro-
5450 gram using this command:
5451
5452 gcc -o pcredemo pcredemo.c -lpcre
5453
5454 If PCRE is installed elsewhere, you may need to add additional options
5455 to the command line. For example, on a Unix-like system that has PCRE
5456 installed in /usr/local, you can compile the demonstration program
5457 using a command like this:
5458
5459 gcc -o pcredemo -I/usr/local/include pcredemo.c \
5460 -L/usr/local/lib -lpcre
5461
5462 Once you have compiled the demonstration program, you can run simple
5463 tests like this:
5464
5465 ./pcredemo 'cat|dog' 'the cat sat on the mat'
5466 ./pcredemo -g 'cat|dog' 'the dog sat on the cat'
5467
5468 Note that there is a much more comprehensive test program, called
5469 pcretest, which supports many more facilities for testing regular
5470 expressions and the PCRE library. The pcredemo program is provided as a
5471 simple coding example.
5472
5473 On some operating systems (e.g. Solaris), when PCRE is not installed in
5474 the standard library directory, you may get an error like this when you
5475 try to run pcredemo:
5476
5477 ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or
5478 directory
5479
5480 This is caused by the way shared library support works on those sys-
5481 tems. You need to add
5482
5483 -R/usr/local/lib
5484
5485 (for example) to the compile command to get round this problem.
5486
5487
5488 AUTHOR
5489
5490 Philip Hazel
5491 University Computing Service
5492 Cambridge CB2 3QH, England.
5493
5494
5495 REVISION
5496
5497 Last updated: 06 March 2007
5498 Copyright (c) 1997-2007 University of Cambridge.
5499 ------------------------------------------------------------------------------
5500 PCRESTACK(3) PCRESTACK(3)
5501
5502
5503 NAME
5504 PCRE - Perl-compatible regular expressions
5505
5506
5507 PCRE DISCUSSION OF STACK USAGE
5508
5509 When you call pcre_exec(), it makes use of an internal function called
5510 match(). This calls itself recursively at branch points in the pattern,
5511 in order to remember the state of the match so that it can back up and
5512 try a different alternative if the first one fails. As matching pro-
5513 ceeds deeper and deeper into the tree of possibilities, the recursion
5514 depth increases.
5515
5516 Not all calls of match() increase the recursion depth; for an item such
5517 as a* it may be called several times at the same level, after matching
5518 different numbers of a's. Furthermore, in a number of cases where the
5519 result of the recursive call would immediately be passed back as the
5520 result of the current call (a "tail recursion"), the function is just
5521 restarted instead.
5522
5523 The pcre_dfa_exec() function operates in an entirely different way, and
5524 hardly uses recursion at all. The limit on its complexity is the amount
5525 of workspace it is given. The comments that follow do NOT apply to
5526 pcre_dfa_exec(); they are relevant only for pcre_exec().
5527
5528 You can set limits on the number of times that match() is called, both
5529 in total and recursively. If the limit is exceeded, an error occurs.
5530 For details, see the section on extra data for pcre_exec() in the
5531 pcreapi documentation.
5532
5533 Each time that match() is actually called recursively, it uses memory
5534 from the process stack. For certain kinds of pattern and data, very
5535 large amounts of stack may be needed, despite the recognition of "tail
5536 recursion". You can often reduce the amount of recursion, and there-
5537 fore the amount of stack used, by modifying the pattern that is being
5538 matched. Consider, for example, this pattern:
5539
5540 ([^<]|<(?!inet))+
5541
5542 It matches from wherever it starts until it encounters "<inet" or the
5543 end of the data, and is the kind of pattern that might be used when
5544 processing an XML file. Each iteration of the outer parentheses matches
5545 either one character that is not "<" or a "<" that is not followed by
5546 "inet". However, each time a parenthesis is processed, a recursion
5547 occurs, so this formulation uses a stack frame for each matched charac-
5548 ter. For a long string, a lot of stack is required. Consider now this
5549 rewritten pattern, which matches exactly the same strings:
5550
5551 ([^<]++|<(?!inet))+
5552
5553 This uses very much less stack, because runs of characters that do not
5554 contain "<" are "swallowed" in one item inside the parentheses. Recur-
5555 sion happens only when a "<" character that is not followed by "inet"
5556 is encountered (and we assume this is relatively rare). A possessive
5557 quantifier is used to stop any backtracking into the runs of non-"<"
5558 characters, but that is not related to stack usage.
5559
5560 This example shows that one way of avoiding stack problems when match-
5561 ing long subject strings is to write repeated parenthesized subpatterns
5562 to match more than one character whenever possible.
5563
5564 In environments where stack memory is constrained, you might want to
5565 compile PCRE to use heap memory instead of stack for remembering back-
5566 up points. This makes it run a lot more slowly, however. Details of how
5567 to do this are given in the pcrebuild documentation.
5568
5569 In Unix-like environments, there is not often a problem with the stack
5570 unless very long strings are involved, though the default limit on
5571 stack size varies from system to system. Values from 8Mb to 64Mb are
5572 common. You can find your default limit by running the command:
5573
5574 ulimit -s
5575
5576 Unfortunately, the effect of running out of stack is often SIGSEGV,
5577 though sometimes a more explicit error message is given. You can nor-
5578 mally increase the limit on stack size by code such as this:
5579
5580 struct rlimit rlim;
5581 getrlimit(RLIMIT_STACK, &rlim);
5582 rlim.rlim_cur = 100*1024*1024;
5583 setrlimit(RLIMIT_STACK, &rlim);
5584
5585 This reads the current limits (soft and hard) using getrlimit(), then
5586 attempts to increase the soft limit to 100Mb using setrlimit(). You
5587 must do this before calling pcre_exec().
5588
5589 PCRE has an internal counter that can be used to limit the depth of
5590 recursion, and thus cause pcre_exec() to give an error code before it
5591 runs out of stack. By default, the limit is very large, and unlikely
5592 ever to operate. It can be changed when PCRE is built, and it can also
5593 be set when pcre_exec() is called. For details of these interfaces, see
5594 the pcrebuild and pcreapi documentation.
5595
5596 As a very rough rule of thumb, you should reckon on about 500 bytes per
5597 recursion. Thus, if you want to limit your stack usage to 8Mb, you
5598 should set the limit at 16000 recursions. A 64Mb stack, on the other
5599 hand, can support around 128000 recursions. The pcretest test program
5600 has a command line option (-S) that can be used to increase the size of
5601 its stack.
5602
5603
5604 AUTHOR
5605
5606 Philip Hazel
5607 University Computing Service
5608 Cambridge CB2 3QH, England.
5609
5610
5611 REVISION
5612
5613 Last updated: 12 March 2007
5614 Copyright (c) 1997-2007 University of Cambridge.
5615 ------------------------------------------------------------------------------
5616
5617

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