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1. Move the comment about version numbers from pcre.h.in to configure.ac 
because that's where they are now set.
2. Update all the man pages to remove the use of .br and .in because this
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at .br/.in.

1 nigel 75 -----------------------------------------------------------------------------
2 nigel 63 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 nigel 41
10 nigel 79 PCRE(3) PCRE(3)
11 nigel 41
12 nigel 79
13 nigel 73 NAME
14     PCRE - Perl-compatible regular expressions
15    
16 nigel 77
17 nigel 75 INTRODUCTION
18 nigel 41
19 nigel 73 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 nigel 93 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 nigel 63
25 nigel 93 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 nigel 77
31 nigel 93 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 nigel 77 included as part of the PCRE distribution. The pcrecpp page has details
41 nigel 93 of this interface. Other people's contributions can be found in the
42 nigel 77 Contrib directory at the primary FTP site, which is:
43 nigel 63
44 nigel 73 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
45 nigel 63
46 nigel 93 Details of exactly which Perl regular expression features are and are
47 nigel 73 not supported by PCRE are given in separate documents. See the pcrepat-
48     tern and pcrecompat pages.
49 nigel 63
50 nigel 93 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 nigel 75 in the source distribution.
56 nigel 63
57 nigel 93 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 nigel 83 any name clashes. In some environments, it is possible to control which
62 nigel 93 external symbols are exported when a shared library is built, and in
63 nigel 83 these cases the undocumented symbols are not exported.
64 nigel 63
65 nigel 77
66 nigel 63 USER DOCUMENTATION
67    
68 nigel 93 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 nigel 75 of searching. The sections are as follows:
73 nigel 63
74 nigel 73 pcre this document
75 nigel 77 pcreapi details of PCRE's native C API
76 nigel 73 pcrebuild options for building PCRE
77     pcrecallout details of the callout feature
78     pcrecompat discussion of Perl compatibility
79 nigel 77 pcrecpp details of the C++ wrapper
80 nigel 73 pcregrep description of the pcregrep command
81 nigel 77 pcrematching discussion of the two matching algorithms
82 nigel 75 pcrepartial details of the partial matching facility
83 nigel 73 pcrepattern syntax and semantics of supported
84     regular expressions
85     pcreperform discussion of performance issues
86 nigel 77 pcreposix the POSIX-compatible C API
87 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
88 nigel 73 pcresample discussion of the sample program
89 nigel 91 pcrestack discussion of stack usage
90 nigel 75 pcretest description of the pcretest testing command
91 nigel 63
92 nigel 93 In addition, in the "man" and HTML formats, there is a short page for
93 nigel 77 each C library function, listing its arguments and results.
94 nigel 63
95    
96     LIMITATIONS
97    
98 nigel 93 There are some size limitations in PCRE but it is hoped that they will
99 nigel 73 never in practice be relevant.
100 nigel 63
101 nigel 93 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
102 nigel 73 is compiled with the default internal linkage size of 2. If you want to
103 nigel 93 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 nigel 63
109 nigel 93 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 nigel 91 30000 bytes. The maximum number of capturing subpatterns is 65535.
112 nigel 63
113 nigel 93 There is no limit to the number of parenthesized subpatterns, but there
114     can be no more than 65535 capturing subpatterns.
115 nigel 63
116 nigel 93 The maximum length of name for a named subpattern is 32 characters, and
117     the maximum number of named subpatterns is 10000.
118 nigel 91
119 nigel 93 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 nigel 77 matching function, PCRE uses recursion to handle subpatterns and indef-
122 nigel 93 inite repetition. This means that the available stack space may limit
123 nigel 77 the size of a subject string that can be processed by certain patterns.
124 nigel 91 For a discussion of stack issues, see the pcrestack documentation.
125 nigel 63
126    
127 nigel 75 UTF-8 AND UNICODE PROPERTY SUPPORT
128 nigel 63
129 nigel 93 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 nigel 75 port for Unicode general category properties was added.
133 nigel 63
134 nigel 93 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 nigel 73 strings instead of just strings of bytes.
139 nigel 63
140 nigel 93 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 nigel 63
145 nigel 75 If PCRE is built with Unicode character property support (which implies
146 nigel 93 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
147 nigel 75 ported. The available properties that can be tested are limited to the
148 nigel 93 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 nigel 87 pcrepattern documentation. Only the short names for properties are sup-
152 nigel 93 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 nigel 87 does not support this.
156 nigel 75
157 nigel 73 The following comments apply when PCRE is running in UTF-8 mode:
158 nigel 63
159 nigel 93 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 nigel 73 If an invalid UTF-8 string is passed, an error return is given. In some
162 nigel 93 situations, you may already know that your strings are valid, and
163 nigel 73 therefore want to skip these checks in order to improve performance. If
164 nigel 93 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 nigel 73 crash.
170 nigel 63
171 nigel 93 2. An unbraced hexadecimal escape sequence (such as \xb3) matches a
172 nigel 87 two-byte UTF-8 character if the value is greater than 127.
173 nigel 63
174 nigel 93 3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
175 nigel 91 characters for values greater than \177.
176    
177 nigel 93 4. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
178 nigel 73 vidual bytes, for example: \x{100}{3}.
179 nigel 63
180 nigel 93 5. The dot metacharacter matches one UTF-8 character instead of a sin-
181 nigel 75 gle byte.
182 nigel 63
183 nigel 93 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 nigel 77 not available in the alternative matching function, pcre_dfa_exec().
186 nigel 63
187 nigel 93 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 nigel 75 before, all with values less than 256. This remains true even when PCRE
191 nigel 93 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 nigel 75 \p{Nd}.
195 nigel 63
196 nigel 93 8. Similarly, characters that match the POSIX named character classes
197 nigel 75 are all low-valued characters.
198 nigel 63
199 nigel 93 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 nigel 87 used only for characters with higher values. Even when Unicode property
205     support is available, PCRE supports case-insensitive matching only when
206 nigel 93 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 nigel 87 ported by PCRE.
209 nigel 63
210    
211     AUTHOR
212    
213 nigel 77 Philip Hazel
214 ph10 99 University Computing Service
215 nigel 93 Cambridge CB2 3QH, England.
216 nigel 63
217 nigel 93 Putting an actual email address here seems to have been a spam magnet,
218 nigel 77 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 nigel 63
222 ph10 99 REVISION
223 nigel 63
224 ph10 99 Last updated: 06 March 2007
225     Copyright (c) 1997-2007 University of Cambridge.
226     ------------------------------------------------------------------------------
227    
228    
229 nigel 79 PCREBUILD(3) PCREBUILD(3)
230 nigel 63
231 nigel 79
232 nigel 73 NAME
233     PCRE - Perl-compatible regular expressions
234    
235 nigel 77
236 nigel 63 PCRE BUILD-TIME OPTIONS
237    
238 nigel 73 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 nigel 75 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 nigel 63
245 nigel 73 ./configure --help
246 nigel 63
247 nigel 73 The following sections describe certain options whose names begin with
248     --enable or --disable. These settings specify changes to the defaults
249     for the configure command. Because of the way that configure works,
250     --enable and --disable always come in pairs, so the complementary
251     option always exists as well, but as it specifies the default, it is
252     not described.
253 nigel 63
254    
255 nigel 83 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 nigel 63 UTF-8 SUPPORT
267    
268 nigel 73 To build PCRE with support for UTF-8 character strings, add
269 nigel 63
270 nigel 73 --enable-utf8
271 nigel 63
272 nigel 73 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 nigel 63
277    
278 nigel 75 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 90K of tables to the
292     PCRE library, approximately doubling its size. Only the general cate-
293     gory properties such as Lu and Nd are supported. Details are given in
294     the pcrepattern documentation.
295    
296    
297 nigel 63 CODE VALUE OF NEWLINE
298    
299 nigel 91 By default, PCRE interprets character 10 (linefeed, LF) as indicating
300     the end of a line. This is the normal newline character on Unix-like
301     systems. You can compile PCRE to use character 13 (carriage return, CR)
302     instead, by adding
303 nigel 63
304 nigel 73 --enable-newline-is-cr
305 nigel 63
306 nigel 91 to the configure command. There is also a --enable-newline-is-lf
307     option, which explicitly specifies linefeed as the newline character.
308 nigel 63
309 nigel 91 Alternatively, you can specify that line endings are to be indicated by
310     the two character sequence CRLF. If you want this, add
311 nigel 63
312 nigel 91 --enable-newline-is-crlf
313    
314 nigel 93 to the configure command. There is a fourth option, specified by
315 nigel 91
316 nigel 93 --enable-newline-is-any
317 nigel 91
318 nigel 93 which causes PCRE to recognize any Unicode newline sequence.
319    
320     Whatever line ending convention is selected when PCRE is built can be
321     overridden when the library functions are called. At build time it is
322     conventional to use the standard for your operating system.
323    
324    
325 nigel 63 BUILDING SHARED AND STATIC LIBRARIES
326    
327 nigel 93 The PCRE building process uses libtool to build both shared and static
328     Unix libraries by default. You can suppress one of these by adding one
329 nigel 73 of
330 nigel 63
331 nigel 73 --disable-shared
332     --disable-static
333 nigel 63
334 nigel 73 to the configure command, as required.
335 nigel 63
336    
337     POSIX MALLOC USAGE
338    
339 nigel 75 When PCRE is called through the POSIX interface (see the pcreposix doc-
340 nigel 93 umentation), additional working storage is required for holding the
341     pointers to capturing substrings, because PCRE requires three integers
342     per substring, whereas the POSIX interface provides only two. If the
343 nigel 73 number of expected substrings is small, the wrapper function uses space
344     on the stack, because this is faster than using malloc() for each call.
345     The default threshold above which the stack is no longer used is 10; it
346     can be changed by adding a setting such as
347 nigel 63
348 nigel 73 --with-posix-malloc-threshold=20
349 nigel 63
350 nigel 73 to the configure command.
351 nigel 63
352    
353     HANDLING VERY LARGE PATTERNS
354    
355 nigel 93 Within a compiled pattern, offset values are used to point from one
356     part to another (for example, from an opening parenthesis to an alter-
357     nation metacharacter). By default, two-byte values are used for these
358     offsets, leading to a maximum size for a compiled pattern of around
359     64K. This is sufficient to handle all but the most gigantic patterns.
360     Nevertheless, some people do want to process enormous patterns, so it
361     is possible to compile PCRE to use three-byte or four-byte offsets by
362 nigel 73 adding a setting such as
363 nigel 63
364 nigel 73 --with-link-size=3
365 nigel 63
366 nigel 93 to the configure command. The value given must be 2, 3, or 4. Using
367     longer offsets slows down the operation of PCRE because it has to load
368 nigel 73 additional bytes when handling them.
369 nigel 63
370 nigel 93 If you build PCRE with an increased link size, test 2 (and test 5 if
371     you are using UTF-8) will fail. Part of the output of these tests is a
372     representation of the compiled pattern, and this changes with the link
373 nigel 73 size.
374 nigel 63
375 nigel 73
376     AVOIDING EXCESSIVE STACK USAGE
377    
378 nigel 77 When matching with the pcre_exec() function, PCRE implements backtrack-
379 nigel 93 ing by making recursive calls to an internal function called match().
380     In environments where the size of the stack is limited, this can se-
381     verely limit PCRE's operation. (The Unix environment does not usually
382 nigel 91 suffer from this problem, but it may sometimes be necessary to increase
383 nigel 93 the maximum stack size. There is a discussion in the pcrestack docu-
384     mentation.) An alternative approach to recursion that uses memory from
385     the heap to remember data, instead of using recursive function calls,
386     has been implemented to work round the problem of limited stack size.
387 nigel 91 If you want to build a version of PCRE that works this way, add
388 nigel 73
389     --disable-stack-for-recursion
390    
391 nigel 93 to the configure command. With this configuration, PCRE will use the
392     pcre_stack_malloc and pcre_stack_free variables to call memory manage-
393     ment functions. Separate functions are provided because the usage is
394     very predictable: the block sizes requested are always the same, and
395     the blocks are always freed in reverse order. A calling program might
396     be able to implement optimized functions that perform better than the
397     standard malloc() and free() functions. PCRE runs noticeably more
398 nigel 77 slowly when built in this way. This option affects only the pcre_exec()
399     function; it is not relevant for the the pcre_dfa_exec() function.
400 nigel 73
401    
402 nigel 91 LIMITING PCRE RESOURCE USAGE
403    
404 nigel 93 Internally, PCRE has a function called match(), which it calls repeat-
405     edly (sometimes recursively) when matching a pattern with the
406     pcre_exec() function. By controlling the maximum number of times this
407     function may be called during a single matching operation, a limit can
408     be placed on the resources used by a single call to pcre_exec(). The
409     limit can be changed at run time, as described in the pcreapi documen-
410     tation. The default is 10 million, but this can be changed by adding a
411 nigel 91 setting such as
412    
413     --with-match-limit=500000
414    
415 nigel 93 to the configure command. This setting has no effect on the
416 nigel 91 pcre_dfa_exec() matching function.
417    
418 nigel 93 In some environments it is desirable to limit the depth of recursive
419 nigel 91 calls of match() more strictly than the total number of calls, in order
420 nigel 93 to restrict the maximum amount of stack (or heap, if --disable-stack-
421 nigel 91 for-recursion is specified) that is used. A second limit controls this;
422 nigel 93 it defaults to the value that is set for --with-match-limit, which
423     imposes no additional constraints. However, you can set a lower limit
424 nigel 91 by adding, for example,
425    
426     --with-match-limit-recursion=10000
427    
428 nigel 93 to the configure command. This value can also be overridden at run
429 nigel 91 time.
430    
431    
432 nigel 73 USING EBCDIC CODE
433    
434 nigel 93 PCRE assumes by default that it will run in an environment where the
435     character code is ASCII (or Unicode, which is a superset of ASCII).
436     PCRE can, however, be compiled to run in an EBCDIC environment by
437 nigel 75 adding
438 nigel 73
439     --enable-ebcdic
440    
441     to the configure command.
442    
443 nigel 93
444     SEE ALSO
445    
446     pcreapi(3), pcre_config(3).
447    
448 nigel 63
449 ph10 99 AUTHOR
450 nigel 63
451 ph10 99 Philip Hazel
452     University Computing Service
453     Cambridge CB2 3QH, England.
454    
455    
456     REVISION
457    
458     Last updated: 06 March 2007
459     Copyright (c) 1997-2007 University of Cambridge.
460     ------------------------------------------------------------------------------
461    
462    
463 nigel 79 PCREMATCHING(3) PCREMATCHING(3)
464 nigel 63
465 nigel 79
466 nigel 77 NAME
467     PCRE - Perl-compatible regular expressions
468 nigel 73
469 nigel 77
470     PCRE MATCHING ALGORITHMS
471    
472     This document describes the two different algorithms that are available
473     in PCRE for matching a compiled regular expression against a given sub-
474     ject string. The "standard" algorithm is the one provided by the
475     pcre_exec() function. This works in the same was as Perl's matching
476     function, and provides a Perl-compatible matching operation.
477    
478     An alternative algorithm is provided by the pcre_dfa_exec() function;
479     this operates in a different way, and is not Perl-compatible. It has
480     advantages and disadvantages compared with the standard algorithm, and
481     these are described below.
482    
483     When there is only one possible way in which a given subject string can
484     match a pattern, the two algorithms give the same answer. A difference
485     arises, however, when there are multiple possibilities. For example, if
486     the pattern
487    
488     ^<.*>
489    
490     is matched against the string
491    
492     <something> <something else> <something further>
493    
494     there are three possible answers. The standard algorithm finds only one
495 nigel 93 of them, whereas the alternative algorithm finds all three.
496 nigel 77
497    
498     REGULAR EXPRESSIONS AS TREES
499    
500     The set of strings that are matched by a regular expression can be rep-
501     resented as a tree structure. An unlimited repetition in the pattern
502     makes the tree of infinite size, but it is still a tree. Matching the
503     pattern to a given subject string (from a given starting point) can be
504 nigel 91 thought of as a search of the tree. There are two ways to search a
505     tree: depth-first and breadth-first, and these correspond to the two
506     matching algorithms provided by PCRE.
507 nigel 77
508    
509     THE STANDARD MATCHING ALGORITHM
510    
511     In the terminology of Jeffrey Friedl's book Mastering Regular Expres-
512     sions, the standard algorithm is an "NFA algorithm". It conducts a
513     depth-first search of the pattern tree. That is, it proceeds along a
514     single path through the tree, checking that the subject matches what is
515     required. When there is a mismatch, the algorithm tries any alterna-
516     tives at the current point, and if they all fail, it backs up to the
517     previous branch point in the tree, and tries the next alternative
518     branch at that level. This often involves backing up (moving to the
519     left) in the subject string as well. The order in which repetition
520     branches are tried is controlled by the greedy or ungreedy nature of
521     the quantifier.
522    
523     If a leaf node is reached, a matching string has been found, and at
524     that point the algorithm stops. Thus, if there is more than one possi-
525     ble match, this algorithm returns the first one that it finds. Whether
526     this is the shortest, the longest, or some intermediate length depends
527     on the way the greedy and ungreedy repetition quantifiers are specified
528     in the pattern.
529    
530     Because it ends up with a single path through the tree, it is rela-
531     tively straightforward for this algorithm to keep track of the sub-
532     strings that are matched by portions of the pattern in parentheses.
533     This provides support for capturing parentheses and back references.
534    
535    
536 nigel 93 THE ALTERNATIVE MATCHING ALGORITHM
537 nigel 77
538 nigel 93 This algorithm conducts a breadth-first search of the tree. Starting
539     from the first matching point in the subject, it scans the subject
540     string from left to right, once, character by character, and as it does
541     this, it remembers all the paths through the tree that represent valid
542     matches. In Friedl's terminology, this is a kind of "DFA algorithm",
543     though it is not implemented as a traditional finite state machine (it
544     keeps multiple states active simultaneously).
545 nigel 77
546 nigel 93 The scan continues until either the end of the subject is reached, or
547     there are no more unterminated paths. At this point, terminated paths
548     represent the different matching possibilities (if there are none, the
549     match has failed). Thus, if there is more than one possible match,
550 nigel 77 this algorithm finds all of them, and in particular, it finds the long-
551 nigel 93 est. In PCRE, there is an option to stop the algorithm after the first
552 nigel 77 match (which is necessarily the shortest) has been found.
553    
554     Note that all the matches that are found start at the same point in the
555     subject. If the pattern
556    
557     cat(er(pillar)?)
558    
559 nigel 93 is matched against the string "the caterpillar catchment", the result
560     will be the three strings "cat", "cater", and "caterpillar" that start
561 nigel 77 at the fourth character of the subject. The algorithm does not automat-
562     ically move on to find matches that start at later positions.
563    
564     There are a number of features of PCRE regular expressions that are not
565 nigel 93 supported by the alternative matching algorithm. They are as follows:
566 nigel 77
567 nigel 93 1. Because the algorithm finds all possible matches, the greedy or
568     ungreedy nature of repetition quantifiers is not relevant. Greedy and
569     ungreedy quantifiers are treated in exactly the same way. However, pos-
570     sessive quantifiers can make a difference when what follows could also
571     match what is quantified, for example in a pattern like this:
572 nigel 77
573 nigel 93 ^a++\w!
574    
575     This pattern matches "aaab!" but not "aaa!", which would be matched by
576     a non-possessive quantifier. Similarly, if an atomic group is present,
577     it is matched as if it were a standalone pattern at the current point,
578     and the longest match is then "locked in" for the rest of the overall
579     pattern.
580    
581 nigel 77 2. When dealing with multiple paths through the tree simultaneously, it
582 nigel 93 is not straightforward to keep track of captured substrings for the
583     different matching possibilities, and PCRE's implementation of this
584 nigel 77 algorithm does not attempt to do this. This means that no captured sub-
585     strings are available.
586    
587 nigel 93 3. Because no substrings are captured, back references within the pat-
588 nigel 77 tern are not supported, and cause errors if encountered.
589    
590 nigel 93 4. For the same reason, conditional expressions that use a backrefer-
591     ence as the condition or test for a specific group recursion are not
592     supported.
593 nigel 77
594     5. Callouts are supported, but the value of the capture_top field is
595     always 1, and the value of the capture_last field is always -1.
596    
597     6. The \C escape sequence, which (in the standard algorithm) matches a
598 nigel 93 single byte, even in UTF-8 mode, is not supported because the alterna-
599     tive algorithm moves through the subject string one character at a
600     time, for all active paths through the tree.
601 nigel 77
602    
603 nigel 93 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
604 nigel 77
605 nigel 93 Using the alternative matching algorithm provides the following advan-
606     tages:
607 nigel 77
608     1. All possible matches (at a single point in the subject) are automat-
609 nigel 93 ically found, and in particular, the longest match is found. To find
610 nigel 77 more than one match using the standard algorithm, you have to do kludgy
611     things with callouts.
612    
613 nigel 93 2. There is much better support for partial matching. The restrictions
614     on the content of the pattern that apply when using the standard algo-
615     rithm for partial matching do not apply to the alternative algorithm.
616     For non-anchored patterns, the starting position of a partial match is
617     available.
618 nigel 77
619 nigel 93 3. Because the alternative algorithm scans the subject string just
620     once, and never needs to backtrack, it is possible to pass very long
621     subject strings to the matching function in several pieces, checking
622     for partial matching each time.
623 nigel 77
624    
625 nigel 93 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
626 nigel 77
627 nigel 93 The alternative algorithm suffers from a number of disadvantages:
628 nigel 77
629 nigel 93 1. It is substantially slower than the standard algorithm. This is
630     partly because it has to search for all possible matches, but is also
631 nigel 77 because it is less susceptible to optimization.
632    
633     2. Capturing parentheses and back references are not supported.
634    
635 nigel 93 3. Although atomic groups are supported, their use does not provide the
636     performance advantage that it does for the standard algorithm.
637 nigel 77
638    
639 ph10 99 AUTHOR
640 nigel 77
641 ph10 99 Philip Hazel
642     University Computing Service
643     Cambridge CB2 3QH, England.
644    
645    
646     REVISION
647    
648     Last updated: 06 March 2007
649     Copyright (c) 1997-2007 University of Cambridge.
650     ------------------------------------------------------------------------------
651    
652    
653 nigel 79 PCREAPI(3) PCREAPI(3)
654 nigel 77
655 nigel 79
656 nigel 73 NAME
657     PCRE - Perl-compatible regular expressions
658    
659 nigel 77
660 nigel 75 PCRE NATIVE API
661 nigel 63
662 nigel 73 #include <pcre.h>
663 nigel 41
664 nigel 73 pcre *pcre_compile(const char *pattern, int options,
665     const char **errptr, int *erroffset,
666     const unsigned char *tableptr);
667 nigel 41
668 nigel 77 pcre *pcre_compile2(const char *pattern, int options,
669     int *errorcodeptr,
670     const char **errptr, int *erroffset,
671     const unsigned char *tableptr);
672    
673 nigel 73 pcre_extra *pcre_study(const pcre *code, int options,
674     const char **errptr);
675 nigel 41
676 nigel 73 int pcre_exec(const pcre *code, const pcre_extra *extra,
677     const char *subject, int length, int startoffset,
678     int options, int *ovector, int ovecsize);
679 nigel 41
680 nigel 77 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
681     const char *subject, int length, int startoffset,
682     int options, int *ovector, int ovecsize,
683     int *workspace, int wscount);
684    
685 nigel 73 int pcre_copy_named_substring(const pcre *code,
686     const char *subject, int *ovector,
687     int stringcount, const char *stringname,
688     char *buffer, int buffersize);
689 nigel 63
690 nigel 73 int pcre_copy_substring(const char *subject, int *ovector,
691     int stringcount, int stringnumber, char *buffer,
692     int buffersize);
693 nigel 41
694 nigel 73 int pcre_get_named_substring(const pcre *code,
695     const char *subject, int *ovector,
696     int stringcount, const char *stringname,
697     const char **stringptr);
698 nigel 63
699 nigel 73 int pcre_get_stringnumber(const pcre *code,
700     const char *name);
701 nigel 63
702 nigel 91 int pcre_get_stringtable_entries(const pcre *code,
703     const char *name, char **first, char **last);
704    
705 nigel 73 int pcre_get_substring(const char *subject, int *ovector,
706     int stringcount, int stringnumber,
707     const char **stringptr);
708 nigel 41
709 nigel 73 int pcre_get_substring_list(const char *subject,
710     int *ovector, int stringcount, const char ***listptr);
711 nigel 41
712 nigel 73 void pcre_free_substring(const char *stringptr);
713 nigel 49
714 nigel 73 void pcre_free_substring_list(const char **stringptr);
715 nigel 49
716 nigel 73 const unsigned char *pcre_maketables(void);
717 nigel 41
718 nigel 73 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
719     int what, void *where);
720 nigel 43
721 nigel 73 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
722 nigel 63
723 nigel 77 int pcre_refcount(pcre *code, int adjust);
724    
725 nigel 73 int pcre_config(int what, void *where);
726 nigel 41
727 nigel 73 char *pcre_version(void);
728 nigel 63
729 nigel 73 void *(*pcre_malloc)(size_t);
730 nigel 41
731 nigel 73 void (*pcre_free)(void *);
732 nigel 41
733 nigel 73 void *(*pcre_stack_malloc)(size_t);
734 nigel 41
735 nigel 73 void (*pcre_stack_free)(void *);
736 nigel 41
737 nigel 73 int (*pcre_callout)(pcre_callout_block *);
738 nigel 41
739 nigel 73
740 nigel 75 PCRE API OVERVIEW
741 nigel 41
742 nigel 73 PCRE has its own native API, which is described in this document. There
743 nigel 93 are also some wrapper functions that correspond to the POSIX regular
744 nigel 77 expression API. These are described in the pcreposix documentation.
745     Both of these APIs define a set of C function calls. A C++ wrapper is
746     distributed with PCRE. It is documented in the pcrecpp page.
747 nigel 43
748 nigel 77 The native API C function prototypes are defined in the header file
749     pcre.h, and on Unix systems the library itself is called libpcre. It
750 nigel 75 can normally be accessed by adding -lpcre to the command for linking an
751     application that uses PCRE. The header file defines the macros
752     PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
753     bers for the library. Applications can use these to include support
754     for different releases of PCRE.
755 nigel 41
756 nigel 77 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
757     pcre_exec() are used for compiling and matching regular expressions in
758     a Perl-compatible manner. A sample program that demonstrates the sim-
759     plest way of using them is provided in the file called pcredemo.c in
760     the source distribution. The pcresample documentation describes how to
761     run it.
762 nigel 49
763 nigel 77 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
764     ble, is also provided. This uses a different algorithm for the match-
765 nigel 91 ing. The alternative algorithm finds all possible matches (at a given
766 nigel 93 point in the subject), and scans the subject just once. However, this
767     algorithm does not return captured substrings. A description of the two
768     matching algorithms and their advantages and disadvantages is given in
769     the pcrematching documentation.
770 nigel 63
771 nigel 77 In addition to the main compiling and matching functions, there are
772     convenience functions for extracting captured substrings from a subject
773     string that is matched by pcre_exec(). They are:
774    
775 nigel 73 pcre_copy_substring()
776     pcre_copy_named_substring()
777     pcre_get_substring()
778     pcre_get_named_substring()
779     pcre_get_substring_list()
780 nigel 75 pcre_get_stringnumber()
781 nigel 91 pcre_get_stringtable_entries()
782 nigel 63
783 nigel 73 pcre_free_substring() and pcre_free_substring_list() are also provided,
784     to free the memory used for extracted strings.
785 nigel 41
786 nigel 77 The function pcre_maketables() is used to build a set of character
787     tables in the current locale for passing to pcre_compile(),
788     pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
789     provided for specialist use. Most commonly, no special tables are
790     passed, in which case internal tables that are generated when PCRE is
791     built are used.
792 nigel 49
793 nigel 75 The function pcre_fullinfo() is used to find out information about a
794     compiled pattern; pcre_info() is an obsolete version that returns only
795     some of the available information, but is retained for backwards com-
796     patibility. The function pcre_version() returns a pointer to a string
797 nigel 73 containing the version of PCRE and its date of release.
798 nigel 41
799 nigel 77 The function pcre_refcount() maintains a reference count in a data
800     block containing a compiled pattern. This is provided for the benefit
801     of object-oriented applications.
802    
803 nigel 75 The global variables pcre_malloc and pcre_free initially contain the
804     entry points of the standard malloc() and free() functions, respec-
805 nigel 73 tively. PCRE calls the memory management functions via these variables,
806 nigel 75 so a calling program can replace them if it wishes to intercept the
807 nigel 73 calls. This should be done before calling any PCRE functions.
808 nigel 41
809 nigel 75 The global variables pcre_stack_malloc and pcre_stack_free are also
810     indirections to memory management functions. These special functions
811     are used only when PCRE is compiled to use the heap for remembering
812 nigel 77 data, instead of recursive function calls, when running the pcre_exec()
813 nigel 91 function. See the pcrebuild documentation for details of how to do
814     this. It is a non-standard way of building PCRE, for use in environ-
815     ments that have limited stacks. Because of the greater use of memory
816     management, it runs more slowly. Separate functions are provided so
817     that special-purpose external code can be used for this case. When
818     used, these functions are always called in a stack-like manner (last
819     obtained, first freed), and always for memory blocks of the same size.
820     There is a discussion about PCRE's stack usage in the pcrestack docu-
821     mentation.
822 nigel 41
823 nigel 73 The global variable pcre_callout initially contains NULL. It can be set
824 nigel 77 by the caller to a "callout" function, which PCRE will then call at
825     specified points during a matching operation. Details are given in the
826 nigel 73 pcrecallout documentation.
827 nigel 41
828 nigel 73
829 nigel 91 NEWLINES
830    
831 nigel 93 PCRE supports four different conventions for indicating line breaks in
832     strings: a single CR (carriage return) character, a single LF (line-
833     feed) character, the two-character sequence CRLF, or any Unicode new-
834     line sequence. The Unicode newline sequences are the three just men-
835     tioned, plus the single characters VT (vertical tab, U+000B), FF (form-
836     feed, U+000C), NEL (next line, U+0085), LS (line separator, U+2028),
837     and PS (paragraph separator, U+2029).
838    
839     Each of the first three conventions is used by at least one operating
840     system as its standard newline sequence. When PCRE is built, a default
841     can be specified. The default default is LF, which is the Unix stan-
842     dard. When PCRE is run, the default can be overridden, either when a
843     pattern is compiled, or when it is matched.
844    
845 nigel 91 In the PCRE documentation the word "newline" is used to mean "the char-
846 nigel 93 acter or pair of characters that indicate a line break". The choice of
847     newline convention affects the handling of the dot, circumflex, and
848     dollar metacharacters, the handling of #-comments in /x mode, and, when
849     CRLF is a recognized line ending sequence, the match position advance-
850     ment for a non-anchored pattern. The choice of newline convention does
851     not affect the interpretation of the \n or \r escape sequences.
852 nigel 91
853    
854 nigel 63 MULTITHREADING
855    
856 nigel 93 The PCRE functions can be used in multi-threading applications, with
857 nigel 73 the proviso that the memory management functions pointed to by
858     pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
859     callout function pointed to by pcre_callout, are shared by all threads.
860 nigel 41
861 nigel 93 The compiled form of a regular expression is not altered during match-
862 nigel 73 ing, so the same compiled pattern can safely be used by several threads
863     at once.
864 nigel 41
865    
866 nigel 75 SAVING PRECOMPILED PATTERNS FOR LATER USE
867    
868     The compiled form of a regular expression can be saved and re-used at a
869 nigel 93 later time, possibly by a different program, and even on a host other
870     than the one on which it was compiled. Details are given in the
871 nigel 75 pcreprecompile documentation.
872    
873    
874 nigel 63 CHECKING BUILD-TIME OPTIONS
875 nigel 41
876 nigel 73 int pcre_config(int what, void *where);
877 nigel 63
878 nigel 93 The function pcre_config() makes it possible for a PCRE client to dis-
879 nigel 73 cover which optional features have been compiled into the PCRE library.
880 nigel 93 The pcrebuild documentation has more details about these optional fea-
881 nigel 73 tures.
882 nigel 63
883 nigel 93 The first argument for pcre_config() is an integer, specifying which
884 nigel 73 information is required; the second argument is a pointer to a variable
885 nigel 93 into which the information is placed. The following information is
886 nigel 73 available:
887 nigel 63
888 nigel 73 PCRE_CONFIG_UTF8
889 nigel 63
890 nigel 93 The output is an integer that is set to one if UTF-8 support is avail-
891 nigel 73 able; otherwise it is set to zero.
892 nigel 63
893 nigel 75 PCRE_CONFIG_UNICODE_PROPERTIES
894    
895 nigel 93 The output is an integer that is set to one if support for Unicode
896 nigel 75 character properties is available; otherwise it is set to zero.
897    
898 nigel 73 PCRE_CONFIG_NEWLINE
899 nigel 63
900 nigel 93 The output is an integer whose value specifies the default character
901     sequence that is recognized as meaning "newline". The four values that
902     are supported are: 10 for LF, 13 for CR, 3338 for CRLF, and -1 for ANY.
903     The default should normally be the standard sequence for your operating
904     system.
905 nigel 63
906 nigel 73 PCRE_CONFIG_LINK_SIZE
907 nigel 63
908 nigel 91 The output is an integer that contains the number of bytes used for
909 nigel 73 internal linkage in compiled regular expressions. The value is 2, 3, or
910 nigel 91 4. Larger values allow larger regular expressions to be compiled, at
911     the expense of slower matching. The default value of 2 is sufficient
912     for all but the most massive patterns, since it allows the compiled
913 nigel 73 pattern to be up to 64K in size.
914 nigel 63
915 nigel 73 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
916 nigel 63
917 nigel 91 The output is an integer that contains the threshold above which the
918     POSIX interface uses malloc() for output vectors. Further details are
919 nigel 73 given in the pcreposix documentation.
920 nigel 63
921 nigel 73 PCRE_CONFIG_MATCH_LIMIT
922 nigel 63
923 nigel 73 The output is an integer that gives the default limit for the number of
924 nigel 91 internal matching function calls in a pcre_exec() execution. Further
925 nigel 73 details are given with pcre_exec() below.
926 nigel 63
927 nigel 87 PCRE_CONFIG_MATCH_LIMIT_RECURSION
928    
929 nigel 91 The output is an integer that gives the default limit for the depth of
930     recursion when calling the internal matching function in a pcre_exec()
931 nigel 87 execution. Further details are given with pcre_exec() below.
932    
933 nigel 73 PCRE_CONFIG_STACKRECURSE
934 nigel 63
935 nigel 91 The output is an integer that is set to one if internal recursion when
936 nigel 77 running pcre_exec() is implemented by recursive function calls that use
937 nigel 91 the stack to remember their state. This is the usual way that PCRE is
938 nigel 77 compiled. The output is zero if PCRE was compiled to use blocks of data
939 nigel 91 on the heap instead of recursive function calls. In this case,
940     pcre_stack_malloc and pcre_stack_free are called to manage memory
941 nigel 77 blocks on the heap, thus avoiding the use of the stack.
942 nigel 73
943    
944 nigel 41 COMPILING A PATTERN
945 nigel 63
946 nigel 73 pcre *pcre_compile(const char *pattern, int options,
947     const char **errptr, int *erroffset,
948     const unsigned char *tableptr);
949 nigel 63
950 nigel 77 pcre *pcre_compile2(const char *pattern, int options,
951     int *errorcodeptr,
952     const char **errptr, int *erroffset,
953     const unsigned char *tableptr);
954 nigel 41
955 nigel 77 Either of the functions pcre_compile() or pcre_compile2() can be called
956     to compile a pattern into an internal form. The only difference between
957 nigel 91 the two interfaces is that pcre_compile2() has an additional argument,
958 nigel 77 errorcodeptr, via which a numerical error code can be returned.
959    
960     The pattern is a C string terminated by a binary zero, and is passed in
961 nigel 91 the pattern argument. A pointer to a single block of memory that is
962     obtained via pcre_malloc is returned. This contains the compiled code
963 nigel 77 and related data. The pcre type is defined for the returned block; this
964     is a typedef for a structure whose contents are not externally defined.
965 nigel 91 It is up to the caller to free the memory (via pcre_free) when it is no
966     longer required.
967 nigel 77
968 nigel 91 Although the compiled code of a PCRE regex is relocatable, that is, it
969 nigel 73 does not depend on memory location, the complete pcre data block is not
970 nigel 91 fully relocatable, because it may contain a copy of the tableptr argu-
971 nigel 75 ment, which is an address (see below).
972 nigel 41
973 nigel 93 The options argument contains various bit settings that affect the com-
974     pilation. It should be zero if no options are required. The available
975 nigel 91 options are described below. Some of them, in particular, those that
976     are compatible with Perl, can also be set and unset from within the
977     pattern (see the detailed description in the pcrepattern documenta-
978     tion). For these options, the contents of the options argument speci-
979     fies their initial settings at the start of compilation and execution.
980     The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
981     of matching as well as at compile time.
982 nigel 41
983 nigel 73 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
984 nigel 91 if compilation of a pattern fails, pcre_compile() returns NULL, and
985 nigel 73 sets the variable pointed to by errptr to point to a textual error mes-
986 nigel 87 sage. This is a static string that is part of the library. You must not
987     try to free it. The offset from the start of the pattern to the charac-
988     ter where the error was discovered is placed in the variable pointed to
989 nigel 91 by erroffset, which must not be NULL. If it is, an immediate error is
990 nigel 73 given.
991 nigel 53
992 nigel 91 If pcre_compile2() is used instead of pcre_compile(), and the error-
993     codeptr argument is not NULL, a non-zero error code number is returned
994     via this argument in the event of an error. This is in addition to the
995 nigel 77 textual error message. Error codes and messages are listed below.
996    
997 nigel 91 If the final argument, tableptr, is NULL, PCRE uses a default set of
998     character tables that are built when PCRE is compiled, using the
999     default C locale. Otherwise, tableptr must be an address that is the
1000     result of a call to pcre_maketables(). This value is stored with the
1001     compiled pattern, and used again by pcre_exec(), unless another table
1002 nigel 75 pointer is passed to it. For more discussion, see the section on locale
1003     support below.
1004 nigel 53
1005 nigel 91 This code fragment shows a typical straightforward call to pcre_com-
1006 nigel 73 pile():
1007 nigel 41
1008 nigel 73 pcre *re;
1009     const char *error;
1010     int erroffset;
1011     re = pcre_compile(
1012     "^A.*Z", /* the pattern */
1013     0, /* default options */
1014     &error, /* for error message */
1015     &erroffset, /* for error offset */
1016     NULL); /* use default character tables */
1017 nigel 41
1018 nigel 91 The following names for option bits are defined in the pcre.h header
1019 nigel 75 file:
1020 nigel 41
1021 nigel 73 PCRE_ANCHORED
1022 nigel 41
1023 nigel 73 If this bit is set, the pattern is forced to be "anchored", that is, it
1024 nigel 91 is constrained to match only at the first matching point in the string
1025     that is being searched (the "subject string"). This effect can also be
1026     achieved by appropriate constructs in the pattern itself, which is the
1027 nigel 73 only way to do it in Perl.
1028 nigel 41
1029 nigel 75 PCRE_AUTO_CALLOUT
1030    
1031     If this bit is set, pcre_compile() automatically inserts callout items,
1032 nigel 91 all with number 255, before each pattern item. For discussion of the
1033 nigel 75 callout facility, see the pcrecallout documentation.
1034    
1035 nigel 73 PCRE_CASELESS
1036 nigel 41
1037 nigel 91 If this bit is set, letters in the pattern match both upper and lower
1038     case letters. It is equivalent to Perl's /i option, and it can be
1039     changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1040     always understands the concept of case for characters whose values are
1041     less than 128, so caseless matching is always possible. For characters
1042     with higher values, the concept of case is supported if PCRE is com-
1043     piled with Unicode property support, but not otherwise. If you want to
1044     use caseless matching for characters 128 and above, you must ensure
1045     that PCRE is compiled with Unicode property support as well as with
1046 nigel 77 UTF-8 support.
1047 nigel 41
1048 nigel 73 PCRE_DOLLAR_ENDONLY
1049 nigel 41
1050 nigel 91 If this bit is set, a dollar metacharacter in the pattern matches only
1051     at the end of the subject string. Without this option, a dollar also
1052     matches immediately before a newline at the end of the string (but not
1053     before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1054     if PCRE_MULTILINE is set. There is no equivalent to this option in
1055     Perl, and no way to set it within a pattern.
1056 nigel 41
1057 nigel 73 PCRE_DOTALL
1058 nigel 41
1059 nigel 73 If this bit is set, a dot metacharater in the pattern matches all char-
1060 nigel 91 acters, including those that indicate newline. Without it, a dot does
1061     not match when the current position is at a newline. This option is
1062     equivalent to Perl's /s option, and it can be changed within a pattern
1063     by a (?s) option setting. A negative class such as [^a] always matches
1064 nigel 93 newline characters, independent of the setting of this option.
1065 nigel 63
1066 nigel 91 PCRE_DUPNAMES
1067    
1068     If this bit is set, names used to identify capturing subpatterns need
1069     not be unique. This can be helpful for certain types of pattern when it
1070     is known that only one instance of the named subpattern can ever be
1071     matched. There are more details of named subpatterns below; see also
1072     the pcrepattern documentation.
1073    
1074 nigel 73 PCRE_EXTENDED
1075 nigel 41
1076 nigel 91 If this bit is set, whitespace data characters in the pattern are
1077 nigel 77 totally ignored except when escaped or inside a character class. White-
1078     space does not include the VT character (code 11). In addition, charac-
1079     ters between an unescaped # outside a character class and the next new-
1080 nigel 91 line, inclusive, are also ignored. This is equivalent to Perl's /x
1081     option, and it can be changed within a pattern by a (?x) option set-
1082     ting.
1083 nigel 41
1084 nigel 91 This option makes it possible to include comments inside complicated
1085     patterns. Note, however, that this applies only to data characters.
1086     Whitespace characters may never appear within special character
1087     sequences in a pattern, for example within the sequence (?( which
1088 nigel 73 introduces a conditional subpattern.
1089 nigel 41
1090 nigel 73 PCRE_EXTRA
1091 nigel 41
1092 nigel 91 This option was invented in order to turn on additional functionality
1093     of PCRE that is incompatible with Perl, but it is currently of very
1094     little use. When set, any backslash in a pattern that is followed by a
1095     letter that has no special meaning causes an error, thus reserving
1096     these combinations for future expansion. By default, as in Perl, a
1097     backslash followed by a letter with no special meaning is treated as a
1098     literal. (Perl can, however, be persuaded to give a warning for this.)
1099     There are at present no other features controlled by this option. It
1100     can also be set by a (?X) option setting within a pattern.
1101 nigel 41
1102 nigel 77 PCRE_FIRSTLINE
1103    
1104 nigel 87 If this option is set, an unanchored pattern is required to match
1105 nigel 91 before or at the first newline in the subject string, though the
1106     matched text may continue over the newline.
1107 nigel 77
1108 nigel 73 PCRE_MULTILINE
1109 nigel 41
1110 nigel 87 By default, PCRE treats the subject string as consisting of a single
1111     line of characters (even if it actually contains newlines). The "start
1112     of line" metacharacter (^) matches only at the start of the string,
1113     while the "end of line" metacharacter ($) matches only at the end of
1114 nigel 75 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1115     is set). This is the same as Perl.
1116 nigel 63
1117 nigel 87 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1118 nigel 91 constructs match immediately following or immediately before internal
1119     newlines in the subject string, respectively, as well as at the very
1120     start and end. This is equivalent to Perl's /m option, and it can be
1121     changed within a pattern by a (?m) option setting. If there are no new-
1122     lines in a subject string, or no occurrences of ^ or $ in a pattern,
1123 nigel 73 setting PCRE_MULTILINE has no effect.
1124 nigel 63
1125 nigel 91 PCRE_NEWLINE_CR
1126     PCRE_NEWLINE_LF
1127     PCRE_NEWLINE_CRLF
1128 nigel 93 PCRE_NEWLINE_ANY
1129 nigel 91
1130     These options override the default newline definition that was chosen
1131     when PCRE was built. Setting the first or the second specifies that a
1132     newline is indicated by a single character (CR or LF, respectively).
1133 nigel 93 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1134     two-character CRLF sequence. Setting PCRE_NEWLINE_ANY specifies that
1135     any Unicode newline sequence should be recognized. The Unicode newline
1136     sequences are the three just mentioned, plus the single characters VT
1137     (vertical tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085),
1138     LS (line separator, U+2028), and PS (paragraph separator, U+2029). The
1139     last two are recognized only in UTF-8 mode.
1140 nigel 91
1141 nigel 93 The newline setting in the options word uses three bits that are
1142     treated as a number, giving eight possibilities. Currently only five
1143     are used (default plus the four values above). This means that if you
1144     set more than one newline option, the combination may or may not be
1145     sensible. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equiva-
1146     lent to PCRE_NEWLINE_CRLF, but other combinations yield unused numbers
1147     and cause an error.
1148 nigel 91
1149 nigel 93 The only time that a line break is specially recognized when compiling
1150     a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1151     character class is encountered. This indicates a comment that lasts
1152     until after the next line break sequence. In other circumstances, line
1153     break sequences are treated as literal data, except that in
1154     PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1155     and are therefore ignored.
1156    
1157     The newline option that is set at compile time becomes the default that
1158     is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1159    
1160 nigel 73 PCRE_NO_AUTO_CAPTURE
1161 nigel 41
1162 nigel 73 If this option is set, it disables the use of numbered capturing paren-
1163 nigel 91 theses in the pattern. Any opening parenthesis that is not followed by
1164     ? behaves as if it were followed by ?: but named parentheses can still
1165     be used for capturing (and they acquire numbers in the usual way).
1166 nigel 73 There is no equivalent of this option in Perl.
1167 nigel 41
1168 nigel 73 PCRE_UNGREEDY
1169 nigel 41
1170 nigel 91 This option inverts the "greediness" of the quantifiers so that they
1171     are not greedy by default, but become greedy if followed by "?". It is
1172     not compatible with Perl. It can also be set by a (?U) option setting
1173 nigel 73 within the pattern.
1174 nigel 41
1175 nigel 73 PCRE_UTF8
1176 nigel 49
1177 nigel 91 This option causes PCRE to regard both the pattern and the subject as
1178     strings of UTF-8 characters instead of single-byte character strings.
1179     However, it is available only when PCRE is built to include UTF-8 sup-
1180     port. If not, the use of this option provokes an error. Details of how
1181     this option changes the behaviour of PCRE are given in the section on
1182 nigel 75 UTF-8 support in the main pcre page.
1183 nigel 71
1184 nigel 73 PCRE_NO_UTF8_CHECK
1185 nigel 71
1186 nigel 73 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1187 nigel 91 automatically checked. If an invalid UTF-8 sequence of bytes is found,
1188     pcre_compile() returns an error. If you already know that your pattern
1189     is valid, and you want to skip this check for performance reasons, you
1190     can set the PCRE_NO_UTF8_CHECK option. When it is set, the effect of
1191 nigel 73 passing an invalid UTF-8 string as a pattern is undefined. It may cause
1192 nigel 91 your program to crash. Note that this option can also be passed to
1193     pcre_exec() and pcre_dfa_exec(), to suppress the UTF-8 validity check-
1194 nigel 77 ing of subject strings.
1195 nigel 71
1196 nigel 73
1197 nigel 77 COMPILATION ERROR CODES
1198    
1199 nigel 91 The following table lists the error codes than may be returned by
1200     pcre_compile2(), along with the error messages that may be returned by
1201 nigel 93 both compiling functions. As PCRE has developed, some error codes have
1202     fallen out of use. To avoid confusion, they have not been re-used.
1203 nigel 77
1204     0 no error
1205     1 \ at end of pattern
1206     2 \c at end of pattern
1207     3 unrecognized character follows \
1208     4 numbers out of order in {} quantifier
1209     5 number too big in {} quantifier
1210     6 missing terminating ] for character class
1211     7 invalid escape sequence in character class
1212     8 range out of order in character class
1213     9 nothing to repeat
1214 nigel 93 10 [this code is not in use]
1215 nigel 77 11 internal error: unexpected repeat
1216     12 unrecognized character after (?
1217     13 POSIX named classes are supported only within a class
1218     14 missing )
1219     15 reference to non-existent subpattern
1220     16 erroffset passed as NULL
1221     17 unknown option bit(s) set
1222     18 missing ) after comment
1223 nigel 93 19 [this code is not in use]
1224 nigel 77 20 regular expression too large
1225     21 failed to get memory
1226     22 unmatched parentheses
1227     23 internal error: code overflow
1228     24 unrecognized character after (?<
1229     25 lookbehind assertion is not fixed length
1230 nigel 91 26 malformed number or name after (?(
1231 nigel 77 27 conditional group contains more than two branches
1232     28 assertion expected after (?(
1233     29 (?R or (?digits must be followed by )
1234     30 unknown POSIX class name
1235     31 POSIX collating elements are not supported
1236     32 this version of PCRE is not compiled with PCRE_UTF8 support
1237 nigel 93 33 [this code is not in use]
1238 nigel 77 34 character value in \x{...} sequence is too large
1239     35 invalid condition (?(0)
1240     36 \C not allowed in lookbehind assertion
1241     37 PCRE does not support \L, \l, \N, \U, or \u
1242     38 number after (?C is > 255
1243     39 closing ) for (?C expected
1244     40 recursive call could loop indefinitely
1245     41 unrecognized character after (?P
1246 nigel 93 42 syntax error in subpattern name (missing terminator)
1247 nigel 91 43 two named subpatterns have the same name
1248 nigel 77 44 invalid UTF-8 string
1249     45 support for \P, \p, and \X has not been compiled
1250     46 malformed \P or \p sequence
1251     47 unknown property name after \P or \p
1252 nigel 91 48 subpattern name is too long (maximum 32 characters)
1253     49 too many named subpatterns (maximum 10,000)
1254     50 repeated subpattern is too long
1255     51 octal value is greater than \377 (not in UTF-8 mode)
1256 nigel 93 52 internal error: overran compiling workspace
1257     53 internal error: previously-checked referenced subpattern not
1258     found
1259     54 DEFINE group contains more than one branch
1260     55 repeating a DEFINE group is not allowed
1261     56 inconsistent NEWLINE options"
1262 nigel 77
1263    
1264 nigel 63 STUDYING A PATTERN
1265 nigel 49
1266 nigel 77 pcre_extra *pcre_study(const pcre *code, int options
1267 nigel 73 const char **errptr);
1268 nigel 63
1269 nigel 91 If a compiled pattern is going to be used several times, it is worth
1270 nigel 75 spending more time analyzing it in order to speed up the time taken for
1271 nigel 91 matching. The function pcre_study() takes a pointer to a compiled pat-
1272 nigel 75 tern as its first argument. If studying the pattern produces additional
1273 nigel 91 information that will help speed up matching, pcre_study() returns a
1274     pointer to a pcre_extra block, in which the study_data field points to
1275 nigel 75 the results of the study.
1276 nigel 41
1277 nigel 75 The returned value from pcre_study() can be passed directly to
1278 nigel 91 pcre_exec(). However, a pcre_extra block also contains other fields
1279     that can be set by the caller before the block is passed; these are
1280 nigel 75 described below in the section on matching a pattern.
1281 nigel 63
1282 nigel 91 If studying the pattern does not produce any additional information
1283 nigel 75 pcre_study() returns NULL. In that circumstance, if the calling program
1284 nigel 91 wants to pass any of the other fields to pcre_exec(), it must set up
1285 nigel 75 its own pcre_extra block.
1286 nigel 41
1287 nigel 91 The second argument of pcre_study() contains option bits. At present,
1288 nigel 75 no options are defined, and this argument should always be zero.
1289    
1290 nigel 91 The third argument for pcre_study() is a pointer for an error message.
1291     If studying succeeds (even if no data is returned), the variable it
1292     points to is set to NULL. Otherwise it is set to point to a textual
1293 nigel 87 error message. This is a static string that is part of the library. You
1294 nigel 91 must not try to free it. You should test the error pointer for NULL
1295 nigel 87 after calling pcre_study(), to be sure that it has run successfully.
1296 nigel 41
1297 nigel 73 This is a typical call to pcre_study():
1298 nigel 53
1299 nigel 73 pcre_extra *pe;
1300     pe = pcre_study(
1301     re, /* result of pcre_compile() */
1302     0, /* no options exist */
1303     &error); /* set to NULL or points to a message */
1304 nigel 53
1305 nigel 73 At present, studying a pattern is useful only for non-anchored patterns
1306 nigel 91 that do not have a single fixed starting character. A bitmap of possi-
1307 nigel 75 ble starting bytes is created.
1308 nigel 41
1309    
1310 nigel 63 LOCALE SUPPORT
1311 nigel 41
1312 nigel 91 PCRE handles caseless matching, and determines whether characters are
1313     letters digits, or whatever, by reference to a set of tables, indexed
1314     by character value. When running in UTF-8 mode, this applies only to
1315     characters with codes less than 128. Higher-valued codes never match
1316     escapes such as \w or \d, but can be tested with \p if PCRE is built
1317     with Unicode character property support. The use of locales with Uni-
1318 nigel 87 code is discouraged.
1319 nigel 41
1320 nigel 91 An internal set of tables is created in the default C locale when PCRE
1321     is built. This is used when the final argument of pcre_compile() is
1322     NULL, and is sufficient for many applications. An alternative set of
1323     tables can, however, be supplied. These may be created in a different
1324     locale from the default. As more and more applications change to using
1325 nigel 75 Unicode, the need for this locale support is expected to die away.
1326 nigel 41
1327 nigel 91 External tables are built by calling the pcre_maketables() function,
1328     which has no arguments, in the relevant locale. The result can then be
1329     passed to pcre_compile() or pcre_exec() as often as necessary. For
1330     example, to build and use tables that are appropriate for the French
1331     locale (where accented characters with values greater than 128 are
1332 nigel 75 treated as letters), the following code could be used:
1333    
1334     setlocale(LC_CTYPE, "fr_FR");
1335 nigel 73 tables = pcre_maketables();
1336     re = pcre_compile(..., tables);
1337 nigel 41
1338 nigel 91 When pcre_maketables() runs, the tables are built in memory that is
1339     obtained via pcre_malloc. It is the caller's responsibility to ensure
1340     that the memory containing the tables remains available for as long as
1341 nigel 75 it is needed.
1342 nigel 41
1343 nigel 75 The pointer that is passed to pcre_compile() is saved with the compiled
1344 nigel 91 pattern, and the same tables are used via this pointer by pcre_study()
1345 nigel 75 and normally also by pcre_exec(). Thus, by default, for any single pat-
1346     tern, compilation, studying and matching all happen in the same locale,
1347     but different patterns can be compiled in different locales.
1348 nigel 41
1349 nigel 91 It is possible to pass a table pointer or NULL (indicating the use of
1350     the internal tables) to pcre_exec(). Although not intended for this
1351     purpose, this facility could be used to match a pattern in a different
1352 nigel 75 locale from the one in which it was compiled. Passing table pointers at
1353     run time is discussed below in the section on matching a pattern.
1354    
1355    
1356 nigel 63 INFORMATION ABOUT A PATTERN
1357 nigel 41
1358 nigel 73 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1359     int what, void *where);
1360 nigel 63
1361 nigel 91 The pcre_fullinfo() function returns information about a compiled pat-
1362 nigel 73 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1363     less retained for backwards compability (and is documented below).
1364 nigel 43
1365 nigel 91 The first argument for pcre_fullinfo() is a pointer to the compiled
1366     pattern. The second argument is the result of pcre_study(), or NULL if
1367     the pattern was not studied. The third argument specifies which piece
1368     of information is required, and the fourth argument is a pointer to a
1369     variable to receive the data. The yield of the function is zero for
1370 nigel 73 success, or one of the following negative numbers:
1371 nigel 41
1372 nigel 73 PCRE_ERROR_NULL the argument code was NULL
1373     the argument where was NULL
1374     PCRE_ERROR_BADMAGIC the "magic number" was not found
1375     PCRE_ERROR_BADOPTION the value of what was invalid
1376 nigel 53
1377 nigel 91 The "magic number" is placed at the start of each compiled pattern as
1378     an simple check against passing an arbitrary memory pointer. Here is a
1379     typical call of pcre_fullinfo(), to obtain the length of the compiled
1380 nigel 75 pattern:
1381 nigel 53
1382 nigel 73 int rc;
1383 nigel 91 size_t length;
1384 nigel 73 rc = pcre_fullinfo(
1385     re, /* result of pcre_compile() */
1386     pe, /* result of pcre_study(), or NULL */
1387     PCRE_INFO_SIZE, /* what is required */
1388     &length); /* where to put the data */
1389 nigel 43
1390 nigel 91 The possible values for the third argument are defined in pcre.h, and
1391 nigel 73 are as follows:
1392 nigel 43
1393 nigel 73 PCRE_INFO_BACKREFMAX
1394 nigel 41
1395 nigel 91 Return the number of the highest back reference in the pattern. The
1396     fourth argument should point to an int variable. Zero is returned if
1397 nigel 73 there are no back references.
1398 nigel 43
1399 nigel 73 PCRE_INFO_CAPTURECOUNT
1400 nigel 43
1401 nigel 91 Return the number of capturing subpatterns in the pattern. The fourth
1402 nigel 73 argument should point to an int variable.
1403 nigel 43
1404 nigel 77 PCRE_INFO_DEFAULT_TABLES
1405 nigel 75
1406 nigel 91 Return a pointer to the internal default character tables within PCRE.
1407     The fourth argument should point to an unsigned char * variable. This
1408 nigel 75 information call is provided for internal use by the pcre_study() func-
1409 nigel 91 tion. External callers can cause PCRE to use its internal tables by
1410 nigel 75 passing a NULL table pointer.
1411    
1412 nigel 73 PCRE_INFO_FIRSTBYTE
1413 nigel 43
1414 nigel 91 Return information about the first byte of any matched string, for a
1415     non-anchored pattern. The fourth argument should point to an int vari-
1416     able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1417     is still recognized for backwards compatibility.)
1418 nigel 41
1419 nigel 91 If there is a fixed first byte, for example, from a pattern such as
1420 nigel 93 (cat|cow|coyote), its value is returned. Otherwise, if either
1421 nigel 41
1422 nigel 87 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1423 nigel 73 branch starts with "^", or
1424 nigel 43
1425 nigel 73 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1426     set (if it were set, the pattern would be anchored),
1427 nigel 41
1428 nigel 87 -1 is returned, indicating that the pattern matches only at the start
1429     of a subject string or after any newline within the string. Otherwise
1430 nigel 73 -2 is returned. For anchored patterns, -2 is returned.
1431 nigel 41
1432 nigel 73 PCRE_INFO_FIRSTTABLE
1433 nigel 41
1434 nigel 87 If the pattern was studied, and this resulted in the construction of a
1435 nigel 73 256-bit table indicating a fixed set of bytes for the first byte in any
1436 nigel 87 matching string, a pointer to the table is returned. Otherwise NULL is
1437     returned. The fourth argument should point to an unsigned char * vari-
1438 nigel 73 able.
1439 nigel 43
1440 nigel 73 PCRE_INFO_LASTLITERAL
1441 nigel 43
1442 nigel 87 Return the value of the rightmost literal byte that must exist in any
1443     matched string, other than at its start, if such a byte has been
1444 nigel 73 recorded. The fourth argument should point to an int variable. If there
1445 nigel 87 is no such byte, -1 is returned. For anchored patterns, a last literal
1446     byte is recorded only if it follows something of variable length. For
1447 nigel 73 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1448     /^a\dz\d/ the returned value is -1.
1449 nigel 63
1450 nigel 73 PCRE_INFO_NAMECOUNT
1451     PCRE_INFO_NAMEENTRYSIZE
1452     PCRE_INFO_NAMETABLE
1453 nigel 63
1454 nigel 87 PCRE supports the use of named as well as numbered capturing parenthe-
1455     ses. The names are just an additional way of identifying the parenthe-
1456 nigel 91 ses, which still acquire numbers. Several convenience functions such as
1457     pcre_get_named_substring() are provided for extracting captured sub-
1458     strings by name. It is also possible to extract the data directly, by
1459     first converting the name to a number in order to access the correct
1460     pointers in the output vector (described with pcre_exec() below). To do
1461     the conversion, you need to use the name-to-number map, which is
1462     described by these three values.
1463 nigel 63
1464 nigel 73 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1465     gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1466 nigel 87 of each entry; both of these return an int value. The entry size
1467     depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1468     a pointer to the first entry of the table (a pointer to char). The
1469 nigel 73 first two bytes of each entry are the number of the capturing parenthe-
1470 nigel 87 sis, most significant byte first. The rest of the entry is the corre-
1471     sponding name, zero terminated. The names are in alphabetical order.
1472 nigel 91 When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1473     theses numbers. For example, consider the following pattern (assume
1474     PCRE_EXTENDED is set, so white space - including newlines - is
1475     ignored):
1476 nigel 63
1477 nigel 93 (?<date> (?<year>(\d\d)?\d\d) -
1478     (?<month>\d\d) - (?<day>\d\d) )
1479 nigel 63
1480 nigel 87 There are four named subpatterns, so the table has four entries, and
1481     each entry in the table is eight bytes long. The table is as follows,
1482 nigel 75 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1483     as ??:
1484 nigel 63
1485 nigel 73 00 01 d a t e 00 ??
1486     00 05 d a y 00 ?? ??
1487     00 04 m o n t h 00
1488     00 02 y e a r 00 ??
1489 nigel 63
1490 nigel 87 When writing code to extract data from named subpatterns using the
1491 nigel 91 name-to-number map, remember that the length of the entries is likely
1492     to be different for each compiled pattern.
1493 nigel 63
1494 nigel 73 PCRE_INFO_OPTIONS
1495 nigel 63
1496 nigel 87 Return a copy of the options with which the pattern was compiled. The
1497     fourth argument should point to an unsigned long int variable. These
1498 nigel 73 option bits are those specified in the call to pcre_compile(), modified
1499     by any top-level option settings within the pattern itself.
1500 nigel 63
1501 nigel 87 A pattern is automatically anchored by PCRE if all of its top-level
1502 nigel 73 alternatives begin with one of the following:
1503 nigel 63
1504 nigel 73 ^ unless PCRE_MULTILINE is set
1505     \A always
1506     \G always
1507     .* if PCRE_DOTALL is set and there are no back
1508     references to the subpattern in which .* appears
1509 nigel 63
1510 nigel 73 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1511     by pcre_fullinfo().
1512 nigel 63
1513 nigel 73 PCRE_INFO_SIZE
1514 nigel 63
1515 nigel 87 Return the size of the compiled pattern, that is, the value that was
1516 nigel 73 passed as the argument to pcre_malloc() when PCRE was getting memory in
1517     which to place the compiled data. The fourth argument should point to a
1518     size_t variable.
1519 nigel 63
1520 nigel 73 PCRE_INFO_STUDYSIZE
1521 nigel 63
1522 nigel 75 Return the size of the data block pointed to by the study_data field in
1523 nigel 87 a pcre_extra block. That is, it is the value that was passed to
1524 nigel 73 pcre_malloc() when PCRE was getting memory into which to place the data
1525 nigel 87 created by pcre_study(). The fourth argument should point to a size_t
1526 nigel 73 variable.
1527 nigel 63
1528 nigel 73
1529 nigel 63 OBSOLETE INFO FUNCTION
1530    
1531 nigel 73 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1532 nigel 63
1533 nigel 87 The pcre_info() function is now obsolete because its interface is too
1534     restrictive to return all the available data about a compiled pattern.
1535     New programs should use pcre_fullinfo() instead. The yield of
1536     pcre_info() is the number of capturing subpatterns, or one of the fol-
1537 nigel 73 lowing negative numbers:
1538 nigel 43
1539 nigel 73 PCRE_ERROR_NULL the argument code was NULL
1540     PCRE_ERROR_BADMAGIC the "magic number" was not found
1541 nigel 43
1542 nigel 87 If the optptr argument is not NULL, a copy of the options with which
1543     the pattern was compiled is placed in the integer it points to (see
1544 nigel 73 PCRE_INFO_OPTIONS above).
1545 nigel 43
1546 nigel 87 If the pattern is not anchored and the firstcharptr argument is not
1547     NULL, it is used to pass back information about the first character of
1548 nigel 73 any matched string (see PCRE_INFO_FIRSTBYTE above).
1549 nigel 43
1550    
1551 nigel 77 REFERENCE COUNTS
1552 nigel 53
1553 nigel 77 int pcre_refcount(pcre *code, int adjust);
1554    
1555 nigel 87 The pcre_refcount() function is used to maintain a reference count in
1556 nigel 77 the data block that contains a compiled pattern. It is provided for the
1557 nigel 87 benefit of applications that operate in an object-oriented manner,
1558 nigel 77 where different parts of the application may be using the same compiled
1559     pattern, but you want to free the block when they are all done.
1560    
1561     When a pattern is compiled, the reference count field is initialized to
1562 nigel 87 zero. It is changed only by calling this function, whose action is to
1563     add the adjust value (which may be positive or negative) to it. The
1564 nigel 77 yield of the function is the new value. However, the value of the count
1565 nigel 87 is constrained to lie between 0 and 65535, inclusive. If the new value
1566 nigel 77 is outside these limits, it is forced to the appropriate limit value.
1567    
1568 nigel 87 Except when it is zero, the reference count is not correctly preserved
1569     if a pattern is compiled on one host and then transferred to a host
1570 nigel 77 whose byte-order is different. (This seems a highly unlikely scenario.)
1571    
1572    
1573     MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1574    
1575 nigel 73 int pcre_exec(const pcre *code, const pcre_extra *extra,
1576     const char *subject, int length, int startoffset,
1577     int options, int *ovector, int ovecsize);
1578 nigel 53
1579 nigel 87 The function pcre_exec() is called to match a subject string against a
1580     compiled pattern, which is passed in the code argument. If the pattern
1581 nigel 75 has been studied, the result of the study should be passed in the extra
1582 nigel 87 argument. This function is the main matching facility of the library,
1583 nigel 77 and it operates in a Perl-like manner. For specialist use there is also
1584 nigel 87 an alternative matching function, which is described below in the sec-
1585 nigel 77 tion about the pcre_dfa_exec() function.
1586 nigel 41
1587 nigel 87 In most applications, the pattern will have been compiled (and option-
1588     ally studied) in the same process that calls pcre_exec(). However, it
1589 nigel 75 is possible to save compiled patterns and study data, and then use them
1590 nigel 87 later in different processes, possibly even on different hosts. For a
1591 nigel 75 discussion about this, see the pcreprecompile documentation.
1592    
1593 nigel 73 Here is an example of a simple call to pcre_exec():
1594 nigel 53
1595 nigel 73 int rc;
1596     int ovector[30];
1597     rc = pcre_exec(
1598     re, /* result of pcre_compile() */
1599     NULL, /* we didn't study the pattern */
1600     "some string", /* the subject string */
1601     11, /* the length of the subject string */
1602     0, /* start at offset 0 in the subject */
1603     0, /* default options */
1604 nigel 75 ovector, /* vector of integers for substring information */
1605 nigel 77 30); /* number of elements (NOT size in bytes) */
1606 nigel 53
1607 nigel 75 Extra data for pcre_exec()
1608 nigel 63
1609 nigel 87 If the extra argument is not NULL, it must point to a pcre_extra data
1610     block. The pcre_study() function returns such a block (when it doesn't
1611     return NULL), but you can also create one for yourself, and pass addi-
1612     tional information in it. The pcre_extra block contains the following
1613     fields (not necessarily in this order):
1614 nigel 75
1615 nigel 73 unsigned long int flags;
1616     void *study_data;
1617     unsigned long int match_limit;
1618 nigel 87 unsigned long int match_limit_recursion;
1619 nigel 73 void *callout_data;
1620 nigel 75 const unsigned char *tables;
1621 nigel 63
1622 nigel 87 The flags field is a bitmap that specifies which of the other fields
1623 nigel 73 are set. The flag bits are:
1624 nigel 63
1625 nigel 73 PCRE_EXTRA_STUDY_DATA
1626     PCRE_EXTRA_MATCH_LIMIT
1627 nigel 87 PCRE_EXTRA_MATCH_LIMIT_RECURSION
1628 nigel 73 PCRE_EXTRA_CALLOUT_DATA
1629 nigel 75 PCRE_EXTRA_TABLES
1630 nigel 63
1631 nigel 87 Other flag bits should be set to zero. The study_data field is set in
1632     the pcre_extra block that is returned by pcre_study(), together with
1633 nigel 75 the appropriate flag bit. You should not set this yourself, but you may
1634 nigel 87 add to the block by setting the other fields and their corresponding
1635 nigel 75 flag bits.
1636 nigel 63
1637 nigel 73 The match_limit field provides a means of preventing PCRE from using up
1638 nigel 87 a vast amount of resources when running patterns that are not going to
1639     match, but which have a very large number of possibilities in their
1640     search trees. The classic example is the use of nested unlimited
1641 nigel 75 repeats.
1642 nigel 63
1643 nigel 87 Internally, PCRE uses a function called match() which it calls repeat-
1644     edly (sometimes recursively). The limit set by match_limit is imposed
1645     on the number of times this function is called during a match, which
1646     has the effect of limiting the amount of backtracking that can take
1647     place. For patterns that are not anchored, the count restarts from zero
1648     for each position in the subject string.
1649 nigel 75
1650 nigel 87 The default value for the limit can be set when PCRE is built; the
1651     default default is 10 million, which handles all but the most extreme
1652     cases. You can override the default by suppling pcre_exec() with a
1653     pcre_extra block in which match_limit is set, and
1654     PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1655 nigel 73 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1656 nigel 63
1657 nigel 87 The match_limit_recursion field is similar to match_limit, but instead
1658     of limiting the total number of times that match() is called, it limits
1659     the depth of recursion. The recursion depth is a smaller number than
1660     the total number of calls, because not all calls to match() are recur-
1661     sive. This limit is of use only if it is set smaller than match_limit.
1662    
1663     Limiting the recursion depth limits the amount of stack that can be
1664     used, or, when PCRE has been compiled to use memory on the heap instead
1665     of the stack, the amount of heap memory that can be used.
1666    
1667     The default value for match_limit_recursion can be set when PCRE is
1668     built; the default default is the same value as the default for
1669     match_limit. You can override the default by suppling pcre_exec() with
1670     a pcre_extra block in which match_limit_recursion is set, and
1671     PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1672     limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1673    
1674     The pcre_callout field is used in conjunction with the "callout" fea-
1675 nigel 73 ture, which is described in the pcrecallout documentation.
1676 nigel 63
1677 nigel 87 The tables field is used to pass a character tables pointer to
1678     pcre_exec(); this overrides the value that is stored with the compiled
1679     pattern. A non-NULL value is stored with the compiled pattern only if
1680     custom tables were supplied to pcre_compile() via its tableptr argu-
1681 nigel 75 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1682 nigel 87 PCRE's internal tables to be used. This facility is helpful when re-
1683     using patterns that have been saved after compiling with an external
1684     set of tables, because the external tables might be at a different
1685     address when pcre_exec() is called. See the pcreprecompile documenta-
1686 nigel 75 tion for a discussion of saving compiled patterns for later use.
1687 nigel 41
1688 nigel 75 Option bits for pcre_exec()
1689 nigel 71
1690 nigel 87 The unused bits of the options argument for pcre_exec() must be zero.
1691 nigel 91 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1692     PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
1693     PCRE_PARTIAL.
1694 nigel 41
1695 nigel 75 PCRE_ANCHORED
1696 nigel 41
1697 nigel 91 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
1698     matching position. If a pattern was compiled with PCRE_ANCHORED, or
1699     turned out to be anchored by virtue of its contents, it cannot be made
1700 nigel 75 unachored at matching time.
1701    
1702 nigel 91 PCRE_NEWLINE_CR
1703     PCRE_NEWLINE_LF
1704     PCRE_NEWLINE_CRLF
1705 nigel 93 PCRE_NEWLINE_ANY
1706 nigel 91
1707     These options override the newline definition that was chosen or
1708     defaulted when the pattern was compiled. For details, see the descrip-
1709 nigel 93 tion of pcre_compile() above. During matching, the newline choice
1710     affects the behaviour of the dot, circumflex, and dollar metacharac-
1711     ters. It may also alter the way the match position is advanced after a
1712     match failure for an unanchored pattern. When PCRE_NEWLINE_CRLF or
1713     PCRE_NEWLINE_ANY is set, and a match attempt fails when the current
1714     position is at a CRLF sequence, the match position is advanced by two
1715     characters instead of one, in other words, to after the CRLF.
1716 nigel 91
1717 nigel 73 PCRE_NOTBOL
1718 nigel 41
1719 nigel 75 This option specifies that first character of the subject string is not
1720 nigel 93 the beginning of a line, so the circumflex metacharacter should not
1721     match before it. Setting this without PCRE_MULTILINE (at compile time)
1722     causes circumflex never to match. This option affects only the behav-
1723 nigel 77 iour of the circumflex metacharacter. It does not affect \A.
1724 nigel 41
1725 nigel 73 PCRE_NOTEOL
1726 nigel 41
1727 nigel 75 This option specifies that the end of the subject string is not the end
1728 nigel 93 of a line, so the dollar metacharacter should not match it nor (except
1729     in multiline mode) a newline immediately before it. Setting this with-
1730 nigel 75 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1731 nigel 93 option affects only the behaviour of the dollar metacharacter. It does
1732 nigel 75 not affect \Z or \z.
1733 nigel 41
1734 nigel 73 PCRE_NOTEMPTY
1735 nigel 41
1736 nigel 73 An empty string is not considered to be a valid match if this option is
1737 nigel 93 set. If there are alternatives in the pattern, they are tried. If all
1738     the alternatives match the empty string, the entire match fails. For
1739 nigel 73 example, if the pattern
1740 nigel 41
1741 nigel 73 a?b?
1742 nigel 41
1743 nigel 93 is applied to a string not beginning with "a" or "b", it matches the
1744     empty string at the start of the subject. With PCRE_NOTEMPTY set, this
1745 nigel 73 match is not valid, so PCRE searches further into the string for occur-
1746     rences of "a" or "b".
1747 nigel 41
1748 nigel 73 Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1749 nigel 93 cial case of a pattern match of the empty string within its split()
1750     function, and when using the /g modifier. It is possible to emulate
1751 nigel 73 Perl's behaviour after matching a null string by first trying the match
1752 nigel 75 again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1753 nigel 93 if that fails by advancing the starting offset (see below) and trying
1754 nigel 75 an ordinary match again. There is some code that demonstrates how to do
1755     this in the pcredemo.c sample program.
1756 nigel 41
1757 nigel 75 PCRE_NO_UTF8_CHECK
1758    
1759     When PCRE_UTF8 is set at compile time, the validity of the subject as a
1760 nigel 93 UTF-8 string is automatically checked when pcre_exec() is subsequently
1761     called. The value of startoffset is also checked to ensure that it
1762     points to the start of a UTF-8 character. If an invalid UTF-8 sequence
1763 nigel 75 of bytes is found, pcre_exec() returns the error PCRE_ERROR_BADUTF8. If
1764 nigel 93 startoffset contains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is
1765 nigel 75 returned.
1766    
1767 nigel 93 If you already know that your subject is valid, and you want to skip
1768     these checks for performance reasons, you can set the
1769     PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
1770     do this for the second and subsequent calls to pcre_exec() if you are
1771     making repeated calls to find all the matches in a single subject
1772     string. However, you should be sure that the value of startoffset
1773     points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1774     set, the effect of passing an invalid UTF-8 string as a subject, or a
1775     value of startoffset that does not point to the start of a UTF-8 char-
1776 nigel 75 acter, is undefined. Your program may crash.
1777    
1778     PCRE_PARTIAL
1779    
1780 nigel 93 This option turns on the partial matching feature. If the subject
1781     string fails to match the pattern, but at some point during the match-
1782     ing process the end of the subject was reached (that is, the subject
1783     partially matches the pattern and the failure to match occurred only
1784     because there were not enough subject characters), pcre_exec() returns
1785     PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1786     used, there are restrictions on what may appear in the pattern. These
1787 nigel 75 are discussed in the pcrepartial documentation.
1788    
1789     The string to be matched by pcre_exec()
1790    
1791 nigel 93 The subject string is passed to pcre_exec() as a pointer in subject, a
1792     length in length, and a starting byte offset in startoffset. In UTF-8
1793     mode, the byte offset must point to the start of a UTF-8 character.
1794     Unlike the pattern string, the subject may contain binary zero bytes.
1795     When the starting offset is zero, the search for a match starts at the
1796 nigel 75 beginning of the subject, and this is by far the most common case.
1797 nigel 63
1798 nigel 93 A non-zero starting offset is useful when searching for another match
1799     in the same subject by calling pcre_exec() again after a previous suc-
1800     cess. Setting startoffset differs from just passing over a shortened
1801     string and setting PCRE_NOTBOL in the case of a pattern that begins
1802 nigel 73 with any kind of lookbehind. For example, consider the pattern
1803 nigel 41
1804 nigel 73 \Biss\B
1805 nigel 41
1806 nigel 93 which finds occurrences of "iss" in the middle of words. (\B matches
1807     only if the current position in the subject is not a word boundary.)
1808     When applied to the string "Mississipi" the first call to pcre_exec()
1809     finds the first occurrence. If pcre_exec() is called again with just
1810     the remainder of the subject, namely "issipi", it does not match,
1811 nigel 73 because \B is always false at the start of the subject, which is deemed
1812 nigel 93 to be a word boundary. However, if pcre_exec() is passed the entire
1813 nigel 75 string again, but with startoffset set to 4, it finds the second occur-
1814 nigel 93 rence of "iss" because it is able to look behind the starting point to
1815 nigel 75 discover that it is preceded by a letter.
1816 nigel 41
1817 nigel 93 If a non-zero starting offset is passed when the pattern is anchored,
1818 nigel 75 one attempt to match at the given offset is made. This can only succeed
1819 nigel 93 if the pattern does not require the match to be at the start of the
1820 nigel 75 subject.
1821 nigel 41
1822 nigel 75 How pcre_exec() returns captured substrings
1823    
1824 nigel 93 In general, a pattern matches a certain portion of the subject, and in
1825     addition, further substrings from the subject may be picked out by
1826     parts of the pattern. Following the usage in Jeffrey Friedl's book,
1827     this is called "capturing" in what follows, and the phrase "capturing
1828     subpattern" is used for a fragment of a pattern that picks out a sub-
1829     string. PCRE supports several other kinds of parenthesized subpattern
1830 nigel 73 that do not cause substrings to be captured.
1831 nigel 65
1832 nigel 93 Captured substrings are returned to the caller via a vector of integer
1833     offsets whose address is passed in ovector. The number of elements in
1834     the vector is passed in ovecsize, which must be a non-negative number.
1835 nigel 75 Note: this argument is NOT the size of ovector in bytes.
1836 nigel 41
1837 nigel 93 The first two-thirds of the vector is used to pass back captured sub-
1838     strings, each substring using a pair of integers. The remaining third
1839     of the vector is used as workspace by pcre_exec() while matching cap-
1840     turing subpatterns, and is not available for passing back information.
1841     The length passed in ovecsize should always be a multiple of three. If
1842 nigel 75 it is not, it is rounded down.
1843    
1844 nigel 93 When a match is successful, information about captured substrings is
1845     returned in pairs of integers, starting at the beginning of ovector,
1846     and continuing up to two-thirds of its length at the most. The first
1847 nigel 73 element of a pair is set to the offset of the first character in a sub-
1848 nigel 93 string, and the second is set to the offset of the first character
1849     after the end of a substring. The first pair, ovector[0] and ovec-
1850     tor[1], identify the portion of the subject string matched by the
1851     entire pattern. The next pair is used for the first capturing subpat-
1852 nigel 91 tern, and so on. The value returned by pcre_exec() is one more than the
1853     highest numbered pair that has been set. For example, if two substrings
1854 nigel 93 have been captured, the returned value is 3. If there are no capturing
1855     subpatterns, the return value from a successful match is 1, indicating
1856 nigel 91 that just the first pair of offsets has been set.
1857 nigel 41
1858 nigel 73 If a capturing subpattern is matched repeatedly, it is the last portion
1859 nigel 75 of the string that it matched that is returned.
1860 nigel 41
1861 nigel 93 If the vector is too small to hold all the captured substring offsets,
1862 nigel 75 it is used as far as possible (up to two-thirds of its length), and the
1863 nigel 93 function returns a value of zero. In particular, if the substring off-
1864 nigel 75 sets are not of interest, pcre_exec() may be called with ovector passed
1865 nigel 93 as NULL and ovecsize as zero. However, if the pattern contains back
1866     references and the ovector is not big enough to remember the related
1867     substrings, PCRE has to get additional memory for use during matching.
1868 nigel 73 Thus it is usually advisable to supply an ovector.
1869 nigel 41
1870 nigel 93 The pcre_info() function can be used to find out how many capturing
1871     subpatterns there are in a compiled pattern. The smallest size for
1872     ovector that will allow for n captured substrings, in addition to the
1873 nigel 91 offsets of the substring matched by the whole pattern, is (n+1)*3.
1874 nigel 41
1875 nigel 93 It is possible for capturing subpattern number n+1 to match some part
1876 nigel 91 of the subject when subpattern n has not been used at all. For example,
1877 nigel 93 if the string "abc" is matched against the pattern (a|(z))(bc) the
1878 nigel 91 return from the function is 4, and subpatterns 1 and 3 are matched, but
1879 nigel 93 2 is not. When this happens, both values in the offset pairs corre-
1880 nigel 91 sponding to unused subpatterns are set to -1.
1881 nigel 75
1882 nigel 93 Offset values that correspond to unused subpatterns at the end of the
1883     expression are also set to -1. For example, if the string "abc" is
1884     matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
1885     matched. The return from the function is 2, because the highest used
1886 nigel 91 capturing subpattern number is 1. However, you can refer to the offsets
1887 nigel 93 for the second and third capturing subpatterns if you wish (assuming
1888 nigel 91 the vector is large enough, of course).
1889    
1890 nigel 93 Some convenience functions are provided for extracting the captured
1891 nigel 91 substrings as separate strings. These are described below.
1892    
1893     Error return values from pcre_exec()
1894    
1895 nigel 93 If pcre_exec() fails, it returns a negative number. The following are
1896 nigel 73 defined in the header file:
1897 nigel 41
1898 nigel 73 PCRE_ERROR_NOMATCH (-1)
1899 nigel 41
1900 nigel 73 The subject string did not match the pattern.
1901 nigel 41
1902 nigel 73 PCRE_ERROR_NULL (-2)
1903 nigel 41
1904 nigel 93 Either code or subject was passed as NULL, or ovector was NULL and
1905 nigel 73 ovecsize was not zero.
1906 nigel 41
1907 nigel 73 PCRE_ERROR_BADOPTION (-3)
1908 nigel 41
1909 nigel 73 An unrecognized bit was set in the options argument.
1910 nigel 41
1911 nigel 73 PCRE_ERROR_BADMAGIC (-4)
1912 nigel 41
1913 nigel 93 PCRE stores a 4-byte "magic number" at the start of the compiled code,
1914 nigel 75 to catch the case when it is passed a junk pointer and to detect when a
1915     pattern that was compiled in an environment of one endianness is run in
1916 nigel 93 an environment with the other endianness. This is the error that PCRE
1917 nigel 75 gives when the magic number is not present.
1918 nigel 41
1919 nigel 93 PCRE_ERROR_UNKNOWN_OPCODE (-5)
1920 nigel 41
1921 nigel 73 While running the pattern match, an unknown item was encountered in the
1922 nigel 93 compiled pattern. This error could be caused by a bug in PCRE or by
1923 nigel 73 overwriting of the compiled pattern.
1924 nigel 41
1925 nigel 73 PCRE_ERROR_NOMEMORY (-6)
1926 nigel 41
1927 nigel 93 If a pattern contains back references, but the ovector that is passed
1928 nigel 73 to pcre_exec() is not big enough to remember the referenced substrings,
1929 nigel 93 PCRE gets a block of memory at the start of matching to use for this
1930     purpose. If the call via pcre_malloc() fails, this error is given. The
1931 nigel 75 memory is automatically freed at the end of matching.
1932 nigel 41
1933 nigel 73 PCRE_ERROR_NOSUBSTRING (-7)
1934 nigel 53
1935 nigel 93 This error is used by the pcre_copy_substring(), pcre_get_substring(),
1936 nigel 73 and pcre_get_substring_list() functions (see below). It is never
1937     returned by pcre_exec().
1938 nigel 63
1939 nigel 73 PCRE_ERROR_MATCHLIMIT (-8)
1940 nigel 63
1941 nigel 93 The backtracking limit, as specified by the match_limit field in a
1942     pcre_extra structure (or defaulted) was reached. See the description
1943 nigel 87 above.
1944    
1945 nigel 73 PCRE_ERROR_CALLOUT (-9)
1946 nigel 63
1947 nigel 73 This error is never generated by pcre_exec() itself. It is provided for
1948 nigel 93 use by callout functions that want to yield a distinctive error code.
1949 nigel 73 See the pcrecallout documentation for details.
1950 nigel 71
1951 nigel 73 PCRE_ERROR_BADUTF8 (-10)
1952 nigel 71
1953 nigel 93 A string that contains an invalid UTF-8 byte sequence was passed as a
1954 nigel 73 subject.
1955    
1956     PCRE_ERROR_BADUTF8_OFFSET (-11)
1957    
1958     The UTF-8 byte sequence that was passed as a subject was valid, but the
1959 nigel 93 value of startoffset did not point to the beginning of a UTF-8 charac-
1960 nigel 73 ter.
1961    
1962 nigel 77 PCRE_ERROR_PARTIAL (-12)
1963 nigel 73
1964 nigel 93 The subject string did not match, but it did match partially. See the
1965 nigel 75 pcrepartial documentation for details of partial matching.
1966    
1967 nigel 77 PCRE_ERROR_BADPARTIAL (-13)
1968 nigel 75
1969 nigel 93 The PCRE_PARTIAL option was used with a compiled pattern containing
1970     items that are not supported for partial matching. See the pcrepartial
1971 nigel 75 documentation for details of partial matching.
1972    
1973 nigel 77 PCRE_ERROR_INTERNAL (-14)
1974 nigel 75
1975 nigel 93 An unexpected internal error has occurred. This error could be caused
1976 nigel 75 by a bug in PCRE or by overwriting of the compiled pattern.
1977    
1978 nigel 77 PCRE_ERROR_BADCOUNT (-15)
1979 nigel 75
1980 nigel 93 This error is given if the value of the ovecsize argument is negative.
1981 nigel 75
1982 nigel 93 PCRE_ERROR_RECURSIONLIMIT (-21)
1983 nigel 75
1984 nigel 93 The internal recursion limit, as specified by the match_limit_recursion
1985     field in a pcre_extra structure (or defaulted) was reached. See the
1986     description above.
1987    
1988     PCRE_ERROR_NULLWSLIMIT (-22)
1989    
1990     When a group that can match an empty substring is repeated with an
1991     unbounded upper limit, the subject position at the start of the group
1992     must be remembered, so that a test for an empty string can be made when
1993     the end of the group is reached. Some workspace is required for this;
1994     if it runs out, this error is given.
1995    
1996     PCRE_ERROR_BADNEWLINE (-23)
1997    
1998     An invalid combination of PCRE_NEWLINE_xxx options was given.
1999    
2000     Error numbers -16 to -20 are not used by pcre_exec().
2001    
2002    
2003 nigel 63 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2004    
2005 nigel 73 int pcre_copy_substring(const char *subject, int *ovector,
2006     int stringcount, int stringnumber, char *buffer,
2007     int buffersize);
2008 nigel 63
2009 nigel 73 int pcre_get_substring(const char *subject, int *ovector,
2010     int stringcount, int stringnumber,
2011     const char **stringptr);
2012 nigel 63
2013 nigel 73 int pcre_get_substring_list(const char *subject,
2014     int *ovector, int stringcount, const char ***listptr);
2015 nigel 63
2016 nigel 93 Captured substrings can be accessed directly by using the offsets
2017     returned by pcre_exec() in ovector. For convenience, the functions
2018 nigel 73 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2019 nigel 93 string_list() are provided for extracting captured substrings as new,
2020     separate, zero-terminated strings. These functions identify substrings
2021     by number. The next section describes functions for extracting named
2022 nigel 91 substrings.
2023 nigel 41
2024 nigel 93 A substring that contains a binary zero is correctly extracted and has
2025     a further zero added on the end, but the result is not, of course, a C
2026     string. However, you can process such a string by referring to the
2027     length that is returned by pcre_copy_substring() and pcre_get_sub-
2028 nigel 91 string(). Unfortunately, the interface to pcre_get_substring_list() is
2029 nigel 93 not adequate for handling strings containing binary zeros, because the
2030 nigel 91 end of the final string is not independently indicated.
2031    
2032 nigel 93 The first three arguments are the same for all three of these func-
2033     tions: subject is the subject string that has just been successfully
2034 nigel 73 matched, ovector is a pointer to the vector of integer offsets that was
2035     passed to pcre_exec(), and stringcount is the number of substrings that
2036 nigel 93 were captured by the match, including the substring that matched the
2037 nigel 75 entire regular expression. This is the value returned by pcre_exec() if
2038 nigel 93 it is greater than zero. If pcre_exec() returned zero, indicating that
2039     it ran out of space in ovector, the value passed as stringcount should
2040 nigel 75 be the number of elements in the vector divided by three.
2041 nigel 41
2042 nigel 93 The functions pcre_copy_substring() and pcre_get_substring() extract a
2043     single substring, whose number is given as stringnumber. A value of
2044     zero extracts the substring that matched the entire pattern, whereas
2045     higher values extract the captured substrings. For pcre_copy_sub-
2046     string(), the string is placed in buffer, whose length is given by
2047     buffersize, while for pcre_get_substring() a new block of memory is
2048     obtained via pcre_malloc, and its address is returned via stringptr.
2049     The yield of the function is the length of the string, not including
2050     the terminating zero, or one of these error codes:
2051 nigel 41
2052 nigel 73 PCRE_ERROR_NOMEMORY (-6)
2053 nigel 41
2054 nigel 93 The buffer was too small for pcre_copy_substring(), or the attempt to
2055 nigel 73 get memory failed for pcre_get_substring().
2056 nigel 41
2057 nigel 73 PCRE_ERROR_NOSUBSTRING (-7)
2058 nigel 41
2059 nigel 73 There is no substring whose number is stringnumber.
2060 nigel 41
2061 nigel 93 The pcre_get_substring_list() function extracts all available sub-
2062     strings and builds a list of pointers to them. All this is done in a
2063 nigel 75 single block of memory that is obtained via pcre_malloc. The address of
2064 nigel 93 the memory block is returned via listptr, which is also the start of
2065     the list of string pointers. The end of the list is marked by a NULL
2066     pointer. The yield of the function is zero if all went well, or the
2067     error code
2068 nigel 41
2069 nigel 73 PCRE_ERROR_NOMEMORY (-6)
2070 nigel 41
2071 nigel 73 if the attempt to get the memory block failed.
2072 nigel 41
2073 nigel 91 When any of these functions encounter a substring that is unset, which
2074     can happen when capturing subpattern number n+1 matches some part of
2075     the subject, but subpattern n has not been used at all, they return an
2076 nigel 73 empty string. This can be distinguished from a genuine zero-length sub-
2077 nigel 91 string by inspecting the appropriate offset in ovector, which is nega-
2078 nigel 73 tive for unset substrings.
2079 nigel 41
2080 nigel 91 The two convenience functions pcre_free_substring() and pcre_free_sub-
2081     string_list() can be used to free the memory returned by a previous
2082 nigel 75 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2083 nigel 91 tively. They do nothing more than call the function pointed to by
2084     pcre_free, which of course could be called directly from a C program.
2085     However, PCRE is used in some situations where it is linked via a spe-
2086     cial interface to another programming language that cannot use
2087     pcre_free directly; it is for these cases that the functions are pro-
2088 nigel 77 vided.
2089 nigel 41
2090 nigel 73
2091 nigel 63 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2092 nigel 41
2093 nigel 75 int pcre_get_stringnumber(const pcre *code,
2094     const char *name);
2095    
2096 nigel 73 int pcre_copy_named_substring(const pcre *code,
2097     const char *subject, int *ovector,
2098     int stringcount, const char *stringname,
2099     char *buffer, int buffersize);
2100 nigel 41
2101 nigel 73 int pcre_get_named_substring(const pcre *code,
2102     const char *subject, int *ovector,
2103     int stringcount, const char *stringname,
2104     const char **stringptr);
2105 nigel 41
2106 nigel 91 To extract a substring by name, you first have to find associated num-
2107 nigel 75 ber. For example, for this pattern
2108 nigel 41
2109 nigel 93 (a+)b(?<xxx>\d+)...
2110 nigel 63
2111 nigel 91 the number of the subpattern called "xxx" is 2. If the name is known to
2112     be unique (PCRE_DUPNAMES was not set), you can find the number from the
2113     name by calling pcre_get_stringnumber(). The first argument is the com-
2114     piled pattern, and the second is the name. The yield of the function is
2115     the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2116     subpattern of that name.
2117 nigel 63
2118 nigel 75 Given the number, you can extract the substring directly, or use one of
2119     the functions described in the previous section. For convenience, there
2120     are also two functions that do the whole job.
2121    
2122 nigel 87 Most of the arguments of pcre_copy_named_substring() and
2123     pcre_get_named_substring() are the same as those for the similarly
2124     named functions that extract by number. As these are described in the
2125     previous section, they are not re-described here. There are just two
2126 nigel 75 differences:
2127 nigel 63
2128 nigel 87 First, instead of a substring number, a substring name is given. Sec-
2129 nigel 73 ond, there is an extra argument, given at the start, which is a pointer
2130 nigel 87 to the compiled pattern. This is needed in order to gain access to the
2131 nigel 73 name-to-number translation table.
2132 nigel 63
2133 nigel 87 These functions call pcre_get_stringnumber(), and if it succeeds, they
2134     then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2135 nigel 73 ate.
2136 nigel 63
2137 nigel 77
2138 nigel 91 DUPLICATE SUBPATTERN NAMES
2139    
2140     int pcre_get_stringtable_entries(const pcre *code,
2141     const char *name, char **first, char **last);
2142    
2143     When a pattern is compiled with the PCRE_DUPNAMES option, names for
2144     subpatterns are not required to be unique. Normally, patterns with
2145     duplicate names are such that in any one match, only one of the named
2146     subpatterns participates. An example is shown in the pcrepattern docu-
2147     mentation. When duplicates are present, pcre_copy_named_substring() and
2148     pcre_get_named_substring() return the first substring corresponding to
2149     the given name that is set. If none are set, an empty string is
2150     returned. The pcre_get_stringnumber() function returns one of the num-
2151     bers that are associated with the name, but it is not defined which it
2152     is.
2153    
2154     If you want to get full details of all captured substrings for a given
2155     name, you must use the pcre_get_stringtable_entries() function. The
2156     first argument is the compiled pattern, and the second is the name. The
2157     third and fourth are pointers to variables which are updated by the
2158     function. After it has run, they point to the first and last entries in
2159     the name-to-number table for the given name. The function itself
2160 nigel 93 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2161     there are none. The format of the table is described above in the sec-
2162     tion entitled Information about a pattern. Given all the relevant
2163     entries for the name, you can extract each of their numbers, and hence
2164     the captured data, if any.
2165 nigel 91
2166    
2167 nigel 77 FINDING ALL POSSIBLE MATCHES
2168    
2169 nigel 91 The traditional matching function uses a similar algorithm to Perl,
2170 nigel 77 which stops when it finds the first match, starting at a given point in
2171 nigel 91 the subject. If you want to find all possible matches, or the longest
2172     possible match, consider using the alternative matching function (see
2173     below) instead. If you cannot use the alternative function, but still
2174     need to find all possible matches, you can kludge it up by making use
2175 nigel 77 of the callout facility, which is described in the pcrecallout documen-
2176     tation.
2177    
2178     What you have to do is to insert a callout right at the end of the pat-
2179 nigel 91 tern. When your callout function is called, extract and save the cur-
2180     rent matched substring. Then return 1, which forces pcre_exec() to
2181     backtrack and try other alternatives. Ultimately, when it runs out of
2182 nigel 77 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2183    
2184    
2185     MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2186    
2187     int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2188     const char *subject, int length, int startoffset,
2189     int options, int *ovector, int ovecsize,
2190     int *workspace, int wscount);
2191    
2192 nigel 91 The function pcre_dfa_exec() is called to match a subject string
2193 nigel 93 against a compiled pattern, using a matching algorithm that scans the
2194     subject string just once, and does not backtrack. This has different
2195     characteristics to the normal algorithm, and is not compatible with
2196     Perl. Some of the features of PCRE patterns are not supported. Never-
2197     theless, there are times when this kind of matching can be useful. For
2198     a discussion of the two matching algorithms, see the pcrematching docu-
2199     mentation.
2200 nigel 77
2201 nigel 93 The arguments for the pcre_dfa_exec() function are the same as for
2202 nigel 77 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2203 nigel 93 ent way, and this is described below. The other common arguments are
2204     used in the same way as for pcre_exec(), so their description is not
2205 nigel 77 repeated here.
2206    
2207 nigel 93 The two additional arguments provide workspace for the function. The
2208     workspace vector should contain at least 20 elements. It is used for
2209 nigel 77 keeping track of multiple paths through the pattern tree. More
2210 nigel 93 workspace will be needed for patterns and subjects where there are a
2211 nigel 91 lot of potential matches.
2212 nigel 77
2213 nigel 87 Here is an example of a simple call to pcre_dfa_exec():
2214 nigel 77
2215     int rc;
2216     int ovector[10];
2217     int wspace[20];
2218 nigel 87 rc = pcre_dfa_exec(
2219 nigel 77 re, /* result of pcre_compile() */
2220     NULL, /* we didn't study the pattern */
2221     "some string", /* the subject string */
2222     11, /* the length of the subject string */
2223     0, /* start at offset 0 in the subject */
2224     0, /* default options */
2225     ovector, /* vector of integers for substring information */
2226     10, /* number of elements (NOT size in bytes) */
2227     wspace, /* working space vector */
2228     20); /* number of elements (NOT size in bytes) */
2229    
2230     Option bits for pcre_dfa_exec()
2231    
2232 nigel 93 The unused bits of the options argument for pcre_dfa_exec() must be
2233     zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2234     LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2235 nigel 91 PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2236     three of these are the same as for pcre_exec(), so their description is
2237     not repeated here.
2238 nigel 77
2239     PCRE_PARTIAL
2240    
2241 nigel 93 This has the same general effect as it does for pcre_exec(), but the
2242     details are slightly different. When PCRE_PARTIAL is set for
2243     pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
2244     PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
2245 nigel 77 been no complete matches, but there is still at least one matching pos-
2246 nigel 93 sibility. The portion of the string that provided the partial match is
2247 nigel 77 set as the first matching string.
2248    
2249     PCRE_DFA_SHORTEST
2250    
2251 nigel 93 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2252     stop as soon as it has found one match. Because of the way the alterna-
2253     tive algorithm works, this is necessarily the shortest possible match
2254     at the first possible matching point in the subject string.
2255 nigel 77
2256     PCRE_DFA_RESTART
2257    
2258 nigel 93 When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
2259     returns a partial match, it is possible to call it again, with addi-
2260     tional subject characters, and have it continue with the same match.
2261     The PCRE_DFA_RESTART option requests this action; when it is set, the
2262     workspace and wscount options must reference the same vector as before
2263     because data about the match so far is left in them after a partial
2264     match. There is more discussion of this facility in the pcrepartial
2265 nigel 77 documentation.
2266    
2267     Successful returns from pcre_dfa_exec()
2268    
2269 nigel 93 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2270 nigel 77 string in the subject. Note, however, that all the matches from one run
2271 nigel 93 of the function start at the same point in the subject. The shorter
2272     matches are all initial substrings of the longer matches. For example,
2273 nigel 77 if the pattern
2274    
2275     <.*>
2276    
2277     is matched against the string
2278    
2279     This is <something> <something else> <something further> no more
2280    
2281     the three matched strings are
2282    
2283     <something>
2284     <something> <something else>
2285     <something> <something else> <something further>
2286    
2287 nigel 93 On success, the yield of the function is a number greater than zero,
2288     which is the number of matched substrings. The substrings themselves
2289     are returned in ovector. Each string uses two elements; the first is
2290     the offset to the start, and the second is the offset to the end. In
2291     fact, all the strings have the same start offset. (Space could have
2292     been saved by giving this only once, but it was decided to retain some
2293     compatibility with the way pcre_exec() returns data, even though the
2294     meaning of the strings is different.)
2295 nigel 77
2296     The strings are returned in reverse order of length; that is, the long-
2297 nigel 93 est matching string is given first. If there were too many matches to
2298     fit into ovector, the yield of the function is zero, and the vector is
2299 nigel 77 filled with the longest matches.
2300    
2301     Error returns from pcre_dfa_exec()
2302    
2303 nigel 93 The pcre_dfa_exec() function returns a negative number when it fails.
2304     Many of the errors are the same as for pcre_exec(), and these are
2305     described above. There are in addition the following errors that are
2306 nigel 77 specific to pcre_dfa_exec():
2307    
2308     PCRE_ERROR_DFA_UITEM (-16)
2309    
2310 nigel 93 This return is given if pcre_dfa_exec() encounters an item in the pat-
2311     tern that it does not support, for instance, the use of \C or a back
2312 nigel 77 reference.
2313    
2314     PCRE_ERROR_DFA_UCOND (-17)
2315    
2316 nigel 93 This return is given if pcre_dfa_exec() encounters a condition item
2317     that uses a back reference for the condition, or a test for recursion
2318     in a specific group. These are not supported.
2319 nigel 77
2320     PCRE_ERROR_DFA_UMLIMIT (-18)
2321    
2322 nigel 93 This return is given if pcre_dfa_exec() is called with an extra block
2323 nigel 77 that contains a setting of the match_limit field. This is not supported
2324     (it is meaningless).
2325    
2326     PCRE_ERROR_DFA_WSSIZE (-19)
2327    
2328 nigel 93 This return is given if pcre_dfa_exec() runs out of space in the
2329 nigel 77 workspace vector.
2330    
2331     PCRE_ERROR_DFA_RECURSE (-20)
2332    
2333 nigel 93 When a recursive subpattern is processed, the matching function calls
2334     itself recursively, using private vectors for ovector and workspace.
2335     This error is given if the output vector is not large enough. This
2336 nigel 77 should be extremely rare, as a vector of size 1000 is used.
2337    
2338 nigel 93
2339     SEE ALSO
2340    
2341     pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2342     tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2343    
2344 nigel 63
2345 ph10 99 AUTHOR
2346 nigel 63
2347 ph10 99 Philip Hazel
2348     University Computing Service
2349     Cambridge CB2 3QH, England.
2350    
2351    
2352     REVISION
2353    
2354     Last updated: 06 March 2007
2355     Copyright (c) 1997-2007 University of Cambridge.
2356     ------------------------------------------------------------------------------
2357    
2358    
2359 nigel 79 PCRECALLOUT(3) PCRECALLOUT(3)
2360 nigel 63
2361 nigel 79
2362 nigel 73 NAME
2363     PCRE - Perl-compatible regular expressions
2364    
2365 nigel 77
2366 nigel 63 PCRE CALLOUTS
2367    
2368 nigel 73 int (*pcre_callout)(pcre_callout_block *);
2369 nigel 63
2370 nigel 73 PCRE provides a feature called "callout", which is a means of temporar-
2371     ily passing control to the caller of PCRE in the middle of pattern
2372     matching. The caller of PCRE provides an external function by putting
2373     its entry point in the global variable pcre_callout. By default, this
2374     variable contains NULL, which disables all calling out.
2375 nigel 63
2376 nigel 73 Within a regular expression, (?C) indicates the points at which the
2377     external function is to be called. Different callout points can be
2378     identified by putting a number less than 256 after the letter C. The
2379     default value is zero. For example, this pattern has two callout
2380     points:
2381 nigel 63
2382 nigel 75 (?C1)eabc(?C2)def
2383 nigel 63
2384 nigel 75 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
2385     called, PCRE automatically inserts callouts, all with number 255,
2386     before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2387     used with the pattern
2388 nigel 63
2389 nigel 75 A(\d{2}|--)
2390    
2391     it is processed as if it were
2392    
2393     (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2394    
2395     Notice that there is a callout before and after each parenthesis and
2396     alternation bar. Automatic callouts can be used for tracking the
2397     progress of pattern matching. The pcretest command has an option that
2398     sets automatic callouts; when it is used, the output indicates how the
2399     pattern is matched. This is useful information when you are trying to
2400     optimize the performance of a particular pattern.
2401    
2402    
2403     MISSING CALLOUTS
2404    
2405     You should be aware that, because of optimizations in the way PCRE
2406     matches patterns, callouts sometimes do not happen. For example, if the
2407     pattern is
2408    
2409     ab(?C4)cd
2410    
2411     PCRE knows that any matching string must contain the letter "d". If the
2412     subject string is "abyz", the lack of "d" means that matching doesn't
2413     ever start, and the callout is never reached. However, with "abyd",
2414     though the result is still no match, the callout is obeyed.
2415    
2416    
2417     THE CALLOUT INTERFACE
2418    
2419     During matching, when PCRE reaches a callout point, the external func-
2420 nigel 77 tion defined by pcre_callout is called (if it is set). This applies to
2421     both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2422     only argument to the callout function is a pointer to a pcre_callout
2423     block. This structure contains the following fields:
2424 nigel 75
2425 nigel 73 int version;
2426     int callout_number;
2427     int *offset_vector;
2428     const char *subject;
2429     int subject_length;
2430     int start_match;
2431     int current_position;
2432     int capture_top;
2433     int capture_last;
2434     void *callout_data;
2435 nigel 75 int pattern_position;
2436     int next_item_length;
2437 nigel 63
2438 nigel 77 The version field is an integer containing the version number of the
2439     block format. The initial version was 0; the current version is 1. The
2440     version number will change again in future if additional fields are
2441 nigel 75 added, but the intention is never to remove any of the existing fields.
2442 nigel 63
2443 nigel 77 The callout_number field contains the number of the callout, as com-
2444     piled into the pattern (that is, the number after ?C for manual call-
2445 nigel 75 outs, and 255 for automatically generated callouts).
2446 nigel 63
2447 nigel 77 The offset_vector field is a pointer to the vector of offsets that was
2448     passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2449     pcre_exec() is used, the contents can be inspected in order to extract
2450     substrings that have been matched so far, in the same way as for
2451     extracting substrings after a match has completed. For pcre_dfa_exec()
2452     this field is not useful.
2453 nigel 63
2454 nigel 75 The subject and subject_length fields contain copies of the values that
2455 nigel 73 were passed to pcre_exec().
2456 nigel 63
2457 nigel 77 The start_match field contains the offset within the subject at which
2458     the current match attempt started. If the pattern is not anchored, the
2459 nigel 75 callout function may be called several times from the same point in the
2460     pattern for different starting points in the subject.
2461 nigel 63
2462 nigel 77 The current_position field contains the offset within the subject of
2463 nigel 73 the current match pointer.
2464 nigel 63
2465 nigel 77 When the pcre_exec() function is used, the capture_top field contains
2466     one more than the number of the highest numbered captured substring so
2467     far. If no substrings have been captured, the value of capture_top is
2468     one. This is always the case when pcre_dfa_exec() is used, because it
2469     does not support captured substrings.
2470 nigel 63
2471 nigel 77 The capture_last field contains the number of the most recently cap-
2472     tured substring. If no substrings have been captured, its value is -1.
2473     This is always the case when pcre_dfa_exec() is used.
2474 nigel 63
2475 nigel 77 The callout_data field contains a value that is passed to pcre_exec()
2476     or pcre_dfa_exec() specifically so that it can be passed back in call-
2477     outs. It is passed in the pcre_callout field of the pcre_extra data
2478     structure. If no such data was passed, the value of callout_data in a
2479     pcre_callout block is NULL. There is a description of the pcre_extra
2480 nigel 73 structure in the pcreapi documentation.
2481 nigel 63
2482 nigel 77 The pattern_position field is present from version 1 of the pcre_call-
2483 nigel 75 out structure. It contains the offset to the next item to be matched in
2484     the pattern string.
2485 nigel 63
2486 nigel 77 The next_item_length field is present from version 1 of the pcre_call-
2487 nigel 75 out structure. It contains the length of the next item to be matched in
2488 nigel 77 the pattern string. When the callout immediately precedes an alterna-
2489     tion bar, a closing parenthesis, or the end of the pattern, the length
2490     is zero. When the callout precedes an opening parenthesis, the length
2491 nigel 75 is that of the entire subpattern.
2492 nigel 73
2493 nigel 77 The pattern_position and next_item_length fields are intended to help
2494     in distinguishing between different automatic callouts, which all have
2495 nigel 75 the same callout number. However, they are set for all callouts.
2496    
2497    
2498 nigel 63 RETURN VALUES
2499    
2500 nigel 77 The external callout function returns an integer to PCRE. If the value
2501     is zero, matching proceeds as normal. If the value is greater than
2502     zero, matching fails at the current point, but the testing of other
2503     matching possibilities goes ahead, just as if a lookahead assertion had
2504     failed. If the value is less than zero, the match is abandoned, and
2505     pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2506 nigel 63
2507 nigel 77 Negative values should normally be chosen from the set of
2508 nigel 73 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2509 nigel 77 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2510     reserved for use by callout functions; it will never be used by PCRE
2511 nigel 73 itself.
2512 nigel 63
2513    
2514 ph10 99 AUTHOR
2515 nigel 63
2516 ph10 99 Philip Hazel
2517     University Computing Service
2518     Cambridge CB2 3QH, England.
2519    
2520    
2521     REVISION
2522    
2523     Last updated: 06 March 2007
2524     Copyright (c) 1997-2007 University of Cambridge.
2525     ------------------------------------------------------------------------------
2526    
2527    
2528 nigel 79 PCRECOMPAT(3) PCRECOMPAT(3)
2529 nigel 63
2530 nigel 79
2531 nigel 73 NAME
2532     PCRE - Perl-compatible regular expressions
2533    
2534 nigel 77
2535 nigel 75 DIFFERENCES BETWEEN PCRE AND PERL
2536 nigel 41
2537 nigel 73 This document describes the differences in the ways that PCRE and Perl
2538 nigel 93 handle regular expressions. The differences described here are mainly
2539     with respect to Perl 5.8, though PCRE version 7.0 contains some fea-
2540     tures that are expected to be in the forthcoming Perl 5.10.
2541 nigel 41
2542 nigel 93 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2543     of what it does have are given in the section on UTF-8 support in the
2544 nigel 87 main pcre page.
2545 nigel 41
2546 nigel 73 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2547 nigel 93 permits them, but they do not mean what you might think. For example,
2548 nigel 73 (?!a){3} does not assert that the next three characters are not "a". It
2549     just asserts that the next character is not "a" three times.
2550 nigel 41
2551 nigel 93 3. Capturing subpatterns that occur inside negative lookahead asser-
2552     tions are counted, but their entries in the offsets vector are never
2553     set. Perl sets its numerical variables from any such patterns that are
2554 nigel 73 matched before the assertion fails to match something (thereby succeed-
2555 nigel 93 ing), but only if the negative lookahead assertion contains just one
2556 nigel 73 branch.
2557 nigel 41
2558 nigel 93 4. Though binary zero characters are supported in the subject string,
2559 nigel 73 they are not allowed in a pattern string because it is passed as a nor-
2560 nigel 75 mal C string, terminated by zero. The escape sequence \0 can be used in
2561     the pattern to represent a binary zero.
2562 nigel 41
2563 nigel 93 5. The following Perl escape sequences are not supported: \l, \u, \L,
2564 nigel 75 \U, and \N. In fact these are implemented by Perl's general string-han-
2565 nigel 93 dling and are not part of its pattern matching engine. If any of these
2566 nigel 75 are encountered by PCRE, an error is generated.
2567 nigel 41
2568 nigel 93 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2569     is built with Unicode character property support. The properties that
2570     can be tested with \p and \P are limited to the general category prop-
2571     erties such as Lu and Nd, script names such as Greek or Han, and the
2572 nigel 87 derived properties Any and L&.
2573 nigel 75
2574     7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2575 nigel 93 ters in between are treated as literals. This is slightly different
2576     from Perl in that $ and @ are also handled as literals inside the
2577     quotes. In Perl, they cause variable interpolation (but of course PCRE
2578 nigel 73 does not have variables). Note the following examples:
2579 nigel 49
2580 nigel 73 Pattern PCRE matches Perl matches
2581 nigel 41
2582 nigel 73 \Qabc$xyz\E abc$xyz abc followed by the
2583     contents of $xyz
2584     \Qabc\$xyz\E abc\$xyz abc\$xyz
2585     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2586 nigel 41
2587 nigel 93 The \Q...\E sequence is recognized both inside and outside character
2588 nigel 73 classes.
2589 nigel 41
2590 nigel 93 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2591     constructions. However, there is support for recursive patterns. This
2592     is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2593     "callout" feature allows an external function to be called during pat-
2594 nigel 75 tern matching. See the pcrecallout documentation for details.
2595 nigel 63
2596 nigel 93 9. Subpatterns that are called recursively or as "subroutines" are
2597     always treated as atomic groups in PCRE. This is like Python, but
2598     unlike Perl.
2599    
2600     10. There are some differences that are concerned with the settings of
2601     captured strings when part of a pattern is repeated. For example,
2602     matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
2603 nigel 73 unset, but in PCRE it is set to "b".
2604 nigel 41
2605 nigel 93 11. PCRE provides some extensions to the Perl regular expression facil-
2606     ities. Perl 5.10 will include new features that are not in earlier
2607     versions, some of which (such as named parentheses) have been in PCRE
2608     for some time. This list is with respect to Perl 5.10:
2609 nigel 41
2610 nigel 93 (a) Although lookbehind assertions must match fixed length strings,
2611 nigel 73 each alternative branch of a lookbehind assertion can match a different
2612     length of string. Perl requires them all to have the same length.
2613 nigel 41
2614 nigel 93 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2615 nigel 73 meta-character matches only at the very end of the string.
2616 nigel 41
2617 nigel 73 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2618 nigel 93 cial meaning is faulted. Otherwise, like Perl, the backslash is
2619 nigel 91 ignored. (Perl can be made to issue a warning.)
2620 nigel 41
2621 nigel 93 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
2622 nigel 73 fiers is inverted, that is, by default they are not greedy, but if fol-
2623     lowed by a question mark they are.
2624 nigel 41
2625 nigel 75 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2626     tried only at the first matching position in the subject string.
2627 nigel 41
2628 nigel 93 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2629 nigel 73 TURE options for pcre_exec() have no Perl equivalents.
2630 nigel 41
2631 nigel 93 (g) The callout facility is PCRE-specific.
2632 nigel 41
2633 nigel 93 (h) The partial matching facility is PCRE-specific.
2634 nigel 43
2635 nigel 93 (i) Patterns compiled by PCRE can be saved and re-used at a later time,
2636 nigel 75 even on different hosts that have the other endianness.
2637    
2638 nigel 93 (j) The alternative matching function (pcre_dfa_exec()) matches in a
2639 nigel 77 different way and is not Perl-compatible.
2640    
2641 nigel 63
2642 ph10 99 AUTHOR
2643 nigel 63
2644 ph10 99 Philip Hazel
2645     University Computing Service
2646     Cambridge CB2 3QH, England.
2647    
2648    
2649     REVISION
2650    
2651     Last updated: 06 March 2007
2652     Copyright (c) 1997-2007 University of Cambridge.
2653     ------------------------------------------------------------------------------
2654    
2655    
2656 nigel 79 PCREPATTERN(3) PCREPATTERN(3)
2657 nigel 63
2658 nigel 79
2659 nigel 73 NAME
2660     PCRE - Perl-compatible regular expressions
2661    
2662 nigel 77
2663 nigel 63 PCRE REGULAR EXPRESSION DETAILS
2664    
2665 nigel 73 The syntax and semantics of the regular expressions supported by PCRE
2666     are described below. Regular expressions are also described in the Perl
2667 nigel 75 documentation and in a number of books, some of which have copious
2668     examples. Jeffrey Friedl's "Mastering Regular Expressions", published
2669     by O'Reilly, covers regular expressions in great detail. This descrip-
2670     tion of PCRE's regular expressions is intended as reference material.
2671 nigel 49
2672 nigel 75 The original operation of PCRE was on strings of one-byte characters.
2673     However, there is now also support for UTF-8 character strings. To use
2674     this, you must build PCRE to include UTF-8 support, and then call
2675     pcre_compile() with the PCRE_UTF8 option. How this affects pattern
2676     matching is mentioned in several places below. There is also a summary
2677     of UTF-8 features in the section on UTF-8 support in the main pcre
2678     page.
2679 nigel 41
2680 nigel 77 The remainder of this document discusses the patterns that are sup-
2681     ported by PCRE when its main matching function, pcre_exec(), is used.
2682     From release 6.0, PCRE offers a second matching function,
2683     pcre_dfa_exec(), which matches using a different algorithm that is not
2684     Perl-compatible. The advantages and disadvantages of the alternative
2685     function, and how it differs from the normal function, are discussed in
2686     the pcrematching page.
2687    
2688 nigel 93
2689     CHARACTERS AND METACHARACTERS
2690    
2691 nigel 75 A regular expression is a pattern that is matched against a subject
2692     string from left to right. Most characters stand for themselves in a
2693     pattern, and match the corresponding characters in the subject. As a
2694 nigel 73 trivial example, the pattern
2695 nigel 41
2696 nigel 73 The quick brown fox
2697 nigel 41
2698 nigel 77 matches a portion of a subject string that is identical to itself. When
2699     caseless matching is specified (the PCRE_CASELESS option), letters are
2700     matched independently of case. In UTF-8 mode, PCRE always understands
2701     the concept of case for characters whose values are less than 128, so
2702     caseless matching is always possible. For characters with higher val-
2703     ues, the concept of case is supported if PCRE is compiled with Unicode
2704     property support, but not otherwise. If you want to use caseless
2705     matching for characters 128 and above, you must ensure that PCRE is
2706     compiled with Unicode property support as well as with UTF-8 support.
2707 nigel 41
2708 nigel 77 The power of regular expressions comes from the ability to include
2709     alternatives and repetitions in the pattern. These are encoded in the
2710     pattern by the use of metacharacters, which do not stand for themselves
2711     but instead are interpreted in some special way.
2712    
2713     There are two different sets of metacharacters: those that are recog-
2714     nized anywhere in the pattern except within square brackets, and those
2715 nigel 93 that are recognized within square brackets. Outside square brackets,
2716     the metacharacters are as follows:
2717 nigel 41
2718 nigel 73 \ general escape character with several uses
2719     ^ assert start of string (or line, in multiline mode)
2720     $ assert end of string (or line, in multiline mode)
2721     . match any character except newline (by default)
2722     [ start character class definition
2723     | start of alternative branch
2724     ( start subpattern
2725     ) end subpattern
2726     ? extends the meaning of (
2727     also 0 or 1 quantifier
2728     also quantifier minimizer
2729     * 0 or more quantifier
2730     + 1 or more quantifier
2731     also "possessive quantifier"
2732     { start min/max quantifier
2733 nigel 41
2734 nigel 77 Part of a pattern that is in square brackets is called a "character
2735 nigel 75 class". In a character class the only metacharacters are:
2736 nigel 41
2737 nigel 73 \ general escape character
2738     ^ negate the class, but only if the first character
2739     - indicates character range
2740     [ POSIX character class (only if followed by POSIX
2741     syntax)
2742     ] terminates the character class
2743 nigel 41
2744 nigel 77 The following sections describe the use of each of the metacharacters.
2745 nigel 41
2746    
2747 nigel 63 BACKSLASH
2748 nigel 41
2749 nigel 73 The backslash character has several uses. Firstly, if it is followed by
2750 nigel 77 a non-alphanumeric character, it takes away any special meaning that
2751     character may have. This use of backslash as an escape character
2752 nigel 73 applies both inside and outside character classes.
2753 nigel 41
2754 nigel 77 For example, if you want to match a * character, you write \* in the
2755     pattern. This escaping action applies whether or not the following
2756     character would otherwise be interpreted as a metacharacter, so it is
2757     always safe to precede a non-alphanumeric with backslash to specify
2758     that it stands for itself. In particular, if you want to match a back-
2759 nigel 75 slash, you write \\.
2760 nigel 41
2761 nigel 77 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
2762     the pattern (other than in a character class) and characters between a
2763 nigel 91 # outside a character class and the next newline are ignored. An escap-
2764     ing backslash can be used to include a whitespace or # character as
2765     part of the pattern.
2766 nigel 41
2767 nigel 77 If you want to remove the special meaning from a sequence of charac-
2768     ters, you can do so by putting them between \Q and \E. This is differ-
2769     ent from Perl in that $ and @ are handled as literals in \Q...\E
2770     sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
2771 nigel 73 tion. Note the following examples:
2772 nigel 63
2773 nigel 73 Pattern PCRE matches Perl matches
2774 nigel 63
2775 nigel 73 \Qabc$xyz\E abc$xyz abc followed by the
2776     contents of $xyz
2777     \Qabc\$xyz\E abc\$xyz abc\$xyz
2778     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2779 nigel 63
2780 nigel 77 The \Q...\E sequence is recognized both inside and outside character
2781 nigel 73 classes.
2782 nigel 63
2783 nigel 75 Non-printing characters
2784    
2785 nigel 73 A second use of backslash provides a way of encoding non-printing char-
2786 nigel 77 acters in patterns in a visible manner. There is no restriction on the
2787     appearance of non-printing characters, apart from the binary zero that
2788     terminates a pattern, but when a pattern is being prepared by text
2789     editing, it is usually easier to use one of the following escape
2790 nigel 73 sequences than the binary character it represents:
2791 nigel 63
2792 nigel 73 \a alarm, that is, the BEL character (hex 07)
2793     \cx "control-x", where x is any character
2794     \e escape (hex 1B)
2795     \f formfeed (hex 0C)
2796     \n newline (hex 0A)
2797     \r carriage return (hex 0D)
2798     \t tab (hex 09)
2799     \ddd character with octal code ddd, or backreference
2800     \xhh character with hex code hh
2801 nigel 87 \x{hhh..} character with hex code hhh..
2802 nigel 41
2803 nigel 77 The precise effect of \cx is as follows: if x is a lower case letter,
2804     it is converted to upper case. Then bit 6 of the character (hex 40) is
2805     inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
2806 nigel 73 becomes hex 7B.
2807 nigel 41
2808 nigel 77 After \x, from zero to two hexadecimal digits are read (letters can be
2809 nigel 87 in upper or lower case). Any number of hexadecimal digits may appear
2810     between \x{ and }, but the value of the character code must be less
2811     than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode (that is,
2812     the maximum hexadecimal value is 7FFFFFFF). If characters other than
2813     hexadecimal digits appear between \x{ and }, or if there is no termi-
2814     nating }, this form of escape is not recognized. Instead, the initial
2815     \x will be interpreted as a basic hexadecimal escape, with no following
2816     digits, giving a character whose value is zero.
2817 nigel 41
2818 nigel 73 Characters whose value is less than 256 can be defined by either of the
2819 nigel 87 two syntaxes for \x. There is no difference in the way they are han-
2820     dled. For example, \xdc is exactly the same as \x{dc}.
2821 nigel 41
2822 nigel 91 After \0 up to two further octal digits are read. If there are fewer
2823     than two digits, just those that are present are used. Thus the
2824     sequence \0\x\07 specifies two binary zeros followed by a BEL character
2825     (code value 7). Make sure you supply two digits after the initial zero
2826     if the pattern character that follows is itself an octal digit.
2827 nigel 63
2828 nigel 73 The handling of a backslash followed by a digit other than 0 is compli-
2829     cated. Outside a character class, PCRE reads it and any following dig-
2830 nigel 91 its as a decimal number. If the number is less than 10, or if there
2831 nigel 73 have been at least that many previous capturing left parentheses in the
2832 nigel 91 expression, the entire sequence is taken as a back reference. A
2833     description of how this works is given later, following the discussion
2834 nigel 73 of parenthesized subpatterns.
2835 nigel 41
2836 nigel 91 Inside a character class, or if the decimal number is greater than 9
2837     and there have not been that many capturing subpatterns, PCRE re-reads
2838 nigel 93 up to three octal digits following the backslash, and uses them to gen-
2839 nigel 91 erate a data character. Any subsequent digits stand for themselves. In
2840     non-UTF-8 mode, the value of a character specified in octal must be
2841     less than \400. In UTF-8 mode, values up to \777 are permitted. For
2842     example:
2843 nigel 41
2844 nigel 73 \040 is another way of writing a space
2845     \40 is the same, provided there are fewer than 40
2846     previous capturing subpatterns
2847     \7 is always a back reference
2848     \11 might be a back reference, or another way of
2849     writing a tab
2850     \011 is always a tab
2851     \0113 is a tab followed by the character "3"
2852     \113 might be a back reference, otherwise the
2853     character with octal code 113
2854     \377 might be a back reference, otherwise
2855     the byte consisting entirely of 1 bits
2856     \81 is either a back reference, or a binary zero
2857     followed by the two characters "8" and "1"
2858 nigel 41
2859 nigel 91 Note that octal values of 100 or greater must not be introduced by a
2860 nigel 73 leading zero, because no more than three octal digits are ever read.
2861 nigel 41
2862 nigel 91 All the sequences that define a single character value can be used both
2863     inside and outside character classes. In addition, inside a character
2864     class, the sequence \b is interpreted as the backspace character (hex
2865 nigel 93 08), and the sequences \R and \X are interpreted as the characters "R"
2866     and "X", respectively. Outside a character class, these sequences have
2867     different meanings (see below).
2868 nigel 43
2869 nigel 93 Absolute and relative back references
2870    
2871     The sequence \g followed by a positive or negative number, optionally
2872     enclosed in braces, is an absolute or relative back reference. Back
2873     references are discussed later, following the discussion of parenthe-
2874     sized subpatterns.
2875    
2876 nigel 75 Generic character types
2877 nigel 41
2878 nigel 93 Another use of backslash is for specifying generic character types. The
2879     following are always recognized:
2880 nigel 75
2881 nigel 73 \d any decimal digit
2882     \D any character that is not a decimal digit
2883     \s any whitespace character
2884     \S any character that is not a whitespace character
2885     \w any "word" character
2886     \W any "non-word" character
2887 nigel 41
2888 nigel 73 Each pair of escape sequences partitions the complete set of characters
2889 nigel 87 into two disjoint sets. Any given character matches one, and only one,
2890 nigel 73 of each pair.
2891 nigel 41
2892 nigel 75 These character type sequences can appear both inside and outside char-
2893 nigel 87 acter classes. They each match one character of the appropriate type.
2894     If the current matching point is at the end of the subject string, all
2895 nigel 75 of them fail, since there is no character to match.
2896 nigel 41
2897 nigel 87 For compatibility with Perl, \s does not match the VT character (code
2898     11). This makes it different from the the POSIX "space" class. The \s
2899 nigel 91 characters are HT (9), LF (10), FF (12), CR (13), and space (32). (If
2900     "use locale;" is included in a Perl script, \s may match the VT charac-
2901     ter. In PCRE, it never does.)
2902 nigel 63
2903 nigel 75 A "word" character is an underscore or any character less than 256 that
2904 nigel 87 is a letter or digit. The definition of letters and digits is con-
2905     trolled by PCRE's low-valued character tables, and may vary if locale-
2906     specific matching is taking place (see "Locale support" in the pcreapi
2907     page). For example, in the "fr_FR" (French) locale, some character
2908     codes greater than 128 are used for accented letters, and these are
2909 nigel 75 matched by \w.
2910 nigel 63
2911 nigel 87 In UTF-8 mode, characters with values greater than 128 never match \d,
2912 nigel 75 \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
2913 nigel 87 code character property support is available. The use of locales with
2914     Unicode is discouraged.
2915 nigel 41
2916 nigel 93 Newline sequences
2917    
2918     Outside a character class, the escape sequence \R matches any Unicode
2919     newline sequence. This is an extension to Perl. In non-UTF-8 mode \R is
2920     equivalent to the following:
2921    
2922     (?>\r\n|\n|\x0b|\f|\r|\x85)
2923    
2924     This is an example of an "atomic group", details of which are given
2925     below. This particular group matches either the two-character sequence
2926     CR followed by LF, or one of the single characters LF (linefeed,
2927     U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
2928     return, U+000D), or NEL (next line, U+0085). The two-character sequence
2929     is treated as a single unit that cannot be split.
2930    
2931     In UTF-8 mode, two additional characters whose codepoints are greater
2932     than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
2933     rator, U+2029). Unicode character property support is not needed for
2934     these characters to be recognized.
2935    
2936     Inside a character class, \R matches the letter "R".
2937    
2938 nigel 75 Unicode character properties
2939    
2940     When PCRE is built with Unicode character property support, three addi-
2941 nigel 87 tional escape sequences to match character properties are available
2942 nigel 75 when UTF-8 mode is selected. They are:
2943    
2944 nigel 87 \p{xx} a character with the xx property
2945     \P{xx} a character without the xx property
2946     \X an extended Unicode sequence
2947 nigel 75
2948 nigel 77 The property names represented by xx above are limited to the Unicode
2949 nigel 87 script names, the general category properties, and "Any", which matches
2950     any character (including newline). Other properties such as "InMusical-
2951     Symbols" are not currently supported by PCRE. Note that \P{Any} does
2952     not match any characters, so always causes a match failure.
2953 nigel 75
2954 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts.
2955     A character from one of these sets can be matched using a script name.
2956     For example:
2957 nigel 75
2958 nigel 87 \p{Greek}
2959     \P{Han}
2960    
2961     Those that are not part of an identified script are lumped together as
2962     "Common". The current list of scripts is:
2963    
2964 nigel 93 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
2965     Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
2966     Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
2967     Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
2968     gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
2969     Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
2970     Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
2971     Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
2972     Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
2973 nigel 87
2974     Each character has exactly one general category property, specified by
2975     a two-letter abbreviation. For compatibility with Perl, negation can be
2976     specified by including a circumflex between the opening brace and the
2977     property name. For example, \p{^Lu} is the same as \P{Lu}.
2978    
2979     If only one letter is specified with \p or \P, it includes all the gen-
2980     eral category properties that start with that letter. In this case, in
2981     the absence of negation, the curly brackets in the escape sequence are
2982     optional; these two examples have the same effect:
2983    
2984 nigel 75 \p{L}
2985     \pL
2986    
2987 nigel 87 The following general category property codes are supported:
2988 nigel 75
2989     C Other
2990     Cc Control
2991     Cf Format
2992     Cn Unassigned
2993     Co Private use
2994     Cs Surrogate
2995    
2996     L Letter
2997     Ll Lower case letter
2998     Lm Modifier letter
2999     Lo Other letter
3000     Lt Title case letter
3001     Lu Upper case letter
3002    
3003     M Mark
3004     Mc Spacing mark
3005     Me Enclosing mark
3006     Mn Non-spacing mark
3007    
3008     N Number
3009     Nd Decimal number
3010     Nl Letter number
3011     No Other number
3012    
3013     P Punctuation
3014     Pc Connector punctuation
3015     Pd Dash punctuation
3016     Pe Close punctuation
3017     Pf Final punctuation
3018     Pi Initial punctuation
3019     Po Other punctuation
3020     Ps Open punctuation
3021    
3022     S Symbol
3023     Sc Currency symbol
3024     Sk Modifier symbol
3025     Sm Mathematical symbol
3026     So Other symbol
3027    
3028     Z Separator
3029     Zl Line separator
3030     Zp Paragraph separator
3031     Zs Space separator
3032    
3033 nigel 87 The special property L& is also supported: it matches a character that
3034     has the Lu, Ll, or Lt property, in other words, a letter that is not
3035     classified as a modifier or "other".
3036 nigel 75
3037 nigel 87 The long synonyms for these properties that Perl supports (such as
3038 nigel 91 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3039 nigel 87 any of these properties with "Is".
3040    
3041     No character that is in the Unicode table has the Cn (unassigned) prop-
3042     erty. Instead, this property is assumed for any code point that is not
3043     in the Unicode table.
3044    
3045 nigel 77 Specifying caseless matching does not affect these escape sequences.
3046 nigel 75 For example, \p{Lu} always matches only upper case letters.
3047    
3048 nigel 77 The \X escape matches any number of Unicode characters that form an
3049 nigel 75 extended Unicode sequence. \X is equivalent to
3050    
3051     (?>\PM\pM*)
3052    
3053 nigel 77 That is, it matches a character without the "mark" property, followed
3054     by zero or more characters with the "mark" property, and treats the
3055     sequence as an atomic group (see below). Characters with the "mark"
3056 nigel 75 property are typically accents that affect the preceding character.
3057    
3058 nigel 77 Matching characters by Unicode property is not fast, because PCRE has
3059     to search a structure that contains data for over fifteen thousand
3060 nigel 75 characters. That is why the traditional escape sequences such as \d and
3061     \w do not use Unicode properties in PCRE.
3062    
3063     Simple assertions
3064    
3065 nigel 93 The final use of backslash is for certain simple assertions. An asser-
3066 nigel 77 tion specifies a condition that has to be met at a particular point in
3067     a match, without consuming any characters from the subject string. The
3068     use of subpatterns for more complicated assertions is described below.
3069 nigel 75 The backslashed assertions are:
3070 nigel 41
3071 nigel 73 \b matches at a word boundary
3072     \B matches when not at a word boundary
3073 nigel 93 \A matches at the start of the subject
3074     \Z matches at the end of the subject
3075     also matches before a newline at the end of the subject
3076     \z matches only at the end of the subject
3077     \G matches at the first matching position in the subject
3078 nigel 41
3079 nigel 77 These assertions may not appear in character classes (but note that \b
3080 nigel 73 has a different meaning, namely the backspace character, inside a char-
3081     acter class).
3082 nigel 41
3083 nigel 77 A word boundary is a position in the subject string where the current
3084     character and the previous character do not both match \w or \W (i.e.
3085     one matches \w and the other matches \W), or the start or end of the
3086 nigel 73 string if the first or last character matches \w, respectively.
3087 nigel 43
3088 nigel 77 The \A, \Z, and \z assertions differ from the traditional circumflex
3089 nigel 75 and dollar (described in the next section) in that they only ever match
3090 nigel 77 at the very start and end of the subject string, whatever options are
3091     set. Thus, they are independent of multiline mode. These three asser-
3092 nigel 75 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3093 nigel 77 affect only the behaviour of the circumflex and dollar metacharacters.
3094     However, if the startoffset argument of pcre_exec() is non-zero, indi-
3095 nigel 75 cating that matching is to start at a point other than the beginning of
3096 nigel 77 the subject, \A can never match. The difference between \Z and \z is
3097 nigel 91 that \Z matches before a newline at the end of the string as well as at
3098     the very end, whereas \z matches only at the end.
3099 nigel 63
3100 nigel 91 The \G assertion is true only when the current matching position is at
3101     the start point of the match, as specified by the startoffset argument
3102     of pcre_exec(). It differs from \A when the value of startoffset is
3103     non-zero. By calling pcre_exec() multiple times with appropriate argu-
3104 nigel 73 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3105     mentation where \G can be useful.
3106 nigel 41
3107 nigel 91 Note, however, that PCRE's interpretation of \G, as the start of the
3108 nigel 73 current match, is subtly different from Perl's, which defines it as the
3109 nigel 91 end of the previous match. In Perl, these can be different when the
3110     previously matched string was empty. Because PCRE does just one match
3111 nigel 73 at a time, it cannot reproduce this behaviour.
3112 nigel 41
3113 nigel 91 If all the alternatives of a pattern begin with \G, the expression is
3114 nigel 73 anchored to the starting match position, and the "anchored" flag is set
3115     in the compiled regular expression.
3116 nigel 63
3117    
3118 nigel 41 CIRCUMFLEX AND DOLLAR
3119 nigel 63
3120 nigel 73 Outside a character class, in the default matching mode, the circumflex
3121 nigel 91 character is an assertion that is true only if the current matching
3122     point is at the start of the subject string. If the startoffset argu-
3123     ment of pcre_exec() is non-zero, circumflex can never match if the
3124     PCRE_MULTILINE option is unset. Inside a character class, circumflex
3125 nigel 73 has an entirely different meaning (see below).
3126 nigel 41
3127 nigel 91 Circumflex need not be the first character of the pattern if a number
3128     of alternatives are involved, but it should be the first thing in each
3129     alternative in which it appears if the pattern is ever to match that
3130     branch. If all possible alternatives start with a circumflex, that is,
3131     if the pattern is constrained to match only at the start of the sub-
3132     ject, it is said to be an "anchored" pattern. (There are also other
3133 nigel 73 constructs that can cause a pattern to be anchored.)
3134 nigel 41
3135 nigel 91 A dollar character is an assertion that is true only if the current
3136     matching point is at the end of the subject string, or immediately
3137     before a newline at the end of the string (by default). Dollar need not
3138     be the last character of the pattern if a number of alternatives are
3139     involved, but it should be the last item in any branch in which it
3140     appears. Dollar has no special meaning in a character class.
3141 nigel 41
3142 nigel 77 The meaning of dollar can be changed so that it matches only at the
3143     very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3144 nigel 73 compile time. This does not affect the \Z assertion.
3145 nigel 41
3146 nigel 73 The meanings of the circumflex and dollar characters are changed if the
3147 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex
3148     matches immediately after internal newlines as well as at the start of
3149     the subject string. It does not match after a newline that ends the
3150     string. A dollar matches before any newlines in the string, as well as
3151     at the very end, when PCRE_MULTILINE is set. When newline is specified
3152     as the two-character sequence CRLF, isolated CR and LF characters do
3153     not indicate newlines.
3154 nigel 41
3155 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3156     (where \n represents a newline) in multiline mode, but not otherwise.
3157     Consequently, patterns that are anchored in single line mode because
3158     all branches start with ^ are not anchored in multiline mode, and a
3159     match for circumflex is possible when the startoffset argument of
3160     pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3161     PCRE_MULTILINE is set.
3162 nigel 41
3163 nigel 91 Note that the sequences \A, \Z, and \z can be used to match the start
3164     and end of the subject in both modes, and if all branches of a pattern
3165     start with \A it is always anchored, whether or not PCRE_MULTILINE is
3166     set.
3167 nigel 41
3168 nigel 91
3169 nigel 63 FULL STOP (PERIOD, DOT)
3170 nigel 41
3171 nigel 73 Outside a character class, a dot in the pattern matches any one charac-
3172 nigel 91 ter in the subject string except (by default) a character that signi-
3173     fies the end of a line. In UTF-8 mode, the matched character may be
3174 nigel 93 more than one byte long.
3175 nigel 41
3176 nigel 93 When a line ending is defined as a single character, dot never matches
3177     that character; when the two-character sequence CRLF is used, dot does
3178     not match CR if it is immediately followed by LF, but otherwise it
3179     matches all characters (including isolated CRs and LFs). When any Uni-
3180     code line endings are being recognized, dot does not match CR or LF or
3181     any of the other line ending characters.
3182 nigel 41
3183 nigel 93 The behaviour of dot with regard to newlines can be changed. If the
3184     PCRE_DOTALL option is set, a dot matches any one character, without
3185     exception. If the two-character sequence CRLF is present in the subject
3186     string, it takes two dots to match it.
3187    
3188     The handling of dot is entirely independent of the handling of circum-
3189     flex and dollar, the only relationship being that they both involve
3190 nigel 91 newlines. Dot has no special meaning in a character class.
3191    
3192    
3193 nigel 63 MATCHING A SINGLE BYTE
3194    
3195 nigel 73 Outside a character class, the escape sequence \C matches any one byte,
3196 nigel 93 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3197     line-ending characters. The feature is provided in Perl in order to
3198     match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3199     acters into individual bytes, what remains in the string may be a mal-
3200     formed UTF-8 string. For this reason, the \C escape sequence is best
3201     avoided.
3202 nigel 63
3203 nigel 77 PCRE does not allow \C to appear in lookbehind assertions (described
3204     below), because in UTF-8 mode this would make it impossible to calcu-
3205 nigel 75 late the length of the lookbehind.
3206 nigel 63
3207    
3208 nigel 75 SQUARE BRACKETS AND CHARACTER CLASSES
3209 nigel 63
3210 nigel 73 An opening square bracket introduces a character class, terminated by a
3211     closing square bracket. A closing square bracket on its own is not spe-
3212     cial. If a closing square bracket is required as a member of the class,
3213 nigel 77 it should be the first data character in the class (after an initial
3214 nigel 73 circumflex, if present) or escaped with a backslash.
3215 nigel 41
3216 nigel 77 A character class matches a single character in the subject. In UTF-8
3217     mode, the character may occupy more than one byte. A matched character
3218 nigel 73 must be in the set of characters defined by the class, unless the first
3219 nigel 77 character in the class definition is a circumflex, in which case the
3220     subject character must not be in the set defined by the class. If a
3221     circumflex is actually required as a member of the class, ensure it is
3222 nigel 73 not the first character, or escape it with a backslash.
3223 nigel 41
3224 nigel 77 For example, the character class [aeiou] matches any lower case vowel,
3225     while [^aeiou] matches any character that is not a lower case vowel.
3226 nigel 73 Note that a circumflex is just a convenient notation for specifying the
3227 nigel 77 characters that are in the class by enumerating those that are not. A
3228     class that starts with a circumflex is not an assertion: it still con-
3229     sumes a character from the subject string, and therefore it fails if
3230 nigel 75 the current pointer is at the end of the string.
3231 nigel 41
3232 nigel 77 In UTF-8 mode, characters with values greater than 255 can be included
3233     in a class as a literal string of bytes, or by using the \x{ escaping
3234 nigel 73 mechanism.
3235 nigel 63
3236 nigel 77 When caseless matching is set, any letters in a class represent both
3237     their upper case and lower case versions, so for example, a caseless
3238     [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3239     match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3240     understands the concept of case for characters whose values are less
3241     than 128, so caseless matching is always possible. For characters with
3242     higher values, the concept of case is supported if PCRE is compiled
3243     with Unicode property support, but not otherwise. If you want to use
3244     caseless matching for characters 128 and above, you must ensure that
3245     PCRE is compiled with Unicode property support as well as with UTF-8
3246     support.
3247 nigel 41
3248 nigel 93 Characters that might indicate line breaks are never treated in any
3249     special way when matching character classes, whatever line-ending
3250     sequence is in use, and whatever setting of the PCRE_DOTALL and
3251     PCRE_MULTILINE options is used. A class such as [^a] always matches one
3252     of these characters.
3253 nigel 41
3254 nigel 75 The minus (hyphen) character can be used to specify a range of charac-
3255     ters in a character class. For example, [d-m] matches any letter
3256     between d and m, inclusive. If a minus character is required in a
3257     class, it must be escaped with a backslash or appear in a position
3258     where it cannot be interpreted as indicating a range, typically as the
3259 nigel 73 first or last character in the class.
3260 nigel 41
3261 nigel 73 It is not possible to have the literal character "]" as the end charac-
3262 nigel 75 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3263     two characters ("W" and "-") followed by a literal string "46]", so it
3264     would match "W46]" or "-46]". However, if the "]" is escaped with a
3265     backslash it is interpreted as the end of range, so [W-\]46] is inter-
3266     preted as a class containing a range followed by two other characters.
3267     The octal or hexadecimal representation of "]" can also be used to end
3268     a range.
3269 nigel 41
3270 nigel 75 Ranges operate in the collating sequence of character values. They can
3271     also be used for characters specified numerically, for example
3272     [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3273 nigel 73 are greater than 255, for example [\x{100}-\x{2ff}].
3274 nigel 63
3275 nigel 73 If a range that includes letters is used when caseless matching is set,
3276     it matches the letters in either case. For example, [W-c] is equivalent
3277 nigel 75 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3278     character tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches
3279     accented E characters in both cases. In UTF-8 mode, PCRE supports the
3280     concept of case for characters with values greater than 128 only when
3281     it is compiled with Unicode property support.
3282 nigel 41
3283 nigel 75 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3284     in a character class, and add the characters that they match to the
3285     class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3286     flex can conveniently be used with the upper case character types to
3287     specify a more restricted set of characters than the matching lower
3288     case type. For example, the class [^\W_] matches any letter or digit,
3289     but not underscore.
3290 nigel 41
3291 nigel 75 The only metacharacters that are recognized in character classes are
3292     backslash, hyphen (only where it can be interpreted as specifying a
3293     range), circumflex (only at the start), opening square bracket (only
3294     when it can be interpreted as introducing a POSIX class name - see the
3295     next section), and the terminating closing square bracket. However,
3296     escaping other non-alphanumeric characters does no harm.
3297 nigel 41
3298 nigel 73
3299 nigel 43 POSIX CHARACTER CLASSES
3300    
3301 nigel 75 Perl supports the POSIX notation for character classes. This uses names
3302     enclosed by [: and :] within the enclosing square brackets. PCRE also
3303     supports this notation. For example,
3304 nigel 63
3305 nigel 73 [01[:alpha:]%]
3306 nigel 43
3307 nigel 73 matches "0", "1", any alphabetic character, or "%". The supported class
3308     names are
3309 nigel 43
3310 nigel 73 alnum letters and digits
3311     alpha letters
3312     ascii character codes 0 - 127
3313     blank space or tab only
3314     cntrl control characters
3315     digit decimal digits (same as \d)
3316     graph printing characters, excluding space
3317     lower lower case letters
3318     print printing characters, including space
3319     punct printing characters, excluding letters and digits
3320     space white space (not quite the same as \s)
3321     upper upper case letters
3322     word "word" characters (same as \w)
3323     xdigit hexadecimal digits
3324 nigel 43
3325 nigel 73 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3326     and space (32). Notice that this list includes the VT character (code
3327     11). This makes "space" different to \s, which does not include VT (for
3328     Perl compatibility).
3329 nigel 43
3330 nigel 73 The name "word" is a Perl extension, and "blank" is a GNU extension
3331     from Perl 5.8. Another Perl extension is negation, which is indicated
3332     by a ^ character after the colon. For example,
3333 nigel 63
3334 nigel 73 [12[:^digit:]]
3335 nigel 43
3336 nigel 73 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3337     POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3338     these are not supported, and an error is given if they are encountered.
3339 nigel 43
3340 nigel 75 In UTF-8 mode, characters with values greater than 128 do not match any
3341 nigel 73 of the POSIX character classes.
3342 nigel 43
3343    
3344 nigel 41 VERTICAL BAR
3345 nigel 63
3346 nigel 73 Vertical bar characters are used to separate alternative patterns. For
3347     example, the pattern
3348 nigel 41
3349 nigel 73 gilbert|sullivan
3350 nigel 41
3351 nigel 73 matches either "gilbert" or "sullivan". Any number of alternatives may
3352     appear, and an empty alternative is permitted (matching the empty
3353 nigel 91 string). The matching process tries each alternative in turn, from left
3354     to right, and the first one that succeeds is used. If the alternatives
3355     are within a subpattern (defined below), "succeeds" means matching the
3356     rest of the main pattern as well as the alternative in the subpattern.
3357 nigel 41
3358    
3359     INTERNAL OPTION SETTING
3360    
3361 nigel 73 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3362     PCRE_EXTENDED options can be changed from within the pattern by a
3363     sequence of Perl option letters enclosed between "(?" and ")". The
3364     option letters are
3365 nigel 63
3366 nigel 73 i for PCRE_CASELESS
3367     m for PCRE_MULTILINE
3368     s for PCRE_DOTALL
3369     x for PCRE_EXTENDED
3370 nigel 41
3371 nigel 73 For example, (?im) sets caseless, multiline matching. It is also possi-
3372     ble to unset these options by preceding the letter with a hyphen, and a
3373     combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3374     LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3375     is also permitted. If a letter appears both before and after the
3376     hyphen, the option is unset.
3377 nigel 41
3378 nigel 73 When an option change occurs at top level (that is, not inside subpat-
3379     tern parentheses), the change applies to the remainder of the pattern
3380     that follows. If the change is placed right at the start of a pattern,
3381     PCRE extracts it into the global options (and it will therefore show up
3382     in data extracted by the pcre_fullinfo() function).
3383 nigel 41
3384 nigel 93 An option change within a subpattern (see below for a description of
3385     subpatterns) affects only that part of the current pattern that follows
3386     it, so
3387 nigel 41
3388 nigel 73 (a(?i)b)c
3389 nigel 41
3390 nigel 73 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3391 nigel 93 used). By this means, options can be made to have different settings
3392     in different parts of the pattern. Any changes made in one alternative
3393     do carry on into subsequent branches within the same subpattern. For
3394 nigel 73 example,
3395 nigel 41
3396 nigel 73 (a(?i)b|c)
3397 nigel 41
3398 nigel 93 matches "ab", "aB", "c", and "C", even though when matching "C" the
3399     first branch is abandoned before the option setting. This is because
3400     the effects of option settings happen at compile time. There would be
3401 nigel 73 some very weird behaviour otherwise.
3402 nigel 41
3403 nigel 93 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3404     can be changed in the same way as the Perl-compatible options by using
3405 nigel 91 the characters J, U and X respectively.
3406 nigel 41
3407    
3408 nigel 63 SUBPATTERNS
3409 nigel 41
3410 nigel 73 Subpatterns are delimited by parentheses (round brackets), which can be
3411 nigel 75 nested. Turning part of a pattern into a subpattern does two things:
3412 nigel 41
3413 nigel 73 1. It localizes a set of alternatives. For example, the pattern
3414 nigel 41
3415 nigel 73 cat(aract|erpillar|)
3416 nigel 41
3417 nigel 93 matches one of the words "cat", "cataract", or "caterpillar". Without
3418     the parentheses, it would match "cataract", "erpillar" or an empty
3419 nigel 73 string.
3420 nigel 41
3421 nigel 93 2. It sets up the subpattern as a capturing subpattern. This means
3422     that, when the whole pattern matches, that portion of the subject
3423 nigel 73 string that matched the subpattern is passed back to the caller via the
3424 nigel 93 ovector argument of pcre_exec(). Opening parentheses are counted from
3425     left to right (starting from 1) to obtain numbers for the capturing
3426 nigel 73 subpatterns.
3427 nigel 41
3428 nigel 93 For example, if the string "the red king" is matched against the pat-
3429 nigel 73 tern
3430 nigel 41
3431 nigel 73 the ((red|white) (king|queen))
3432 nigel 41
3433 nigel 73 the captured substrings are "red king", "red", and "king", and are num-
3434     bered 1, 2, and 3, respectively.
3435 nigel 41
3436 nigel 93 The fact that plain parentheses fulfil two functions is not always
3437     helpful. There are often times when a grouping subpattern is required
3438     without a capturing requirement. If an opening parenthesis is followed
3439     by a question mark and a colon, the subpattern does not do any captur-
3440     ing, and is not counted when computing the number of any subsequent
3441     capturing subpatterns. For example, if the string "the white queen" is
3442 nigel 73 matched against the pattern
3443 nigel 41
3444 nigel 73 the ((?:red|white) (king|queen))
3445 nigel 41
3446 nigel 73 the captured substrings are "white queen" and "queen", and are numbered
3447 nigel 93 1 and 2. The maximum number of capturing subpatterns is 65535.