/[pcre]/code/trunk/doc/pcre.txt
ViewVC logotype

Contents of /code/trunk/doc/pcre.txt

Parent Directory Parent Directory | Revision Log Revision Log


Revision 243 - (show annotations)
Thu Sep 13 09:28:14 2007 UTC (12 years ago) by ph10
File MIME type: text/plain
File size: 301449 byte(s)
Error occurred while calculating annotation data.
Detrailed files for 7.4-RC1 test release.
1 -----------------------------------------------------------------------------
2 This file contains a concatenation of the PCRE man pages, converted to plain
3 text format for ease of searching with a text editor, or for use on systems
4 that do not have a man page processor. The small individual files that give
5 synopses of each function in the library have not been included. There are
6 separate text files for the pcregrep and pcretest commands.
7 -----------------------------------------------------------------------------
8
9
10 PCRE(3) PCRE(3)
11
12
13 NAME
14 PCRE - Perl-compatible regular expressions
15
16
17 INTRODUCTION
18
19 The PCRE library is a set of functions that implement regular expres-
20 sion pattern matching using the same syntax and semantics as Perl, with
21 just a few differences. (Certain features that appeared in Python and
22 PCRE before they appeared in Perl are also available using the Python
23 syntax.)
24
25 The current implementation of PCRE (release 7.x) corresponds approxi-
26 mately with Perl 5.10, including support for UTF-8 encoded strings and
27 Unicode general category properties. However, UTF-8 and Unicode support
28 has to be explicitly enabled; it is not the default. The Unicode tables
29 correspond to Unicode release 5.0.0.
30
31 In addition to the Perl-compatible matching function, PCRE contains an
32 alternative matching function that matches the same compiled patterns
33 in a different way. In certain circumstances, the alternative function
34 has some advantages. For a discussion of the two matching algorithms,
35 see the pcrematching page.
36
37 PCRE is written in C and released as a C library. A number of people
38 have written wrappers and interfaces of various kinds. In particular,
39 Google Inc. have provided a comprehensive C++ wrapper. This is now
40 included as part of the PCRE distribution. The pcrecpp page has details
41 of this interface. Other people's contributions can be found in the
42 Contrib directory at the primary FTP site, which is:
43
44 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
45
46 Details of exactly which Perl regular expression features are and are
47 not supported by PCRE are given in separate documents. See the pcrepat-
48 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
49 page.
50
51 Some features of PCRE can be included, excluded, or changed when the
52 library is built. The pcre_config() function makes it possible for a
53 client to discover which features are available. The features them-
54 selves are described in the pcrebuild page. Documentation about build-
55 ing PCRE for various operating systems can be found in the README file
56 in the source distribution.
57
58 The library contains a number of undocumented internal functions and
59 data tables that are used by more than one of the exported external
60 functions, but which are not intended for use by external callers.
61 Their names all begin with "_pcre_", which hopefully will not provoke
62 any name clashes. In some environments, it is possible to control which
63 external symbols are exported when a shared library is built, and in
64 these cases the undocumented symbols are not exported.
65
66
67 USER DOCUMENTATION
68
69 The user documentation for PCRE comprises a number of different sec-
70 tions. In the "man" format, each of these is a separate "man page". In
71 the HTML format, each is a separate page, linked from the index page.
72 In the plain text format, all the sections are concatenated, for ease
73 of searching. The sections are as follows:
74
75 pcre this document
76 pcre-config show PCRE installation configuration information
77 pcreapi details of PCRE's native C API
78 pcrebuild options for building PCRE
79 pcrecallout details of the callout feature
80 pcrecompat discussion of Perl compatibility
81 pcrecpp details of the C++ wrapper
82 pcregrep description of the pcregrep command
83 pcrematching discussion of the two matching algorithms
84 pcrepartial details of the partial matching facility
85 pcrepattern syntax and semantics of supported
86 regular expressions
87 pcresyntax quick syntax reference
88 pcreperform discussion of performance issues
89 pcreposix the POSIX-compatible C API
90 pcreprecompile details of saving and re-using precompiled patterns
91 pcresample discussion of the sample program
92 pcrestack discussion of stack usage
93 pcretest description of the pcretest testing command
94
95 In addition, in the "man" and HTML formats, there is a short page for
96 each C library function, listing its arguments and results.
97
98
99 LIMITATIONS
100
101 There are some size limitations in PCRE but it is hoped that they will
102 never in practice be relevant.
103
104 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
105 is compiled with the default internal linkage size of 2. If you want to
106 process regular expressions that are truly enormous, you can compile
107 PCRE with an internal linkage size of 3 or 4 (see the README file in
108 the source distribution and the pcrebuild documentation for details).
109 In these cases the limit is substantially larger. However, the speed
110 of execution is slower.
111
112 All values in repeating quantifiers must be less than 65536.
113
114 There is no limit to the number of parenthesized subpatterns, but there
115 can be no more than 65535 capturing subpatterns.
116
117 The maximum length of name for a named subpattern is 32 characters, and
118 the maximum number of named subpatterns is 10000.
119
120 The maximum length of a subject string is the largest positive number
121 that an integer variable can hold. However, when using the traditional
122 matching function, PCRE uses recursion to handle subpatterns and indef-
123 inite repetition. This means that the available stack space may limit
124 the size of a subject string that can be processed by certain patterns.
125 For a discussion of stack issues, see the pcrestack documentation.
126
127
128 UTF-8 AND UNICODE PROPERTY SUPPORT
129
130 From release 3.3, PCRE has had some support for character strings
131 encoded in the UTF-8 format. For release 4.0 this was greatly extended
132 to cover most common requirements, and in release 5.0 additional sup-
133 port for Unicode general category properties was added.
134
135 In order process UTF-8 strings, you must build PCRE to include UTF-8
136 support in the code, and, in addition, you must call pcre_compile()
137 with the PCRE_UTF8 option flag. When you do this, both the pattern and
138 any subject strings that are matched against it are treated as UTF-8
139 strings instead of just strings of bytes.
140
141 If you compile PCRE with UTF-8 support, but do not use it at run time,
142 the library will be a bit bigger, but the additional run time overhead
143 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
144 very big.
145
146 If PCRE is built with Unicode character property support (which implies
147 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
148 ported. The available properties that can be tested are limited to the
149 general category properties such as Lu for an upper case letter or Nd
150 for a decimal number, the Unicode script names such as Arabic or Han,
151 and the derived properties Any and L&. A full list is given in the
152 pcrepattern documentation. Only the short names for properties are sup-
153 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
154 ter}, is not supported. Furthermore, in Perl, many properties may
155 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
156 does not support this.
157
158 Validity of UTF-8 strings
159
160 When you set the PCRE_UTF8 flag, the strings passed as patterns and
161 subjects are (by default) checked for validity on entry to the relevant
162 functions. From release 7.3 of PCRE, the check is according the rules
163 of RFC 3629, which are themselves derived from the Unicode specifica-
164 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
165 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
166 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
167 to U+DFFF.
168
169 The excluded code points are the "Low Surrogate Area" of Unicode, of
170 which the Unicode Standard says this: "The Low Surrogate Area does not
171 contain any character assignments, consequently no character code
172 charts or namelists are provided for this area. Surrogates are reserved
173 for use with UTF-16 and then must be used in pairs." The code points
174 that are encoded by UTF-16 pairs are available as independent code
175 points in the UTF-8 encoding. (In other words, the whole surrogate
176 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
177
178 If an invalid UTF-8 string is passed to PCRE, an error return
179 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
180 that your strings are valid, and therefore want to skip these checks in
181 order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
182 compile time or at run time, PCRE assumes that the pattern or subject
183 it is given (respectively) contains only valid UTF-8 codes. In this
184 case, it does not diagnose an invalid UTF-8 string.
185
186 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
187 what happens depends on why the string is invalid. If the string con-
188 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
189 string of characters in the range 0 to 0x7FFFFFFF. In other words,
190 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
191 strings according to the more liberal rules of RFC 2279. However, if
192 the string does not even conform to RFC 2279, the result is undefined.
193 Your program may crash.
194
195 If you want to process strings of values in the full range 0 to
196 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
197 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
198 this situation, you will have to apply your own validity check.
199
200 General comments about UTF-8 mode
201
202 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
203 two-byte UTF-8 character if the value is greater than 127.
204
205 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
206 characters for values greater than \177.
207
208 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
209 vidual bytes, for example: \x{100}{3}.
210
211 4. The dot metacharacter matches one UTF-8 character instead of a sin-
212 gle byte.
213
214 5. The escape sequence \C can be used to match a single byte in UTF-8
215 mode, but its use can lead to some strange effects. This facility is
216 not available in the alternative matching function, pcre_dfa_exec().
217
218 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
219 test characters of any code value, but the characters that PCRE recog-
220 nizes as digits, spaces, or word characters remain the same set as
221 before, all with values less than 256. This remains true even when PCRE
222 includes Unicode property support, because to do otherwise would slow
223 down PCRE in many common cases. If you really want to test for a wider
224 sense of, say, "digit", you must use Unicode property tests such as
225 \p{Nd}.
226
227 7. Similarly, characters that match the POSIX named character classes
228 are all low-valued characters.
229
230 8. However, the Perl 5.10 horizontal and vertical whitespace matching
231 escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
232 acters.
233
234 9. Case-insensitive matching applies only to characters whose values
235 are less than 128, unless PCRE is built with Unicode property support.
236 Even when Unicode property support is available, PCRE still uses its
237 own character tables when checking the case of low-valued characters,
238 so as not to degrade performance. The Unicode property information is
239 used only for characters with higher values. Even when Unicode property
240 support is available, PCRE supports case-insensitive matching only when
241 there is a one-to-one mapping between a letter's cases. There are a
242 small number of many-to-one mappings in Unicode; these are not sup-
243 ported by PCRE.
244
245
246 AUTHOR
247
248 Philip Hazel
249 University Computing Service
250 Cambridge CB2 3QH, England.
251
252 Putting an actual email address here seems to have been a spam magnet,
253 so I've taken it away. If you want to email me, use my two initials,
254 followed by the two digits 10, at the domain cam.ac.uk.
255
256
257 REVISION
258
259 Last updated: 09 August 2007
260 Copyright (c) 1997-2007 University of Cambridge.
261 ------------------------------------------------------------------------------
262
263
264 PCREBUILD(3) PCREBUILD(3)
265
266
267 NAME
268 PCRE - Perl-compatible regular expressions
269
270
271 PCRE BUILD-TIME OPTIONS
272
273 This document describes the optional features of PCRE that can be
274 selected when the library is compiled. They are all selected, or dese-
275 lected, by providing options to the configure script that is run before
276 the make command. The complete list of options for configure (which
277 includes the standard ones such as the selection of the installation
278 directory) can be obtained by running
279
280 ./configure --help
281
282 The following sections include descriptions of options whose names
283 begin with --enable or --disable. These settings specify changes to the
284 defaults for the configure command. Because of the way that configure
285 works, --enable and --disable always come in pairs, so the complemen-
286 tary option always exists as well, but as it specifies the default, it
287 is not described.
288
289
290 C++ SUPPORT
291
292 By default, the configure script will search for a C++ compiler and C++
293 header files. If it finds them, it automatically builds the C++ wrapper
294 library for PCRE. You can disable this by adding
295
296 --disable-cpp
297
298 to the configure command.
299
300
301 UTF-8 SUPPORT
302
303 To build PCRE with support for UTF-8 character strings, add
304
305 --enable-utf8
306
307 to the configure command. Of itself, this does not make PCRE treat
308 strings as UTF-8. As well as compiling PCRE with this option, you also
309 have have to set the PCRE_UTF8 option when you call the pcre_compile()
310 function.
311
312
313 UNICODE CHARACTER PROPERTY SUPPORT
314
315 UTF-8 support allows PCRE to process character values greater than 255
316 in the strings that it handles. On its own, however, it does not pro-
317 vide any facilities for accessing the properties of such characters. If
318 you want to be able to use the pattern escapes \P, \p, and \X, which
319 refer to Unicode character properties, you must add
320
321 --enable-unicode-properties
322
323 to the configure command. This implies UTF-8 support, even if you have
324 not explicitly requested it.
325
326 Including Unicode property support adds around 30K of tables to the
327 PCRE library. Only the general category properties such as Lu and Nd
328 are supported. Details are given in the pcrepattern documentation.
329
330
331 CODE VALUE OF NEWLINE
332
333 By default, PCRE interprets character 10 (linefeed, LF) as indicating
334 the end of a line. This is the normal newline character on Unix-like
335 systems. You can compile PCRE to use character 13 (carriage return, CR)
336 instead, by adding
337
338 --enable-newline-is-cr
339
340 to the configure command. There is also a --enable-newline-is-lf
341 option, which explicitly specifies linefeed as the newline character.
342
343 Alternatively, you can specify that line endings are to be indicated by
344 the two character sequence CRLF. If you want this, add
345
346 --enable-newline-is-crlf
347
348 to the configure command. There is a fourth option, specified by
349
350 --enable-newline-is-anycrlf
351
352 which causes PCRE to recognize any of the three sequences CR, LF, or
353 CRLF as indicating a line ending. Finally, a fifth option, specified by
354
355 --enable-newline-is-any
356
357 causes PCRE to recognize any Unicode newline sequence.
358
359 Whatever line ending convention is selected when PCRE is built can be
360 overridden when the library functions are called. At build time it is
361 conventional to use the standard for your operating system.
362
363
364 WHAT \R MATCHES
365
366 By default, the sequence \R in a pattern matches any Unicode newline
367 sequence, whatever has been selected as the line ending sequence. If
368 you specify
369
370 --enable-bsr-anycrlf
371
372 the default is changed so that \R matches only CR, LF, or CRLF. What-
373 ever is selected when PCRE is built can be overridden when the library
374 functions are called.
375
376
377 BUILDING SHARED AND STATIC LIBRARIES
378
379 The PCRE building process uses libtool to build both shared and static
380 Unix libraries by default. You can suppress one of these by adding one
381 of
382
383 --disable-shared
384 --disable-static
385
386 to the configure command, as required.
387
388
389 POSIX MALLOC USAGE
390
391 When PCRE is called through the POSIX interface (see the pcreposix doc-
392 umentation), additional working storage is required for holding the
393 pointers to capturing substrings, because PCRE requires three integers
394 per substring, whereas the POSIX interface provides only two. If the
395 number of expected substrings is small, the wrapper function uses space
396 on the stack, because this is faster than using malloc() for each call.
397 The default threshold above which the stack is no longer used is 10; it
398 can be changed by adding a setting such as
399
400 --with-posix-malloc-threshold=20
401
402 to the configure command.
403
404
405 HANDLING VERY LARGE PATTERNS
406
407 Within a compiled pattern, offset values are used to point from one
408 part to another (for example, from an opening parenthesis to an alter-
409 nation metacharacter). By default, two-byte values are used for these
410 offsets, leading to a maximum size for a compiled pattern of around
411 64K. This is sufficient to handle all but the most gigantic patterns.
412 Nevertheless, some people do want to process enormous patterns, so it
413 is possible to compile PCRE to use three-byte or four-byte offsets by
414 adding a setting such as
415
416 --with-link-size=3
417
418 to the configure command. The value given must be 2, 3, or 4. Using
419 longer offsets slows down the operation of PCRE because it has to load
420 additional bytes when handling them.
421
422
423 AVOIDING EXCESSIVE STACK USAGE
424
425 When matching with the pcre_exec() function, PCRE implements backtrack-
426 ing by making recursive calls to an internal function called match().
427 In environments where the size of the stack is limited, this can se-
428 verely limit PCRE's operation. (The Unix environment does not usually
429 suffer from this problem, but it may sometimes be necessary to increase
430 the maximum stack size. There is a discussion in the pcrestack docu-
431 mentation.) An alternative approach to recursion that uses memory from
432 the heap to remember data, instead of using recursive function calls,
433 has been implemented to work round the problem of limited stack size.
434 If you want to build a version of PCRE that works this way, add
435
436 --disable-stack-for-recursion
437
438 to the configure command. With this configuration, PCRE will use the
439 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
440 ment functions. By default these point to malloc() and free(), but you
441 can replace the pointers so that your own functions are used.
442
443 Separate functions are provided rather than using pcre_malloc and
444 pcre_free because the usage is very predictable: the block sizes
445 requested are always the same, and the blocks are always freed in
446 reverse order. A calling program might be able to implement optimized
447 functions that perform better than malloc() and free(). PCRE runs
448 noticeably more slowly when built in this way. This option affects only
449 the pcre_exec() function; it is not relevant for the the
450 pcre_dfa_exec() function.
451
452
453 LIMITING PCRE RESOURCE USAGE
454
455 Internally, PCRE has a function called match(), which it calls repeat-
456 edly (sometimes recursively) when matching a pattern with the
457 pcre_exec() function. By controlling the maximum number of times this
458 function may be called during a single matching operation, a limit can
459 be placed on the resources used by a single call to pcre_exec(). The
460 limit can be changed at run time, as described in the pcreapi documen-
461 tation. The default is 10 million, but this can be changed by adding a
462 setting such as
463
464 --with-match-limit=500000
465
466 to the configure command. This setting has no effect on the
467 pcre_dfa_exec() matching function.
468
469 In some environments it is desirable to limit the depth of recursive
470 calls of match() more strictly than the total number of calls, in order
471 to restrict the maximum amount of stack (or heap, if --disable-stack-
472 for-recursion is specified) that is used. A second limit controls this;
473 it defaults to the value that is set for --with-match-limit, which
474 imposes no additional constraints. However, you can set a lower limit
475 by adding, for example,
476
477 --with-match-limit-recursion=10000
478
479 to the configure command. This value can also be overridden at run
480 time.
481
482
483 CREATING CHARACTER TABLES AT BUILD TIME
484
485 PCRE uses fixed tables for processing characters whose code values are
486 less than 256. By default, PCRE is built with a set of tables that are
487 distributed in the file pcre_chartables.c.dist. These tables are for
488 ASCII codes only. If you add
489
490 --enable-rebuild-chartables
491
492 to the configure command, the distributed tables are no longer used.
493 Instead, a program called dftables is compiled and run. This outputs
494 the source for new set of tables, created in the default locale of your
495 C runtime system. (This method of replacing the tables does not work if
496 you are cross compiling, because dftables is run on the local host. If
497 you need to create alternative tables when cross compiling, you will
498 have to do so "by hand".)
499
500
501 USING EBCDIC CODE
502
503 PCRE assumes by default that it will run in an environment where the
504 character code is ASCII (or Unicode, which is a superset of ASCII).
505 This is the case for most computer operating systems. PCRE can, how-
506 ever, be compiled to run in an EBCDIC environment by adding
507
508 --enable-ebcdic
509
510 to the configure command. This setting implies --enable-rebuild-charta-
511 bles. You should only use it if you know that you are in an EBCDIC
512 environment (for example, an IBM mainframe operating system).
513
514
515 SEE ALSO
516
517 pcreapi(3), pcre_config(3).
518
519
520 AUTHOR
521
522 Philip Hazel
523 University Computing Service
524 Cambridge CB2 3QH, England.
525
526
527 REVISION
528
529 Last updated: 11 September 2007
530 Copyright (c) 1997-2007 University of Cambridge.
531 ------------------------------------------------------------------------------
532
533
534 PCREMATCHING(3) PCREMATCHING(3)
535
536
537 NAME
538 PCRE - Perl-compatible regular expressions
539
540
541 PCRE MATCHING ALGORITHMS
542
543 This document describes the two different algorithms that are available
544 in PCRE for matching a compiled regular expression against a given sub-
545 ject string. The "standard" algorithm is the one provided by the
546 pcre_exec() function. This works in the same was as Perl's matching
547 function, and provides a Perl-compatible matching operation.
548
549 An alternative algorithm is provided by the pcre_dfa_exec() function;
550 this operates in a different way, and is not Perl-compatible. It has
551 advantages and disadvantages compared with the standard algorithm, and
552 these are described below.
553
554 When there is only one possible way in which a given subject string can
555 match a pattern, the two algorithms give the same answer. A difference
556 arises, however, when there are multiple possibilities. For example, if
557 the pattern
558
559 ^<.*>
560
561 is matched against the string
562
563 <something> <something else> <something further>
564
565 there are three possible answers. The standard algorithm finds only one
566 of them, whereas the alternative algorithm finds all three.
567
568
569 REGULAR EXPRESSIONS AS TREES
570
571 The set of strings that are matched by a regular expression can be rep-
572 resented as a tree structure. An unlimited repetition in the pattern
573 makes the tree of infinite size, but it is still a tree. Matching the
574 pattern to a given subject string (from a given starting point) can be
575 thought of as a search of the tree. There are two ways to search a
576 tree: depth-first and breadth-first, and these correspond to the two
577 matching algorithms provided by PCRE.
578
579
580 THE STANDARD MATCHING ALGORITHM
581
582 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
583 sions", the standard algorithm is an "NFA algorithm". It conducts a
584 depth-first search of the pattern tree. That is, it proceeds along a
585 single path through the tree, checking that the subject matches what is
586 required. When there is a mismatch, the algorithm tries any alterna-
587 tives at the current point, and if they all fail, it backs up to the
588 previous branch point in the tree, and tries the next alternative
589 branch at that level. This often involves backing up (moving to the
590 left) in the subject string as well. The order in which repetition
591 branches are tried is controlled by the greedy or ungreedy nature of
592 the quantifier.
593
594 If a leaf node is reached, a matching string has been found, and at
595 that point the algorithm stops. Thus, if there is more than one possi-
596 ble match, this algorithm returns the first one that it finds. Whether
597 this is the shortest, the longest, or some intermediate length depends
598 on the way the greedy and ungreedy repetition quantifiers are specified
599 in the pattern.
600
601 Because it ends up with a single path through the tree, it is rela-
602 tively straightforward for this algorithm to keep track of the sub-
603 strings that are matched by portions of the pattern in parentheses.
604 This provides support for capturing parentheses and back references.
605
606
607 THE ALTERNATIVE MATCHING ALGORITHM
608
609 This algorithm conducts a breadth-first search of the tree. Starting
610 from the first matching point in the subject, it scans the subject
611 string from left to right, once, character by character, and as it does
612 this, it remembers all the paths through the tree that represent valid
613 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
614 though it is not implemented as a traditional finite state machine (it
615 keeps multiple states active simultaneously).
616
617 The scan continues until either the end of the subject is reached, or
618 there are no more unterminated paths. At this point, terminated paths
619 represent the different matching possibilities (if there are none, the
620 match has failed). Thus, if there is more than one possible match,
621 this algorithm finds all of them, and in particular, it finds the long-
622 est. In PCRE, there is an option to stop the algorithm after the first
623 match (which is necessarily the shortest) has been found.
624
625 Note that all the matches that are found start at the same point in the
626 subject. If the pattern
627
628 cat(er(pillar)?)
629
630 is matched against the string "the caterpillar catchment", the result
631 will be the three strings "cat", "cater", and "caterpillar" that start
632 at the fourth character of the subject. The algorithm does not automat-
633 ically move on to find matches that start at later positions.
634
635 There are a number of features of PCRE regular expressions that are not
636 supported by the alternative matching algorithm. They are as follows:
637
638 1. Because the algorithm finds all possible matches, the greedy or
639 ungreedy nature of repetition quantifiers is not relevant. Greedy and
640 ungreedy quantifiers are treated in exactly the same way. However, pos-
641 sessive quantifiers can make a difference when what follows could also
642 match what is quantified, for example in a pattern like this:
643
644 ^a++\w!
645
646 This pattern matches "aaab!" but not "aaa!", which would be matched by
647 a non-possessive quantifier. Similarly, if an atomic group is present,
648 it is matched as if it were a standalone pattern at the current point,
649 and the longest match is then "locked in" for the rest of the overall
650 pattern.
651
652 2. When dealing with multiple paths through the tree simultaneously, it
653 is not straightforward to keep track of captured substrings for the
654 different matching possibilities, and PCRE's implementation of this
655 algorithm does not attempt to do this. This means that no captured sub-
656 strings are available.
657
658 3. Because no substrings are captured, back references within the pat-
659 tern are not supported, and cause errors if encountered.
660
661 4. For the same reason, conditional expressions that use a backrefer-
662 ence as the condition or test for a specific group recursion are not
663 supported.
664
665 5. Because many paths through the tree may be active, the \K escape
666 sequence, which resets the start of the match when encountered (but may
667 be on some paths and not on others), is not supported. It causes an
668 error if encountered.
669
670 6. Callouts are supported, but the value of the capture_top field is
671 always 1, and the value of the capture_last field is always -1.
672
673 7. The \C escape sequence, which (in the standard algorithm) matches a
674 single byte, even in UTF-8 mode, is not supported because the alterna-
675 tive algorithm moves through the subject string one character at a
676 time, for all active paths through the tree.
677
678 8. None of the backtracking control verbs such as (*PRUNE) are sup-
679 ported.
680
681
682 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
683
684 Using the alternative matching algorithm provides the following advan-
685 tages:
686
687 1. All possible matches (at a single point in the subject) are automat-
688 ically found, and in particular, the longest match is found. To find
689 more than one match using the standard algorithm, you have to do kludgy
690 things with callouts.
691
692 2. There is much better support for partial matching. The restrictions
693 on the content of the pattern that apply when using the standard algo-
694 rithm for partial matching do not apply to the alternative algorithm.
695 For non-anchored patterns, the starting position of a partial match is
696 available.
697
698 3. Because the alternative algorithm scans the subject string just
699 once, and never needs to backtrack, it is possible to pass very long
700 subject strings to the matching function in several pieces, checking
701 for partial matching each time.
702
703
704 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
705
706 The alternative algorithm suffers from a number of disadvantages:
707
708 1. It is substantially slower than the standard algorithm. This is
709 partly because it has to search for all possible matches, but is also
710 because it is less susceptible to optimization.
711
712 2. Capturing parentheses and back references are not supported.
713
714 3. Although atomic groups are supported, their use does not provide the
715 performance advantage that it does for the standard algorithm.
716
717
718 AUTHOR
719
720 Philip Hazel
721 University Computing Service
722 Cambridge CB2 3QH, England.
723
724
725 REVISION
726
727 Last updated: 08 August 2007
728 Copyright (c) 1997-2007 University of Cambridge.
729 ------------------------------------------------------------------------------
730
731
732 PCREAPI(3) PCREAPI(3)
733
734
735 NAME
736 PCRE - Perl-compatible regular expressions
737
738
739 PCRE NATIVE API
740
741 #include <pcre.h>
742
743 pcre *pcre_compile(const char *pattern, int options,
744 const char **errptr, int *erroffset,
745 const unsigned char *tableptr);
746
747 pcre *pcre_compile2(const char *pattern, int options,
748 int *errorcodeptr,
749 const char **errptr, int *erroffset,
750 const unsigned char *tableptr);
751
752 pcre_extra *pcre_study(const pcre *code, int options,
753 const char **errptr);
754
755 int pcre_exec(const pcre *code, const pcre_extra *extra,
756 const char *subject, int length, int startoffset,
757 int options, int *ovector, int ovecsize);
758
759 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
760 const char *subject, int length, int startoffset,
761 int options, int *ovector, int ovecsize,
762 int *workspace, int wscount);
763
764 int pcre_copy_named_substring(const pcre *code,
765 const char *subject, int *ovector,
766 int stringcount, const char *stringname,
767 char *buffer, int buffersize);
768
769 int pcre_copy_substring(const char *subject, int *ovector,
770 int stringcount, int stringnumber, char *buffer,
771 int buffersize);
772
773 int pcre_get_named_substring(const pcre *code,
774 const char *subject, int *ovector,
775 int stringcount, const char *stringname,
776 const char **stringptr);
777
778 int pcre_get_stringnumber(const pcre *code,
779 const char *name);
780
781 int pcre_get_stringtable_entries(const pcre *code,
782 const char *name, char **first, char **last);
783
784 int pcre_get_substring(const char *subject, int *ovector,
785 int stringcount, int stringnumber,
786 const char **stringptr);
787
788 int pcre_get_substring_list(const char *subject,
789 int *ovector, int stringcount, const char ***listptr);
790
791 void pcre_free_substring(const char *stringptr);
792
793 void pcre_free_substring_list(const char **stringptr);
794
795 const unsigned char *pcre_maketables(void);
796
797 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
798 int what, void *where);
799
800 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
801
802 int pcre_refcount(pcre *code, int adjust);
803
804 int pcre_config(int what, void *where);
805
806 char *pcre_version(void);
807
808 void *(*pcre_malloc)(size_t);
809
810 void (*pcre_free)(void *);
811
812 void *(*pcre_stack_malloc)(size_t);
813
814 void (*pcre_stack_free)(void *);
815
816 int (*pcre_callout)(pcre_callout_block *);
817
818
819 PCRE API OVERVIEW
820
821 PCRE has its own native API, which is described in this document. There
822 are also some wrapper functions that correspond to the POSIX regular
823 expression API. These are described in the pcreposix documentation.
824 Both of these APIs define a set of C function calls. A C++ wrapper is
825 distributed with PCRE. It is documented in the pcrecpp page.
826
827 The native API C function prototypes are defined in the header file
828 pcre.h, and on Unix systems the library itself is called libpcre. It
829 can normally be accessed by adding -lpcre to the command for linking an
830 application that uses PCRE. The header file defines the macros
831 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
832 bers for the library. Applications can use these to include support
833 for different releases of PCRE.
834
835 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
836 pcre_exec() are used for compiling and matching regular expressions in
837 a Perl-compatible manner. A sample program that demonstrates the sim-
838 plest way of using them is provided in the file called pcredemo.c in
839 the source distribution. The pcresample documentation describes how to
840 run it.
841
842 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
843 ble, is also provided. This uses a different algorithm for the match-
844 ing. The alternative algorithm finds all possible matches (at a given
845 point in the subject), and scans the subject just once. However, this
846 algorithm does not return captured substrings. A description of the two
847 matching algorithms and their advantages and disadvantages is given in
848 the pcrematching documentation.
849
850 In addition to the main compiling and matching functions, there are
851 convenience functions for extracting captured substrings from a subject
852 string that is matched by pcre_exec(). They are:
853
854 pcre_copy_substring()
855 pcre_copy_named_substring()
856 pcre_get_substring()
857 pcre_get_named_substring()
858 pcre_get_substring_list()
859 pcre_get_stringnumber()
860 pcre_get_stringtable_entries()
861
862 pcre_free_substring() and pcre_free_substring_list() are also provided,
863 to free the memory used for extracted strings.
864
865 The function pcre_maketables() is used to build a set of character
866 tables in the current locale for passing to pcre_compile(),
867 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
868 provided for specialist use. Most commonly, no special tables are
869 passed, in which case internal tables that are generated when PCRE is
870 built are used.
871
872 The function pcre_fullinfo() is used to find out information about a
873 compiled pattern; pcre_info() is an obsolete version that returns only
874 some of the available information, but is retained for backwards com-
875 patibility. The function pcre_version() returns a pointer to a string
876 containing the version of PCRE and its date of release.
877
878 The function pcre_refcount() maintains a reference count in a data
879 block containing a compiled pattern. This is provided for the benefit
880 of object-oriented applications.
881
882 The global variables pcre_malloc and pcre_free initially contain the
883 entry points of the standard malloc() and free() functions, respec-
884 tively. PCRE calls the memory management functions via these variables,
885 so a calling program can replace them if it wishes to intercept the
886 calls. This should be done before calling any PCRE functions.
887
888 The global variables pcre_stack_malloc and pcre_stack_free are also
889 indirections to memory management functions. These special functions
890 are used only when PCRE is compiled to use the heap for remembering
891 data, instead of recursive function calls, when running the pcre_exec()
892 function. See the pcrebuild documentation for details of how to do
893 this. It is a non-standard way of building PCRE, for use in environ-
894 ments that have limited stacks. Because of the greater use of memory
895 management, it runs more slowly. Separate functions are provided so
896 that special-purpose external code can be used for this case. When
897 used, these functions are always called in a stack-like manner (last
898 obtained, first freed), and always for memory blocks of the same size.
899 There is a discussion about PCRE's stack usage in the pcrestack docu-
900 mentation.
901
902 The global variable pcre_callout initially contains NULL. It can be set
903 by the caller to a "callout" function, which PCRE will then call at
904 specified points during a matching operation. Details are given in the
905 pcrecallout documentation.
906
907
908 NEWLINES
909
910 PCRE supports five different conventions for indicating line breaks in
911 strings: a single CR (carriage return) character, a single LF (line-
912 feed) character, the two-character sequence CRLF, any of the three pre-
913 ceding, or any Unicode newline sequence. The Unicode newline sequences
914 are the three just mentioned, plus the single characters VT (vertical
915 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
916 separator, U+2028), and PS (paragraph separator, U+2029).
917
918 Each of the first three conventions is used by at least one operating
919 system as its standard newline sequence. When PCRE is built, a default
920 can be specified. The default default is LF, which is the Unix stan-
921 dard. When PCRE is run, the default can be overridden, either when a
922 pattern is compiled, or when it is matched.
923
924 At compile time, the newline convention can be specified by the options
925 argument of pcre_compile(), or it can be specified by special text at
926 the start of the pattern itself; this overrides any other settings. See
927 the pcrepattern page for details of the special character sequences.
928
929 In the PCRE documentation the word "newline" is used to mean "the char-
930 acter or pair of characters that indicate a line break". The choice of
931 newline convention affects the handling of the dot, circumflex, and
932 dollar metacharacters, the handling of #-comments in /x mode, and, when
933 CRLF is a recognized line ending sequence, the match position advance-
934 ment for a non-anchored pattern. There is more detail about this in the
935 section on pcre_exec() options below.
936
937 The choice of newline convention does not affect the interpretation of
938 the \n or \r escape sequences, nor does it affect what \R matches,
939 which is controlled in a similar way, but by separate options.
940
941
942 MULTITHREADING
943
944 The PCRE functions can be used in multi-threading applications, with
945 the proviso that the memory management functions pointed to by
946 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
947 callout function pointed to by pcre_callout, are shared by all threads.
948
949 The compiled form of a regular expression is not altered during match-
950 ing, so the same compiled pattern can safely be used by several threads
951 at once.
952
953
954 SAVING PRECOMPILED PATTERNS FOR LATER USE
955
956 The compiled form of a regular expression can be saved and re-used at a
957 later time, possibly by a different program, and even on a host other
958 than the one on which it was compiled. Details are given in the
959 pcreprecompile documentation. However, compiling a regular expression
960 with one version of PCRE for use with a different version is not guar-
961 anteed to work and may cause crashes.
962
963
964 CHECKING BUILD-TIME OPTIONS
965
966 int pcre_config(int what, void *where);
967
968 The function pcre_config() makes it possible for a PCRE client to dis-
969 cover which optional features have been compiled into the PCRE library.
970 The pcrebuild documentation has more details about these optional fea-
971 tures.
972
973 The first argument for pcre_config() is an integer, specifying which
974 information is required; the second argument is a pointer to a variable
975 into which the information is placed. The following information is
976 available:
977
978 PCRE_CONFIG_UTF8
979
980 The output is an integer that is set to one if UTF-8 support is avail-
981 able; otherwise it is set to zero.
982
983 PCRE_CONFIG_UNICODE_PROPERTIES
984
985 The output is an integer that is set to one if support for Unicode
986 character properties is available; otherwise it is set to zero.
987
988 PCRE_CONFIG_NEWLINE
989
990 The output is an integer whose value specifies the default character
991 sequence that is recognized as meaning "newline". The four values that
992 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
993 and -1 for ANY. The default should normally be the standard sequence
994 for your operating system.
995
996 PCRE_CONFIG_BSR
997
998 The output is an integer whose value indicates what character sequences
999 the \R escape sequence matches by default. A value of 0 means that \R
1000 matches any Unicode line ending sequence; a value of 1 means that \R
1001 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1002 tern is compiled or matched.
1003
1004 PCRE_CONFIG_LINK_SIZE
1005
1006 The output is an integer that contains the number of bytes used for
1007 internal linkage in compiled regular expressions. The value is 2, 3, or
1008 4. Larger values allow larger regular expressions to be compiled, at
1009 the expense of slower matching. The default value of 2 is sufficient
1010 for all but the most massive patterns, since it allows the compiled
1011 pattern to be up to 64K in size.
1012
1013 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1014
1015 The output is an integer that contains the threshold above which the
1016 POSIX interface uses malloc() for output vectors. Further details are
1017 given in the pcreposix documentation.
1018
1019 PCRE_CONFIG_MATCH_LIMIT
1020
1021 The output is an integer that gives the default limit for the number of
1022 internal matching function calls in a pcre_exec() execution. Further
1023 details are given with pcre_exec() below.
1024
1025 PCRE_CONFIG_MATCH_LIMIT_RECURSION
1026
1027 The output is an integer that gives the default limit for the depth of
1028 recursion when calling the internal matching function in a pcre_exec()
1029 execution. Further details are given with pcre_exec() below.
1030
1031 PCRE_CONFIG_STACKRECURSE
1032
1033 The output is an integer that is set to one if internal recursion when
1034 running pcre_exec() is implemented by recursive function calls that use
1035 the stack to remember their state. This is the usual way that PCRE is
1036 compiled. The output is zero if PCRE was compiled to use blocks of data
1037 on the heap instead of recursive function calls. In this case,
1038 pcre_stack_malloc and pcre_stack_free are called to manage memory
1039 blocks on the heap, thus avoiding the use of the stack.
1040
1041
1042 COMPILING A PATTERN
1043
1044 pcre *pcre_compile(const char *pattern, int options,
1045 const char **errptr, int *erroffset,
1046 const unsigned char *tableptr);
1047
1048 pcre *pcre_compile2(const char *pattern, int options,
1049 int *errorcodeptr,
1050 const char **errptr, int *erroffset,
1051 const unsigned char *tableptr);
1052
1053 Either of the functions pcre_compile() or pcre_compile2() can be called
1054 to compile a pattern into an internal form. The only difference between
1055 the two interfaces is that pcre_compile2() has an additional argument,
1056 errorcodeptr, via which a numerical error code can be returned.
1057
1058 The pattern is a C string terminated by a binary zero, and is passed in
1059 the pattern argument. A pointer to a single block of memory that is
1060 obtained via pcre_malloc is returned. This contains the compiled code
1061 and related data. The pcre type is defined for the returned block; this
1062 is a typedef for a structure whose contents are not externally defined.
1063 It is up to the caller to free the memory (via pcre_free) when it is no
1064 longer required.
1065
1066 Although the compiled code of a PCRE regex is relocatable, that is, it
1067 does not depend on memory location, the complete pcre data block is not
1068 fully relocatable, because it may contain a copy of the tableptr argu-
1069 ment, which is an address (see below).
1070
1071 The options argument contains various bit settings that affect the com-
1072 pilation. It should be zero if no options are required. The available
1073 options are described below. Some of them, in particular, those that
1074 are compatible with Perl, can also be set and unset from within the
1075 pattern (see the detailed description in the pcrepattern documenta-
1076 tion). For these options, the contents of the options argument speci-
1077 fies their initial settings at the start of compilation and execution.
1078 The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
1079 of matching as well as at compile time.
1080
1081 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1082 if compilation of a pattern fails, pcre_compile() returns NULL, and
1083 sets the variable pointed to by errptr to point to a textual error mes-
1084 sage. This is a static string that is part of the library. You must not
1085 try to free it. The offset from the start of the pattern to the charac-
1086 ter where the error was discovered is placed in the variable pointed to
1087 by erroffset, which must not be NULL. If it is, an immediate error is
1088 given.
1089
1090 If pcre_compile2() is used instead of pcre_compile(), and the error-
1091 codeptr argument is not NULL, a non-zero error code number is returned
1092 via this argument in the event of an error. This is in addition to the
1093 textual error message. Error codes and messages are listed below.
1094
1095 If the final argument, tableptr, is NULL, PCRE uses a default set of
1096 character tables that are built when PCRE is compiled, using the
1097 default C locale. Otherwise, tableptr must be an address that is the
1098 result of a call to pcre_maketables(). This value is stored with the
1099 compiled pattern, and used again by pcre_exec(), unless another table
1100 pointer is passed to it. For more discussion, see the section on locale
1101 support below.
1102
1103 This code fragment shows a typical straightforward call to pcre_com-
1104 pile():
1105
1106 pcre *re;
1107 const char *error;
1108 int erroffset;
1109 re = pcre_compile(
1110 "^A.*Z", /* the pattern */
1111 0, /* default options */
1112 &error, /* for error message */
1113 &erroffset, /* for error offset */
1114 NULL); /* use default character tables */
1115
1116 The following names for option bits are defined in the pcre.h header
1117 file:
1118
1119 PCRE_ANCHORED
1120
1121 If this bit is set, the pattern is forced to be "anchored", that is, it
1122 is constrained to match only at the first matching point in the string
1123 that is being searched (the "subject string"). This effect can also be
1124 achieved by appropriate constructs in the pattern itself, which is the
1125 only way to do it in Perl.
1126
1127 PCRE_AUTO_CALLOUT
1128
1129 If this bit is set, pcre_compile() automatically inserts callout items,
1130 all with number 255, before each pattern item. For discussion of the
1131 callout facility, see the pcrecallout documentation.
1132
1133 PCRE_BSR_ANYCRLF
1134 PCRE_BSR_UNICODE
1135
1136 These options (which are mutually exclusive) control what the \R escape
1137 sequence matches. The choice is either to match only CR, LF, or CRLF,
1138 or to match any Unicode newline sequence. The default is specified when
1139 PCRE is built. It can be overridden from within the pattern, or by set-
1140 ting an option when a compiled pattern is matched.
1141
1142 PCRE_CASELESS
1143
1144 If this bit is set, letters in the pattern match both upper and lower
1145 case letters. It is equivalent to Perl's /i option, and it can be
1146 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1147 always understands the concept of case for characters whose values are
1148 less than 128, so caseless matching is always possible. For characters
1149 with higher values, the concept of case is supported if PCRE is com-
1150 piled with Unicode property support, but not otherwise. If you want to
1151 use caseless matching for characters 128 and above, you must ensure
1152 that PCRE is compiled with Unicode property support as well as with
1153 UTF-8 support.
1154
1155 PCRE_DOLLAR_ENDONLY
1156
1157 If this bit is set, a dollar metacharacter in the pattern matches only
1158 at the end of the subject string. Without this option, a dollar also
1159 matches immediately before a newline at the end of the string (but not
1160 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1161 if PCRE_MULTILINE is set. There is no equivalent to this option in
1162 Perl, and no way to set it within a pattern.
1163
1164 PCRE_DOTALL
1165
1166 If this bit is set, a dot metacharater in the pattern matches all char-
1167 acters, including those that indicate newline. Without it, a dot does
1168 not match when the current position is at a newline. This option is
1169 equivalent to Perl's /s option, and it can be changed within a pattern
1170 by a (?s) option setting. A negative class such as [^a] always matches
1171 newline characters, independent of the setting of this option.
1172
1173 PCRE_DUPNAMES
1174
1175 If this bit is set, names used to identify capturing subpatterns need
1176 not be unique. This can be helpful for certain types of pattern when it
1177 is known that only one instance of the named subpattern can ever be
1178 matched. There are more details of named subpatterns below; see also
1179 the pcrepattern documentation.
1180
1181 PCRE_EXTENDED
1182
1183 If this bit is set, whitespace data characters in the pattern are
1184 totally ignored except when escaped or inside a character class. White-
1185 space does not include the VT character (code 11). In addition, charac-
1186 ters between an unescaped # outside a character class and the next new-
1187 line, inclusive, are also ignored. This is equivalent to Perl's /x
1188 option, and it can be changed within a pattern by a (?x) option set-
1189 ting.
1190
1191 This option makes it possible to include comments inside complicated
1192 patterns. Note, however, that this applies only to data characters.
1193 Whitespace characters may never appear within special character
1194 sequences in a pattern, for example within the sequence (?( which
1195 introduces a conditional subpattern.
1196
1197 PCRE_EXTRA
1198
1199 This option was invented in order to turn on additional functionality
1200 of PCRE that is incompatible with Perl, but it is currently of very
1201 little use. When set, any backslash in a pattern that is followed by a
1202 letter that has no special meaning causes an error, thus reserving
1203 these combinations for future expansion. By default, as in Perl, a
1204 backslash followed by a letter with no special meaning is treated as a
1205 literal. (Perl can, however, be persuaded to give a warning for this.)
1206 There are at present no other features controlled by this option. It
1207 can also be set by a (?X) option setting within a pattern.
1208
1209 PCRE_FIRSTLINE
1210
1211 If this option is set, an unanchored pattern is required to match
1212 before or at the first newline in the subject string, though the
1213 matched text may continue over the newline.
1214
1215 PCRE_MULTILINE
1216
1217 By default, PCRE treats the subject string as consisting of a single
1218 line of characters (even if it actually contains newlines). The "start
1219 of line" metacharacter (^) matches only at the start of the string,
1220 while the "end of line" metacharacter ($) matches only at the end of
1221 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1222 is set). This is the same as Perl.
1223
1224 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1225 constructs match immediately following or immediately before internal
1226 newlines in the subject string, respectively, as well as at the very
1227 start and end. This is equivalent to Perl's /m option, and it can be
1228 changed within a pattern by a (?m) option setting. If there are no new-
1229 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1230 setting PCRE_MULTILINE has no effect.
1231
1232 PCRE_NEWLINE_CR
1233 PCRE_NEWLINE_LF
1234 PCRE_NEWLINE_CRLF
1235 PCRE_NEWLINE_ANYCRLF
1236 PCRE_NEWLINE_ANY
1237
1238 These options override the default newline definition that was chosen
1239 when PCRE was built. Setting the first or the second specifies that a
1240 newline is indicated by a single character (CR or LF, respectively).
1241 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1242 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1243 that any of the three preceding sequences should be recognized. Setting
1244 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1245 recognized. The Unicode newline sequences are the three just mentioned,
1246 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1247 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1248 (paragraph separator, U+2029). The last two are recognized only in
1249 UTF-8 mode.
1250
1251 The newline setting in the options word uses three bits that are
1252 treated as a number, giving eight possibilities. Currently only six are
1253 used (default plus the five values above). This means that if you set
1254 more than one newline option, the combination may or may not be sensi-
1255 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1256 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1257 cause an error.
1258
1259 The only time that a line break is specially recognized when compiling
1260 a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1261 character class is encountered. This indicates a comment that lasts
1262 until after the next line break sequence. In other circumstances, line
1263 break sequences are treated as literal data, except that in
1264 PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1265 and are therefore ignored.
1266
1267 The newline option that is set at compile time becomes the default that
1268 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1269
1270 PCRE_NO_AUTO_CAPTURE
1271
1272 If this option is set, it disables the use of numbered capturing paren-
1273 theses in the pattern. Any opening parenthesis that is not followed by
1274 ? behaves as if it were followed by ?: but named parentheses can still
1275 be used for capturing (and they acquire numbers in the usual way).
1276 There is no equivalent of this option in Perl.
1277
1278 PCRE_UNGREEDY
1279
1280 This option inverts the "greediness" of the quantifiers so that they
1281 are not greedy by default, but become greedy if followed by "?". It is
1282 not compatible with Perl. It can also be set by a (?U) option setting
1283 within the pattern.
1284
1285 PCRE_UTF8
1286
1287 This option causes PCRE to regard both the pattern and the subject as
1288 strings of UTF-8 characters instead of single-byte character strings.
1289 However, it is available only when PCRE is built to include UTF-8 sup-
1290 port. If not, the use of this option provokes an error. Details of how
1291 this option changes the behaviour of PCRE are given in the section on
1292 UTF-8 support in the main pcre page.
1293
1294 PCRE_NO_UTF8_CHECK
1295
1296 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1297 automatically checked. There is a discussion about the validity of
1298 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1299 bytes is found, pcre_compile() returns an error. If you already know
1300 that your pattern is valid, and you want to skip this check for perfor-
1301 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1302 set, the effect of passing an invalid UTF-8 string as a pattern is
1303 undefined. It may cause your program to crash. Note that this option
1304 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1305 UTF-8 validity checking of subject strings.
1306
1307
1308 COMPILATION ERROR CODES
1309
1310 The following table lists the error codes than may be returned by
1311 pcre_compile2(), along with the error messages that may be returned by
1312 both compiling functions. As PCRE has developed, some error codes have
1313 fallen out of use. To avoid confusion, they have not been re-used.
1314
1315 0 no error
1316 1 \ at end of pattern
1317 2 \c at end of pattern
1318 3 unrecognized character follows \
1319 4 numbers out of order in {} quantifier
1320 5 number too big in {} quantifier
1321 6 missing terminating ] for character class
1322 7 invalid escape sequence in character class
1323 8 range out of order in character class
1324 9 nothing to repeat
1325 10 [this code is not in use]
1326 11 internal error: unexpected repeat
1327 12 unrecognized character after (?
1328 13 POSIX named classes are supported only within a class
1329 14 missing )
1330 15 reference to non-existent subpattern
1331 16 erroffset passed as NULL
1332 17 unknown option bit(s) set
1333 18 missing ) after comment
1334 19 [this code is not in use]
1335 20 regular expression too large
1336 21 failed to get memory
1337 22 unmatched parentheses
1338 23 internal error: code overflow
1339 24 unrecognized character after (?<
1340 25 lookbehind assertion is not fixed length
1341 26 malformed number or name after (?(
1342 27 conditional group contains more than two branches
1343 28 assertion expected after (?(
1344 29 (?R or (?[+-]digits must be followed by )
1345 30 unknown POSIX class name
1346 31 POSIX collating elements are not supported
1347 32 this version of PCRE is not compiled with PCRE_UTF8 support
1348 33 [this code is not in use]
1349 34 character value in \x{...} sequence is too large
1350 35 invalid condition (?(0)
1351 36 \C not allowed in lookbehind assertion
1352 37 PCRE does not support \L, \l, \N, \U, or \u
1353 38 number after (?C is > 255
1354 39 closing ) for (?C expected
1355 40 recursive call could loop indefinitely
1356 41 unrecognized character after (?P
1357 42 syntax error in subpattern name (missing terminator)
1358 43 two named subpatterns have the same name
1359 44 invalid UTF-8 string
1360 45 support for \P, \p, and \X has not been compiled
1361 46 malformed \P or \p sequence
1362 47 unknown property name after \P or \p
1363 48 subpattern name is too long (maximum 32 characters)
1364 49 too many named subpatterns (maximum 10,000)
1365 50 [this code is not in use]
1366 51 octal value is greater than \377 (not in UTF-8 mode)
1367 52 internal error: overran compiling workspace
1368 53 internal error: previously-checked referenced subpattern not
1369 found
1370 54 DEFINE group contains more than one branch
1371 55 repeating a DEFINE group is not allowed
1372 56 inconsistent NEWLINE options
1373 57 \g is not followed by a braced name or an optionally braced
1374 non-zero number
1375 58 (?+ or (?- or (?(+ or (?(- must be followed by a non-zero number
1376
1377
1378 STUDYING A PATTERN
1379
1380 pcre_extra *pcre_study(const pcre *code, int options
1381 const char **errptr);
1382
1383 If a compiled pattern is going to be used several times, it is worth
1384 spending more time analyzing it in order to speed up the time taken for
1385 matching. The function pcre_study() takes a pointer to a compiled pat-
1386 tern as its first argument. If studying the pattern produces additional
1387 information that will help speed up matching, pcre_study() returns a
1388 pointer to a pcre_extra block, in which the study_data field points to
1389 the results of the study.
1390
1391 The returned value from pcre_study() can be passed directly to
1392 pcre_exec(). However, a pcre_extra block also contains other fields
1393 that can be set by the caller before the block is passed; these are
1394 described below in the section on matching a pattern.
1395
1396 If studying the pattern does not produce any additional information
1397 pcre_study() returns NULL. In that circumstance, if the calling program
1398 wants to pass any of the other fields to pcre_exec(), it must set up
1399 its own pcre_extra block.
1400
1401 The second argument of pcre_study() contains option bits. At present,
1402 no options are defined, and this argument should always be zero.
1403
1404 The third argument for pcre_study() is a pointer for an error message.
1405 If studying succeeds (even if no data is returned), the variable it
1406 points to is set to NULL. Otherwise it is set to point to a textual
1407 error message. This is a static string that is part of the library. You
1408 must not try to free it. You should test the error pointer for NULL
1409 after calling pcre_study(), to be sure that it has run successfully.
1410
1411 This is a typical call to pcre_study():
1412
1413 pcre_extra *pe;
1414 pe = pcre_study(
1415 re, /* result of pcre_compile() */
1416 0, /* no options exist */
1417 &error); /* set to NULL or points to a message */
1418
1419 At present, studying a pattern is useful only for non-anchored patterns
1420 that do not have a single fixed starting character. A bitmap of possi-
1421 ble starting bytes is created.
1422
1423
1424 LOCALE SUPPORT
1425
1426 PCRE handles caseless matching, and determines whether characters are
1427 letters, digits, or whatever, by reference to a set of tables, indexed
1428 by character value. When running in UTF-8 mode, this applies only to
1429 characters with codes less than 128. Higher-valued codes never match
1430 escapes such as \w or \d, but can be tested with \p if PCRE is built
1431 with Unicode character property support. The use of locales with Uni-
1432 code is discouraged. If you are handling characters with codes greater
1433 than 128, you should either use UTF-8 and Unicode, or use locales, but
1434 not try to mix the two.
1435
1436 PCRE contains an internal set of tables that are used when the final
1437 argument of pcre_compile() is NULL. These are sufficient for many
1438 applications. Normally, the internal tables recognize only ASCII char-
1439 acters. However, when PCRE is built, it is possible to cause the inter-
1440 nal tables to be rebuilt in the default "C" locale of the local system,
1441 which may cause them to be different.
1442
1443 The internal tables can always be overridden by tables supplied by the
1444 application that calls PCRE. These may be created in a different locale
1445 from the default. As more and more applications change to using Uni-
1446 code, the need for this locale support is expected to die away.
1447
1448 External tables are built by calling the pcre_maketables() function,
1449 which has no arguments, in the relevant locale. The result can then be
1450 passed to pcre_compile() or pcre_exec() as often as necessary. For
1451 example, to build and use tables that are appropriate for the French
1452 locale (where accented characters with values greater than 128 are
1453 treated as letters), the following code could be used:
1454
1455 setlocale(LC_CTYPE, "fr_FR");
1456 tables = pcre_maketables();
1457 re = pcre_compile(..., tables);
1458
1459 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1460 if you are using Windows, the name for the French locale is "french".
1461
1462 When pcre_maketables() runs, the tables are built in memory that is
1463 obtained via pcre_malloc. It is the caller's responsibility to ensure
1464 that the memory containing the tables remains available for as long as
1465 it is needed.
1466
1467 The pointer that is passed to pcre_compile() is saved with the compiled
1468 pattern, and the same tables are used via this pointer by pcre_study()
1469 and normally also by pcre_exec(). Thus, by default, for any single pat-
1470 tern, compilation, studying and matching all happen in the same locale,
1471 but different patterns can be compiled in different locales.
1472
1473 It is possible to pass a table pointer or NULL (indicating the use of
1474 the internal tables) to pcre_exec(). Although not intended for this
1475 purpose, this facility could be used to match a pattern in a different
1476 locale from the one in which it was compiled. Passing table pointers at
1477 run time is discussed below in the section on matching a pattern.
1478
1479
1480 INFORMATION ABOUT A PATTERN
1481
1482 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1483 int what, void *where);
1484
1485 The pcre_fullinfo() function returns information about a compiled pat-
1486 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1487 less retained for backwards compability (and is documented below).
1488
1489 The first argument for pcre_fullinfo() is a pointer to the compiled
1490 pattern. The second argument is the result of pcre_study(), or NULL if
1491 the pattern was not studied. The third argument specifies which piece
1492 of information is required, and the fourth argument is a pointer to a
1493 variable to receive the data. The yield of the function is zero for
1494 success, or one of the following negative numbers:
1495
1496 PCRE_ERROR_NULL the argument code was NULL
1497 the argument where was NULL
1498 PCRE_ERROR_BADMAGIC the "magic number" was not found
1499 PCRE_ERROR_BADOPTION the value of what was invalid
1500
1501 The "magic number" is placed at the start of each compiled pattern as
1502 an simple check against passing an arbitrary memory pointer. Here is a
1503 typical call of pcre_fullinfo(), to obtain the length of the compiled
1504 pattern:
1505
1506 int rc;
1507 size_t length;
1508 rc = pcre_fullinfo(
1509 re, /* result of pcre_compile() */
1510 pe, /* result of pcre_study(), or NULL */
1511 PCRE_INFO_SIZE, /* what is required */
1512 &length); /* where to put the data */
1513
1514 The possible values for the third argument are defined in pcre.h, and
1515 are as follows:
1516
1517 PCRE_INFO_BACKREFMAX
1518
1519 Return the number of the highest back reference in the pattern. The
1520 fourth argument should point to an int variable. Zero is returned if
1521 there are no back references.
1522
1523 PCRE_INFO_CAPTURECOUNT
1524
1525 Return the number of capturing subpatterns in the pattern. The fourth
1526 argument should point to an int variable.
1527
1528 PCRE_INFO_DEFAULT_TABLES
1529
1530 Return a pointer to the internal default character tables within PCRE.
1531 The fourth argument should point to an unsigned char * variable. This
1532 information call is provided for internal use by the pcre_study() func-
1533 tion. External callers can cause PCRE to use its internal tables by
1534 passing a NULL table pointer.
1535
1536 PCRE_INFO_FIRSTBYTE
1537
1538 Return information about the first byte of any matched string, for a
1539 non-anchored pattern. The fourth argument should point to an int vari-
1540 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1541 is still recognized for backwards compatibility.)
1542
1543 If there is a fixed first byte, for example, from a pattern such as
1544 (cat|cow|coyote), its value is returned. Otherwise, if either
1545
1546 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1547 branch starts with "^", or
1548
1549 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1550 set (if it were set, the pattern would be anchored),
1551
1552 -1 is returned, indicating that the pattern matches only at the start
1553 of a subject string or after any newline within the string. Otherwise
1554 -2 is returned. For anchored patterns, -2 is returned.
1555
1556 PCRE_INFO_FIRSTTABLE
1557
1558 If the pattern was studied, and this resulted in the construction of a
1559 256-bit table indicating a fixed set of bytes for the first byte in any
1560 matching string, a pointer to the table is returned. Otherwise NULL is
1561 returned. The fourth argument should point to an unsigned char * vari-
1562 able.
1563
1564 PCRE_INFO_HASCRORLF
1565
1566 Return 1 if the pattern contains any explicit matches for CR or LF
1567 characters, otherwise 0. The fourth argument should point to an int
1568 variable. An explicit match is either a literal CR or LF character, or
1569 \r or \n.
1570
1571 PCRE_INFO_JCHANGED
1572
1573 Return 1 if the (?J) option setting is used in the pattern, otherwise
1574 0. The fourth argument should point to an int variable. The (?J) inter-
1575 nal option setting changes the local PCRE_DUPNAMES option.
1576
1577 PCRE_INFO_LASTLITERAL
1578
1579 Return the value of the rightmost literal byte that must exist in any
1580 matched string, other than at its start, if such a byte has been
1581 recorded. The fourth argument should point to an int variable. If there
1582 is no such byte, -1 is returned. For anchored patterns, a last literal
1583 byte is recorded only if it follows something of variable length. For
1584 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1585 /^a\dz\d/ the returned value is -1.
1586
1587 PCRE_INFO_NAMECOUNT
1588 PCRE_INFO_NAMEENTRYSIZE
1589 PCRE_INFO_NAMETABLE
1590
1591 PCRE supports the use of named as well as numbered capturing parenthe-
1592 ses. The names are just an additional way of identifying the parenthe-
1593 ses, which still acquire numbers. Several convenience functions such as
1594 pcre_get_named_substring() are provided for extracting captured sub-
1595 strings by name. It is also possible to extract the data directly, by
1596 first converting the name to a number in order to access the correct
1597 pointers in the output vector (described with pcre_exec() below). To do
1598 the conversion, you need to use the name-to-number map, which is
1599 described by these three values.
1600
1601 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1602 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1603 of each entry; both of these return an int value. The entry size
1604 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1605 a pointer to the first entry of the table (a pointer to char). The
1606 first two bytes of each entry are the number of the capturing parenthe-
1607 sis, most significant byte first. The rest of the entry is the corre-
1608 sponding name, zero terminated. The names are in alphabetical order.
1609 When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1610 theses numbers. For example, consider the following pattern (assume
1611 PCRE_EXTENDED is set, so white space - including newlines - is
1612 ignored):
1613
1614 (?<date> (?<year>(\d\d)?\d\d) -
1615 (?<month>\d\d) - (?<day>\d\d) )
1616
1617 There are four named subpatterns, so the table has four entries, and
1618 each entry in the table is eight bytes long. The table is as follows,
1619 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1620 as ??:
1621
1622 00 01 d a t e 00 ??
1623 00 05 d a y 00 ?? ??
1624 00 04 m o n t h 00
1625 00 02 y e a r 00 ??
1626
1627 When writing code to extract data from named subpatterns using the
1628 name-to-number map, remember that the length of the entries is likely
1629 to be different for each compiled pattern.
1630
1631 PCRE_INFO_OKPARTIAL
1632
1633 Return 1 if the pattern can be used for partial matching, otherwise 0.
1634 The fourth argument should point to an int variable. The pcrepartial
1635 documentation lists the restrictions that apply to patterns when par-
1636 tial matching is used.
1637
1638 PCRE_INFO_OPTIONS
1639
1640 Return a copy of the options with which the pattern was compiled. The
1641 fourth argument should point to an unsigned long int variable. These
1642 option bits are those specified in the call to pcre_compile(), modified
1643 by any top-level option settings at the start of the pattern itself. In
1644 other words, they are the options that will be in force when matching
1645 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1646 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1647 and PCRE_EXTENDED.
1648
1649 A pattern is automatically anchored by PCRE if all of its top-level
1650 alternatives begin with one of the following:
1651
1652 ^ unless PCRE_MULTILINE is set
1653 \A always
1654 \G always
1655 .* if PCRE_DOTALL is set and there are no back
1656 references to the subpattern in which .* appears
1657
1658 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1659 by pcre_fullinfo().
1660
1661 PCRE_INFO_SIZE
1662
1663 Return the size of the compiled pattern, that is, the value that was
1664 passed as the argument to pcre_malloc() when PCRE was getting memory in
1665 which to place the compiled data. The fourth argument should point to a
1666 size_t variable.
1667
1668 PCRE_INFO_STUDYSIZE
1669
1670 Return the size of the data block pointed to by the study_data field in
1671 a pcre_extra block. That is, it is the value that was passed to
1672 pcre_malloc() when PCRE was getting memory into which to place the data
1673 created by pcre_study(). The fourth argument should point to a size_t
1674 variable.
1675
1676
1677 OBSOLETE INFO FUNCTION
1678
1679 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1680
1681 The pcre_info() function is now obsolete because its interface is too
1682 restrictive to return all the available data about a compiled pattern.
1683 New programs should use pcre_fullinfo() instead. The yield of
1684 pcre_info() is the number of capturing subpatterns, or one of the fol-
1685 lowing negative numbers:
1686
1687 PCRE_ERROR_NULL the argument code was NULL
1688 PCRE_ERROR_BADMAGIC the "magic number" was not found
1689
1690 If the optptr argument is not NULL, a copy of the options with which
1691 the pattern was compiled is placed in the integer it points to (see
1692 PCRE_INFO_OPTIONS above).
1693
1694 If the pattern is not anchored and the firstcharptr argument is not
1695 NULL, it is used to pass back information about the first character of
1696 any matched string (see PCRE_INFO_FIRSTBYTE above).
1697
1698
1699 REFERENCE COUNTS
1700
1701 int pcre_refcount(pcre *code, int adjust);
1702
1703 The pcre_refcount() function is used to maintain a reference count in
1704 the data block that contains a compiled pattern. It is provided for the
1705 benefit of applications that operate in an object-oriented manner,
1706 where different parts of the application may be using the same compiled
1707 pattern, but you want to free the block when they are all done.
1708
1709 When a pattern is compiled, the reference count field is initialized to
1710 zero. It is changed only by calling this function, whose action is to
1711 add the adjust value (which may be positive or negative) to it. The
1712 yield of the function is the new value. However, the value of the count
1713 is constrained to lie between 0 and 65535, inclusive. If the new value
1714 is outside these limits, it is forced to the appropriate limit value.
1715
1716 Except when it is zero, the reference count is not correctly preserved
1717 if a pattern is compiled on one host and then transferred to a host
1718 whose byte-order is different. (This seems a highly unlikely scenario.)
1719
1720
1721 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1722
1723 int pcre_exec(const pcre *code, const pcre_extra *extra,
1724 const char *subject, int length, int startoffset,
1725 int options, int *ovector, int ovecsize);
1726
1727 The function pcre_exec() is called to match a subject string against a
1728 compiled pattern, which is passed in the code argument. If the pattern
1729 has been studied, the result of the study should be passed in the extra
1730 argument. This function is the main matching facility of the library,
1731 and it operates in a Perl-like manner. For specialist use there is also
1732 an alternative matching function, which is described below in the sec-
1733 tion about the pcre_dfa_exec() function.
1734
1735 In most applications, the pattern will have been compiled (and option-
1736 ally studied) in the same process that calls pcre_exec(). However, it
1737 is possible to save compiled patterns and study data, and then use them
1738 later in different processes, possibly even on different hosts. For a
1739 discussion about this, see the pcreprecompile documentation.
1740
1741 Here is an example of a simple call to pcre_exec():
1742
1743 int rc;
1744 int ovector[30];
1745 rc = pcre_exec(
1746 re, /* result of pcre_compile() */
1747 NULL, /* we didn't study the pattern */
1748 "some string", /* the subject string */
1749 11, /* the length of the subject string */
1750 0, /* start at offset 0 in the subject */
1751 0, /* default options */
1752 ovector, /* vector of integers for substring information */
1753 30); /* number of elements (NOT size in bytes) */
1754
1755 Extra data for pcre_exec()
1756
1757 If the extra argument is not NULL, it must point to a pcre_extra data
1758 block. The pcre_study() function returns such a block (when it doesn't
1759 return NULL), but you can also create one for yourself, and pass addi-
1760 tional information in it. The pcre_extra block contains the following
1761 fields (not necessarily in this order):
1762
1763 unsigned long int flags;
1764 void *study_data;
1765 unsigned long int match_limit;
1766 unsigned long int match_limit_recursion;
1767 void *callout_data;
1768 const unsigned char *tables;
1769
1770 The flags field is a bitmap that specifies which of the other fields
1771 are set. The flag bits are:
1772
1773 PCRE_EXTRA_STUDY_DATA
1774 PCRE_EXTRA_MATCH_LIMIT
1775 PCRE_EXTRA_MATCH_LIMIT_RECURSION
1776 PCRE_EXTRA_CALLOUT_DATA
1777 PCRE_EXTRA_TABLES
1778
1779 Other flag bits should be set to zero. The study_data field is set in
1780 the pcre_extra block that is returned by pcre_study(), together with
1781 the appropriate flag bit. You should not set this yourself, but you may
1782 add to the block by setting the other fields and their corresponding
1783 flag bits.
1784
1785 The match_limit field provides a means of preventing PCRE from using up
1786 a vast amount of resources when running patterns that are not going to
1787 match, but which have a very large number of possibilities in their
1788 search trees. The classic example is the use of nested unlimited
1789 repeats.
1790
1791 Internally, PCRE uses a function called match() which it calls repeat-
1792 edly (sometimes recursively). The limit set by match_limit is imposed
1793 on the number of times this function is called during a match, which
1794 has the effect of limiting the amount of backtracking that can take
1795 place. For patterns that are not anchored, the count restarts from zero
1796 for each position in the subject string.
1797
1798 The default value for the limit can be set when PCRE is built; the
1799 default default is 10 million, which handles all but the most extreme
1800 cases. You can override the default by suppling pcre_exec() with a
1801 pcre_extra block in which match_limit is set, and
1802 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1803 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1804
1805 The match_limit_recursion field is similar to match_limit, but instead
1806 of limiting the total number of times that match() is called, it limits
1807 the depth of recursion. The recursion depth is a smaller number than
1808 the total number of calls, because not all calls to match() are recur-
1809 sive. This limit is of use only if it is set smaller than match_limit.
1810
1811 Limiting the recursion depth limits the amount of stack that can be
1812 used, or, when PCRE has been compiled to use memory on the heap instead
1813 of the stack, the amount of heap memory that can be used.
1814
1815 The default value for match_limit_recursion can be set when PCRE is
1816 built; the default default is the same value as the default for
1817 match_limit. You can override the default by suppling pcre_exec() with
1818 a pcre_extra block in which match_limit_recursion is set, and
1819 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1820 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1821
1822 The pcre_callout field is used in conjunction with the "callout" fea-
1823 ture, which is described in the pcrecallout documentation.
1824
1825 The tables field is used to pass a character tables pointer to
1826 pcre_exec(); this overrides the value that is stored with the compiled
1827 pattern. A non-NULL value is stored with the compiled pattern only if
1828 custom tables were supplied to pcre_compile() via its tableptr argu-
1829 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1830 PCRE's internal tables to be used. This facility is helpful when re-
1831 using patterns that have been saved after compiling with an external
1832 set of tables, because the external tables might be at a different
1833 address when pcre_exec() is called. See the pcreprecompile documenta-
1834 tion for a discussion of saving compiled patterns for later use.
1835
1836 Option bits for pcre_exec()
1837
1838 The unused bits of the options argument for pcre_exec() must be zero.
1839 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1840 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
1841 PCRE_PARTIAL.
1842
1843 PCRE_ANCHORED
1844
1845 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
1846 matching position. If a pattern was compiled with PCRE_ANCHORED, or
1847 turned out to be anchored by virtue of its contents, it cannot be made
1848 unachored at matching time.
1849
1850 PCRE_BSR_ANYCRLF
1851 PCRE_BSR_UNICODE
1852
1853 These options (which are mutually exclusive) control what the \R escape
1854 sequence matches. The choice is either to match only CR, LF, or CRLF,
1855 or to match any Unicode newline sequence. These options override the
1856 choice that was made or defaulted when the pattern was compiled.
1857
1858 PCRE_NEWLINE_CR
1859 PCRE_NEWLINE_LF
1860 PCRE_NEWLINE_CRLF
1861 PCRE_NEWLINE_ANYCRLF
1862 PCRE_NEWLINE_ANY
1863
1864 These options override the newline definition that was chosen or
1865 defaulted when the pattern was compiled. For details, see the descrip-
1866 tion of pcre_compile() above. During matching, the newline choice
1867 affects the behaviour of the dot, circumflex, and dollar metacharac-
1868 ters. It may also alter the way the match position is advanced after a
1869 match failure for an unanchored pattern.
1870
1871 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
1872 set, and a match attempt for an unanchored pattern fails when the cur-
1873 rent position is at a CRLF sequence, and the pattern contains no
1874 explicit matches for CR or LF characters, the match position is
1875 advanced by two characters instead of one, in other words, to after the
1876 CRLF.
1877
1878 The above rule is a compromise that makes the most common cases work as
1879 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
1880 option is not set), it does not match the string "\r\nA" because, after
1881 failing at the start, it skips both the CR and the LF before retrying.
1882 However, the pattern [\r\n]A does match that string, because it con-
1883 tains an explicit CR or LF reference, and so advances only by one char-
1884 acter after the first failure.
1885
1886 An explicit match for CR of LF is either a literal appearance of one of
1887 those characters, or one of the \r or \n escape sequences. Implicit
1888 matches such as [^X] do not count, nor does \s (which includes CR and
1889 LF in the characters that it matches).
1890
1891 Notwithstanding the above, anomalous effects may still occur when CRLF
1892 is a valid newline sequence and explicit \r or \n escapes appear in the
1893 pattern.
1894
1895 PCRE_NOTBOL
1896
1897 This option specifies that first character of the subject string is not
1898 the beginning of a line, so the circumflex metacharacter should not
1899 match before it. Setting this without PCRE_MULTILINE (at compile time)
1900 causes circumflex never to match. This option affects only the behav-
1901 iour of the circumflex metacharacter. It does not affect \A.
1902
1903 PCRE_NOTEOL
1904
1905 This option specifies that the end of the subject string is not the end
1906 of a line, so the dollar metacharacter should not match it nor (except
1907 in multiline mode) a newline immediately before it. Setting this with-
1908 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1909 option affects only the behaviour of the dollar metacharacter. It does
1910 not affect \Z or \z.
1911
1912 PCRE_NOTEMPTY
1913
1914 An empty string is not considered to be a valid match if this option is
1915 set. If there are alternatives in the pattern, they are tried. If all
1916 the alternatives match the empty string, the entire match fails. For
1917 example, if the pattern
1918
1919 a?b?
1920
1921 is applied to a string not beginning with "a" or "b", it matches the
1922 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
1923 match is not valid, so PCRE searches further into the string for occur-
1924 rences of "a" or "b".
1925
1926 Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
1927 cial case of a pattern match of the empty string within its split()
1928 function, and when using the /g modifier. It is possible to emulate
1929 Perl's behaviour after matching a null string by first trying the match
1930 again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
1931 if that fails by advancing the starting offset (see below) and trying
1932 an ordinary match again. There is some code that demonstrates how to do
1933 this in the pcredemo.c sample program.
1934
1935 PCRE_NO_UTF8_CHECK
1936
1937 When PCRE_UTF8 is set at compile time, the validity of the subject as a
1938 UTF-8 string is automatically checked when pcre_exec() is subsequently
1939 called. The value of startoffset is also checked to ensure that it
1940 points to the start of a UTF-8 character. There is a discussion about
1941 the validity of UTF-8 strings in the section on UTF-8 support in the
1942 main pcre page. If an invalid UTF-8 sequence of bytes is found,
1943 pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con-
1944 tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
1945
1946 If you already know that your subject is valid, and you want to skip
1947 these checks for performance reasons, you can set the
1948 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
1949 do this for the second and subsequent calls to pcre_exec() if you are
1950 making repeated calls to find all the matches in a single subject
1951 string. However, you should be sure that the value of startoffset
1952 points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
1953 set, the effect of passing an invalid UTF-8 string as a subject, or a
1954 value of startoffset that does not point to the start of a UTF-8 char-
1955 acter, is undefined. Your program may crash.
1956
1957 PCRE_PARTIAL
1958
1959 This option turns on the partial matching feature. If the subject
1960 string fails to match the pattern, but at some point during the match-
1961 ing process the end of the subject was reached (that is, the subject
1962 partially matches the pattern and the failure to match occurred only
1963 because there were not enough subject characters), pcre_exec() returns
1964 PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
1965 used, there are restrictions on what may appear in the pattern. These
1966 are discussed in the pcrepartial documentation.
1967
1968 The string to be matched by pcre_exec()
1969
1970 The subject string is passed to pcre_exec() as a pointer in subject, a
1971 length in length, and a starting byte offset in startoffset. In UTF-8
1972 mode, the byte offset must point to the start of a UTF-8 character.
1973 Unlike the pattern string, the subject may contain binary zero bytes.
1974 When the starting offset is zero, the search for a match starts at the
1975 beginning of the subject, and this is by far the most common case.
1976
1977 A non-zero starting offset is useful when searching for another match
1978 in the same subject by calling pcre_exec() again after a previous suc-
1979 cess. Setting startoffset differs from just passing over a shortened
1980 string and setting PCRE_NOTBOL in the case of a pattern that begins
1981 with any kind of lookbehind. For example, consider the pattern
1982
1983 \Biss\B
1984
1985 which finds occurrences of "iss" in the middle of words. (\B matches
1986 only if the current position in the subject is not a word boundary.)
1987 When applied to the string "Mississipi" the first call to pcre_exec()
1988 finds the first occurrence. If pcre_exec() is called again with just
1989 the remainder of the subject, namely "issipi", it does not match,
1990 because \B is always false at the start of the subject, which is deemed
1991 to be a word boundary. However, if pcre_exec() is passed the entire
1992 string again, but with startoffset set to 4, it finds the second occur-
1993 rence of "iss" because it is able to look behind the starting point to
1994 discover that it is preceded by a letter.
1995
1996 If a non-zero starting offset is passed when the pattern is anchored,
1997 one attempt to match at the given offset is made. This can only succeed
1998 if the pattern does not require the match to be at the start of the
1999 subject.
2000
2001 How pcre_exec() returns captured substrings
2002
2003 In general, a pattern matches a certain portion of the subject, and in
2004 addition, further substrings from the subject may be picked out by
2005 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2006 this is called "capturing" in what follows, and the phrase "capturing
2007 subpattern" is used for a fragment of a pattern that picks out a sub-
2008 string. PCRE supports several other kinds of parenthesized subpattern
2009 that do not cause substrings to be captured.
2010
2011 Captured substrings are returned to the caller via a vector of integer
2012 offsets whose address is passed in ovector. The number of elements in
2013 the vector is passed in ovecsize, which must be a non-negative number.
2014 Note: this argument is NOT the size of ovector in bytes.
2015
2016 The first two-thirds of the vector is used to pass back captured sub-
2017 strings, each substring using a pair of integers. The remaining third
2018 of the vector is used as workspace by pcre_exec() while matching cap-
2019 turing subpatterns, and is not available for passing back information.
2020 The length passed in ovecsize should always be a multiple of three. If
2021 it is not, it is rounded down.
2022
2023 When a match is successful, information about captured substrings is
2024 returned in pairs of integers, starting at the beginning of ovector,
2025 and continuing up to two-thirds of its length at the most. The first
2026 element of a pair is set to the offset of the first character in a sub-
2027 string, and the second is set to the offset of the first character
2028 after the end of a substring. The first pair, ovector[0] and ovec-
2029 tor[1], identify the portion of the subject string matched by the
2030 entire pattern. The next pair is used for the first capturing subpat-
2031 tern, and so on. The value returned by pcre_exec() is one more than the
2032 highest numbered pair that has been set. For example, if two substrings
2033 have been captured, the returned value is 3. If there are no capturing
2034 subpatterns, the return value from a successful match is 1, indicating
2035 that just the first pair of offsets has been set.
2036
2037 If a capturing subpattern is matched repeatedly, it is the last portion
2038 of the string that it matched that is returned.
2039
2040 If the vector is too small to hold all the captured substring offsets,
2041 it is used as far as possible (up to two-thirds of its length), and the
2042 function returns a value of zero. In particular, if the substring off-
2043 sets are not of interest, pcre_exec() may be called with ovector passed
2044 as NULL and ovecsize as zero. However, if the pattern contains back
2045 references and the ovector is not big enough to remember the related
2046 substrings, PCRE has to get additional memory for use during matching.
2047 Thus it is usually advisable to supply an ovector.
2048
2049 The pcre_info() function can be used to find out how many capturing
2050 subpatterns there are in a compiled pattern. The smallest size for
2051 ovector that will allow for n captured substrings, in addition to the
2052 offsets of the substring matched by the whole pattern, is (n+1)*3.
2053
2054 It is possible for capturing subpattern number n+1 to match some part
2055 of the subject when subpattern n has not been used at all. For example,
2056 if the string "abc" is matched against the pattern (a|(z))(bc) the
2057 return from the function is 4, and subpatterns 1 and 3 are matched, but
2058 2 is not. When this happens, both values in the offset pairs corre-
2059 sponding to unused subpatterns are set to -1.
2060
2061 Offset values that correspond to unused subpatterns at the end of the
2062 expression are also set to -1. For example, if the string "abc" is
2063 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2064 matched. The return from the function is 2, because the highest used
2065 capturing subpattern number is 1. However, you can refer to the offsets
2066 for the second and third capturing subpatterns if you wish (assuming
2067 the vector is large enough, of course).
2068
2069 Some convenience functions are provided for extracting the captured
2070 substrings as separate strings. These are described below.
2071
2072 Error return values from pcre_exec()
2073
2074 If pcre_exec() fails, it returns a negative number. The following are
2075 defined in the header file:
2076
2077 PCRE_ERROR_NOMATCH (-1)
2078
2079 The subject string did not match the pattern.
2080
2081 PCRE_ERROR_NULL (-2)
2082
2083 Either code or subject was passed as NULL, or ovector was NULL and
2084 ovecsize was not zero.
2085
2086 PCRE_ERROR_BADOPTION (-3)
2087
2088 An unrecognized bit was set in the options argument.
2089
2090 PCRE_ERROR_BADMAGIC (-4)
2091
2092 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2093 to catch the case when it is passed a junk pointer and to detect when a
2094 pattern that was compiled in an environment of one endianness is run in
2095 an environment with the other endianness. This is the error that PCRE
2096 gives when the magic number is not present.
2097
2098 PCRE_ERROR_UNKNOWN_OPCODE (-5)
2099
2100 While running the pattern match, an unknown item was encountered in the
2101 compiled pattern. This error could be caused by a bug in PCRE or by
2102 overwriting of the compiled pattern.
2103
2104 PCRE_ERROR_NOMEMORY (-6)
2105
2106 If a pattern contains back references, but the ovector that is passed
2107 to pcre_exec() is not big enough to remember the referenced substrings,
2108 PCRE gets a block of memory at the start of matching to use for this
2109 purpose. If the call via pcre_malloc() fails, this error is given. The
2110 memory is automatically freed at the end of matching.
2111
2112 PCRE_ERROR_NOSUBSTRING (-7)
2113
2114 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2115 and pcre_get_substring_list() functions (see below). It is never
2116 returned by pcre_exec().
2117
2118 PCRE_ERROR_MATCHLIMIT (-8)
2119
2120 The backtracking limit, as specified by the match_limit field in a
2121 pcre_extra structure (or defaulted) was reached. See the description
2122 above.
2123
2124 PCRE_ERROR_CALLOUT (-9)
2125
2126 This error is never generated by pcre_exec() itself. It is provided for
2127 use by callout functions that want to yield a distinctive error code.
2128 See the pcrecallout documentation for details.
2129
2130 PCRE_ERROR_BADUTF8 (-10)
2131
2132 A string that contains an invalid UTF-8 byte sequence was passed as a
2133 subject.
2134
2135 PCRE_ERROR_BADUTF8_OFFSET (-11)
2136
2137 The UTF-8 byte sequence that was passed as a subject was valid, but the
2138 value of startoffset did not point to the beginning of a UTF-8 charac-
2139 ter.
2140
2141 PCRE_ERROR_PARTIAL (-12)
2142
2143 The subject string did not match, but it did match partially. See the
2144 pcrepartial documentation for details of partial matching.
2145
2146 PCRE_ERROR_BADPARTIAL (-13)
2147
2148 The PCRE_PARTIAL option was used with a compiled pattern containing
2149 items that are not supported for partial matching. See the pcrepartial
2150 documentation for details of partial matching.
2151
2152 PCRE_ERROR_INTERNAL (-14)
2153
2154 An unexpected internal error has occurred. This error could be caused
2155 by a bug in PCRE or by overwriting of the compiled pattern.
2156
2157 PCRE_ERROR_BADCOUNT (-15)
2158
2159 This error is given if the value of the ovecsize argument is negative.
2160
2161 PCRE_ERROR_RECURSIONLIMIT (-21)
2162
2163 The internal recursion limit, as specified by the match_limit_recursion
2164 field in a pcre_extra structure (or defaulted) was reached. See the
2165 description above.
2166
2167 PCRE_ERROR_BADNEWLINE (-23)
2168
2169 An invalid combination of PCRE_NEWLINE_xxx options was given.
2170
2171 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2172
2173
2174 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2175
2176 int pcre_copy_substring(const char *subject, int *ovector,
2177 int stringcount, int stringnumber, char *buffer,
2178 int buffersize);
2179
2180 int pcre_get_substring(const char *subject, int *ovector,
2181 int stringcount, int stringnumber,
2182 const char **stringptr);
2183
2184 int pcre_get_substring_list(const char *subject,
2185 int *ovector, int stringcount, const char ***listptr);
2186
2187 Captured substrings can be accessed directly by using the offsets
2188 returned by pcre_exec() in ovector. For convenience, the functions
2189 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2190 string_list() are provided for extracting captured substrings as new,
2191 separate, zero-terminated strings. These functions identify substrings
2192 by number. The next section describes functions for extracting named
2193 substrings.
2194
2195 A substring that contains a binary zero is correctly extracted and has
2196 a further zero added on the end, but the result is not, of course, a C
2197 string. However, you can process such a string by referring to the
2198 length that is returned by pcre_copy_substring() and pcre_get_sub-
2199 string(). Unfortunately, the interface to pcre_get_substring_list() is
2200 not adequate for handling strings containing binary zeros, because the
2201 end of the final string is not independently indicated.
2202
2203 The first three arguments are the same for all three of these func-
2204 tions: subject is the subject string that has just been successfully
2205 matched, ovector is a pointer to the vector of integer offsets that was
2206 passed to pcre_exec(), and stringcount is the number of substrings that
2207 were captured by the match, including the substring that matched the
2208 entire regular expression. This is the value returned by pcre_exec() if
2209 it is greater than zero. If pcre_exec() returned zero, indicating that
2210 it ran out of space in ovector, the value passed as stringcount should
2211 be the number of elements in the vector divided by three.
2212
2213 The functions pcre_copy_substring() and pcre_get_substring() extract a
2214 single substring, whose number is given as stringnumber. A value of
2215 zero extracts the substring that matched the entire pattern, whereas
2216 higher values extract the captured substrings. For pcre_copy_sub-
2217 string(), the string is placed in buffer, whose length is given by
2218 buffersize, while for pcre_get_substring() a new block of memory is
2219 obtained via pcre_malloc, and its address is returned via stringptr.
2220 The yield of the function is the length of the string, not including
2221 the terminating zero, or one of these error codes:
2222
2223 PCRE_ERROR_NOMEMORY (-6)
2224
2225 The buffer was too small for pcre_copy_substring(), or the attempt to
2226 get memory failed for pcre_get_substring().
2227
2228 PCRE_ERROR_NOSUBSTRING (-7)
2229
2230 There is no substring whose number is stringnumber.
2231
2232 The pcre_get_substring_list() function extracts all available sub-
2233 strings and builds a list of pointers to them. All this is done in a
2234 single block of memory that is obtained via pcre_malloc. The address of
2235 the memory block is returned via listptr, which is also the start of
2236 the list of string pointers. The end of the list is marked by a NULL
2237 pointer. The yield of the function is zero if all went well, or the
2238 error code
2239
2240 PCRE_ERROR_NOMEMORY (-6)
2241
2242 if the attempt to get the memory block failed.
2243
2244 When any of these functions encounter a substring that is unset, which
2245 can happen when capturing subpattern number n+1 matches some part of
2246 the subject, but subpattern n has not been used at all, they return an
2247 empty string. This can be distinguished from a genuine zero-length sub-
2248 string by inspecting the appropriate offset in ovector, which is nega-
2249 tive for unset substrings.
2250
2251 The two convenience functions pcre_free_substring() and pcre_free_sub-
2252 string_list() can be used to free the memory returned by a previous
2253 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2254 tively. They do nothing more than call the function pointed to by
2255 pcre_free, which of course could be called directly from a C program.
2256 However, PCRE is used in some situations where it is linked via a spe-
2257 cial interface to another programming language that cannot use
2258 pcre_free directly; it is for these cases that the functions are pro-
2259 vided.
2260
2261
2262 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2263
2264 int pcre_get_stringnumber(const pcre *code,
2265 const char *name);
2266
2267 int pcre_copy_named_substring(const pcre *code,
2268 const char *subject, int *ovector,
2269 int stringcount, const char *stringname,
2270 char *buffer, int buffersize);
2271
2272 int pcre_get_named_substring(const pcre *code,
2273 const char *subject, int *ovector,
2274 int stringcount, const char *stringname,
2275 const char **stringptr);
2276
2277 To extract a substring by name, you first have to find associated num-
2278 ber. For example, for this pattern
2279
2280 (a+)b(?<xxx>\d+)...
2281
2282 the number of the subpattern called "xxx" is 2. If the name is known to
2283 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2284 name by calling pcre_get_stringnumber(). The first argument is the com-
2285 piled pattern, and the second is the name. The yield of the function is
2286 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2287 subpattern of that name.
2288
2289 Given the number, you can extract the substring directly, or use one of
2290 the functions described in the previous section. For convenience, there
2291 are also two functions that do the whole job.
2292
2293 Most of the arguments of pcre_copy_named_substring() and
2294 pcre_get_named_substring() are the same as those for the similarly
2295 named functions that extract by number. As these are described in the
2296 previous section, they are not re-described here. There are just two
2297 differences:
2298
2299 First, instead of a substring number, a substring name is given. Sec-
2300 ond, there is an extra argument, given at the start, which is a pointer
2301 to the compiled pattern. This is needed in order to gain access to the
2302 name-to-number translation table.
2303
2304 These functions call pcre_get_stringnumber(), and if it succeeds, they
2305 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2306 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2307 behaviour may not be what you want (see the next section).
2308
2309
2310 DUPLICATE SUBPATTERN NAMES
2311
2312 int pcre_get_stringtable_entries(const pcre *code,
2313 const char *name, char **first, char **last);
2314
2315 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2316 subpatterns are not required to be unique. Normally, patterns with
2317 duplicate names are such that in any one match, only one of the named
2318 subpatterns participates. An example is shown in the pcrepattern docu-
2319 mentation.
2320
2321 When duplicates are present, pcre_copy_named_substring() and
2322 pcre_get_named_substring() return the first substring corresponding to
2323 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2324 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2325 function returns one of the numbers that are associated with the name,
2326 but it is not defined which it is.
2327
2328 If you want to get full details of all captured substrings for a given
2329 name, you must use the pcre_get_stringtable_entries() function. The
2330 first argument is the compiled pattern, and the second is the name. The
2331 third and fourth are pointers to variables which are updated by the
2332 function. After it has run, they point to the first and last entries in
2333 the name-to-number table for the given name. The function itself
2334 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2335 there are none. The format of the table is described above in the sec-
2336 tion entitled Information about a pattern. Given all the relevant
2337 entries for the name, you can extract each of their numbers, and hence
2338 the captured data, if any.
2339
2340
2341 FINDING ALL POSSIBLE MATCHES
2342
2343 The traditional matching function uses a similar algorithm to Perl,
2344 which stops when it finds the first match, starting at a given point in
2345 the subject. If you want to find all possible matches, or the longest
2346 possible match, consider using the alternative matching function (see
2347 below) instead. If you cannot use the alternative function, but still
2348 need to find all possible matches, you can kludge it up by making use
2349 of the callout facility, which is described in the pcrecallout documen-
2350 tation.
2351
2352 What you have to do is to insert a callout right at the end of the pat-
2353 tern. When your callout function is called, extract and save the cur-
2354 rent matched substring. Then return 1, which forces pcre_exec() to
2355 backtrack and try other alternatives. Ultimately, when it runs out of
2356 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2357
2358
2359 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2360
2361 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2362 const char *subject, int length, int startoffset,
2363 int options, int *ovector, int ovecsize,
2364 int *workspace, int wscount);
2365
2366 The function pcre_dfa_exec() is called to match a subject string
2367 against a compiled pattern, using a matching algorithm that scans the
2368 subject string just once, and does not backtrack. This has different
2369 characteristics to the normal algorithm, and is not compatible with
2370 Perl. Some of the features of PCRE patterns are not supported. Never-
2371 theless, there are times when this kind of matching can be useful. For
2372 a discussion of the two matching algorithms, see the pcrematching docu-
2373 mentation.
2374
2375 The arguments for the pcre_dfa_exec() function are the same as for
2376 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2377 ent way, and this is described below. The other common arguments are
2378 used in the same way as for pcre_exec(), so their description is not
2379 repeated here.
2380
2381 The two additional arguments provide workspace for the function. The
2382 workspace vector should contain at least 20 elements. It is used for
2383 keeping track of multiple paths through the pattern tree. More
2384 workspace will be needed for patterns and subjects where there are a
2385 lot of potential matches.
2386
2387 Here is an example of a simple call to pcre_dfa_exec():
2388
2389 int rc;
2390 int ovector[10];
2391 int wspace[20];
2392 rc = pcre_dfa_exec(
2393 re, /* result of pcre_compile() */
2394 NULL, /* we didn't study the pattern */
2395 "some string", /* the subject string */
2396 11, /* the length of the subject string */
2397 0, /* start at offset 0 in the subject */
2398 0, /* default options */
2399 ovector, /* vector of integers for substring information */
2400 10, /* number of elements (NOT size in bytes) */
2401 wspace, /* working space vector */
2402 20); /* number of elements (NOT size in bytes) */
2403
2404 Option bits for pcre_dfa_exec()
2405
2406 The unused bits of the options argument for pcre_dfa_exec() must be
2407 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2408 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2409 PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2410 three of these are the same as for pcre_exec(), so their description is
2411 not repeated here.
2412
2413 PCRE_PARTIAL
2414
2415 This has the same general effect as it does for pcre_exec(), but the
2416 details are slightly different. When PCRE_PARTIAL is set for
2417 pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
2418 PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
2419 been no complete matches, but there is still at least one matching pos-
2420 sibility. The portion of the string that provided the partial match is
2421 set as the first matching string.
2422
2423 PCRE_DFA_SHORTEST
2424
2425 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2426 stop as soon as it has found one match. Because of the way the alterna-
2427 tive algorithm works, this is necessarily the shortest possible match
2428 at the first possible matching point in the subject string.
2429
2430 PCRE_DFA_RESTART
2431
2432 When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
2433 returns a partial match, it is possible to call it again, with addi-
2434 tional subject characters, and have it continue with the same match.
2435 The PCRE_DFA_RESTART option requests this action; when it is set, the
2436 workspace and wscount options must reference the same vector as before
2437 because data about the match so far is left in them after a partial
2438 match. There is more discussion of this facility in the pcrepartial
2439 documentation.
2440
2441 Successful returns from pcre_dfa_exec()
2442
2443 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2444 string in the subject. Note, however, that all the matches from one run
2445 of the function start at the same point in the subject. The shorter
2446 matches are all initial substrings of the longer matches. For example,
2447 if the pattern
2448
2449 <.*>
2450
2451 is matched against the string
2452
2453 This is <something> <something else> <something further> no more
2454
2455 the three matched strings are
2456
2457 <something>
2458 <something> <something else>
2459 <something> <something else> <something further>
2460
2461 On success, the yield of the function is a number greater than zero,
2462 which is the number of matched substrings. The substrings themselves
2463 are returned in ovector. Each string uses two elements; the first is
2464 the offset to the start, and the second is the offset to the end. In
2465 fact, all the strings have the same start offset. (Space could have
2466 been saved by giving this only once, but it was decided to retain some
2467 compatibility with the way pcre_exec() returns data, even though the
2468 meaning of the strings is different.)
2469
2470 The strings are returned in reverse order of length; that is, the long-
2471 est matching string is given first. If there were too many matches to
2472 fit into ovector, the yield of the function is zero, and the vector is
2473 filled with the longest matches.
2474
2475 Error returns from pcre_dfa_exec()
2476
2477 The pcre_dfa_exec() function returns a negative number when it fails.
2478 Many of the errors are the same as for pcre_exec(), and these are
2479 described above. There are in addition the following errors that are
2480 specific to pcre_dfa_exec():
2481
2482 PCRE_ERROR_DFA_UITEM (-16)
2483
2484 This return is given if pcre_dfa_exec() encounters an item in the pat-
2485 tern that it does not support, for instance, the use of \C or a back
2486 reference.
2487
2488 PCRE_ERROR_DFA_UCOND (-17)
2489
2490 This return is given if pcre_dfa_exec() encounters a condition item
2491 that uses a back reference for the condition, or a test for recursion
2492 in a specific group. These are not supported.
2493
2494 PCRE_ERROR_DFA_UMLIMIT (-18)
2495
2496 This return is given if pcre_dfa_exec() is called with an extra block
2497 that contains a setting of the match_limit field. This is not supported
2498 (it is meaningless).
2499
2500 PCRE_ERROR_DFA_WSSIZE (-19)
2501
2502 This return is given if pcre_dfa_exec() runs out of space in the
2503 workspace vector.
2504
2505 PCRE_ERROR_DFA_RECURSE (-20)
2506
2507 When a recursive subpattern is processed, the matching function calls
2508 itself recursively, using private vectors for ovector and workspace.
2509 This error is given if the output vector is not large enough. This
2510 should be extremely rare, as a vector of size 1000 is used.
2511
2512
2513 SEE ALSO
2514
2515 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2516 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2517
2518
2519 AUTHOR
2520
2521 Philip Hazel
2522 University Computing Service
2523 Cambridge CB2 3QH, England.
2524
2525
2526 REVISION
2527
2528 Last updated: 11 September 2007
2529 Copyright (c) 1997-2007 University of Cambridge.
2530 ------------------------------------------------------------------------------
2531
2532
2533 PCRECALLOUT(3) PCRECALLOUT(3)
2534
2535
2536 NAME
2537 PCRE - Perl-compatible regular expressions
2538
2539
2540 PCRE CALLOUTS
2541
2542 int (*pcre_callout)(pcre_callout_block *);
2543
2544 PCRE provides a feature called "callout", which is a means of temporar-
2545 ily passing control to the caller of PCRE in the middle of pattern
2546 matching. The caller of PCRE provides an external function by putting
2547 its entry point in the global variable pcre_callout. By default, this
2548 variable contains NULL, which disables all calling out.
2549
2550 Within a regular expression, (?C) indicates the points at which the
2551 external function is to be called. Different callout points can be
2552 identified by putting a number less than 256 after the letter C. The
2553 default value is zero. For example, this pattern has two callout
2554 points:
2555
2556 (?C1)abc(?C2)def
2557
2558 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
2559 called, PCRE automatically inserts callouts, all with number 255,
2560 before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2561 used with the pattern
2562
2563 A(\d{2}|--)
2564
2565 it is processed as if it were
2566
2567 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2568
2569 Notice that there is a callout before and after each parenthesis and
2570 alternation bar. Automatic callouts can be used for tracking the
2571 progress of pattern matching. The pcretest command has an option that
2572 sets automatic callouts; when it is used, the output indicates how the
2573 pattern is matched. This is useful information when you are trying to
2574 optimize the performance of a particular pattern.
2575
2576
2577 MISSING CALLOUTS
2578
2579 You should be aware that, because of optimizations in the way PCRE
2580 matches patterns, callouts sometimes do not happen. For example, if the
2581 pattern is
2582
2583 ab(?C4)cd
2584
2585 PCRE knows that any matching string must contain the letter "d". If the
2586 subject string is "abyz", the lack of "d" means that matching doesn't
2587 ever start, and the callout is never reached. However, with "abyd",
2588 though the result is still no match, the callout is obeyed.
2589
2590
2591 THE CALLOUT INTERFACE
2592
2593 During matching, when PCRE reaches a callout point, the external func-
2594 tion defined by pcre_callout is called (if it is set). This applies to
2595 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2596 only argument to the callout function is a pointer to a pcre_callout
2597 block. This structure contains the following fields:
2598
2599 int version;
2600 int callout_number;
2601 int *offset_vector;
2602 const char *subject;
2603 int subject_length;
2604 int start_match;
2605 int current_position;
2606 int capture_top;
2607 int capture_last;
2608 void *callout_data;
2609 int pattern_position;
2610 int next_item_length;
2611
2612 The version field is an integer containing the version number of the
2613 block format. The initial version was 0; the current version is 1. The
2614 version number will change again in future if additional fields are
2615 added, but the intention is never to remove any of the existing fields.
2616
2617 The callout_number field contains the number of the callout, as com-
2618 piled into the pattern (that is, the number after ?C for manual call-
2619 outs, and 255 for automatically generated callouts).
2620
2621 The offset_vector field is a pointer to the vector of offsets that was
2622 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2623 pcre_exec() is used, the contents can be inspected in order to extract
2624 substrings that have been matched so far, in the same way as for
2625 extracting substrings after a match has completed. For pcre_dfa_exec()
2626 this field is not useful.
2627
2628 The subject and subject_length fields contain copies of the values that
2629 were passed to pcre_exec().
2630
2631 The start_match field normally contains the offset within the subject
2632 at which the current match attempt started. However, if the escape
2633 sequence \K has been encountered, this value is changed to reflect the
2634 modified starting point. If the pattern is not anchored, the callout
2635 function may be called several times from the same point in the pattern
2636 for different starting points in the subject.
2637
2638 The current_position field contains the offset within the subject of
2639 the current match pointer.
2640
2641 When the pcre_exec() function is used, the capture_top field contains
2642 one more than the number of the highest numbered captured substring so
2643 far. If no substrings have been captured, the value of capture_top is
2644 one. This is always the case when pcre_dfa_exec() is used, because it
2645 does not support captured substrings.
2646
2647 The capture_last field contains the number of the most recently cap-
2648 tured substring. If no substrings have been captured, its value is -1.
2649 This is always the case when pcre_dfa_exec() is used.
2650
2651 The callout_data field contains a value that is passed to pcre_exec()
2652 or pcre_dfa_exec() specifically so that it can be passed back in call-
2653 outs. It is passed in the pcre_callout field of the pcre_extra data
2654 structure. If no such data was passed, the value of callout_data in a
2655 pcre_callout block is NULL. There is a description of the pcre_extra
2656 structure in the pcreapi documentation.
2657
2658 The pattern_position field is present from version 1 of the pcre_call-
2659 out structure. It contains the offset to the next item to be matched in
2660 the pattern string.
2661
2662 The next_item_length field is present from version 1 of the pcre_call-
2663 out structure. It contains the length of the next item to be matched in
2664 the pattern string. When the callout immediately precedes an alterna-
2665 tion bar, a closing parenthesis, or the end of the pattern, the length
2666 is zero. When the callout precedes an opening parenthesis, the length
2667 is that of the entire subpattern.
2668
2669 The pattern_position and next_item_length fields are intended to help
2670 in distinguishing between different automatic callouts, which all have
2671 the same callout number. However, they are set for all callouts.
2672
2673
2674 RETURN VALUES
2675
2676 The external callout function returns an integer to PCRE. If the value
2677 is zero, matching proceeds as normal. If the value is greater than
2678 zero, matching fails at the current point, but the testing of other
2679 matching possibilities goes ahead, just as if a lookahead assertion had
2680 failed. If the value is less than zero, the match is abandoned, and
2681 pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2682
2683 Negative values should normally be chosen from the set of
2684 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2685 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2686 reserved for use by callout functions; it will never be used by PCRE
2687 itself.
2688
2689
2690 AUTHOR
2691
2692 Philip Hazel
2693 University Computing Service
2694 Cambridge CB2 3QH, England.
2695
2696
2697 REVISION
2698
2699 Last updated: 29 May 2007
2700 Copyright (c) 1997-2007 University of Cambridge.
2701 ------------------------------------------------------------------------------
2702
2703
2704 PCRECOMPAT(3) PCRECOMPAT(3)
2705
2706
2707 NAME
2708 PCRE - Perl-compatible regular expressions
2709
2710
2711 DIFFERENCES BETWEEN PCRE AND PERL
2712
2713 This document describes the differences in the ways that PCRE and Perl
2714 handle regular expressions. The differences described here are mainly
2715 with respect to Perl 5.8, though PCRE versions 7.0 and later contain
2716 some features that are expected to be in the forthcoming Perl 5.10.
2717
2718 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2719 of what it does have are given in the section on UTF-8 support in the
2720 main pcre page.
2721
2722 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2723 permits them, but they do not mean what you might think. For example,
2724 (?!a){3} does not assert that the next three characters are not "a". It
2725 just asserts that the next character is not "a" three times.
2726
2727 3. Capturing subpatterns that occur inside negative lookahead asser-
2728 tions are counted, but their entries in the offsets vector are never
2729 set. Perl sets its numerical variables from any such patterns that are
2730 matched before the assertion fails to match something (thereby succeed-
2731 ing), but only if the negative lookahead assertion contains just one
2732 branch.
2733
2734 4. Though binary zero characters are supported in the subject string,
2735 they are not allowed in a pattern string because it is passed as a nor-
2736 mal C string, terminated by zero. The escape sequence \0 can be used in
2737 the pattern to represent a binary zero.
2738
2739 5. The following Perl escape sequences are not supported: \l, \u, \L,
2740 \U, and \N. In fact these are implemented by Perl's general string-han-
2741 dling and are not part of its pattern matching engine. If any of these
2742 are encountered by PCRE, an error is generated.
2743
2744 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2745 is built with Unicode character property support. The properties that
2746 can be tested with \p and \P are limited to the general category prop-
2747 erties such as Lu and Nd, script names such as Greek or Han, and the
2748 derived properties Any and L&.
2749
2750 7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2751 ters in between are treated as literals. This is slightly different
2752 from Perl in that $ and @ are also handled as literals inside the
2753 quotes. In Perl, they cause variable interpolation (but of course PCRE
2754 does not have variables). Note the following examples:
2755
2756 Pattern PCRE matches Perl matches
2757
2758 \Qabc$xyz\E abc$xyz abc followed by the
2759 contents of $xyz
2760 \Qabc\$xyz\E abc\$xyz abc\$xyz
2761 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2762
2763 The \Q...\E sequence is recognized both inside and outside character
2764 classes.
2765
2766 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2767 constructions. However, there is support for recursive patterns. This
2768 is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2769 "callout" feature allows an external function to be called during pat-
2770 tern matching. See the pcrecallout documentation for details.
2771
2772 9. Subpatterns that are called recursively or as "subroutines" are
2773 always treated as atomic groups in PCRE. This is like Python, but
2774 unlike Perl.
2775
2776 10. There are some differences that are concerned with the settings of
2777 captured strings when part of a pattern is repeated. For example,
2778 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
2779 unset, but in PCRE it is set to "b".
2780
2781 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT),
2782 (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
2783 the forms without an argument. PCRE does not support (*MARK). If
2784 (*ACCEPT) is within capturing parentheses, PCRE does not set that cap-
2785 ture group; this is different to Perl.
2786
2787 12. PCRE provides some extensions to the Perl regular expression facil-
2788 ities. Perl 5.10 will include new features that are not in earlier
2789 versions, some of which (such as named parentheses) have been in PCRE
2790 for some time. This list is with respect to Perl 5.10:
2791
2792 (a) Although lookbehind assertions must match fixed length strings,
2793 each alternative branch of a lookbehind assertion can match a different
2794 length of string. Perl requires them all to have the same length.
2795
2796 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2797 meta-character matches only at the very end of the string.
2798
2799 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2800 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
2801 ignored. (Perl can be made to issue a warning.)
2802
2803 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
2804 fiers is inverted, that is, by default they are not greedy, but if fol-
2805 lowed by a question mark they are.
2806
2807 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2808 tried only at the first matching position in the subject string.
2809
2810 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2811 TURE options for pcre_exec() have no Perl equivalents.
2812
2813 (g) The \R escape sequence can be restricted to match only CR, LF, or
2814 CRLF by the PCRE_BSR_ANYCRLF option.
2815
2816 (h) The callout facility is PCRE-specific.
2817
2818 (i) The partial matching facility is PCRE-specific.
2819
2820 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
2821 even on different hosts that have the other endianness.
2822
2823 (k) The alternative matching function (pcre_dfa_exec()) matches in a
2824 different way and is not Perl-compatible.
2825
2826 (l) PCRE recognizes some special sequences such as (*CR) at the start
2827 of a pattern that set overall options that cannot be changed within the
2828 pattern.
2829
2830
2831 AUTHOR
2832
2833 Philip Hazel
2834 University Computing Service
2835 Cambridge CB2 3QH, England.
2836
2837
2838 REVISION
2839
2840 Last updated: 11 September 2007
2841 Copyright (c) 1997-2007 University of Cambridge.
2842 ------------------------------------------------------------------------------
2843
2844
2845 PCREPATTERN(3) PCREPATTERN(3)
2846
2847
2848 NAME
2849 PCRE - Perl-compatible regular expressions
2850
2851
2852 PCRE REGULAR EXPRESSION DETAILS
2853
2854 The syntax and semantics of the regular expressions that are supported
2855 by PCRE are described in detail below. There is a quick-reference syn-
2856 tax summary in the pcresyntax page. Perl's regular expressions are
2857 described in its own documentation, and regular expressions in general
2858 are covered in a number of books, some of which have copious examples.
2859 Jeffrey Friedl's "Mastering Regular Expressions", published by
2860 O'Reilly, covers regular expressions in great detail. This description
2861 of PCRE's regular expressions is intended as reference material.
2862
2863 The original operation of PCRE was on strings of one-byte characters.
2864 However, there is now also support for UTF-8 character strings. To use
2865 this, you must build PCRE to include UTF-8 support, and then call
2866 pcre_compile() with the PCRE_UTF8 option. How this affects pattern
2867 matching is mentioned in several places below. There is also a summary
2868 of UTF-8 features in the section on UTF-8 support in the main pcre
2869 page.
2870
2871 The remainder of this document discusses the patterns that are sup-
2872 ported by PCRE when its main matching function, pcre_exec(), is used.
2873 From release 6.0, PCRE offers a second matching function,
2874 pcre_dfa_exec(), which matches using a different algorithm that is not
2875 Perl-compatible. Some of the features discussed below are not available
2876 when pcre_dfa_exec() is used. The advantages and disadvantages of the
2877 alternative function, and how it differs from the normal function, are
2878 discussed in the pcrematching page.
2879
2880
2881 NEWLINE CONVENTIONS
2882
2883 PCRE supports five different conventions for indicating line breaks in
2884 strings: a single CR (carriage return) character, a single LF (line-
2885 feed) character, the two-character sequence CRLF, any of the three pre-
2886 ceding, or any Unicode newline sequence. The pcreapi page has further
2887 discussion about newlines, and shows how to set the newline convention
2888 in the options arguments for the compiling and matching functions.
2889
2890 It is also possible to specify a newline convention by starting a pat-
2891 tern string with one of the following five sequences:
2892
2893 (*CR) carriage return
2894 (*LF) linefeed
2895 (*CRLF) carriage return, followed by linefeed
2896 (*ANYCRLF) any of the three above
2897 (*ANY) all Unicode newline sequences
2898
2899 These override the default and the options given to pcre_compile(). For
2900 example, on a Unix system where LF is the default newline sequence, the
2901 pattern
2902
2903 (*CR)a.b
2904
2905 changes the convention to CR. That pattern matches "a\nb" because LF is
2906 no longer a newline. Note that these special settings, which are not
2907 Perl-compatible, are recognized only at the very start of a pattern,
2908 and that they must be in upper case. If more than one of them is
2909 present, the last one is used.
2910
2911 The newline convention does not affect what the \R escape sequence
2912 matches. By default, this is any Unicode newline sequence, for Perl
2913 compatibility. However, this can be changed; see the description of \R
2914 in the section entitled "Newline sequences" below.
2915
2916
2917 CHARACTERS AND METACHARACTERS
2918
2919 A regular expression is a pattern that is matched against a subject
2920 string from left to right. Most characters stand for themselves in a
2921 pattern, and match the corresponding characters in the subject. As a
2922 trivial example, the pattern
2923
2924 The quick brown fox
2925
2926 matches a portion of a subject string that is identical to itself. When
2927 caseless matching is specified (the PCRE_CASELESS option), letters are
2928 matched independently of case. In UTF-8 mode, PCRE always understands
2929 the concept of case for characters whose values are less than 128, so
2930 caseless matching is always possible. For characters with higher val-
2931 ues, the concept of case is supported if PCRE is compiled with Unicode
2932 property support, but not otherwise. If you want to use caseless
2933 matching for characters 128 and above, you must ensure that PCRE is
2934 compiled with Unicode property support as well as with UTF-8 support.
2935
2936 The power of regular expressions comes from the ability to include
2937 alternatives and repetitions in the pattern. These are encoded in the
2938 pattern by the use of metacharacters, which do not stand for themselves
2939 but instead are interpreted in some special way.
2940
2941 There are two different sets of metacharacters: those that are recog-
2942 nized anywhere in the pattern except within square brackets, and those
2943 that are recognized within square brackets. Outside square brackets,
2944 the metacharacters are as follows:
2945
2946 \ general escape character with several uses
2947 ^ assert start of string (or line, in multiline mode)
2948 $ assert end of string (or line, in multiline mode)
2949 . match any character except newline (by default)
2950 [ start character class definition
2951 | start of alternative branch
2952 ( start subpattern
2953 ) end subpattern
2954 ? extends the meaning of (
2955 also 0 or 1 quantifier
2956 also quantifier minimizer
2957 * 0 or more quantifier
2958 + 1 or more quantifier
2959 also "possessive quantifier"
2960 { start min/max quantifier
2961
2962 Part of a pattern that is in square brackets is called a "character
2963 class". In a character class the only metacharacters are:
2964
2965 \ general escape character
2966 ^ negate the class, but only if the first character
2967 - indicates character range
2968 [ POSIX character class (only if followed by POSIX
2969 syntax)
2970 ] terminates the character class
2971
2972 The following sections describe the use of each of the metacharacters.
2973
2974
2975 BACKSLASH
2976
2977 The backslash character has several uses. Firstly, if it is followed by
2978 a non-alphanumeric character, it takes away any special meaning that
2979 character may have. This use of backslash as an escape character
2980 applies both inside and outside character classes.
2981
2982 For example, if you want to match a * character, you write \* in the
2983 pattern. This escaping action applies whether or not the following
2984 character would otherwise be interpreted as a metacharacter, so it is
2985 always safe to precede a non-alphanumeric with backslash to specify
2986 that it stands for itself. In particular, if you want to match a back-
2987 slash, you write \\.
2988
2989 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
2990 the pattern (other than in a character class) and characters between a
2991 # outside a character class and the next newline are ignored. An escap-
2992 ing backslash can be used to include a whitespace or # character as
2993 part of the pattern.
2994
2995 If you want to remove the special meaning from a sequence of charac-
2996 ters, you can do so by putting them between \Q and \E. This is differ-
2997 ent from Perl in that $ and @ are handled as literals in \Q...\E
2998 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
2999 tion. Note the following examples:
3000
3001 Pattern PCRE matches Perl matches
3002
3003 \Qabc$xyz\E abc$xyz abc followed by the
3004 contents of $xyz
3005 \Qabc\$xyz\E abc\$xyz abc\$xyz
3006 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3007
3008 The \Q...\E sequence is recognized both inside and outside character
3009 classes.
3010
3011 Non-printing characters
3012
3013 A second use of backslash provides a way of encoding non-printing char-
3014 acters in patterns in a visible manner. There is no restriction on the
3015 appearance of non-printing characters, apart from the binary zero that
3016 terminates a pattern, but when a pattern is being prepared by text
3017 editing, it is usually easier to use one of the following escape
3018 sequences than the binary character it represents:
3019
3020 \a alarm, that is, the BEL character (hex 07)
3021 \cx "control-x", where x is any character
3022 \e escape (hex 1B)
3023 \f formfeed (hex 0C)
3024 \n linefeed (hex 0A)
3025 \r carriage return (hex 0D)
3026 \t tab (hex 09)
3027 \ddd character with octal code ddd, or backreference
3028 \xhh character with hex code hh
3029 \x{hhh..} character with hex code hhh..
3030
3031 The precise effect of \cx is as follows: if x is a lower case letter,
3032 it is converted to upper case. Then bit 6 of the character (hex 40) is
3033 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
3034 becomes hex 7B.
3035
3036 After \x, from zero to two hexadecimal digits are read (letters can be
3037 in upper or lower case). Any number of hexadecimal digits may appear
3038 between \x{ and }, but the value of the character code must be less
3039 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3040 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3041 than the largest Unicode code point, which is 10FFFF.
3042
3043 If characters other than hexadecimal digits appear between \x{ and },
3044 or if there is no terminating }, this form of escape is not recognized.
3045 Instead, the initial \x will be interpreted as a basic hexadecimal
3046 escape, with no following digits, giving a character whose value is
3047 zero.
3048
3049 Characters whose value is less than 256 can be defined by either of the
3050 two syntaxes for \x. There is no difference in the way they are han-
3051 dled. For example, \xdc is exactly the same as \x{dc}.
3052
3053 After \0 up to two further octal digits are read. If there are fewer
3054 than two digits, just those that are present are used. Thus the
3055 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3056 (code value 7). Make sure you supply two digits after the initial zero
3057 if the pattern character that follows is itself an octal digit.
3058
3059 The handling of a backslash followed by a digit other than 0 is compli-
3060 cated. Outside a character class, PCRE reads it and any following dig-
3061 its as a decimal number. If the number is less than 10, or if there
3062 have been at least that many previous capturing left parentheses in the
3063 expression, the entire sequence is taken as a back reference. A
3064 description of how this works is given later, following the discussion
3065 of parenthesized subpatterns.
3066
3067 Inside a character class, or if the decimal number is greater than 9
3068 and there have not been that many capturing subpatterns, PCRE re-reads
3069 up to three octal digits following the backslash, and uses them to gen-
3070 erate a data character. Any subsequent digits stand for themselves. In
3071 non-UTF-8 mode, the value of a character specified in octal must be
3072 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3073 example:
3074
3075 \040 is another way of writing a space
3076 \40 is the same, provided there are fewer than 40
3077 previous capturing subpatterns
3078 \7 is always a back reference
3079 \11 might be a back reference, or another way of
3080 writing a tab
3081 \011 is always a tab
3082 \0113 is a tab followed by the character "3"
3083 \113 might be a back reference, otherwise the
3084 character with octal code 113
3085 \377 might be a back reference, otherwise
3086 the byte consisting entirely of 1 bits
3087 \81 is either a back reference, or a binary zero
3088 followed by the two characters "8" and "1"
3089
3090 Note that octal values of 100 or greater must not be introduced by a
3091 leading zero, because no more than three octal digits are ever read.
3092
3093 All the sequences that define a single character value can be used both
3094 inside and outside character classes. In addition, inside a character
3095 class, the sequence \b is interpreted as the backspace character (hex
3096 08), and the sequences \R and \X are interpreted as the characters "R"
3097 and "X", respectively. Outside a character class, these sequences have
3098 different meanings (see below).
3099
3100 Absolute and relative back references
3101
3102 The sequence \g followed by an unsigned or a negative number, option-
3103 ally enclosed in braces, is an absolute or relative back reference. A
3104 named back reference can be coded as \g{name}. Back references are dis-
3105 cussed later, following the discussion of parenthesized subpatterns.
3106
3107 Generic character types
3108
3109 Another use of backslash is for specifying generic character types. The
3110 following are always recognized:
3111
3112 \d any decimal digit
3113 \D any character that is not a decimal digit
3114 \h any horizontal whitespace character
3115 \H any character that is not a horizontal whitespace character
3116 \s any whitespace character
3117 \S any character that is not a whitespace character
3118 \v any vertical whitespace character
3119 \V any character that is not a vertical whitespace character
3120 \w any "word" character
3121 \W any "non-word" character
3122
3123 Each pair of escape sequences partitions the complete set of characters
3124 into two disjoint sets. Any given character matches one, and only one,
3125 of each pair.
3126
3127 These character type sequences can appear both inside and outside char-
3128 acter classes. They each match one character of the appropriate type.
3129 If the current matching point is at the end of the subject string, all
3130 of them fail, since there is no character to match.
3131
3132 For compatibility with Perl, \s does not match the VT character (code
3133 11). This makes it different from the the POSIX "space" class. The \s
3134 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3135 "use locale;" is included in a Perl script, \s may match the VT charac-
3136 ter. In PCRE, it never does.
3137
3138 In UTF-8 mode, characters with values greater than 128 never match \d,
3139 \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3140 code character property support is available. These sequences retain
3141 their original meanings from before UTF-8 support was available, mainly
3142 for efficiency reasons.
3143
3144 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3145 the other sequences, these do match certain high-valued codepoints in
3146 UTF-8 mode. The horizontal space characters are:
3147
3148 U+0009 Horizontal tab
3149 U+0020 Space
3150 U+00A0 Non-break space
3151 U+1680 Ogham space mark
3152 U+180E Mongolian vowel separator
3153 U+2000 En quad
3154 U+2001 Em quad
3155 U+2002 En space
3156 U+2003 Em space
3157 U+2004 Three-per-em space
3158 U+2005 Four-per-em space
3159 U+2006 Six-per-em space
3160 U+2007 Figure space
3161 U+2008 Punctuation space
3162 U+2009 Thin space
3163 U+200A Hair space
3164 U+202F Narrow no-break space
3165 U+205F Medium mathematical space
3166 U+3000 Ideographic space
3167
3168 The vertical space characters are:
3169
3170 U+000A Linefeed
3171 U+000B Vertical tab
3172 U+000C Formfeed
3173 U+000D Carriage return
3174 U+0085 Next line
3175 U+2028 Line separator
3176 U+2029 Paragraph separator
3177
3178 A "word" character is an underscore or any character less than 256 that
3179 is a letter or digit. The definition of letters and digits is con-
3180 trolled by PCRE's low-valued character tables, and may vary if locale-
3181 specific matching is taking place (see "Locale support" in the pcreapi
3182 page). For example, in a French locale such as "fr_FR" in Unix-like
3183 systems, or "french" in Windows, some character codes greater than 128
3184 are used for accented letters, and these are matched by \w. The use of
3185 locales with Unicode is discouraged.
3186
3187 Newline sequences
3188
3189 Outside a character class, by default, the escape sequence \R matches
3190 any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3191 mode \R is equivalent to the following:
3192
3193 (?>\r\n|\n|\x0b|\f|\r|\x85)
3194
3195 This is an example of an "atomic group", details of which are given
3196 below. This particular group matches either the two-character sequence
3197 CR followed by LF, or one of the single characters LF (linefeed,
3198 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3199 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3200 is treated as a single unit that cannot be split.
3201
3202 In UTF-8 mode, two additional characters whose codepoints are greater
3203 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3204 rator, U+2029). Unicode character property support is not needed for
3205 these characters to be recognized.
3206
3207 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3208 the complete set of Unicode line endings) by setting the option
3209 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3210 This can be made the default when PCRE is built; if this is the case,
3211 the other behaviour can be requested via the PCRE_BSR_UNICODE option.
3212 It is also possible to specify these settings by starting a pattern
3213 string with one of the following sequences:
3214
3215 (*BSR_ANYCRLF) CR, LF, or CRLF only
3216 (*BSR_UNICODE) any Unicode newline sequence
3217
3218 These override the default and the options given to pcre_compile(), but
3219 they can be overridden by options given to pcre_exec(). Note that these
3220 special settings, which are not Perl-compatible, are recognized only at
3221 the very start of a pattern, and that they must be in upper case. If
3222 more than one of them is present, the last one is used.
3223
3224 Inside a character class, \R matches the letter "R".
3225
3226 Unicode character properties
3227
3228 When PCRE is built with Unicode character property support, three addi-
3229 tional escape sequences that match characters with specific properties
3230 are available. When not in UTF-8 mode, these sequences are of course
3231 limited to testing characters whose codepoints are less than 256, but
3232 they do work in this mode. The extra escape sequences are:
3233
3234 \p{xx} a character with the xx property
3235 \P{xx} a character without the xx property
3236 \X an extended Unicode sequence
3237
3238 The property names represented by xx above are limited to the Unicode
3239 script names, the general category properties, and "Any", which matches
3240 any character (including newline). Other properties such as "InMusical-
3241 Symbols" are not currently supported by PCRE. Note that \P{Any} does
3242 not match any characters, so always causes a match failure.
3243
3244 Sets of Unicode characters are defined as belonging to certain scripts.
3245 A character from one of these sets can be matched using a script name.
3246 For example:
3247
3248 \p{Greek}
3249 \P{Han}
3250
3251 Those that are not part of an identified script are lumped together as
3252 "Common". The current list of scripts is:
3253
3254 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
3255 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
3256 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3257 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
3258 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
3259 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
3260 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
3261 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
3262 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3263
3264 Each character has exactly one general category property, specified by
3265 a two-letter abbreviation. For compatibility with Perl, negation can be
3266 specified by including a circumflex between the opening brace and the
3267 property name. For example, \p{^Lu} is the same as \P{Lu}.
3268
3269 If only one letter is specified with \p or \P, it includes all the gen-
3270 eral category properties that start with that letter. In this case, in
3271 the absence of negation, the curly brackets in the escape sequence are
3272 optional; these two examples have the same effect:
3273
3274 \p{L}
3275 \pL
3276
3277 The following general category property codes are supported:
3278
3279 C Other
3280 Cc Control
3281 Cf Format
3282 Cn Unassigned
3283 Co Private use
3284 Cs Surrogate
3285
3286 L Letter
3287 Ll Lower case letter
3288 Lm Modifier letter
3289 Lo Other letter
3290 Lt Title case letter
3291 Lu Upper case letter
3292
3293 M Mark
3294 Mc Spacing mark
3295 Me Enclosing mark
3296 Mn Non-spacing mark
3297
3298 N Number
3299 Nd Decimal number
3300 Nl Letter number
3301 No Other number
3302
3303 P Punctuation
3304 Pc Connector punctuation
3305 Pd Dash punctuation
3306 Pe Close punctuation
3307 Pf Final punctuation
3308 Pi Initial punctuation
3309 Po Other punctuation
3310 Ps Open punctuation
3311
3312 S Symbol
3313 Sc Currency symbol
3314 Sk Modifier symbol
3315 Sm Mathematical symbol
3316 So Other symbol
3317
3318 Z Separator
3319 Zl Line separator
3320 Zp Paragraph separator
3321 Zs Space separator
3322
3323 The special property L& is also supported: it matches a character that
3324 has the Lu, Ll, or Lt property, in other words, a letter that is not
3325 classified as a modifier or "other".
3326
3327 The Cs (Surrogate) property applies only to characters in the range
3328 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3329 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3330 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3331 the pcreapi page).
3332
3333 The long synonyms for these properties that Perl supports (such as
3334 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3335 any of these properties with "Is".
3336
3337 No character that is in the Unicode table has the Cn (unassigned) prop-
3338 erty. Instead, this property is assumed for any code point that is not
3339 in the Unicode table.
3340
3341 Specifying caseless matching does not affect these escape sequences.
3342 For example, \p{Lu} always matches only upper case letters.
3343
3344 The \X escape matches any number of Unicode characters that form an
3345 extended Unicode sequence. \X is equivalent to
3346
3347 (?>\PM\pM*)
3348
3349 That is, it matches a character without the "mark" property, followed
3350 by zero or more characters with the "mark" property, and treats the
3351 sequence as an atomic group (see below). Characters with the "mark"
3352 property are typically accents that affect the preceding character.
3353 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3354 matches any one character.
3355
3356 Matching characters by Unicode property is not fast, because PCRE has
3357 to search a structure that contains data for over fifteen thousand
3358 characters. That is why the traditional escape sequences such as \d and
3359 \w do not use Unicode properties in PCRE.
3360
3361 Resetting the match start
3362
3363 The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3364 ously matched characters not to be included in the final matched
3365 sequence. For example, the pattern:
3366
3367 foo\Kbar
3368
3369 matches "foobar", but reports that it has matched "bar". This feature
3370 is similar to a lookbehind assertion (described below). However, in
3371 this case, the part of the subject before the real match does not have
3372 to be of fixed length, as lookbehind assertions do. The use of \K does
3373 not interfere with the setting of captured substrings. For example,
3374 when the pattern
3375
3376 (foo)\Kbar
3377
3378 matches "foobar", the first substring is still set to "foo".
3379
3380 Simple assertions
3381
3382 The final use of backslash is for certain simple assertions. An asser-
3383 tion specifies a condition that has to be met at a particular point in
3384 a match, without consuming any characters from the subject string. The
3385 use of subpatterns for more complicated assertions is described below.
3386 The backslashed assertions are:
3387
3388 \b matches at a word boundary
3389 \B matches when not at a word boundary
3390 \A matches at the start of the subject
3391 \Z matches at the end of the subject
3392 also matches before a newline at the end of the subject
3393 \z matches only at the end of the subject
3394 \G matches at the first matching position in the subject
3395
3396 These assertions may not appear in character classes (but note that \b
3397 has a different meaning, namely the backspace character, inside a char-
3398 acter class).
3399
3400 A word boundary is a position in the subject string where the current
3401 character and the previous character do not both match \w or \W (i.e.
3402 one matches \w and the other matches \W), or the start or end of the
3403 string if the first or last character matches \w, respectively.
3404
3405 The \A, \Z, and \z assertions differ from the traditional circumflex
3406 and dollar (described in the next section) in that they only ever match
3407 at the very start and end of the subject string, whatever options are
3408 set. Thus, they are independent of multiline mode. These three asser-
3409 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3410 affect only the behaviour of the circumflex and dollar metacharacters.
3411 However, if the startoffset argument of pcre_exec() is non-zero, indi-
3412 cating that matching is to start at a point other than the beginning of
3413 the subject, \A can never match. The difference between \Z and \z is
3414 that \Z matches before a newline at the end of the string as well as at
3415 the very end, whereas \z matches only at the end.
3416
3417 The \G assertion is true only when the current matching position is at
3418 the start point of the match, as specified by the startoffset argument
3419 of pcre_exec(). It differs from \A when the value of startoffset is
3420 non-zero. By calling pcre_exec() multiple times with appropriate argu-
3421 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3422 mentation where \G can be useful.
3423
3424 Note, however, that PCRE's interpretation of \G, as the start of the
3425 current match, is subtly different from Perl's, which defines it as the
3426 end of the previous match. In Perl, these can be different when the
3427 previously matched string was empty. Because PCRE does just one match
3428 at a time, it cannot reproduce this behaviour.
3429
3430 If all the alternatives of a pattern begin with \G, the expression is
3431 anchored to the starting match position, and the "anchored" flag is set
3432 in the compiled regular expression.
3433
3434
3435 CIRCUMFLEX AND DOLLAR
3436
3437 Outside a character class, in the default matching mode, the circumflex
3438 character is an assertion that is true only if the current matching
3439 point is at the start of the subject string. If the startoffset argu-
3440 ment of pcre_exec() is non-zero, circumflex can never match if the
3441 PCRE_MULTILINE option is unset. Inside a character class, circumflex
3442 has an entirely different meaning (see below).
3443
3444 Circumflex need not be the first character of the pattern if a number
3445 of alternatives are involved, but it should be the first thing in each
3446 alternative in which it appears if the pattern is ever to match that
3447 branch. If all possible alternatives start with a circumflex, that is,
3448 if the pattern is constrained to match only at the start of the sub-
3449 ject, it is said to be an "anchored" pattern. (There are also other
3450 constructs that can cause a pattern to be anchored.)
3451
3452 A dollar character is an assertion that is true only if the current
3453 matching point is at the end of the subject string, or immediately
3454 before a newline at the end of the string (by default). Dollar need not
3455 be the last character of the pattern if a number of alternatives are
3456 involved, but it should be the last item in any branch in which it
3457 appears. Dollar has no special meaning in a character class.
3458
3459 The meaning of dollar can be changed so that it matches only at the
3460 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3461 compile time. This does not affect the \Z assertion.
3462
3463 The meanings of the circumflex and dollar characters are changed if the
3464 PCRE_MULTILINE option is set. When this is the case, a circumflex
3465 matches immediately after internal newlines as well as at the start of
3466 the subject string. It does not match after a newline that ends the
3467 string. A dollar matches before any newlines in the string, as well as
3468 at the very end, when PCRE_MULTILINE is set. When newline is specified
3469 as the two-character sequence CRLF, isolated CR and LF characters do
3470 not indicate newlines.
3471
3472 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3473 (where \n represents a newline) in multiline mode, but not otherwise.
3474 Consequently, patterns that are anchored in single line mode because
3475 all branches start with ^ are not anchored in multiline mode, and a
3476 match for circumflex is possible when the startoffset argument of
3477 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3478 PCRE_MULTILINE is set.
3479
3480 Note that the sequences \A, \Z, and \z can be used to match the start
3481 and end of the subject in both modes, and if all branches of a pattern
3482 start with \A it is always anchored, whether or not PCRE_MULTILINE is
3483 set.
3484
3485
3486 FULL STOP (PERIOD, DOT)
3487
3488 Outside a character class, a dot in the pattern matches any one charac-
3489 ter in the subject string except (by default) a character that signi-
3490 fies the end of a line. In UTF-8 mode, the matched character may be
3491 more than one byte long.
3492
3493 When a line ending is defined as a single character, dot never matches
3494 that character; when the two-character sequence CRLF is used, dot does
3495 not match CR if it is immediately followed by LF, but otherwise it
3496 matches all characters (including isolated CRs and LFs). When any Uni-
3497 code line endings are being recognized, dot does not match CR or LF or
3498 any of the other line ending characters.
3499
3500 The behaviour of dot with regard to newlines can be changed. If the
3501 PCRE_DOTALL option is set, a dot matches any one character, without
3502 exception. If the two-character sequence CRLF is present in the subject
3503 string, it takes two dots to match it.
3504
3505 The handling of dot is entirely independent of the handling of circum-
3506 flex and dollar, the only relationship being that they both involve
3507 newlines. Dot has no special meaning in a character class.
3508
3509
3510 MATCHING A SINGLE BYTE
3511
3512 Outside a character class, the escape sequence \C matches any one byte,
3513 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3514 line-ending characters. The feature is provided in Perl in order to
3515 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3516 acters into individual bytes, what remains in the string may be a mal-
3517 formed UTF-8 string. For this reason, the \C escape sequence is best
3518 avoided.
3519
3520 PCRE does not allow \C to appear in lookbehind assertions (described
3521 below), because in UTF-8 mode this would make it impossible to calcu-
3522 late the length of the lookbehind.
3523
3524
3525 SQUARE BRACKETS AND CHARACTER CLASSES
3526
3527 An opening square bracket introduces a character class, terminated by a
3528 closing square bracket. A closing square bracket on its own is not spe-
3529 cial. If a closing square bracket is required as a member of the class,
3530 it should be the first data character in the class (after an initial
3531 circumflex, if present) or escaped with a backslash.
3532
3533 A character class matches a single character in the subject. In UTF-8
3534 mode, the character may occupy more than one byte. A matched character
3535 must be in the set of characters defined by the class, unless the first
3536 character in the class definition is a circumflex, in which case the
3537 subject character must not be in the set defined by the class. If a
3538 circumflex is actually required as a member of the class, ensure it is
3539 not the first character, or escape it with a backslash.
3540
3541 For example, the character class [aeiou] matches any lower case vowel,
3542 while [^aeiou] matches any character that is not a lower case vowel.
3543 Note that a circumflex is just a convenient notation for specifying the
3544 characters that are in the class by enumerating those that are not. A
3545 class that starts with a circumflex is not an assertion: it still con-
3546 sumes a character from the subject string, and therefore it fails if
3547 the current pointer is at the end of the string.
3548
3549 In UTF-8 mode, characters with values greater than 255 can be included
3550 in a class as a literal string of bytes, or by using the \x{ escaping
3551 mechanism.
3552
3553 When caseless matching is set, any letters in a class represent both
3554 their upper case and lower case versions, so for example, a caseless
3555 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3556 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3557 understands the concept of case for characters whose values are less
3558 than 128, so caseless matching is always possible. For characters with
3559 higher values, the concept of case is supported if PCRE is compiled
3560 with Unicode property support, but not otherwise. If you want to use
3561 caseless matching for characters 128 and above, you must ensure that
3562 PCRE is compiled with Unicode property support as well as with UTF-8
3563 support.
3564
3565 Characters that might indicate line breaks are never treated in any
3566 special way when matching character classes, whatever line-ending
3567 sequence is in use, and whatever setting of the PCRE_DOTALL and
3568 PCRE_MULTILINE options is used. A class such as [^a] always matches one
3569 of these characters.
3570
3571 The minus (hyphen) character can be used to specify a range of charac-
3572 ters in a character class. For example, [d-m] matches any letter
3573 between d and m, inclusive. If a minus character is required in a
3574 class, it must be escaped with a backslash or appear in a position
3575 where it cannot be interpreted as indicating a range, typically as the
3576 first or last character in the class.
3577
3578 It is not possible to have the literal character "]" as the end charac-
3579 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3580 two characters ("W" and "-") followed by a literal string "46]", so it
3581 would match "W46]" or "-46]". However, if the "]" is escaped with a
3582 backslash it is interpreted as the end of range, so [W-\]46] is inter-
3583 preted as a class containing a range followed by two other characters.
3584 The octal or hexadecimal representation of "]" can also be used to end
3585 a range.
3586
3587 Ranges operate in the collating sequence of character values. They can
3588 also be used for characters specified numerically, for example
3589 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3590 are greater than 255, for example [\x{100}-\x{2ff}].
3591
3592 If a range that includes letters is used when caseless matching is set,
3593 it matches the letters in either case. For example, [W-c] is equivalent
3594 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3595 character tables for a French locale are in use, [\xc8-\xcb] matches
3596 accented E characters in both cases. In UTF-8 mode, PCRE supports the
3597 concept of case for characters with values greater than 128 only when
3598 it is compiled with Unicode property support.
3599
3600 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3601 in a character class, and add the characters that they match to the
3602 class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3603 flex can conveniently be used with the upper case character types to
3604 specify a more restricted set of characters than the matching lower
3605 case type. For example, the class [^\W_] matches any letter or digit,
3606 but not underscore.
3607
3608 The only metacharacters that are recognized in character classes are
3609 backslash, hyphen (only where it can be interpreted as specifying a
3610 range), circumflex (only at the start), opening square bracket (only
3611 when it can be interpreted as introducing a POSIX class name - see the
3612 next section), and the terminating closing square bracket. However,
3613 escaping other non-alphanumeric characters does no harm.
3614
3615
3616 POSIX CHARACTER CLASSES
3617
3618 Perl supports the POSIX notation for character classes. This uses names
3619 enclosed by [: and :] within the enclosing square brackets. PCRE also
3620 supports this notation. For example,
3621
3622 [01[:alpha:]%]
3623
3624 matches "0", "1", any alphabetic character, or "%". The supported class
3625 names are
3626
3627 alnum letters and digits
3628 alpha letters
3629 ascii character codes 0 - 127
3630 blank space or tab only
3631 cntrl control characters
3632 digit decimal digits (same as \d)
3633 graph printing characters, excluding space
3634 lower lower case letters
3635 print printing characters, including space
3636 punct printing characters, excluding letters and digits
3637 space white space (not quite the same as \s)
3638 upper upper case letters
3639 word "word" characters (same as \w)
3640 xdigit hexadecimal digits
3641
3642 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3643 and space (32). Notice that this list includes the VT character (code
3644 11). This makes "space" different to \s, which does not include VT (for
3645 Perl compatibility).
3646
3647 The name "word" is a Perl extension, and "blank" is a GNU extension
3648 from Perl 5.8. Another Perl extension is negation, which is indicated
3649 by a ^ character after the colon. For example,
3650
3651 [12[:^digit:]]
3652
3653 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3654 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3655 these are not supported, and an error is given if they are encountered.
3656
3657 In UTF-8 mode, characters with values greater than 128 do not match any
3658 of the POSIX character classes.
3659
3660
3661 VERTICAL BAR
3662
3663 Vertical bar characters are used to separate alternative patterns. For
3664 example, the pattern
3665
3666 gilbert|sullivan
3667
3668 matches either "gilbert" or "sullivan". Any number of alternatives may
3669 appear, and an empty alternative is permitted (matching the empty
3670 string). The matching process tries each alternative in turn, from left
3671 to right, and the first one that succeeds is used. If the alternatives
3672 are within a subpattern (defined below), "succeeds" means matching the
3673 rest of the main pattern as well as the alternative in the subpattern.
3674
3675
3676 INTERNAL OPTION SETTING
3677
3678 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3679 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
3680 within the pattern by a sequence of Perl option letters enclosed
3681 between "(?" and ")". The option letters are
3682
3683 i for PCRE_CASELESS
3684 m for PCRE_MULTILINE
3685 s for PCRE_DOTALL
3686 x for PCRE_EXTENDED
3687
3688 For example, (?im) sets caseless, multiline matching. It is also possi-
3689 ble to unset these options by preceding the letter with a hyphen, and a
3690 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3691 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3692 is also permitted. If a letter appears both before and after the
3693 hyphen, the option is unset.
3694
3695 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3696 can be changed in the same way as the Perl-compatible options by using
3697 the characters J, U and X respectively.
3698
3699 When an option change occurs at top level (that is, not inside subpat-
3700 tern parentheses), the change applies to the remainder of the pattern
3701 that follows. If the change is placed right at the start of a pattern,
3702 PCRE extracts it into the global options (and it will therefore show up
3703 in data extracted by the pcre_fullinfo() function).
3704
3705 An option change within a subpattern (see below for a description of
3706 subpatterns) affects only that part of the current pattern that follows
3707 it, so
3708
3709 (a(?i)b)c
3710
3711 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3712 used). By this means, options can be made to have different settings
3713 in different parts of the pattern. Any changes made in one alternative
3714 do carry on into subsequent branches within the same subpattern. For
3715 example,
3716
3717 (a(?i)b|c)
3718
3719 matches "ab", "aB", "c", and "C", even though when matching "C" the
3720 first branch is abandoned before the option setting. This is because
3721 the effects of option settings happen at compile time. There would be
3722 some very weird behaviour otherwise.
3723
3724
3725 SUBPATTERNS
3726
3727 Subpatterns are delimited by parentheses (round brackets), which can be
3728 nested. Turning part of a pattern into a subpattern does two things:
3729
3730 1. It localizes a set of alternatives. For example, the pattern
3731
3732 cat(aract|erpillar|)
3733
3734 matches one of the words "cat", "cataract", or "caterpillar". Without
3735 the parentheses, it would match "cataract", "erpillar" or an empty
3736 string.
3737
3738 2. It sets up the subpattern as a capturing subpattern. This means
3739 that, when the whole pattern matches, that portion of the subject
3740 string that matched the subpattern is passed back to the caller via the
3741 ovector argument of pcre_exec(). Opening parentheses are counted from
3742 left to right (starting from 1) to obtain numbers for the capturing
3743 subpatterns.
3744
3745 For example, if the string "the red king" is matched against the pat-
3746 tern
3747
3748 the ((red|white) (king|queen))
3749
3750 the captured substrings are "red king", "red", and "king", and are num-
3751 bered 1, 2, and 3, respectively.
3752
3753 The fact that plain parentheses fulfil two functions is not always
3754 helpful. There are often times when a grouping subpattern is required
3755 without a capturing requirement. If an opening parenthesis is followed
3756 by a question mark and a colon, the subpattern does not do any captur-
3757 ing, and is not counted when computing the number of any subsequent
3758 capturing subpatterns. For example, if the string "the white queen" is
3759 matched against the pattern
3760
3761 the ((?:red|white) (king|queen))
3762
3763 the captured substrings are "white queen" and "queen", and are numbered
3764 1 and 2. The maximum number of capturing subpatterns is 65535.
3765
3766 As a convenient shorthand, if any option settings are required at the
3767 start of a non-capturing subpattern, the option letters may appear
3768 between the "?" and the ":". Thus the two patterns
3769
3770 (?i:saturday|sunday)
3771 (?:(?i)saturday|sunday)
3772
3773 match exactly the same set of strings. Because alternative branches are
3774 tried from left to right, and options are not reset until the end of
3775 the subpattern is reached, an option setting in one branch does affect
3776 subsequent branches, so the above patterns match "SUNDAY" as well as
3777 "Saturday".
3778
3779
3780 DUPLICATE SUBPATTERN NUMBERS
3781
3782 Perl 5.10 introduced a feature whereby each alternative in a subpattern
3783 uses the same numbers for its capturing parentheses. Such a subpattern
3784 starts with (?| and is itself a non-capturing subpattern. For example,
3785 consider this pattern:
3786
3787 (?|(Sat)ur|(Sun))day
3788
3789 Because the two alternatives are inside a (?| group, both sets of cap-
3790 turing parentheses are numbered one. Thus, when the pattern matches,
3791 you can look at captured substring number one, whichever alternative
3792 matched. This construct is useful when you want to capture part, but
3793 not all, of one of a number of alternatives. Inside a (?| group, paren-
3794 theses are numbered as usual, but the number is reset at the start of
3795 each branch. The numbers of any capturing buffers that follow the sub-
3796 pattern start after the highest number used in any branch. The follow-
3797 ing example is taken from the Perl documentation. The numbers under-
3798 neath show in which buffer the captured content will be stored.
3799
3800 # before ---------------branch-reset----------- after
3801 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3802 # 1 2 2 3 2 3 4
3803
3804 A backreference or a recursive call to a numbered subpattern always
3805 refers to the first one in the pattern with the given number.
3806
3807 An alternative approach to using this "branch reset" feature is to use
3808 duplicate named subpatterns, as described in the next section.
3809
3810
3811 NAMED SUBPATTERNS
3812
3813 Identifying capturing parentheses by number is simple, but it can be
3814 very hard to keep track of the numbers in complicated regular expres-
3815 sions. Furthermore, if an expression is modified, the numbers may
3816 change. To help with this difficulty, PCRE supports the naming of sub-
3817 patterns. This feature was not added to Perl until release 5.10. Python
3818 had the feature earlier, and PCRE introduced it at release 4.0, using
3819 the Python syntax. PCRE now supports both the Perl and the Python syn-
3820 tax.
3821
3822 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
3823 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
3824 to capturing parentheses from other parts of the pattern, such as back-
3825 references, recursion, and conditions, can be made by name as well as
3826 by number.
3827
3828 Names consist of up to 32 alphanumeric characters and underscores.
3829 Named capturing parentheses are still allocated numbers as well as
3830 names, exactly as if the names were not present. The PCRE API provides
3831 function calls for extracting the name-to-number translation table from
3832 a compiled pattern. There is also a convenience function for extracting
3833 a captured substring by name.
3834
3835 By default, a name must be unique within a pattern, but it is possible
3836 to relax this constraint by setting the PCRE_DUPNAMES option at compile
3837 time. This can be useful for patterns where only one instance of the
3838 named parentheses can match. Suppose you want to match the name of a
3839 weekday, either as a 3-letter abbreviation or as the full name, and in
3840 both cases you want to extract the abbreviation. This pattern (ignoring
3841 the line breaks) does the job:
3842
3843 (?<DN>Mon|Fri|Sun)(?:day)?|
3844 (?<DN>Tue)(?:sday)?|
3845 (?<DN>Wed)(?:nesday)?|
3846 (?<DN>Thu)(?:rsday)?|
3847 (?<DN>Sat)(?:urday)?
3848
3849 There are five capturing substrings, but only one is ever set after a
3850 match. (An alternative way of solving this problem is to use a "branch
3851 reset" subpattern, as described in the previous section.)
3852
3853 The convenience function for extracting the data by name returns the
3854 substring for the first (and in this example, the only) subpattern of
3855 that name that matched. This saves searching to find which numbered
3856 subpattern it was. If you make a reference to a non-unique named sub-
3857 pattern from elsewhere in the pattern, the one that corresponds to the
3858 lowest number is used. For further details of the interfaces for han-
3859 dling named subpatterns, see the pcreapi documentation.
3860
3861
3862 REPETITION
3863
3864 Repetition is specified by quantifiers, which can follow any of the
3865 following items:
3866
3867 a literal data character
3868 the dot metacharacter
3869 the \C escape sequence
3870 the \X escape sequence (in UTF-8 mode with Unicode properties)
3871 the \R escape sequence
3872 an escape such as \d that matches a single character
3873 a character class
3874 a back reference (see next section)
3875 a parenthesized subpattern (unless it is an assertion)
3876
3877 The general repetition quantifier specifies a minimum and maximum num-
3878 ber of permitted matches, by giving the two numbers in curly brackets
3879 (braces), separated by a comma. The numbers must be less than 65536,
3880 and the first must be less than or equal to the second. For example:
3881
3882 z{2,4}
3883
3884 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
3885 special character. If the second number is omitted, but the comma is
3886 present, there is no upper limit; if the second number and the comma
3887 are both omitted, the quantifier specifies an exact number of required
3888 matches. Thus
3889
3890 [aeiou]{3,}
3891
3892 matches at least 3 successive vowels, but may match many more, while
3893
3894 \d{8}
3895
3896 matches exactly 8 digits. An opening curly bracket that appears in a
3897 position where a quantifier is not allowed, or one that does not match
3898 the syntax of a quantifier, is taken as a literal character. For exam-
3899 ple, {,6} is not a quantifier, but a literal string of four characters.
3900
3901 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
3902 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
3903 acters, each of which is represented by a two-byte sequence. Similarly,
3904 when Unicode property support is available, \X{3} matches three Unicode
3905 extended sequences, each of which may be several bytes long (and they
3906 may be of different lengths).
3907
3908 The quantifier {0} is permitted, causing the expression to behave as if
3909 the previous item and the quantifier were not present.
3910
3911 For convenience, the three most common quantifiers have single-charac-
3912 ter abbreviations:
3913
3914 * is equivalent to {0,}
3915 + is equivalent to {1,}
3916 ? is equivalent to {0,1}
3917
3918 It is possible to construct infinite loops by following a subpattern
3919 that can match no characters with a quantifier that has no upper limit,
3920 for example:
3921
3922 (a?)*
3923
3924 Earlier versions of Perl and PCRE used to give an error at compile time
3925 for such patterns. However, because there are cases where this can be
3926 useful, such patterns are now accepted, but if any repetition of the
3927 subpattern does in fact match no characters, the loop is forcibly bro-
3928 ken.
3929
3930 By default, the quantifiers are "greedy", that is, they match as much
3931 as possible (up to the maximum number of permitted times), without
3932 causing the rest of the pattern to fail. The classic example of where
3933 this gives problems is in trying to match comments in C programs. These
3934 appear between /* and */ and within the comment, individual * and /
3935 characters may appear. An attempt to match C comments by applying the
3936 pattern
3937
3938 /\*.*\*/
3939
3940 to the string
3941
3942 /* first comment */ not comment /* second comment */
3943
3944 fails, because it matches the entire string owing to the greediness of
3945 the .* item.
3946
3947 However, if a quantifier is followed by a question mark, it ceases to
3948 be greedy, and instead matches the minimum number of times possible, so
3949 the pattern
3950
3951 /\*.*?\*/
3952
3953 does the right thing with the C comments. The meaning of the various
3954 quantifiers is not otherwise changed, just the preferred number of
3955 matches. Do not confuse this use of question mark with its use as a
3956 quantifier in its own right. Because it has two uses, it can sometimes
3957 appear doubled, as in
3958
3959 \d??\d
3960
3961 which matches one digit by preference, but can match two if that is the
3962 only way the rest of the pattern matches.
3963
3964 If the PCRE_UNGREEDY option is set (an option that is not available in
3965 Perl), the quantifiers are not greedy by default, but individual ones
3966 can be made greedy by following them with a question mark. In other
3967 words, it inverts the default behaviour.
3968
3969 When a parenthesized subpattern is quantified with a minimum repeat
3970 count that is greater than 1 or with a limited maximum, more memory is
3971 required for the compiled pattern, in proportion to the size of the
3972 minimum or maximum.
3973
3974 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
3975 alent to Perl's /s) is set, thus allowing the dot to match newlines,
3976 the pattern is implicitly anchored, because whatever follows will be
3977 tried against every character position in the subject string, so there
3978 is no point in retrying the overall match at any position after the
3979 first. PCRE normally treats such a pattern as though it were preceded
3980 by \A.
3981
3982 In cases where it is known that the subject string contains no new-
3983 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
3984 mization, or alternatively using ^ to indicate anchoring explicitly.
3985
3986 However, there is one situation where the optimization cannot be used.
3987 When .* is inside capturing parentheses that are the subject of a
3988 backreference elsewhere in the pattern, a match at the start may fail
3989 where a later one succeeds. Consider, for example:
3990
3991 (.*)abc\1
3992
3993 If the subject is "xyz123abc123" the match point is the fourth charac-
3994 ter. For this reason, such a pattern is not implicitly anchored.
3995
3996 When a capturing subpattern is repeated, the value captured is the sub-
3997 string that matched the final iteration. For example, after
3998
3999 (tweedle[dume]{3}\s*)+
4000
4001 has matched "tweedledum tweedledee" the value of the captured substring
4002 is "tweedledee". However, if there are nested capturing subpatterns,
4003 the corresponding captured values may have been set in previous itera-
4004 tions. For example, after
4005
4006 /(a|(b))+/
4007
4008 matches "aba" the value of the second captured substring is "b".
4009
4010
4011 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4012
4013 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4014 repetition, failure of what follows normally causes the repeated item
4015 to be re-evaluated to see if a different number of repeats allows the
4016 rest of the pattern to match. Sometimes it is useful to prevent this,
4017 either to change the nature of the match, or to cause it fail earlier
4018 than it otherwise might, when the author of the pattern knows there is
4019 no point in carrying on.
4020
4021 Consider, for example, the pattern \d+foo when applied to the subject
4022 line
4023
4024 123456bar
4025
4026 After matching all 6 digits and then failing to match "foo", the normal
4027 action of the matcher is to try again with only 5 digits matching the
4028 \d+ item, and then with 4, and so on, before ultimately failing.
4029 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4030 the means for specifying that once a subpattern has matched, it is not
4031 to be re-evaluated in this way.
4032
4033 If we use atomic grouping for the previous example, the matcher gives
4034 up immediately on failing to match "foo" the first time. The notation
4035 is a kind of special parenthesis, starting with (?> as in this example:
4036
4037 (?>\d+)foo
4038
4039 This kind of parenthesis "locks up" the part of the pattern it con-
4040 tains once it has matched, and a failure further into the pattern is
4041 prevented from backtracking into it. Backtracking past it to previous
4042 items, however, works as normal.
4043
4044 An alternative description is that a subpattern of this type matches
4045 the string of characters that an identical standalone pattern would
4046 match, if anchored at the current point in the subject string.
4047
4048 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4049 such as the above example can be thought of as a maximizing repeat that
4050 must swallow everything it can. So, while both \d+ and \d+? are pre-
4051 pared to adjust the number of digits they match in order to make the
4052 rest of the pattern match, (?>\d+) can only match an entire sequence of
4053 digits.
4054
4055 Atomic groups in general can of course contain arbitrarily complicated
4056 subpatterns, and can be nested. However, when the subpattern for an
4057 atomic group is just a single repeated item, as in the example above, a
4058 simpler notation, called a "possessive quantifier" can be used. This
4059 consists of an additional + character following a quantifier. Using
4060 this notation, the previous example can be rewritten as
4061
4062 \d++foo
4063
4064 Note that a possessive quantifier can be used with an entire group, for
4065 example:
4066
4067 (abc|xyz){2,3}+
4068
4069 Possessive quantifiers are always greedy; the setting of the
4070 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4071 simpler forms of atomic group. However, there is no difference in the
4072 meaning of a possessive quantifier and the equivalent atomic group,
4073 though there may be a performance difference; possessive quantifiers
4074 should be slightly faster.
4075
4076 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4077 tax. Jeffrey Friedl originated the idea (and the name) in the first
4078 edition of his book. Mike McCloskey liked it, so implemented it when he
4079 built Sun's Java package, and PCRE copied it from there. It ultimately
4080 found its way into Perl at release 5.10.
4081
4082 PCRE has an optimization that automatically "possessifies" certain sim-
4083 ple pattern constructs. For example, the sequence A+B is treated as
4084 A++B because there is no point in backtracking into a sequence of A's
4085 when B must follow.
4086
4087 When a pattern contains an unlimited repeat inside a subpattern that
4088 can itself be repeated an unlimited number of times, the use of an
4089 atomic group is the only way to avoid some failing matches taking a
4090 very long time indeed. The pattern
4091
4092 (\D+|<\d+>)*[!?]
4093
4094 matches an unlimited number of substrings that either consist of non-
4095 digits, or digits enclosed in <>, followed by either ! or ?. When it
4096 matches, it runs quickly. However, if it is applied to
4097
4098 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4099
4100 it takes a long time before reporting failure. This is because the
4101 string can be divided between the internal \D+ repeat and the external
4102 * repeat in a large number of ways, and all have to be tried. (The
4103 example uses [!?] rather than a single character at the end, because
4104 both PCRE and Perl have an optimization that allows for fast failure
4105 when a single character is used. They remember the last single charac-
4106 ter that is required for a match, and fail early if it is not present
4107 in the string.) If the pattern is changed so that it uses an atomic
4108 group, like this:
4109
4110 ((?>\D+)|<\d+>)*[!?]
4111
4112 sequences of non-digits cannot be broken, and failure happens quickly.
4113
4114
4115 BACK REFERENCES
4116
4117 Outside a character class, a backslash followed by a digit greater than
4118 0 (and possibly further digits) is a back reference to a capturing sub-
4119 pattern earlier (that is, to its left) in the pattern, provided there
4120 have been that many previous capturing left parentheses.
4121
4122 However, if the decimal number following the backslash is less than 10,
4123 it is always taken as a back reference, and causes an error only if
4124 there are not that many capturing left parentheses in the entire pat-
4125 tern. In other words, the parentheses that are referenced need not be
4126 to the left of the reference for numbers less than 10. A "forward back
4127 reference" of this type can make sense when a repetition is involved
4128 and the subpattern to the right has participated in an earlier itera-
4129 tion.
4130
4131 It is not possible to have a numerical "forward back reference" to a
4132 subpattern whose number is 10 or more using this syntax because a
4133 sequence such as \50 is interpreted as a character defined in octal.
4134 See the subsection entitled "Non-printing characters" above for further
4135 details of the handling of digits following a backslash. There is no
4136 such problem when named parentheses are used. A back reference to any
4137 subpattern is possible using named parentheses (see below).
4138
4139 Another way of avoiding the ambiguity inherent in the use of digits
4140 following a backslash is to use the \g escape sequence, which is a fea-
4141 ture introduced in Perl 5.10. This escape must be followed by an
4142 unsigned number or a negative number, optionally enclosed in braces.
4143 These examples are all identical:
4144
4145 (ring), \1
4146 (ring), \g1
4147 (ring), \g{1}
4148
4149 An unsigned number specifies an absolute reference without the ambigu-
4150 ity that is present in the older syntax. It is also useful when literal
4151 digits follow the reference. A negative number is a relative reference.
4152 Consider this example:
4153
4154 (abc(def)ghi)\g{-1}
4155
4156 The sequence \g{-1} is a reference to the most recently started captur-
4157 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
4158 \g{-2} would be equivalent to \1. The use of relative references can be
4159 helpful in long patterns, and also in patterns that are created by
4160 joining together fragments that contain references within themselves.
4161
4162 A back reference matches whatever actually matched the capturing sub-
4163 pattern in the current subject string, rather than anything matching
4164 the subpattern itself (see "Subpatterns as subroutines" below for a way
4165 of doing that). So the pattern
4166
4167 (sens|respons)e and \1ibility
4168
4169 matches "sense and sensibility" and "response and responsibility", but
4170 not "sense and responsibility". If caseful matching is in force at the
4171 time of the back reference, the case of letters is relevant. For exam-
4172 ple,
4173
4174 ((?i)rah)\s+\1
4175
4176 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4177 original capturing subpattern is matched caselessly.
4178
4179 There are several different ways of writing back references to named
4180 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4181 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4182 unified back reference syntax, in which \g can be used for both numeric
4183 and named references, is also supported. We could rewrite the above
4184 example in any of the following ways:
4185
4186 (?<p1>(?i)rah)\s+\k<p1>
4187 (?'p1'(?i)rah)\s+\k{p1}
4188 (?P<p1>(?i)rah)\s+(?P=p1)
4189 (?<p1>(?i)rah)\s+\g{p1}
4190
4191 A subpattern that is referenced by name may appear in the pattern
4192 before or after the reference.
4193
4194 There may be more than one back reference to the same subpattern. If a
4195 subpattern has not actually been used in a particular match, any back
4196 references to it always fail. For example, the pattern
4197
4198 (a|(bc))\2
4199
4200 always fails if it starts to match "a" rather than "bc". Because there
4201 may be many capturing parentheses in a pattern, all digits following
4202 the backslash are taken as part of a potential back reference number.
4203 If the pattern continues with a digit character, some delimiter must be
4204 used to terminate the back reference. If the PCRE_EXTENDED option is
4205 set, this can be whitespace. Otherwise an empty comment (see "Com-
4206 ments" below) can be used.
4207
4208 A back reference that occurs inside the parentheses to which it refers
4209 fails when the subpattern is first used, so, for example, (a\1) never
4210 matches. However, such references can be useful inside repeated sub-
4211 patterns. For example, the pattern
4212
4213 (a|b\1)+
4214
4215 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4216 ation of the subpattern, the back reference matches the character
4217 string corresponding to the previous iteration. In order for this to
4218 work, the pattern must be such that the first iteration does not need
4219 to match the back reference. This can be done using alternation, as in
4220 the example above, or by a quantifier with a minimum of zero.
4221
4222
4223 ASSERTIONS
4224
4225 An assertion is a test on the characters following or preceding the
4226 current matching point that does not actually consume any characters.
4227 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
4228 described above.
4229
4230 More complicated assertions are coded as subpatterns. There are two
4231 kinds: those that look ahead of the current position in the subject
4232 string, and those that look behind it. An assertion subpattern is
4233 matched in the normal way, except that it does not cause the current
4234 matching position to be changed.
4235
4236 Assertion subpatterns are not capturing subpatterns, and may not be
4237 repeated, because it makes no sense to assert the same thing several
4238 times. If any kind of assertion contains capturing subpatterns within
4239 it, these are counted for the purposes of numbering the capturing sub-
4240 patterns in the whole pattern. However, substring capturing is carried
4241 out only for positive assertions, because it does not make sense for
4242 negative assertions.
4243
4244 Lookahead assertions
4245
4246 Lookahead assertions start with (?= for positive assertions and (?! for
4247 negative assertions. For example,
4248
4249 \w+(?=;)
4250
4251 matches a word followed by a semicolon, but does not include the semi-
4252 colon in the match, and
4253
4254 foo(?!bar)
4255
4256 matches any occurrence of "foo" that is not followed by "bar". Note
4257 that the apparently similar pattern
4258
4259 (?!foo)bar
4260
4261 does not find an occurrence of "bar" that is preceded by something
4262 other than "foo"; it finds any occurrence of "bar" whatsoever, because
4263 the assertion (?!foo) is always true when the next three characters are
4264 "bar". A lookbehind assertion is needed to achieve the other effect.
4265
4266 If you want to force a matching failure at some point in a pattern, the
4267 most convenient way to do it is with (?!) because an empty string
4268 always matches, so an assertion that requires there not to be an empty
4269 string must always fail.
4270
4271 Lookbehind assertions
4272
4273 Lookbehind assertions start with (?<= for positive assertions and (?<!
4274 for negative assertions. For example,
4275
4276 (?<!foo)bar
4277
4278 does find an occurrence of "bar" that is not preceded by "foo". The
4279 contents of a lookbehind assertion are restricted such that all the
4280 strings it matches must have a fixed length. However, if there are sev-
4281 eral top-level alternatives, they do not all have to have the same
4282 fixed length. Thus
4283
4284 (?<=bullock|donkey)
4285
4286 is permitted, but
4287
4288 (?<!dogs?|cats?)
4289
4290 causes an error at compile time. Branches that match different length
4291 strings are permitted only at the top level of a lookbehind assertion.
4292 This is an extension compared with Perl (at least for 5.8), which
4293 requires all branches to match the same length of string. An assertion
4294 such as
4295
4296 (?<=ab(c|de))
4297
4298 is not permitted, because its single top-level branch can match two
4299 different lengths, but it is acceptable if rewritten to use two top-
4300 level branches:
4301
4302 (?<=abc|abde)
4303
4304 In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4305 instead of a lookbehind assertion; this is not restricted to a fixed-
4306 length.
4307
4308 The implementation of lookbehind assertions is, for each alternative,
4309 to temporarily move the current position back by the fixed length and
4310 then try to match. If there are insufficient characters before the cur-
4311 rent position, the assertion fails.
4312
4313 PCRE does not allow the \C escape (which matches a single byte in UTF-8
4314 mode) to appear in lookbehind assertions, because it makes it impossi-
4315 ble to calculate the length of the lookbehind. The \X and \R escapes,
4316 which can match different numbers of bytes, are also not permitted.
4317
4318 Possessive quantifiers can be used in conjunction with lookbehind
4319 assertions to specify efficient matching at the end of the subject
4320 string. Consider a simple pattern such as
4321
4322 abcd$
4323
4324 when applied to a long string that does not match. Because matching
4325 proceeds from left to right, PCRE will look for each "a" in the subject
4326 and then see if what follows matches the rest of the pattern. If the
4327 pattern is specified as
4328
4329 ^.*abcd$
4330
4331 the initial .* matches the entire string at first, but when this fails
4332 (because there is no following "a"), it backtracks to match all but the
4333 last character, then all but the last two characters, and so on. Once
4334 again the search for "a" covers the entire string, from right to left,
4335 so we are no better off. However, if the pattern is written as
4336
4337 ^.*+(?<=abcd)
4338
4339 there can be no backtracking for the .*+ item; it can match only the
4340 entire string. The subsequent lookbehind assertion does a single test
4341 on the last four characters. If it fails, the match fails immediately.
4342 For long strings, this approach makes a significant difference to the
4343 processing time.
4344
4345 Using multiple assertions
4346
4347 Several assertions (of any sort) may occur in succession. For example,
4348
4349 (?<=\d{3})(?<!999)foo
4350
4351 matches "foo" preceded by three digits that are not "999". Notice that
4352 each of the assertions is applied independently at the same point in
4353 the subject string. First there is a check that the previous three
4354 characters are all digits, and then there is a check that the same
4355 three characters are not "999". This pattern does not match "foo" pre-
4356 ceded by six characters, the first of which are digits and the last
4357 three of which are not "999". For example, it doesn't match "123abc-
4358 foo". A pattern to do that is
4359
4360 (?<=\d{3}...)(?<!999)foo
4361
4362 This time the first assertion looks at the preceding six characters,
4363 checking that the first three are digits, and then the second assertion
4364 checks that the preceding three characters are not "999".
4365
4366 Assertions can be nested in any combination. For example,
4367
4368 (?<=(?<!foo)bar)baz
4369
4370 matches an occurrence of "baz" that is preceded by "bar" which in turn
4371 is not preceded by "foo", while
4372
4373 (?<=\d{3}(?!999)...)foo
4374
4375 is another pattern that matches "foo" preceded by three digits and any
4376 three characters that are not "999".
4377
4378
4379 CONDITIONAL SUBPATTERNS
4380
4381 It is possible to cause the matching process to obey a subpattern con-
4382 ditionally or to choose between two alternative subpatterns, depending
4383 on the result of an assertion, or whether a previous capturing subpat-
4384 tern matched or not. The two possible forms of conditional subpattern
4385 are
4386
4387 (?(condition)yes-pattern)
4388 (?(condition)yes-pattern|no-pattern)
4389
4390 If the condition is satisfied, the yes-pattern is used; otherwise the
4391 no-pattern (if present) is used. If there are more than two alterna-
4392 tives in the subpattern, a compile-time error occurs.
4393
4394 There are four kinds of condition: references to subpatterns, refer-
4395 ences to recursion, a pseudo-condition called DEFINE, and assertions.
4396
4397 Checking for a used subpattern by number
4398
4399 If the text between the parentheses consists of a sequence of digits,
4400 the condition is true if the capturing subpattern of that number has
4401 previously matched. An alternative notation is to precede the digits
4402 with a plus or minus sign. In this case, the subpattern number is rela-
4403 tive rather than absolute. The most recently opened parentheses can be
4404 referenced by (?(-1), the next most recent by (?(-2), and so on. In
4405 looping constructs it can also make sense to refer to subsequent groups
4406 with constructs such as (?(+2).
4407
4408 Consider the following pattern, which contains non-significant white
4409 space to make it more readable (assume the PCRE_EXTENDED option) and to
4410 divide it into three parts for ease of discussion:
4411
4412 ( \( )? [^()]+ (?(1) \) )
4413
4414 The first part matches an optional opening parenthesis, and if that
4415 character is present, sets it as the first captured substring. The sec-
4416 ond part matches one or more characters that are not parentheses. The
4417 third part is a conditional subpattern that tests whether the first set
4418 of parentheses matched or not. If they did, that is, if subject started
4419 with an opening parenthesis, the condition is true, and so the yes-pat-
4420 tern is executed and a closing parenthesis is required. Otherwise,
4421 since no-pattern is not present, the subpattern matches nothing. In
4422 other words, this pattern matches a sequence of non-parentheses,
4423 optionally enclosed in parentheses.
4424
4425 If you were embedding this pattern in a larger one, you could use a
4426 relative reference:
4427
4428 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
4429
4430 This makes the fragment independent of the parentheses in the larger
4431 pattern.
4432
4433 Checking for a used subpattern by name
4434
4435 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
4436 used subpattern by name. For compatibility with earlier versions of
4437 PCRE, which had this facility before Perl, the syntax (?(name)...) is
4438 also recognized. However, there is a possible ambiguity with this syn-
4439 tax, because subpattern names may consist entirely of digits. PCRE
4440 looks first for a named subpattern; if it cannot find one and the name
4441 consists entirely of digits, PCRE looks for a subpattern of that num-
4442 ber, which must be greater than zero. Using subpattern names that con-
4443 sist entirely of digits is not recommended.
4444
4445 Rewriting the above example to use a named subpattern gives this:
4446
4447 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
4448
4449
4450 Checking for pattern recursion
4451
4452 If the condition is the string (R), and there is no subpattern with the
4453 name R, the condition is true if a recursive call to the whole pattern
4454 or any subpattern has been made. If digits or a name preceded by amper-
4455 sand follow the letter R, for example:
4456
4457 (?(R3)...) or (?(R&name)...)
4458
4459 the condition is true if the most recent recursion is into the subpat-
4460 tern whose number or name is given. This condition does not check the
4461 entire recursion stack.
4462
4463 At "top level", all these recursion test conditions are false. Recur-
4464 sive patterns are described below.
4465
4466 Defining subpatterns for use by reference only
4467
4468 If the condition is the string (DEFINE), and there is no subpattern
4469 with the name DEFINE, the condition is always false. In this case,
4470 there may be only one alternative in the subpattern. It is always
4471 skipped if control reaches this point in the pattern; the idea of
4472 DEFINE is that it can be used to define "subroutines" that can be ref-
4473 erenced from elsewhere. (The use of "subroutines" is described below.)
4474 For example, a pattern to match an IPv4 address could be written like
4475 this (ignore whitespace and line breaks):
4476
4477 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4478 \b (?&byte) (\.(?&byte)){3} \b
4479
4480 The first part of the pattern is a DEFINE group inside which a another
4481 group named "byte" is defined. This matches an individual component of
4482 an IPv4 address (a number less than 256). When matching takes place,
4483 this part of the pattern is skipped because DEFINE acts like a false
4484 condition.
4485
4486 The rest of the pattern uses references to the named group to match the
4487 four dot-separated components of an IPv4 address, insisting on a word
4488 boundary at each end.
4489
4490 Assertion conditions
4491
4492 If the condition is not in any of the above formats, it must be an
4493 assertion. This may be a positive or negative lookahead or lookbehind
4494 assertion. Consider this pattern, again containing non-significant
4495 white space, and with the two alternatives on the second line:
4496
4497 (?(?=[^a-z]*[a-z])
4498 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
4499
4500 The condition is a positive lookahead assertion that matches an
4501 optional sequence of non-letters followed by a letter. In other words,
4502 it tests for the presence of at least one letter in the subject. If a
4503 letter is found, the subject is matched against the first alternative;
4504 otherwise it is matched against the second. This pattern matches
4505 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4506 letters and dd are digits.
4507
4508
4509 COMMENTS
4510
4511 The sequence (?# marks the start of a comment that continues up to the
4512 next closing parenthesis. Nested parentheses are not permitted. The
4513 characters that make up a comment play no part in the pattern matching
4514 at all.
4515
4516 If the PCRE_EXTENDED option is set, an unescaped # character outside a
4517 character class introduces a comment that continues to immediately
4518 after the next newline in the pattern.
4519
4520
4521 RECURSIVE PATTERNS
4522
4523 Consider the problem of matching a string in parentheses, allowing for
4524 unlimited nested parentheses. Without the use of recursion, the best
4525 that can be done is to use a pattern that matches up to some fixed
4526 depth of nesting. It is not possible to handle an arbitrary nesting
4527 depth.
4528
4529 For some time, Perl has provided a facility that allows regular expres-
4530 sions to recurse (amongst other things). It does this by interpolating
4531 Perl code in the expression at run time, and the code can refer to the
4532 expression itself. A Perl pattern using code interpolation to solve the
4533 parentheses problem can be created like this:
4534
4535 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4536
4537 The (?p{...}) item interpolates Perl code at run time, and in this case
4538 refers recursively to the pattern in which it appears.
4539
4540 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4541 it supports special syntax for recursion of the entire pattern, and
4542 also for individual subpattern recursion. After its introduction in
4543 PCRE and Python, this kind of recursion was introduced into Perl at
4544 release 5.10.
4545
4546 A special item that consists of (? followed by a number greater than
4547 zero and a closing parenthesis is a recursive call of the subpattern of
4548 the given number, provided that it occurs inside that subpattern. (If
4549 not, it is a "subroutine" call, which is described in the next sec-
4550 tion.) The special item (?R) or (?0) is a recursive call of the entire
4551 regular expression.
4552
4553 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
4554 always treated as an atomic group. That is, once it has matched some of
4555 the subject string, it is never re-entered, even if it contains untried
4556 alternatives and there is a subsequent matching failure.
4557
4558 This PCRE pattern solves the nested parentheses problem (assume the
4559 PCRE_EXTENDED option is set so that white space is ignored):
4560
4561 \( ( (?>[^()]+) | (?R) )* \)
4562
4563 First it matches an opening parenthesis. Then it matches any number of
4564 substrings which can either be a sequence of non-parentheses, or a
4565 recursive match of the pattern itself (that is, a correctly parenthe-
4566 sized substring). Finally there is a closing parenthesis.
4567
4568 If this were part of a larger pattern, you would not want to recurse
4569 the entire pattern, so instead you could use this:
4570
4571 ( \( ( (?>[^()]+) | (?1) )* \) )
4572
4573 We have put the pattern into parentheses, and caused the recursion to
4574 refer to them instead of the whole pattern.
4575
4576 In a larger pattern, keeping track of parenthesis numbers can be
4577 tricky. This is made easier by the use of relative references. (A Perl
4578 5.10 feature.) Instead of (?1) in the pattern above you can write
4579 (?-2) to refer to the second most recently opened parentheses preceding
4580 the recursion. In other words, a negative number counts capturing
4581 parentheses leftwards from the point at which it is encountered.
4582
4583 It is also possible to refer to subsequently opened parentheses, by
4584 writing references such as (?+2). However, these cannot be recursive
4585 because the reference is not inside the parentheses that are refer-
4586 enced. They are always "subroutine" calls, as described in the next
4587 section.
4588
4589 An alternative approach is to use named parentheses instead. The Perl
4590 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
4591 supported. We could rewrite the above example as follows:
4592
4593 (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4594
4595 If there is more than one subpattern with the same name, the earliest
4596 one is used.
4597
4598 This particular example pattern that we have been looking at contains
4599 nested unlimited repeats, and so the use of atomic grouping for match-
4600 ing strings of non-parentheses is important when applying the pattern
4601 to strings that do not match. For example, when this pattern is applied
4602 to
4603
4604 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4605
4606 it yields "no match" quickly. However, if atomic grouping is not used,
4607 the match runs for a very long time indeed because there are so many
4608 different ways the + and * repeats can carve up the subject, and all
4609 have to be tested before failure can be reported.
4610
4611 At the end of a match, the values set for any capturing subpatterns are
4612 those from the outermost level of the recursion at which the subpattern
4613 value is set. If you want to obtain intermediate values, a callout
4614 function can be used (see below and the pcrecallout documentation). If
4615 the pattern above is matched against
4616
4617 (ab(cd)ef)
4618
4619 the value for the capturing parentheses is "ef", which is the last
4620 value taken on at the top level. If additional parentheses are added,
4621 giving
4622
4623 \( ( ( (?>[^()]+) | (?R) )* ) \)
4624 ^ ^
4625 ^ ^
4626
4627 the string they capture is "ab(cd)ef", the contents of the top level
4628 parentheses. If there are more than 15 capturing parentheses in a pat-
4629 tern, PCRE has to obtain extra memory to store data during a recursion,
4630 which it does by using pcre_malloc, freeing it via pcre_free after-
4631 wards. If no memory can be obtained, the match fails with the
4632 PCRE_ERROR_NOMEMORY error.
4633
4634 Do not confuse the (?R) item with the condition (R), which tests for
4635 recursion. Consider this pattern, which matches text in angle brack-
4636 ets, allowing for arbitrary nesting. Only digits are allowed in nested
4637 brackets (that is, when recursing), whereas any characters are permit-
4638 ted at the outer level.
4639
4640 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
4641
4642 In this pattern, (?(R) is the start of a conditional subpattern, with
4643 two different alternatives for the recursive and non-recursive cases.
4644 The (?R) item is the actual recursive call.
4645
4646
4647 SUBPATTERNS AS SUBROUTINES
4648
4649 If the syntax for a recursive subpattern reference (either by number or
4650 by name) is used outside the parentheses to which it refers, it oper-
4651 ates like a subroutine in a programming language. The "called" subpat-
4652 tern may be defined before or after the reference. A numbered reference
4653 can be absolute or relative, as in these examples:
4654
4655 (...(absolute)...)...(?2)...
4656 (...(relative)...)...(?-1)...
4657 (...(?+1)...(relative)...
4658
4659 An earlier example pointed out that the pattern
4660
4661 (sens|respons)e and \1ibility
4662
4663 matches "sense and sensibility" and "response and responsibility", but
4664 not "sense and responsibility". If instead the pattern
4665
4666 (sens|respons)e and (?1)ibility
4667
4668 is used, it does match "sense and responsibility" as well as the other
4669 two strings. Another example is given in the discussion of DEFINE
4670 above.
4671
4672 Like recursive subpatterns, a "subroutine" call is always treated as an
4673 atomic group. That is, once it has matched some of the subject string,
4674 it is never re-entered, even if it contains untried alternatives and
4675 there is a subsequent matching failure.
4676
4677 When a subpattern is used as a subroutine, processing options such as
4678 case-independence are fixed when the subpattern is defined. They cannot
4679 be changed for different calls. For example, consider this pattern:
4680
4681 (abc)(?i:(?-1))
4682
4683 It matches "abcabc". It does not match "abcABC" because the change of
4684 processing option does not affect the called subpattern.
4685
4686
4687 CALLOUTS
4688
4689 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4690 Perl code to be obeyed in the middle of matching a regular expression.
4691 This makes it possible, amongst other things, to extract different sub-
4692 strings that match the same pair of parentheses when there is a repeti-
4693 tion.
4694
4695 PCRE provides a similar feature, but of course it cannot obey arbitrary
4696 Perl code. The feature is called "callout". The caller of PCRE provides
4697 an external function by putting its entry point in the global variable
4698 pcre_callout. By default, this variable contains NULL, which disables
4699 all calling out.
4700
4701 Within a regular expression, (?C) indicates the points at which the
4702 external function is to be called. If you want to identify different
4703 callout points, you can put a number less than 256 after the letter C.
4704 The default value is zero. For example, this pattern has two callout
4705 points:
4706
4707 (?C1)abc(?C2)def
4708
4709 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4710 automatically installed before each item in the pattern. They are all
4711 numbered 255.
4712
4713 During matching, when PCRE reaches a callout point (and pcre_callout is
4714 set), the external function is called. It is provided with the number
4715 of the callout, the position in the pattern, and, optionally, one item
4716 of data originally supplied by the caller of pcre_exec(). The callout
4717 function may cause matching to proceed, to backtrack, or to fail alto-
4718 gether. A complete description of the interface to the callout function
4719 is given in the pcrecallout documentation.
4720
4721
4722 BACKTRACKING CONTROL
4723
4724 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
4725 which are described in the Perl documentation as "experimental and sub-
4726 ject to change or removal in a future version of Perl". It goes on to
4727 say: "Their usage in production code should be noted to avoid problems
4728 during upgrades." The same remarks apply to the PCRE features described
4729 in this section.
4730
4731 Since these verbs are specifically related to backtracking, they can be
4732 used only when the pattern is to be matched using pcre_exec(), which
4733 uses a backtracking algorithm. They cause an error if encountered by
4734 pcre_dfa_exec().
4735
4736 The new verbs make use of what was previously invalid syntax: an open-
4737 ing parenthesis followed by an asterisk. In Perl, they are generally of
4738 the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4739 its general form is just (*VERB). Any number of these verbs may occur
4740 in a pattern. There are two kinds:
4741
4742 Verbs that act immediately
4743
4744 The following verbs act as soon as they are encountered:
4745
4746 (*ACCEPT)
4747
4748 This verb causes the match to end successfully, skipping the remainder
4749 of the pattern. When inside a recursion, only the innermost pattern is
4750 ended immediately. PCRE differs from Perl in what happens if the
4751 (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is
4752 captured: in PCRE no data is captured. For example:
4753
4754 A(A|B(*ACCEPT)|C)D
4755
4756 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data
4757 is captured.
4758
4759 (*FAIL) or (*F)
4760
4761 This verb causes the match to fail, forcing backtracking to occur. It
4762 is equivalent to (?!) but easier to read. The Perl documentation notes
4763 that it is probably useful only when combined with (?{}) or (??{}).
4764 Those are, of course, Perl features that are not present in PCRE. The
4765 nearest equivalent is the callout feature, as for example in this pat-
4766 tern:
4767
4768 a+(?C)(*FAIL)
4769
4770 A match with the string "aaaa" always fails, but the callout is taken
4771 before each backtrack happens (in this example, 10 times).
4772
4773 Verbs that act after backtracking
4774
4775 The following verbs do nothing when they are encountered. Matching con-
4776 tinues with what follows, but if there is no subsequent match, a fail-
4777 ure is forced. The verbs differ in exactly what kind of failure
4778 occurs.
4779
4780 (*COMMIT)
4781
4782 This verb causes the whole match to fail outright if the rest of the
4783 pattern does not match. Even if the pattern is unanchored, no further
4784 attempts to find a match by advancing the start point take place. Once
4785 (*COMMIT) has been passed, pcre_exec() is committed to finding a match
4786 at the current starting point, or not at all. For example:
4787
4788 a+(*COMMIT)b
4789
4790 This matches "xxaab" but not "aacaab". It can be thought of as a kind
4791 of dynamic anchor, or "I've started, so I must finish."
4792
4793 (*PRUNE)
4794
4795 This verb causes the match to fail at the current position if the rest
4796 of the pattern does not match. If the pattern is unanchored, the normal
4797 "bumpalong" advance to the next starting character then happens. Back-
4798 tracking can occur as usual to the left of (*PRUNE), or when matching
4799 to the right of (*PRUNE), but if there is no match to the right, back-
4800 tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
4801 is just an alternative to an atomic group or possessive quantifier, but
4802 there are some uses of (*PRUNE) that cannot be expressed in any other
4803 way.
4804
4805 (*SKIP)
4806
4807 This verb is like (*PRUNE), except that if the pattern is unanchored,
4808 the "bumpalong" advance is not to the next character, but to the posi-
4809 tion in the subject where (*SKIP) was encountered. (*SKIP) signifies
4810 that whatever text was matched leading up to it cannot be part of a
4811 successful match. Consider:
4812
4813 a+(*SKIP)b
4814
4815 If the subject is "aaaac...", after the first match attempt fails
4816 (starting at the first character in the string), the starting point
4817 skips on to start the next attempt at "c". Note that a possessive quan-
4818 tifer does not have the same effect in this example; although it would
4819 suppress backtracking during the first match attempt, the second
4820 attempt would start at the second character instead of skipping on to
4821 "c".
4822
4823 (*THEN)
4824
4825 This verb causes a skip to the next alternation if the rest of the pat-
4826 tern does not match. That is, it cancels pending backtracking, but only
4827 within the current alternation. Its name comes from the observation
4828 that it can be used for a pattern-based if-then-else block:
4829
4830 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
4831
4832 If the COND1 pattern matches, FOO is tried (and possibly further items
4833 after the end of the group if FOO succeeds); on failure the matcher
4834 skips to the second alternative and tries COND2, without backtracking
4835 into COND1. If (*THEN) is used outside of any alternation, it acts
4836 exactly like (*PRUNE).
4837
4838
4839 SEE ALSO
4840
4841 pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
4842
4843
4844 AUTHOR
4845
4846 Philip Hazel
4847 University Computing Service
4848 Cambridge CB2 3QH, England.
4849
4850
4851 REVISION
4852
4853 Last updated: 11 September 2007
4854 Copyright (c) 1997-2007 University of Cambridge.
4855 ------------------------------------------------------------------------------
4856
4857
4858 PCRESYNTAX(3) PCRESYNTAX(3)
4859
4860
4861 NAME
4862 PCRE - Perl-compatible regular expressions
4863
4864
4865 PCRE REGULAR EXPRESSION SYNTAX SUMMARY
4866
4867 The full syntax and semantics of the regular expressions that are sup-
4868 ported by PCRE are described in the pcrepattern documentation. This
4869 document contains just a quick-reference summary of the syntax.
4870
4871
4872 QUOTING
4873
4874 \x where x is non-alphanumeric is a literal x
4875 \Q...\E treat enclosed characters as literal
4876
4877
4878 CHARACTERS
4879
4880 \a alarm, that is, the BEL character (hex 07)
4881 \cx "control-x", where x is any character
4882 \e escape (hex 1B)
4883 \f formfeed (hex 0C)
4884 \n newline (hex 0A)
4885 \r carriage return (hex 0D)
4886 \t tab (hex 09)
4887 \ddd character with octal code ddd, or backreference
4888 \xhh character with hex code hh
4889 \x{hhh..} character with hex code hhh..
4890
4891
4892 CHARACTER TYPES
4893
4894 . any character except newline;
4895 in dotall mode, any character whatsoever
4896 \C one byte, even in UTF-8 mode (best avoided)
4897 \d a decimal digit
4898 \D a character that is not a decimal digit
4899 \h a horizontal whitespace character
4900 \H a character that is not a horizontal whitespace character
4901 \p{xx} a character with the xx property
4902 \P{xx} a character without the xx property
4903 \R a newline sequence
4904 \s a whitespace character
4905 \S a character that is not a whitespace character
4906 \v a vertical whitespace character
4907 \V a character that is not a vertical whitespace character
4908 \w a "word" character
4909 \W a "non-word" character
4910 \X an extended Unicode sequence
4911
4912 In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.
4913
4914
4915 GENERAL CATEGORY PROPERTY CODES FOR \p and \P
4916
4917 C Other
4918 Cc Control
4919 Cf Format
4920 Cn Unassigned
4921 Co Private use
4922 Cs Surrogate
4923
4924 L Letter
4925 Ll Lower case letter
4926 Lm Modifier letter
4927 Lo Other letter
4928 Lt Title case letter
4929 Lu Upper case letter
4930 L& Ll, Lu, or Lt
4931
4932 M Mark
4933 Mc Spacing mark
4934 Me Enclosing mark
4935 Mn Non-spacing mark
4936
4937 N Number
4938 Nd Decimal number
4939 Nl Letter number
4940 No Other number
4941
4942 P Punctuation
4943 Pc Connector punctuation
4944 Pd Dash punctuation
4945 Pe Close punctuation
4946 Pf Final punctuation
4947 Pi Initial punctuation
4948 Po Other punctuation
4949 Ps Open punctuation
4950
4951 S Symbol
4952 Sc Currency symbol
4953 Sk Modifier symbol
4954 Sm Mathematical symbol
4955 So Other symbol
4956
4957 Z Separator
4958 Zl Line separator
4959 Zp Paragraph separator
4960 Zs Space separator
4961
4962
4963 SCRIPT NAMES FOR \p AND \P
4964
4965 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
4966 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
4967 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
4968 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
4969 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
4970 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
4971 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
4972 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
4973 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
4974
4975
4976 CHARACTER CLASSES
4977
4978 [...] positive character class
4979 [^...] negative character class
4980 [x-y] range (can be used for hex characters)
4981 [[:xxx:]] positive POSIX named set
4982 [[^:xxx:]] negative POSIX named set
4983
4984 alnum alphanumeric
4985 alpha alphabetic
4986 ascii 0-127
4987 blank space or tab
4988 cntrl control character
4989 digit decimal digit
4990 graph printing, excluding space
4991 lower lower case letter
4992 print printing, including space
4993 punct printing, excluding alphanumeric
4994 space whitespace
4995 upper upper case letter
4996 word same as \w
4997 xdigit hexadecimal digit
4998
4999 In PCRE, POSIX character set names recognize only ASCII characters. You
5000 can use \Q...\E inside a character class.
5001
5002
5003 QUANTIFIERS
5004
5005 ? 0 or 1, greedy
5006 ?+ 0 or 1, possessive
5007 ?? 0 or 1, lazy
5008 * 0 or more, greedy
5009 *+ 0 or more, possessive
5010 *? 0 or more, lazy
5011 + 1 or more, greedy
5012 ++ 1 or more, possessive
5013 +? 1 or more, lazy
5014 {n} exactly n
5015 {n,m} at least n, no more than m, greedy
5016 {n,m}+ at least n, no more than m, possessive
5017 {n,m}? at least n, no more than m, lazy
5018 {n,} n or more, greedy
5019 {n,}+ n or more, possessive
5020 {n,}? n or more, lazy
5021
5022
5023 ANCHORS AND SIMPLE ASSERTIONS
5024
5025 \b word boundary
5026 \B not a word boundary
5027 ^ start of subject
5028 also after internal newline in multiline mode
5029 \A start of subject
5030 $ end of subject
5031 also before newline at end of subject
5032 also before internal newline in multiline mode
5033 \Z end of subject
5034 also before newline at end of subject
5035 \z end of subject
5036 \G first matching position in subject
5037
5038
5039 MATCH POINT RESET
5040
5041 \K reset start of match
5042
5043
5044 ALTERNATION
5045
5046 expr|expr|expr...
5047
5048
5049 CAPTURING
5050
5051 (...) capturing group
5052 (?<name>...) named capturing group (Perl)
5053 (?'name'...) named capturing group (Perl)
5054 (?P<name>...) named capturing group (Python)
5055 (?:...) non-capturing group
5056 (?|...) non-capturing group; reset group numbers for
5057 capturing groups in each alternative
5058
5059
5060 ATOMIC GROUPS
5061
5062 (?>...) atomic, non-capturing group
5063
5064
5065 COMMENT
5066
5067 (?#....) comment (not nestable)
5068
5069
5070 OPTION SETTING
5071
5072 (?i) caseless
5073 (?J) allow duplicate names
5074 (?m) multiline
5075 (?s) single line (dotall)
5076 (?U) default ungreedy (lazy)
5077 (?x) extended (ignore white space)
5078 (?-...) unset option(s)
5079
5080
5081 LOOKAHEAD AND LOOKBEHIND ASSERTIONS
5082
5083 (?=...) positive look ahead
5084 (?!...) negative look ahead
5085 (?<=...) positive look behind
5086 (?<!...) negative look behind
5087
5088 Each top-level branch of a look behind must be of a fixed length.
5089
5090
5091 BACKREFERENCES
5092
5093 \n reference by number (can be ambiguous)
5094 \gn reference by number
5095 \g{n} reference by number
5096 \g{-n} relative reference by number
5097 \k<name> reference by name (Perl)
5098 \k'name' reference by name (Perl)
5099 \g{name} reference by name (Perl)
5100 \k{name} reference by name (.NET)
5101 (?P=name) reference by name (Python)
5102
5103
5104 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
5105
5106 (?R) recurse whole pattern
5107 (?n) call subpattern by absolute number
5108 (?+n) call subpattern by relative number
5109 (?-n) call subpattern by relative number
5110 (?&name) call subpattern by name (Perl)
5111 (?P>name) call subpattern by name (Python)
5112
5113
5114 CONDITIONAL PATTERNS
5115
5116 (?(condition)yes-pattern)
5117 (?(condition)yes-pattern|no-pattern)
5118
5119 (?(n)... absolute reference condition
5120 (?(+n)... relative reference condition
5121 (?(-n)... relative reference condition
5122 (?(<name>)... named reference condition (Perl)
5123 (?('name')... named reference condition (Perl)
5124 (?(name)... named reference condition (PCRE)
5125 (?(R)... overall recursion condition
5126 (?(Rn)... specific group recursion condition
5127 (?(R&name)... specific recursion condition
5128 (?(DEFINE)... define subpattern for reference
5129 (?(assert)... assertion condition
5130
5131
5132 BACKTRACKING CONTROL
5133
5134 The following act immediately they are reached:
5135
5136 (*ACCEPT) force successful match
5137 (*FAIL) force backtrack; synonym (*F)
5138
5139 The following act only when a subsequent match failure causes a back-
5140 track to reach them. They all force a match failure, but they differ in
5141 what happens afterwards. Those that advance the start-of-match point do
5142 so only if the pattern is not anchored.
5143
5144 (*COMMIT) overall failure, no advance of starting point
5145 (*PRUNE) advance to next starting character
5146 (*SKIP) advance start to current matching position
5147 (*THEN) local failure, backtrack to next alternation
5148
5149
5150 NEWLINE CONVENTIONS
5151
5152 These are recognized only at the very start of a pattern.
5153
5154 (*CR)
5155 (*LF)
5156 (*CRLF)
5157 (*ANYCRLF)
5158 (*ANY)
5159
5160
5161 WHAT \R MATCHES
5162
5163 These are recognized only at the very start of a pattern.
5164
5165 (*BSR_ANYCRLF)
5166 (*BSR_UNICODE)
5167
5168
5169 CALLOUTS
5170
5171 (?C) callout
5172 (?Cn) callout with data n
5173
5174
5175 SEE ALSO
5176
5177 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
5178
5179
5180 AUTHOR
5181
5182 Philip Hazel
5183 University Computing Service
5184 Cambridge CB2 3QH, England.
5185
5186
5187 REVISION
5188
5189 Last updated: 11 September 2007
5190 Copyright (c) 1997-2007 University of Cambridge.
5191 ------------------------------------------------------------------------------
5192
5193
5194 PCREPARTIAL(3) PCREPARTIAL(3)
5195
5196
5197 NAME
5198 PCRE - Perl-compatible regular expressions
5199
5200
5201 PARTIAL MATCHING IN PCRE
5202
5203 In normal use of PCRE, if the subject string that is passed to
5204 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
5205 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
5206 There are circumstances where it might be helpful to distinguish this
5207 case from other cases in which there is no match.
5208
5209 Consider, for example, an application where a human is required to type
5210 in data for a field with specific formatting requirements. An example
5211 might be a date in the form ddmmmyy, defined by this pattern:
5212
5213 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
5214
5215 If the application sees the user's keystrokes one by one, and can check
5216 that what has been typed so far is potentially valid, it is able to
5217 raise an error as soon as a mistake is made, possibly beeping and not
5218 reflecting the character that has been typed. This immediate feedback
5219 is likely to be a better user interface than a check that is delayed
5220 until the entire string has been entered.
5221
5222 PCRE supports the concept of partial matching by means of the PCRE_PAR-
5223 TIAL option, which can be set when calling pcre_exec() or
5224 pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
5225 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
5226 during the matching process the last part of the subject string matched
5227 part of the pattern. Unfortunately, for non-anchored matching, it is
5228 not possible to obtain the position of the start of the partial match.
5229 No captured data is set when PCRE_ERROR_PARTIAL is returned.
5230
5231 When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code
5232 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of
5233 the subject is reached, there have been no complete matches, but there
5234 is still at least one matching possibility. The portion of the string
5235 that provided the partial match is set as the first matching string.
5236
5237 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
5238 the last literal byte in a pattern, and abandons matching immediately
5239 if such a byte is not present in the subject string. This optimization
5240 cannot be used for a subject string that might match only partially.
5241
5242
5243 RESTRICTED PATTERNS FOR PCRE_PARTIAL
5244
5245 Because of the way certain internal optimizations are implemented in
5246 the pcre_exec() function, the PCRE_PARTIAL option cannot be used with
5247 all patterns. These restrictions do not apply when pcre_dfa_exec() is
5248 used. For pcre_exec(), repeated single characters such as
5249
5250 a{2,4}
5251
5252 and repeated single metasequences such as
5253
5254 \d+
5255
5256 are not permitted if the maximum number of occurrences is greater than
5257 one. Optional items such as \d? (where the maximum is one) are permit-
5258 ted. Quantifiers with any values are permitted after parentheses, so
5259 the invalid examples above can be coded thus:
5260
5261 (a){2,4}
5262 (\d)+
5263
5264 These constructions run more slowly, but for the kinds of application
5265 that are envisaged for this facility, this is not felt to be a major
5266 restriction.
5267
5268 If PCRE_PARTIAL is set for a pattern that does not conform to the
5269 restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
5270 (-13). You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to
5271 find out if a compiled pattern can be used for partial matching.
5272
5273
5274 EXAMPLE OF PARTIAL MATCHING USING PCRETEST
5275
5276 If the escape sequence \P is present in a pcretest data line, the
5277 PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
5278 uses the date example quoted above:
5279
5280 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5281 data> 25jun04\P
5282 0: 25jun04
5283 1: jun
5284 data> 25dec3\P
5285 Partial match
5286 data> 3ju\P
5287 Partial match
5288 data> 3juj\P
5289 No match
5290 data> j\P
5291 No match
5292
5293 The first data string is matched completely, so pcretest shows the
5294 matched substrings. The remaining four strings do not match the com-
5295 plete pattern, but the first two are partial matches. The same test,
5296 using pcre_dfa_exec() matching (by means of the \D escape sequence),
5297 produces the following output:
5298
5299 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5300 data> 25jun04\P\D
5301 0: 25jun04
5302 data> 23dec3\P\D
5303 Partial match: 23dec3
5304 data> 3ju\P\D
5305 Partial match: 3ju
5306 data> 3juj\P\D
5307 No match
5308 data> j\P\D
5309 No match
5310
5311 Notice that in this case the portion of the string that was matched is
5312 made available.
5313
5314
5315 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
5316
5317 When a partial match has been found using pcre_dfa_exec(), it is possi-
5318 ble to continue the match by providing additional subject data and
5319 calling pcre_dfa_exec() again with the same compiled regular expres-
5320 sion, this time setting the PCRE_DFA_RESTART option. You must also pass
5321 the same working space as before, because this is where details of the
5322 previous partial match are stored. Here is an example using pcretest,
5323 using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
5324 \D are as above):
5325
5326 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5327 data> 23ja\P\D
5328 Partial match: 23ja
5329 data> n05\R\D
5330 0: n05
5331
5332 The first call has "23ja" as the subject, and requests partial match-
5333 ing; the second call has "n05" as the subject for the continued
5334 (restarted) match. Notice that when the match is complete, only the
5335 last part is shown; PCRE does not retain the previously partially-
5336 matched string. It is up to the calling program to do that if it needs
5337 to.
5338
5339 You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial
5340 matching over multiple segments. This facility can be used to pass very
5341 long subject strings to pcre_dfa_exec(). However, some care is needed
5342 for certain types of pattern.
5343
5344 1. If the pattern contains tests for the beginning or end of a line,
5345 you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
5346 ate, when the subject string for any call does not contain the begin-
5347 ning or end of a line.
5348
5349 2. If the pattern contains backward assertions (including \b or \B),
5350 you need to arrange for some overlap in the subject strings to allow
5351 for this. For example, you could pass the subject in chunks that are
5352 500 bytes long, but in a buffer of 700 bytes, with the starting offset
5353 set to 200 and the previous 200 bytes at the start of the buffer.
5354
5355 3. Matching a subject string that is split into multiple segments does
5356 not always produce exactly the same result as matching over one single
5357 long string. The difference arises when there are multiple matching
5358 possibilities, because a partial match result is given only when there
5359 are no completed matches in a call to pcre_dfa_exec(). This means that
5360 as soon as the shortest match has been found, continuation to a new
5361 subject segment is no longer possible. Consider this pcretest example:
5362
5363 re> /dog(sbody)?/
5364 data> do\P\D
5365 Partial match: do
5366 data> gsb\R\P\D
5367 0: g
5368 data> dogsbody\D
5369 0: dogsbody
5370 1: dog
5371
5372 The pattern matches the words "dog" or "dogsbody". When the subject is
5373 presented in several parts ("do" and "gsb" being the first two) the
5374 match stops when "dog" has been found, and it is not possible to con-
5375 tinue. On the other hand, if "dogsbody" is presented as a single
5376 string, both matches are found.
5377
5378 Because of this phenomenon, it does not usually make sense to end a
5379 pattern that is going to be matched in this way with a variable repeat.
5380
5381 4. Patterns that contain alternatives at the top level which do not all
5382 start with the same pattern item may not work as expected. For example,
5383 consider this pattern:
5384
5385 1234|3789
5386
5387 If the first part of the subject is "ABC123", a partial match of the
5388 first alternative is found at offset 3. There is no partial match for
5389 the second alternative, because such a match does not start at the same
5390 point in the subject string. Attempting to continue with the string
5391 "789" does not yield a match because only those alternatives that match
5392 at one point in the subject are remembered. The problem arises because
5393 the start of the second alternative matches within the first alterna-
5394 tive. There is no problem with anchored patterns or patterns such as:
5395
5396 1234|ABCD
5397
5398 where no string can be a partial match for both alternatives.
5399
5400
5401 AUTHOR
5402
5403 Philip Hazel
5404 University Computing Service
5405 Cambridge CB2 3QH, England.
5406
5407
5408 REVISION
5409
5410 Last updated: 04 June 2007
5411 Copyright (c) 1997-2007 University of Cambridge.
5412 ------------------------------------------------------------------------------
5413
5414
5415 PCREPRECOMPILE(3) PCREPRECOMPILE(3)
5416
5417
5418 NAME
5419 PCRE - Perl-compatible regular expressions
5420
5421
5422 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
5423
5424 If you are running an application that uses a large number of regular
5425 expression patterns, it may be useful to store them in a precompiled
5426 form instead of having to compile them every time the application is
5427 run. If you are not using any private character tables (see the
5428 pcre_maketables() documentation), this is relatively straightforward.
5429 If you are using private tables, it is a little bit more complicated.
5430
5431 If you save compiled patterns to a file, you can copy them to a differ-
5432 ent host and run them there. This works even if the new host has the
5433 opposite endianness to the one on which the patterns were compiled.
5434 There may be a small performance penalty, but it should be insignifi-
5435 cant. However, compiling regular expressions with one version of PCRE
5436 for use with a different version is not guaranteed to work and may
5437 cause crashes.
5438
5439
5440 SAVING A COMPILED PATTERN
5441 The value returned by pcre_compile() points to a single block of memory
5442 that holds the compiled pattern and associated data. You can find the
5443 length of this block in bytes by calling pcre_fullinfo() with an argu-
5444 ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
5445 manner. Here is sample code that compiles a pattern and writes it to a
5446 file. It assumes that the variable fd refers to a file that is open for
5447 output:
5448
5449 int erroroffset, rc, size;
5450 char *error;
5451 pcre *re;
5452
5453 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
5454 if (re == NULL) { ... handle errors ... }
5455 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
5456 if (rc < 0) { ... handle errors ... }
5457 rc = fwrite(re, 1, size, fd);
5458 if (rc != size) { ... handle errors ... }
5459
5460 In this example, the bytes that comprise the compiled pattern are
5461 copied exactly. Note that this is binary data that may contain any of
5462 the 256 possible byte values. On systems that make a distinction
5463 between binary and non-binary data, be sure that the file is opened for
5464 binary output.
5465
5466 If you want to write more than one pattern to a file, you will have to
5467 devise a way of separating them. For binary data, preceding each pat-
5468 tern with its length is probably the most straightforward approach.
5469 Another possibility is to write out the data in hexadecimal instead of
5470 binary, one pattern to a line.
5471
5472 Saving compiled patterns in a file is only one possible way of storing
5473 them for later use. They could equally well be saved in a database, or
5474 in the memory of some daemon process that passes them via sockets to
5475 the processes that want them.
5476
5477 If the pattern has been studied, it is also possible to save the study
5478 data in a similar way to the compiled pattern itself. When studying
5479 generates additional information, pcre_study() returns a pointer to a
5480 pcre_extra data block. Its format is defined in the section on matching
5481 a pattern in the pcreapi documentation. The study_data field points to
5482 the binary study data, and this is what you must save (not the
5483 pcre_extra block itself). The length of the study data can be obtained
5484 by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
5485 Remember to check that pcre_study() did return a non-NULL value before
5486 trying to save the study data.
5487
5488
5489 RE-USING A PRECOMPILED PATTERN
5490
5491 Re-using a precompiled pattern is straightforward. Having reloaded it
5492 into main memory, you pass its pointer to pcre_exec() or
5493 pcre_dfa_exec() in the usual way. This should work even on another
5494 host, and even if that host has the opposite endianness to the one
5495 where the pattern was compiled.
5496
5497 However, if you passed a pointer to custom character tables when the
5498 pattern was compiled (the tableptr argument of pcre_compile()), you
5499 must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
5500 because the value saved with the compiled pattern will obviously be
5501 nonsense. A field in a pcre_extra() block is used to pass this data, as
5502 described in the section on matching a pattern in the pcreapi documen-
5503 tation.
5504
5505 If you did not provide custom character tables when the pattern was
5506 compiled, the pointer in the compiled pattern is NULL, which causes
5507 pcre_exec() to use PCRE's internal tables. Thus, you do not need to
5508 take any special action at run time in this case.
5509
5510 If you saved study data with the compiled pattern, you need to create
5511 your own pcre_extra data block and set the study_data field to point to
5512 the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
5513 bit in the flags field to indicate that study data is present. Then
5514 pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
5515 usual way.
5516
5517
5518 COMPATIBILITY WITH DIFFERENT PCRE RELEASES
5519
5520 In general, it is safest to recompile all saved patterns when you
5521 update to a new PCRE release, though not all updates actually require
5522 this. Recompiling is definitely needed for release 7.2.
5523
5524
5525 AUTHOR
5526
5527 Philip Hazel
5528 University Computing Service
5529 Cambridge CB2 3QH, England.
5530
5531
5532 REVISION
5533
5534 Last updated: 13 June 2007
5535 Copyright (c) 1997-2007 University of Cambridge.
5536 ------------------------------------------------------------------------------
5537
5538
5539 PCREPERFORM(3) PCREPERFORM(3)
5540
5541
5542 NAME
5543 PCRE - Perl-compatible regular expressions
5544
5545
5546 PCRE PERFORMANCE
5547
5548 Two aspects of performance are discussed below: memory usage and pro-
5549 cessing time. The way you express your pattern as a regular expression
5550 can affect both of them.
5551
5552
5553 MEMORY USAGE
5554
5555 Patterns are compiled by PCRE into a reasonably efficient byte code, so
5556 that most simple patterns do not use much memory. However, there is one
5557 case where memory usage can be unexpectedly large. When a parenthesized
5558 subpattern has a quantifier with a minimum greater than 1 and/or a lim-
5559 ited maximum, the whole subpattern is repeated in the compiled code.
5560 For example, the pattern
5561
5562 (abc|def){2,4}
5563
5564 is compiled as if it were
5565
5566 (abc|def)(abc|def)((abc|def)(abc|def)?)?
5567
5568 (Technical aside: It is done this way so that backtrack points within
5569 each of the repetitions can be independently maintained.)
5570
5571 For regular expressions whose quantifiers use only small numbers, this
5572 is not usually a problem. However, if the numbers are large, and par-
5573 ticularly if such repetitions are nested, the memory usage can become
5574 an embarrassment. For example, the very simple pattern
5575
5576 ((ab){1,1000}c){1,3}
5577
5578 uses 51K bytes when compiled. When PCRE is compiled with its default
5579 internal pointer size of two bytes, the size limit on a compiled pat-
5580 tern is 64K, and this is reached with the above pattern if the outer
5581 repetition is increased from 3 to 4. PCRE can be compiled to use larger
5582 internal pointers and thus handle larger compiled patterns, but it is
5583 better to try to rewrite your pattern to use less memory if you can.
5584
5585 One way of reducing the memory usage for such patterns is to make use
5586 of PCRE's "subroutine" facility. Re-writing the above pattern as
5587
5588 ((ab)(?2){0,999}c)(?1){0,2}
5589
5590 reduces the memory requirements to 18K, and indeed it remains under 20K
5591 even with the outer repetition increased to 100. However, this pattern
5592 is not exactly equivalent, because the "subroutine" calls are treated
5593 as atomic groups into which there can be no backtracking if there is a
5594 subsequent matching failure. Therefore, PCRE cannot do this kind of
5595 rewriting automatically. Furthermore, there is a noticeable loss of
5596 speed when executing the modified pattern. Nevertheless, if the atomic
5597 grouping is not a problem and the loss of speed is acceptable, this
5598 kind of rewriting will allow you to process patterns that PCRE cannot
5599 otherwise handle.
5600
5601
5602 PROCESSING TIME
5603
5604 Certain items in regular expression patterns are processed more effi-
5605 ciently than others. It is more efficient to use a character class like
5606 [aeiou] than a set of single-character alternatives such as
5607 (a|e|i|o|u). In general, the simplest construction that provides the
5608 required behaviour is usually the most efficient. Jeffrey Friedl's book
5609 contains a lot of useful general discussion about optimizing regular
5610 expressions for efficient performance. This document contains a few
5611 observations about PCRE.
5612
5613 Using Unicode character properties (the \p, \P, and \X escapes) is
5614 slow, because PCRE has to scan a structure that contains data for over
5615 fifteen thousand characters whenever it needs a character's property.
5616 If you can find an alternative pattern that does not use character
5617 properties, it will probably be faster.
5618
5619 When a pattern begins with .* not in parentheses, or in parentheses
5620 that are not the subject of a backreference, and the PCRE_DOTALL option
5621 is set, the pattern is implicitly anchored by PCRE, since it can match
5622 only at the start of a subject string. However, if PCRE_DOTALL is not
5623 set, PCRE cannot make this optimization, because the . metacharacter
5624 does not then match a newline, and if the subject string contains new-
5625 lines, the pattern may match from the character immediately following
5626 one of them instead of from the very start. For example, the pattern
5627
5628 .*second
5629
5630 matches the subject "first\nand second" (where \n stands for a newline
5631 character), with the match starting at the seventh character. In order
5632 to do this, PCRE has to retry the match starting after every newline in
5633 the subject.
5634
5635 If you are using such a pattern with subject strings that do not con-
5636 tain newlines, the best performance is obtained by setting PCRE_DOTALL,
5637 or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
5638 ing. That saves PCRE from having to scan along the subject looking for
5639 a newline to restart at.
5640
5641 Beware of patterns that contain nested indefinite repeats. These can
5642 take a long time to run when applied to a string that does not match.
5643 Consider the pattern fragment
5644
5645 ^(a+)*
5646
5647 This can match "aaaa" in 16 different ways, and this number increases
5648 very rapidly as the string gets longer. (The * repeat can match 0, 1,
5649 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
5650 repeats can match different numbers of times.) When the remainder of
5651 the pattern is such that the entire match is going to fail, PCRE has in
5652 principle to try every possible variation, and this can take an
5653 extremely long time, even for relatively short strings.
5654
5655 An optimization catches some of the more simple cases such as
5656
5657 (a+)*b
5658
5659 where a literal character follows. Before embarking on the standard
5660 matching procedure, PCRE checks that there is a "b" later in the sub-
5661 ject string, and if there is not, it fails the match immediately. How-
5662 ever, when there is no following literal this optimization cannot be
5663 used. You can see the difference by comparing the behaviour of
5664
5665 (a+)*\d
5666
5667 with the pattern above. The former gives a failure almost instantly
5668 when applied to a whole line of "a" characters, whereas the latter
5669 takes an appreciable time with strings longer than about 20 characters.
5670
5671 In many cases, the solution to this kind of performance issue is to use
5672 an atomic group or a possessive quantifier.
5673
5674
5675 AUTHOR
5676
5677 Philip Hazel
5678 University Computing Service
5679 Cambridge CB2 3QH, England.
5680
5681
5682 REVISION
5683
5684 Last updated: 06 March 2007
5685 Copyright (c) 1997-2007 University of Cambridge.
5686 ------------------------------------------------------------------------------
5687
5688
5689 PCREPOSIX(3) PCREPOSIX(3)
5690
5691
5692 NAME
5693 PCRE - Perl-compatible regular expressions.
5694
5695
5696 SYNOPSIS OF POSIX API
5697
5698 #include <pcreposix.h>
5699
5700 int regcomp(regex_t *preg, const char *pattern,
5701 int cflags);
5702
5703 int regexec(regex_t *preg, const char *string,
5704 size_t nmatch, regmatch_t pmatch[], int eflags);
5705
5706 size_t regerror(int errcode, const regex_t *preg,
5707 char *errbuf, size_t errbuf_size);
5708
5709 void regfree(regex_t *preg);
5710
5711
5712 DESCRIPTION
5713
5714 This set of functions provides a POSIX-style API to the PCRE regular
5715 expression package. See the pcreapi documentation for a description of
5716 PCRE's native API, which contains much additional functionality.
5717
5718 The functions described here are just wrapper functions that ultimately
5719 call the PCRE native API. Their prototypes are defined in the
5720 pcreposix.h header file, and on Unix systems the library itself is
5721 called pcreposix.a, so can be accessed by adding -lpcreposix to the
5722 command for linking an application that uses them. Because the POSIX
5723 functions call the native ones, it is also necessary to add -lpcre.
5724
5725 I have implemented only those option bits that can be reasonably mapped
5726 to PCRE native options. In addition, the option REG_EXTENDED is defined
5727 with the value zero. This has no effect, but since programs that are
5728 written to the POSIX interface often use it, this makes it easier to
5729 slot in PCRE as a replacement library. Other POSIX options are not even
5730 defined.
5731
5732 When PCRE is called via these functions, it is only the API that is
5733 POSIX-like in style. The syntax and semantics of the regular expres-
5734 sions themselves are still those of Perl, subject to the setting of
5735 various PCRE options, as described below. "POSIX-like in style" means
5736 that the API approximates to the POSIX definition; it is not fully
5737 POSIX-compatible, and in multi-byte encoding domains it is probably
5738 even less compatible.
5739
5740 The header for these functions is supplied as pcreposix.h to avoid any
5741 potential clash with other POSIX libraries. It can, of course, be
5742 renamed or aliased as regex.h, which is the "correct" name. It provides
5743 two structure types, regex_t for compiled internal forms, and reg-
5744 match_t for returning captured substrings. It also defines some con-
5745 stants whose names start with "REG_"; these are used for setting
5746 options and identifying error codes.
5747
5748
5749 COMPILING A PATTERN
5750
5751 The function regcomp() is called to compile a pattern into an internal
5752 form. The pattern is a C string terminated by a binary zero, and is
5753 passed in the argument pattern. The preg argument is a pointer to a
5754 regex_t structure that is used as a base for storing information about
5755 the compiled regular expression.
5756
5757 The argument cflags is either zero, or contains one or more of the bits
5758 defined by the following macros:
5759
5760 REG_DOTALL
5761
5762 The PCRE_DOTALL option is set when the regular expression is passed for
5763 compilation to the native function. Note that REG_DOTALL is not part of
5764 the POSIX standard.
5765
5766 REG_ICASE
5767
5768 The PCRE_CASELESS option is set when the regular expression is passed
5769 for compilation to the native function.
5770
5771 REG_NEWLINE
5772
5773 The PCRE_MULTILINE option is set when the regular expression is passed
5774 for compilation to the native function. Note that this does not mimic
5775 the defined POSIX behaviour for REG_NEWLINE (see the following sec-
5776 tion).
5777
5778 REG_NOSUB
5779
5780 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
5781 passed for compilation to the native function. In addition, when a pat-
5782 tern that is compiled with this flag is passed to regexec() for match-
5783 ing, the nmatch and pmatch arguments are ignored, and no captured
5784 strings are returned.
5785
5786 REG_UTF8
5787
5788 The PCRE_UTF8 option is set when the regular expression is passed for
5789 compilation to the native function. This causes the pattern itself and
5790 all data strings used for matching it to be treated as UTF-8 strings.
5791 Note that REG_UTF8 is not part of the POSIX standard.
5792
5793 In the absence of these flags, no options are passed to the native
5794 function. This means the the regex is compiled with PCRE default
5795 semantics. In particular, the way it handles newline characters in the
5796 subject string is the Perl way, not the POSIX way. Note that setting
5797 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
5798 It does not affect the way newlines are matched by . (they aren't) or
5799 by a negative class such as [^a] (they are).
5800
5801 The yield of regcomp() is zero on success, and non-zero otherwise. The
5802 preg structure is filled in on success, and one member of the structure
5803 is public: re_nsub contains the number of capturing subpatterns in the
5804 regular expression. Various error codes are defined in the header file.
5805
5806
5807 MATCHING NEWLINE CHARACTERS
5808
5809 This area is not simple, because POSIX and Perl take different views of
5810 things. It is not possible to get PCRE to obey POSIX semantics, but
5811 then PCRE was never intended to be a POSIX engine. The following table
5812 lists the different possibilities for matching newline characters in
5813 PCRE:
5814
5815 Default Change with
5816
5817 . matches newline no PCRE_DOTALL
5818 newline matches [^a] yes not changeable
5819 $ matches \n at end yes PCRE_DOLLARENDONLY
5820 $ matches \n in middle no PCRE_MULTILINE
5821 ^ matches \n in middle no PCRE_MULTILINE
5822
5823 This is the equivalent table for POSIX:
5824
5825 Default Change with
5826
5827 . matches newline yes REG_NEWLINE
5828 newline matches [^a] yes REG_NEWLINE
5829 $ matches \n at end no REG_NEWLINE
5830 $ matches \n in middle no REG_NEWLINE
5831 ^ matches \n in middle no REG_NEWLINE
5832
5833 PCRE's behaviour is the same as Perl's, except that there is no equiva-
5834 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
5835 no way to stop newline from matching [^a].
5836
5837 The default POSIX newline handling can be obtained by setting
5838 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
5839 behave exactly as for the REG_NEWLINE action.
5840
5841
5842 MATCHING A PATTERN
5843
5844 The function regexec() is called to match a compiled pattern preg
5845 against a given string, which is terminated by a zero byte, subject to
5846 the options in eflags. These can be:
5847
5848 REG_NOTBOL
5849
5850 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
5851 function.
5852
5853 REG_NOTEOL
5854
5855 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
5856 function.
5857
5858 If the pattern was compiled with the REG_NOSUB flag, no data about any
5859 matched strings is returned. The nmatch and pmatch arguments of
5860 regexec() are ignored.
5861
5862 Otherwise,the portion of the string that was matched, and also any cap-
5863 tured substrings, are returned via the pmatch argument, which points to
5864 an array of nmatch structures of type regmatch_t, containing the mem-
5865 bers rm_so and rm_eo. These contain the offset to the first character
5866 of each substring and the offset to the first character after the end
5867 of each substring, respectively. The 0th element of the vector relates
5868 to the entire portion of string that was matched; subsequent elements
5869 relate to the capturing subpatterns of the regular expression. Unused
5870 entries in the array have both structure members set to -1.
5871
5872 A successful match yields a zero return; various error codes are
5873 defined in the header file, of which REG_NOMATCH is the "expected"
5874 failure code.
5875
5876
5877 ERROR MESSAGES
5878
5879 The regerror() function maps a non-zero errorcode from either regcomp()
5880 or regexec() to a printable message. If preg is not NULL, the error
5881 should have arisen from the use of that structure. A message terminated
5882 by a binary zero is placed in errbuf. The length of the message,
5883 including the zero, is limited to errbuf_size. The yield of the func-
5884 tion is the size of buffer needed to hold the whole message.
5885
5886
5887 MEMORY USAGE
5888
5889 Compiling a regular expression causes memory to be allocated and asso-
5890 ciated with the preg structure. The function regfree() frees all such
5891 memory, after which preg may no longer be used as a compiled expres-
5892 sion.
5893
5894
5895 AUTHOR
5896
5897 Philip Hazel
5898 University Computing Service
5899 Cambridge CB2 3QH, England.
5900
5901
5902 REVISION
5903
5904 Last updated: 06 March 2007
5905 Copyright (c) 1997-2007 University of Cambridge.
5906 ------------------------------------------------------------------------------
5907
5908
5909 PCRECPP(3) PCRECPP(3)
5910
5911
5912 NAME
5913 PCRE - Perl-compatible regular expressions.
5914
5915
5916 SYNOPSIS OF C++ WRAPPER
5917
5918 #include <pcrecpp.h>
5919
5920
5921 DESCRIPTION
5922
5923 The C++ wrapper for PCRE was provided by Google Inc. Some additional
5924 functionality was added by Giuseppe Maxia. This brief man page was con-
5925 structed from the notes in the pcrecpp.h file, which should be con-
5926 sulted for further details.
5927
5928
5929 MATCHING INTERFACE
5930
5931 The "FullMatch" operation checks that supplied text matches a supplied
5932 pattern exactly. If pointer arguments are supplied, it copies matched
5933 sub-strings that match sub-patterns into them.
5934
5935 Example: successful match
5936 pcrecpp::RE re("h.*o");
5937 re.FullMatch("hello");
5938
5939 Example: unsuccessful match (requires full match):
5940 pcrecpp::RE re("e");
5941 !re.FullMatch("hello");
5942
5943 Example: creating a temporary RE object:
5944 pcrecpp::RE("h.*o").FullMatch("hello");
5945
5946 You can pass in a "const char*" or a "string" for "text". The examples
5947 below tend to use a const char*. You can, as in the different examples
5948 above, store the RE object explicitly in a variable or use a temporary
5949 RE object. The examples below use one mode or the other arbitrarily.
5950 Either could correctly be used for any of these examples.
5951
5952 You must supply extra pointer arguments to extract matched subpieces.
5953
5954 Example: extracts "ruby" into "s" and 1234 into "i"
5955 int i;
5956 string s;
5957 pcrecpp::RE re("(\\w+):(\\d+)");
5958 re.FullMatch("ruby:1234", &s, &i);
5959
5960 Example: does not try to extract any extra sub-patterns
5961 re.FullMatch("ruby:1234", &s);
5962
5963 Example: does not try to extract into NULL
5964 re.FullMatch("ruby:1234", NULL, &i);
5965
5966 Example: integer overflow causes failure
5967 !re.FullMatch("ruby:1234567891234", NULL, &i);
5968
5969 Example: fails because there aren't enough sub-patterns:
5970 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
5971
5972 Example: fails because string cannot be stored in integer
5973 !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
5974
5975 The provided pointer arguments can be pointers to any scalar numeric
5976 type, or one of:
5977
5978 string (matched piece is copied to string)
5979 StringPiece (StringPiece is mutated to point to matched piece)
5980 T (where "bool T::ParseFrom(const char*, int)" exists)
5981 NULL (the corresponding matched sub-pattern is not copied)
5982
5983 The function returns true iff all of the following conditions are sat-
5984 isfied:
5985
5986 a. "text" matches "pattern" exactly;
5987
5988 b. The number of matched sub-patterns is >= number of supplied
5989 pointers;
5990
5991 c. The "i"th argument has a suitable type for holding the
5992 string captured as the "i"th sub-pattern. If you pass in
5993 NULL for the "i"th argument, or pass fewer arguments than
5994 number of sub-patterns, "i"th captured sub-pattern is
5995 ignored.
5996
5997 CAVEAT: An optional sub-pattern that does not exist in the matched
5998 string is assigned the empty string. Therefore, the following will
5999 return false (because the empty string is not a valid number):
6000
6001 int number;
6002 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
6003
6004 The matching interface supports at most 16 arguments per call. If you
6005 need more, consider using the more general interface
6006 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
6007
6008
6009 QUOTING METACHARACTERS
6010
6011 You can use the "QuoteMeta" operation to insert backslashes before all
6012 potentially meaningful characters in a string. The returned string,
6013 used as a regular expression, will exactly match the original string.
6014
6015 Example:
6016 string quoted = RE::QuoteMeta(unquoted);
6017
6018 Note that it's legal to escape a character even if it has no special
6019 meaning in a regular expression -- so this function does that. (This
6020 also makes it identical to the perl function of the same name; see
6021 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
6022 "1\.5\-2\.0\?".
6023
6024
6025 PARTIAL MATCHES
6026
6027 You can use the "PartialMatch" operation when you want the pattern to
6028 match any substring of the text.
6029
6030 Example: simple search for a string:
6031 pcrecpp::RE("ell").PartialMatch("hello");
6032
6033 Example: find first number in a string:
6034 int number;
6035 pcrecpp::RE re("(\\d+)");
6036 re.PartialMatch("x*100 + 20", &number);
6037 assert(number == 100);
6038
6039
6040 UTF-8 AND THE MATCHING INTERFACE
6041
6042 By default, pattern and text are plain text, one byte per character.
6043 The UTF8 flag, passed to the constructor, causes both pattern and
6044 string to be treated as UTF-8 text, still a byte stream but potentially
6045 multiple bytes per character. In practice, the text is likelier to be
6046 UTF-8 than the pattern, but the match returned may depend on the UTF8
6047 flag, so always use it when matching UTF8 text. For example, "." will
6048 match one byte normally but with UTF8 set may match up to three bytes
6049 of a multi-byte character.
6050
6051 Example:
6052 pcrecpp::RE_Options options;
6053 options.set_utf8();
6054 pcrecpp::RE re(utf8_pattern, options);
6055 re.FullMatch(utf8_string);
6056
6057 Example: using the convenience function UTF8():
6058 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
6059 re.FullMatch(utf8_string);
6060
6061 NOTE: The UTF8 flag is ignored if pcre was not configured with the
6062 --enable-utf8 flag.
6063
6064
6065 PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
6066
6067 PCRE defines some modifiers to change the behavior of the regular
6068 expression engine. The C++ wrapper defines an auxiliary class,
6069 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
6070 rently, the following modifiers are supported:
6071
6072 modifier description Perl corresponding
6073
6074 PCRE_CASELESS case insensitive match /i
6075 PCRE_MULTILINE multiple lines match /m
6076 PCRE_DOTALL dot matches newlines /s
6077 PCRE_DOLLAR_ENDONLY $ matches only at end N/A
6078 PCRE_EXTRA strict escape parsing N/A
6079 PCRE_EXTENDED ignore whitespaces /x
6080 PCRE_UTF8 handles UTF8 chars built-in
6081 PCRE_UNGREEDY reverses * and *? N/A
6082 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
6083
6084 (*) Both Perl and PCRE allow non capturing parentheses by means of the
6085 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
6086 ture, while (ab|cd) does.
6087
6088 For a full account on how each modifier works, please check the PCRE
6089 API reference page.
6090
6091 For each modifier, there are two member functions whose name is made
6092 out of the modifier in lowercase, without the "PCRE_" prefix. For
6093 instance, PCRE_CASELESS is handled by
6094
6095 bool caseless()
6096
6097 which returns true if the modifier is set, and
6098
6099 RE_Options & set_caseless(bool)
6100
6101 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
6102 be accessed through the set_match_limit() and match_limit() member
6103 functions. Setting match_limit to a non-zero value will limit the exe-
6104 cution of pcre to keep it from doing bad things like blowing the stack
6105 or taking an eternity to return a result. A value of 5000 is good
6106 enough to stop stack blowup in a 2MB thread stack. Setting match_limit
6107 to zero disables match limiting. Alternatively, you can call
6108 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
6109 limit how much PCRE recurses. match_limit() limits the number of
6110 matches PCRE does; match_limit_recursion() limits the depth of internal
6111