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1 -----------------------------------------------------------------------------
2 This file contains a concatenation of the PCRE man pages, converted to plain
3 text format for ease of searching with a text editor, or for use on systems
4 that do not have a man page processor. The small individual files that give
5 synopses of each function in the library have not been included. Neither has
6 the pcredemo program. There are separate text files for the pcregrep and
7 pcretest commands.
8 -----------------------------------------------------------------------------
9
10
11 PCRE(3) PCRE(3)
12
13
14 NAME
15 PCRE - Perl-compatible regular expressions
16
17
18 INTRODUCTION
19
20 The PCRE library is a set of functions that implement regular expres-
21 sion pattern matching using the same syntax and semantics as Perl, with
22 just a few differences. Some features that appeared in Python and PCRE
23 before they appeared in Perl are also available using the Python syn-
24 tax, there is some support for one or two .NET and Oniguruma syntax
25 items, and there is an option for requesting some minor changes that
26 give better JavaScript compatibility.
27
28 Starting with release 8.30, it is possible to compile two separate PCRE
29 libraries: the original, which supports 8-bit character strings
30 (including UTF-8 strings), and a second library that supports 16-bit
31 character strings (including UTF-16 strings). The build process allows
32 either one or both to be built. The majority of the work to make this
33 possible was done by Zoltan Herczeg.
34
35 The two libraries contain identical sets of functions, except that the
36 names in the 16-bit library start with pcre16_ instead of pcre_. To
37 avoid over-complication and reduce the documentation maintenance load,
38 most of the documentation describes the 8-bit library, with the differ-
39 ences for the 16-bit library described separately in the pcre16 page.
40 References to functions or structures of the form pcre[16]_xxx should
41 be read as meaning "pcre_xxx when using the 8-bit library and
42 pcre16_xxx when using the 16-bit library".
43
44 The current implementation of PCRE corresponds approximately with Perl
45 5.12, including support for UTF-8/16 encoded strings and Unicode gen-
46 eral category properties. However, UTF-8/16 and Unicode support has to
47 be explicitly enabled; it is not the default. The Unicode tables corre-
48 spond to Unicode release 6.0.0.
49
50 In addition to the Perl-compatible matching function, PCRE contains an
51 alternative function that matches the same compiled patterns in a dif-
52 ferent way. In certain circumstances, the alternative function has some
53 advantages. For a discussion of the two matching algorithms, see the
54 pcrematching page.
55
56 PCRE is written in C and released as a C library. A number of people
57 have written wrappers and interfaces of various kinds. In particular,
58 Google Inc. have provided a comprehensive C++ wrapper for the 8-bit
59 library. This is now included as part of the PCRE distribution. The
60 pcrecpp page has details of this interface. Other people's contribu-
61 tions can be found in the Contrib directory at the primary FTP site,
62 which is:
63
64 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
65
66 Details of exactly which Perl regular expression features are and are
67 not supported by PCRE are given in separate documents. See the pcrepat-
68 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
69 page.
70
71 Some features of PCRE can be included, excluded, or changed when the
72 library is built. The pcre_config() function makes it possible for a
73 client to discover which features are available. The features them-
74 selves are described in the pcrebuild page. Documentation about build-
75 ing PCRE for various operating systems can be found in the README and
76 NON-UNIX-USE files in the source distribution.
77
78 The libraries contains a number of undocumented internal functions and
79 data tables that are used by more than one of the exported external
80 functions, but which are not intended for use by external callers.
81 Their names all begin with "_pcre_" or "_pcre16_", which hopefully will
82 not provoke any name clashes. In some environments, it is possible to
83 control which external symbols are exported when a shared library is
84 built, and in these cases the undocumented symbols are not exported.
85
86
87 USER DOCUMENTATION
88
89 The user documentation for PCRE comprises a number of different sec-
90 tions. In the "man" format, each of these is a separate "man page". In
91 the HTML format, each is a separate page, linked from the index page.
92 In the plain text format, all the sections, except the pcredemo sec-
93 tion, are concatenated, for ease of searching. The sections are as fol-
94 lows:
95
96 pcre this document
97 pcre16 details of the 16-bit library
98 pcre-config show PCRE installation configuration information
99 pcreapi details of PCRE's native C API
100 pcrebuild options for building PCRE
101 pcrecallout details of the callout feature
102 pcrecompat discussion of Perl compatibility
103 pcrecpp details of the C++ wrapper for the 8-bit library
104 pcredemo a demonstration C program that uses PCRE
105 pcregrep description of the pcregrep command (8-bit only)
106 pcrejit discussion of the just-in-time optimization support
107 pcrelimits details of size and other limits
108 pcrematching discussion of the two matching algorithms
109 pcrepartial details of the partial matching facility
110 pcrepattern syntax and semantics of supported
111 regular expressions
112 pcreperform discussion of performance issues
113 pcreposix the POSIX-compatible C API for the 8-bit library
114 pcreprecompile details of saving and re-using precompiled patterns
115 pcresample discussion of the pcredemo program
116 pcrestack discussion of stack usage
117 pcresyntax quick syntax reference
118 pcretest description of the pcretest testing command
119 pcreunicode discussion of Unicode and UTF-8/16 support
120
121 In addition, in the "man" and HTML formats, there is a short page for
122 each 8-bit C library function, listing its arguments and results.
123
124
125 AUTHOR
126
127 Philip Hazel
128 University Computing Service
129 Cambridge CB2 3QH, England.
130
131 Putting an actual email address here seems to have been a spam magnet,
132 so I've taken it away. If you want to email me, use my two initials,
133 followed by the two digits 10, at the domain cam.ac.uk.
134
135
136 REVISION
137
138 Last updated: 10 January 2012
139 Copyright (c) 1997-2012 University of Cambridge.
140 ------------------------------------------------------------------------------
141
142
143 PCRE(3) PCRE(3)
144
145
146 NAME
147 PCRE - Perl-compatible regular expressions
148
149 #include <pcre.h>
150
151
152 PCRE 16-BIT API BASIC FUNCTIONS
153
154 pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options,
155 const char **errptr, int *erroffset,
156 const unsigned char *tableptr);
157
158 pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options,
159 int *errorcodeptr,
160 const char **errptr, int *erroffset,
161 const unsigned char *tableptr);
162
163 pcre16_extra *pcre16_study(const pcre16 *code, int options,
164 const char **errptr);
165
166 void pcre16_free_study(pcre16_extra *extra);
167
168 int pcre16_exec(const pcre16 *code, const pcre16_extra *extra,
169 PCRE_SPTR16 subject, int length, int startoffset,
170 int options, int *ovector, int ovecsize);
171
172 int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra,
173 PCRE_SPTR16 subject, int length, int startoffset,
174 int options, int *ovector, int ovecsize,
175 int *workspace, int wscount);
176
177
178 PCRE 16-BIT API STRING EXTRACTION FUNCTIONS
179
180 int pcre16_copy_named_substring(const pcre16 *code,
181 PCRE_SPTR16 subject, int *ovector,
182 int stringcount, PCRE_SPTR16 stringname,
183 PCRE_UCHAR16 *buffer, int buffersize);
184
185 int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector,
186 int stringcount, int stringnumber, PCRE_UCHAR16 *buffer,
187 int buffersize);
188
189 int pcre16_get_named_substring(const pcre16 *code,
190 PCRE_SPTR16 subject, int *ovector,
191 int stringcount, PCRE_SPTR16 stringname,
192 PCRE_SPTR16 *stringptr);
193
194 int pcre16_get_stringnumber(const pcre16 *code,
195 PCRE_SPTR16 name);
196
197 int pcre16_get_stringtable_entries(const pcre16 *code,
198 PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last);
199
200 int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector,
201 int stringcount, int stringnumber,
202 PCRE_SPTR16 *stringptr);
203
204 int pcre16_get_substring_list(PCRE_SPTR16 subject,
205 int *ovector, int stringcount, PCRE_SPTR16 **listptr);
206
207 void pcre16_free_substring(PCRE_SPTR16 stringptr);
208
209 void pcre16_free_substring_list(PCRE_SPTR16 *stringptr);
210
211
212 PCRE 16-BIT API AUXILIARY FUNCTIONS
213
214 pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize);
215
216 void pcre16_jit_stack_free(pcre16_jit_stack *stack);
217
218 void pcre16_assign_jit_stack(pcre16_extra *extra,
219 pcre16_jit_callback callback, void *data);
220
221 const unsigned char *pcre16_maketables(void);
222
223 int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra,
224 int what, void *where);
225
226 int pcre16_refcount(pcre16 *code, int adjust);
227
228 int pcre16_config(int what, void *where);
229
230 const char *pcre16_version(void);
231
232 int pcre16_pattern_to_host_byte_order(pcre16 *code,
233 pcre16_extra *extra, const unsigned char *tables);
234
235
236 PCRE 16-BIT API INDIRECTED FUNCTIONS
237
238 void *(*pcre16_malloc)(size_t);
239
240 void (*pcre16_free)(void *);
241
242 void *(*pcre16_stack_malloc)(size_t);
243
244 void (*pcre16_stack_free)(void *);
245
246 int (*pcre16_callout)(pcre16_callout_block *);
247
248
249 PCRE 16-BIT API 16-BIT-ONLY FUNCTION
250
251 int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output,
252 PCRE_SPTR16 input, int length, int *byte_order,
253 int keep_boms);
254
255
256 THE PCRE 16-BIT LIBRARY
257
258 Starting with release 8.30, it is possible to compile a PCRE library
259 that supports 16-bit character strings, including UTF-16 strings, as
260 well as or instead of the original 8-bit library. The majority of the
261 work to make this possible was done by Zoltan Herczeg. The two
262 libraries contain identical sets of functions, used in exactly the same
263 way. Only the names of the functions and the data types of their argu-
264 ments and results are different. To avoid over-complication and reduce
265 the documentation maintenance load, most of the PCRE documentation
266 describes the 8-bit library, with only occasional references to the
267 16-bit library. This page describes what is different when you use the
268 16-bit library.
269
270 WARNING: A single application can be linked with both libraries, but
271 you must take care when processing any particular pattern to use func-
272 tions from just one library. For example, if you want to study a pat-
273 tern that was compiled with pcre16_compile(), you must do so with
274 pcre16_study(), not pcre_study(), and you must free the study data with
275 pcre16_free_study().
276
277
278 THE HEADER FILE
279
280 There is only one header file, pcre.h. It contains prototypes for all
281 the functions in both libraries, as well as definitions of flags,
282 structures, error codes, etc.
283
284
285 THE LIBRARY NAME
286
287 In Unix-like systems, the 16-bit library is called libpcre16, and can
288 normally be accesss by adding -lpcre16 to the command for linking an
289 application that uses PCRE.
290
291
292 STRING TYPES
293
294 In the 8-bit library, strings are passed to PCRE library functions as
295 vectors of bytes with the C type "char *". In the 16-bit library,
296 strings are passed as vectors of unsigned 16-bit quantities. The macro
297 PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is
298 defined as "const PCRE_UCHAR16 *". In very many environments, "short
299 int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16
300 as "short int", but checks that it really is a 16-bit data type. If it
301 is not, the build fails with an error message telling the maintainer to
302 modify the definition appropriately.
303
304
305 STRUCTURE TYPES
306
307 The types of the opaque structures that are used for compiled 16-bit
308 patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively.
309 The type of the user-accessible structure that is returned by
310 pcre16_study() is pcre16_extra, and the type of the structure that is
311 used for passing data to a callout function is pcre16_callout_block.
312 These structures contain the same fields, with the same names, as their
313 8-bit counterparts. The only difference is that pointers to character
314 strings are 16-bit instead of 8-bit types.
315
316
317 16-BIT FUNCTIONS
318
319 For every function in the 8-bit library there is a corresponding func-
320 tion in the 16-bit library with a name that starts with pcre16_ instead
321 of pcre_. The prototypes are listed above. In addition, there is one
322 extra function, pcre16_utf16_to_host_byte_order(). This is a utility
323 function that converts a UTF-16 character string to host byte order if
324 necessary. The other 16-bit functions expect the strings they are
325 passed to be in host byte order.
326
327 The input and output arguments of pcre16_utf16_to_host_byte_order() may
328 point to the same address, that is, conversion in place is supported.
329 The output buffer must be at least as long as the input.
330
331 The length argument specifies the number of 16-bit data units in the
332 input string; a negative value specifies a zero-terminated string.
333
334 If byte_order is NULL, it is assumed that the string starts off in host
335 byte order. This may be changed by byte-order marks (BOMs) anywhere in
336 the string (commonly as the first character).
337
338 If byte_order is not NULL, a non-zero value of the integer to which it
339 points means that the input starts off in host byte order, otherwise
340 the opposite order is assumed. Again, BOMs in the string can change
341 this. The final byte order is passed back at the end of processing.
342
343 If keep_boms is not zero, byte-order mark characters (0xfeff) are
344 copied into the output string. Otherwise they are discarded.
345
346 The result of the function is the number of 16-bit units placed into
347 the output buffer, including the zero terminator if the string was
348 zero-terminated.
349
350
351 SUBJECT STRING OFFSETS
352
353 The offsets within subject strings that are returned by the matching
354 functions are in 16-bit units rather than bytes.
355
356
357 NAMED SUBPATTERNS
358
359 The name-to-number translation table that is maintained for named sub-
360 patterns uses 16-bit characters. The pcre16_get_stringtable_entries()
361 function returns the length of each entry in the table as the number of
362 16-bit data units.
363
364
365 OPTION NAMES
366
367 There are two new general option names, PCRE_UTF16 and
368 PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and
369 PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options
370 define the same bits in the options word. There is a discussion about
371 the validity of UTF-16 strings in the pcreunicode page.
372
373 For the pcre16_config() function there is an option PCRE_CONFIG_UTF16
374 that returns 1 if UTF-16 support is configured, otherwise 0. If this
375 option is given to pcre_config(), or if the PCRE_CONFIG_UTF8 option is
376 given to pcre16_config(), the result is the PCRE_ERROR_BADOPTION error.
377
378
379 CHARACTER CODES
380
381 In 16-bit mode, when PCRE_UTF16 is not set, character values are
382 treated in the same way as in 8-bit, non UTF-8 mode, except, of course,
383 that they can range from 0 to 0xffff instead of 0 to 0xff. Character
384 types for characters less than 0xff can therefore be influenced by the
385 locale in the same way as before. Characters greater than 0xff have
386 only one case, and no "type" (such as letter or digit).
387
388 In UTF-16 mode, the character code is Unicode, in the range 0 to
389 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff
390 because those are "surrogate" values that are used in pairs to encode
391 values greater than 0xffff.
392
393 A UTF-16 string can indicate its endianness by special code knows as a
394 byte-order mark (BOM). The PCRE functions do not handle this, expecting
395 strings to be in host byte order. A utility function called
396 pcre16_utf16_to_host_byte_order() is provided to help with this (see
397 above).
398
399
400 ERROR NAMES
401
402 The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre-
403 spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is
404 given when a compiled pattern is passed to a function that processes
405 patterns in the other mode, for example, if a pattern compiled with
406 pcre_compile() is passed to pcre16_exec().
407
408 There are new error codes whose names begin with PCRE_UTF16_ERR for
409 invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for
410 UTF-8 strings that are described in the section entitled "Reason codes
411 for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors
412 are:
413
414 PCRE_UTF16_ERR1 Missing low surrogate at end of string
415 PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate
416 PCRE_UTF16_ERR3 Isolated low surrogate
417 PCRE_UTF16_ERR4 Invalid character 0xfffe
418
419
420 ERROR TEXTS
421
422 If there is an error while compiling a pattern, the error text that is
423 passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit
424 character string, zero-terminated.
425
426
427 CALLOUTS
428
429 The subject and mark fields in the callout block that is passed to a
430 callout function point to 16-bit vectors.
431
432
433 TESTING
434
435 The pcretest program continues to operate with 8-bit input and output
436 files, but it can be used for testing the 16-bit library. If it is run
437 with the command line option -16, patterns and subject strings are con-
438 verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit
439 library functions are used instead of the 8-bit ones. Returned 16-bit
440 strings are converted to 8-bit for output. If the 8-bit library was not
441 compiled, pcretest defaults to 16-bit and the -16 option is ignored.
442
443 When PCRE is being built, the RunTest script that is called by "make
444 check" uses the pcretest -C option to discover which of the 8-bit and
445 16-bit libraries has been built, and runs the tests appropriately.
446
447
448 NOT SUPPORTED IN 16-BIT MODE
449
450 Not all the features of the 8-bit library are available with the 16-bit
451 library. The C++ and POSIX wrapper functions support only the 8-bit
452 library, and the pcregrep program is at present 8-bit only.
453
454
455 AUTHOR
456
457 Philip Hazel
458 University Computing Service
459 Cambridge CB2 3QH, England.
460
461
462 REVISION
463
464 Last updated: 14 April 2012
465 Copyright (c) 1997-2012 University of Cambridge.
466 ------------------------------------------------------------------------------
467
468
469 PCREBUILD(3) PCREBUILD(3)
470
471
472 NAME
473 PCRE - Perl-compatible regular expressions
474
475
476 PCRE BUILD-TIME OPTIONS
477
478 This document describes the optional features of PCRE that can be
479 selected when the library is compiled. It assumes use of the configure
480 script, where the optional features are selected or deselected by pro-
481 viding options to configure before running the make command. However,
482 the same options can be selected in both Unix-like and non-Unix-like
483 environments using the GUI facility of cmake-gui if you are using CMake
484 instead of configure to build PCRE.
485
486 There is a lot more information about building PCRE in non-Unix-like
487 environments in the file called NON_UNIX_USE, which is part of the PCRE
488 distribution. You should consult this file as well as the README file
489 if you are building in a non-Unix-like environment.
490
491 The complete list of options for configure (which includes the standard
492 ones such as the selection of the installation directory) can be
493 obtained by running
494
495 ./configure --help
496
497 The following sections include descriptions of options whose names
498 begin with --enable or --disable. These settings specify changes to the
499 defaults for the configure command. Because of the way that configure
500 works, --enable and --disable always come in pairs, so the complemen-
501 tary option always exists as well, but as it specifies the default, it
502 is not described.
503
504
505 BUILDING 8-BIT and 16-BIT LIBRARIES
506
507 By default, a library called libpcre is built, containing functions
508 that take string arguments contained in vectors of bytes, either as
509 single-byte characters, or interpreted as UTF-8 strings. You can also
510 build a separate library, called libpcre16, in which strings are con-
511 tained in vectors of 16-bit data units and interpreted either as sin-
512 gle-unit characters or UTF-16 strings, by adding
513
514 --enable-pcre16
515
516 to the configure command. If you do not want the 8-bit library, add
517
518 --disable-pcre8
519
520 as well. At least one of the two libraries must be built. Note that the
521 C++ and POSIX wrappers are for the 8-bit library only, and that pcre-
522 grep is an 8-bit program. None of these are built if you select only
523 the 16-bit library.
524
525
526 BUILDING SHARED AND STATIC LIBRARIES
527
528 The PCRE building process uses libtool to build both shared and static
529 Unix libraries by default. You can suppress one of these by adding one
530 of
531
532 --disable-shared
533 --disable-static
534
535 to the configure command, as required.
536
537
538 C++ SUPPORT
539
540 By default, if the 8-bit library is being built, the configure script
541 will search for a C++ compiler and C++ header files. If it finds them,
542 it automatically builds the C++ wrapper library (which supports only
543 8-bit strings). You can disable this by adding
544
545 --disable-cpp
546
547 to the configure command.
548
549
550 UTF-8 and UTF-16 SUPPORT
551
552 To build PCRE with support for UTF Unicode character strings, add
553
554 --enable-utf
555
556 to the configure command. This setting applies to both libraries,
557 adding support for UTF-8 to the 8-bit library and support for UTF-16 to
558 the 16-bit library. There are no separate options for enabling UTF-8
559 and UTF-16 independently because that would allow ridiculous settings
560 such as requesting UTF-16 support while building only the 8-bit
561 library. It is not possible to build one library with UTF support and
562 the other without in the same configuration. (For backwards compatibil-
563 ity, --enable-utf8 is a synonym of --enable-utf.)
564
565 Of itself, this setting does not make PCRE treat strings as UTF-8 or
566 UTF-16. As well as compiling PCRE with this option, you also have have
567 to set the PCRE_UTF8 or PCRE_UTF16 option when you call one of the pat-
568 tern compiling functions.
569
570 If you set --enable-utf when compiling in an EBCDIC environment, PCRE
571 expects its input to be either ASCII or UTF-8 (depending on the run-
572 time option). It is not possible to support both EBCDIC and UTF-8 codes
573 in the same version of the library. Consequently, --enable-utf and
574 --enable-ebcdic are mutually exclusive.
575
576
577 UNICODE CHARACTER PROPERTY SUPPORT
578
579 UTF support allows the libraries to process character codepoints up to
580 0x10ffff in the strings that they handle. On its own, however, it does
581 not provide any facilities for accessing the properties of such charac-
582 ters. If you want to be able to use the pattern escapes \P, \p, and \X,
583 which refer to Unicode character properties, you must add
584
585 --enable-unicode-properties
586
587 to the configure command. This implies UTF support, even if you have
588 not explicitly requested it.
589
590 Including Unicode property support adds around 30K of tables to the
591 PCRE library. Only the general category properties such as Lu and Nd
592 are supported. Details are given in the pcrepattern documentation.
593
594
595 JUST-IN-TIME COMPILER SUPPORT
596
597 Just-in-time compiler support is included in the build by specifying
598
599 --enable-jit
600
601 This support is available only for certain hardware architectures. If
602 this option is set for an unsupported architecture, a compile time
603 error occurs. See the pcrejit documentation for a discussion of JIT
604 usage. When JIT support is enabled, pcregrep automatically makes use of
605 it, unless you add
606
607 --disable-pcregrep-jit
608
609 to the "configure" command.
610
611
612 CODE VALUE OF NEWLINE
613
614 By default, PCRE interprets the linefeed (LF) character as indicating
615 the end of a line. This is the normal newline character on Unix-like
616 systems. You can compile PCRE to use carriage return (CR) instead, by
617 adding
618
619 --enable-newline-is-cr
620
621 to the configure command. There is also a --enable-newline-is-lf
622 option, which explicitly specifies linefeed as the newline character.
623
624 Alternatively, you can specify that line endings are to be indicated by
625 the two character sequence CRLF. If you want this, add
626
627 --enable-newline-is-crlf
628
629 to the configure command. There is a fourth option, specified by
630
631 --enable-newline-is-anycrlf
632
633 which causes PCRE to recognize any of the three sequences CR, LF, or
634 CRLF as indicating a line ending. Finally, a fifth option, specified by
635
636 --enable-newline-is-any
637
638 causes PCRE to recognize any Unicode newline sequence.
639
640 Whatever line ending convention is selected when PCRE is built can be
641 overridden when the library functions are called. At build time it is
642 conventional to use the standard for your operating system.
643
644
645 WHAT \R MATCHES
646
647 By default, the sequence \R in a pattern matches any Unicode newline
648 sequence, whatever has been selected as the line ending sequence. If
649 you specify
650
651 --enable-bsr-anycrlf
652
653 the default is changed so that \R matches only CR, LF, or CRLF. What-
654 ever is selected when PCRE is built can be overridden when the library
655 functions are called.
656
657
658 POSIX MALLOC USAGE
659
660 When the 8-bit library is called through the POSIX interface (see the
661 pcreposix documentation), additional working storage is required for
662 holding the pointers to capturing substrings, because PCRE requires
663 three integers per substring, whereas the POSIX interface provides only
664 two. If the number of expected substrings is small, the wrapper func-
665 tion uses space on the stack, because this is faster than using mal-
666 loc() for each call. The default threshold above which the stack is no
667 longer used is 10; it can be changed by adding a setting such as
668
669 --with-posix-malloc-threshold=20
670
671 to the configure command.
672
673
674 HANDLING VERY LARGE PATTERNS
675
676 Within a compiled pattern, offset values are used to point from one
677 part to another (for example, from an opening parenthesis to an alter-
678 nation metacharacter). By default, two-byte values are used for these
679 offsets, leading to a maximum size for a compiled pattern of around
680 64K. This is sufficient to handle all but the most gigantic patterns.
681 Nevertheless, some people do want to process truly enormous patterns,
682 so it is possible to compile PCRE to use three-byte or four-byte off-
683 sets by adding a setting such as
684
685 --with-link-size=3
686
687 to the configure command. The value given must be 2, 3, or 4. For the
688 16-bit library, a value of 3 is rounded up to 4. Using longer offsets
689 slows down the operation of PCRE because it has to load additional data
690 when handling them.
691
692
693 AVOIDING EXCESSIVE STACK USAGE
694
695 When matching with the pcre_exec() function, PCRE implements backtrack-
696 ing by making recursive calls to an internal function called match().
697 In environments where the size of the stack is limited, this can se-
698 verely limit PCRE's operation. (The Unix environment does not usually
699 suffer from this problem, but it may sometimes be necessary to increase
700 the maximum stack size. There is a discussion in the pcrestack docu-
701 mentation.) An alternative approach to recursion that uses memory from
702 the heap to remember data, instead of using recursive function calls,
703 has been implemented to work round the problem of limited stack size.
704 If you want to build a version of PCRE that works this way, add
705
706 --disable-stack-for-recursion
707
708 to the configure command. With this configuration, PCRE will use the
709 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
710 ment functions. By default these point to malloc() and free(), but you
711 can replace the pointers so that your own functions are used instead.
712
713 Separate functions are provided rather than using pcre_malloc and
714 pcre_free because the usage is very predictable: the block sizes
715 requested are always the same, and the blocks are always freed in
716 reverse order. A calling program might be able to implement optimized
717 functions that perform better than malloc() and free(). PCRE runs
718 noticeably more slowly when built in this way. This option affects only
719 the pcre_exec() function; it is not relevant for pcre_dfa_exec().
720
721
722 LIMITING PCRE RESOURCE USAGE
723
724 Internally, PCRE has a function called match(), which it calls repeat-
725 edly (sometimes recursively) when matching a pattern with the
726 pcre_exec() function. By controlling the maximum number of times this
727 function may be called during a single matching operation, a limit can
728 be placed on the resources used by a single call to pcre_exec(). The
729 limit can be changed at run time, as described in the pcreapi documen-
730 tation. The default is 10 million, but this can be changed by adding a
731 setting such as
732
733 --with-match-limit=500000
734
735 to the configure command. This setting has no effect on the
736 pcre_dfa_exec() matching function.
737
738 In some environments it is desirable to limit the depth of recursive
739 calls of match() more strictly than the total number of calls, in order
740 to restrict the maximum amount of stack (or heap, if --disable-stack-
741 for-recursion is specified) that is used. A second limit controls this;
742 it defaults to the value that is set for --with-match-limit, which
743 imposes no additional constraints. However, you can set a lower limit
744 by adding, for example,
745
746 --with-match-limit-recursion=10000
747
748 to the configure command. This value can also be overridden at run
749 time.
750
751
752 CREATING CHARACTER TABLES AT BUILD TIME
753
754 PCRE uses fixed tables for processing characters whose code values are
755 less than 256. By default, PCRE is built with a set of tables that are
756 distributed in the file pcre_chartables.c.dist. These tables are for
757 ASCII codes only. If you add
758
759 --enable-rebuild-chartables
760
761 to the configure command, the distributed tables are no longer used.
762 Instead, a program called dftables is compiled and run. This outputs
763 the source for new set of tables, created in the default locale of your
764 C run-time system. (This method of replacing the tables does not work
765 if you are cross compiling, because dftables is run on the local host.
766 If you need to create alternative tables when cross compiling, you will
767 have to do so "by hand".)
768
769
770 USING EBCDIC CODE
771
772 PCRE assumes by default that it will run in an environment where the
773 character code is ASCII (or Unicode, which is a superset of ASCII).
774 This is the case for most computer operating systems. PCRE can, how-
775 ever, be compiled to run in an EBCDIC environment by adding
776
777 --enable-ebcdic
778
779 to the configure command. This setting implies --enable-rebuild-charta-
780 bles. You should only use it if you know that you are in an EBCDIC
781 environment (for example, an IBM mainframe operating system). The
782 --enable-ebcdic option is incompatible with --enable-utf.
783
784
785 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
786
787 By default, pcregrep reads all files as plain text. You can build it so
788 that it recognizes files whose names end in .gz or .bz2, and reads them
789 with libz or libbz2, respectively, by adding one or both of
790
791 --enable-pcregrep-libz
792 --enable-pcregrep-libbz2
793
794 to the configure command. These options naturally require that the rel-
795 evant libraries are installed on your system. Configuration will fail
796 if they are not.
797
798
799 PCREGREP BUFFER SIZE
800
801 pcregrep uses an internal buffer to hold a "window" on the file it is
802 scanning, in order to be able to output "before" and "after" lines when
803 it finds a match. The size of the buffer is controlled by a parameter
804 whose default value is 20K. The buffer itself is three times this size,
805 but because of the way it is used for holding "before" lines, the long-
806 est line that is guaranteed to be processable is the parameter size.
807 You can change the default parameter value by adding, for example,
808
809 --with-pcregrep-bufsize=50K
810
811 to the configure command. The caller of pcregrep can, however, override
812 this value by specifying a run-time option.
813
814
815 PCRETEST OPTION FOR LIBREADLINE SUPPORT
816
817 If you add
818
819 --enable-pcretest-libreadline
820
821 to the configure command, pcretest is linked with the libreadline
822 library, and when its input is from a terminal, it reads it using the
823 readline() function. This provides line-editing and history facilities.
824 Note that libreadline is GPL-licensed, so if you distribute a binary of
825 pcretest linked in this way, there may be licensing issues.
826
827 Setting this option causes the -lreadline option to be added to the
828 pcretest build. In many operating environments with a sytem-installed
829 libreadline this is sufficient. However, in some environments (e.g. if
830 an unmodified distribution version of readline is in use), some extra
831 configuration may be necessary. The INSTALL file for libreadline says
832 this:
833
834 "Readline uses the termcap functions, but does not link with the
835 termcap or curses library itself, allowing applications which link
836 with readline the to choose an appropriate library."
837
838 If your environment has not been set up so that an appropriate library
839 is automatically included, you may need to add something like
840
841 LIBS="-ncurses"
842
843 immediately before the configure command.
844
845
846 SEE ALSO
847
848 pcreapi(3), pcre16, pcre_config(3).
849
850
851 AUTHOR
852
853 Philip Hazel
854 University Computing Service
855 Cambridge CB2 3QH, England.
856
857
858 REVISION
859
860 Last updated: 07 January 2012
861 Copyright (c) 1997-2012 University of Cambridge.
862 ------------------------------------------------------------------------------
863
864
865 PCREMATCHING(3) PCREMATCHING(3)
866
867
868 NAME
869 PCRE - Perl-compatible regular expressions
870
871
872 PCRE MATCHING ALGORITHMS
873
874 This document describes the two different algorithms that are available
875 in PCRE for matching a compiled regular expression against a given sub-
876 ject string. The "standard" algorithm is the one provided by the
877 pcre_exec() and pcre16_exec() functions. These work in the same was as
878 Perl's matching function, and provide a Perl-compatible matching opera-
879 tion. The just-in-time (JIT) optimization that is described in the
880 pcrejit documentation is compatible with these functions.
881
882 An alternative algorithm is provided by the pcre_dfa_exec() and
883 pcre16_dfa_exec() functions; they operate in a different way, and are
884 not Perl-compatible. This alternative has advantages and disadvantages
885 compared with the standard algorithm, and these are described below.
886
887 When there is only one possible way in which a given subject string can
888 match a pattern, the two algorithms give the same answer. A difference
889 arises, however, when there are multiple possibilities. For example, if
890 the pattern
891
892 ^<.*>
893
894 is matched against the string
895
896 <something> <something else> <something further>
897
898 there are three possible answers. The standard algorithm finds only one
899 of them, whereas the alternative algorithm finds all three.
900
901
902 REGULAR EXPRESSIONS AS TREES
903
904 The set of strings that are matched by a regular expression can be rep-
905 resented as a tree structure. An unlimited repetition in the pattern
906 makes the tree of infinite size, but it is still a tree. Matching the
907 pattern to a given subject string (from a given starting point) can be
908 thought of as a search of the tree. There are two ways to search a
909 tree: depth-first and breadth-first, and these correspond to the two
910 matching algorithms provided by PCRE.
911
912
913 THE STANDARD MATCHING ALGORITHM
914
915 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
916 sions", the standard algorithm is an "NFA algorithm". It conducts a
917 depth-first search of the pattern tree. That is, it proceeds along a
918 single path through the tree, checking that the subject matches what is
919 required. When there is a mismatch, the algorithm tries any alterna-
920 tives at the current point, and if they all fail, it backs up to the
921 previous branch point in the tree, and tries the next alternative
922 branch at that level. This often involves backing up (moving to the
923 left) in the subject string as well. The order in which repetition
924 branches are tried is controlled by the greedy or ungreedy nature of
925 the quantifier.
926
927 If a leaf node is reached, a matching string has been found, and at
928 that point the algorithm stops. Thus, if there is more than one possi-
929 ble match, this algorithm returns the first one that it finds. Whether
930 this is the shortest, the longest, or some intermediate length depends
931 on the way the greedy and ungreedy repetition quantifiers are specified
932 in the pattern.
933
934 Because it ends up with a single path through the tree, it is rela-
935 tively straightforward for this algorithm to keep track of the sub-
936 strings that are matched by portions of the pattern in parentheses.
937 This provides support for capturing parentheses and back references.
938
939
940 THE ALTERNATIVE MATCHING ALGORITHM
941
942 This algorithm conducts a breadth-first search of the tree. Starting
943 from the first matching point in the subject, it scans the subject
944 string from left to right, once, character by character, and as it does
945 this, it remembers all the paths through the tree that represent valid
946 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
947 though it is not implemented as a traditional finite state machine (it
948 keeps multiple states active simultaneously).
949
950 Although the general principle of this matching algorithm is that it
951 scans the subject string only once, without backtracking, there is one
952 exception: when a lookaround assertion is encountered, the characters
953 following or preceding the current point have to be independently
954 inspected.
955
956 The scan continues until either the end of the subject is reached, or
957 there are no more unterminated paths. At this point, terminated paths
958 represent the different matching possibilities (if there are none, the
959 match has failed). Thus, if there is more than one possible match,
960 this algorithm finds all of them, and in particular, it finds the long-
961 est. The matches are returned in decreasing order of length. There is
962 an option to stop the algorithm after the first match (which is neces-
963 sarily the shortest) is found.
964
965 Note that all the matches that are found start at the same point in the
966 subject. If the pattern
967
968 cat(er(pillar)?)?
969
970 is matched against the string "the caterpillar catchment", the result
971 will be the three strings "caterpillar", "cater", and "cat" that start
972 at the fifth character of the subject. The algorithm does not automati-
973 cally move on to find matches that start at later positions.
974
975 There are a number of features of PCRE regular expressions that are not
976 supported by the alternative matching algorithm. They are as follows:
977
978 1. Because the algorithm finds all possible matches, the greedy or
979 ungreedy nature of repetition quantifiers is not relevant. Greedy and
980 ungreedy quantifiers are treated in exactly the same way. However, pos-
981 sessive quantifiers can make a difference when what follows could also
982 match what is quantified, for example in a pattern like this:
983
984 ^a++\w!
985
986 This pattern matches "aaab!" but not "aaa!", which would be matched by
987 a non-possessive quantifier. Similarly, if an atomic group is present,
988 it is matched as if it were a standalone pattern at the current point,
989 and the longest match is then "locked in" for the rest of the overall
990 pattern.
991
992 2. When dealing with multiple paths through the tree simultaneously, it
993 is not straightforward to keep track of captured substrings for the
994 different matching possibilities, and PCRE's implementation of this
995 algorithm does not attempt to do this. This means that no captured sub-
996 strings are available.
997
998 3. Because no substrings are captured, back references within the pat-
999 tern are not supported, and cause errors if encountered.
1000
1001 4. For the same reason, conditional expressions that use a backrefer-
1002 ence as the condition or test for a specific group recursion are not
1003 supported.
1004
1005 5. Because many paths through the tree may be active, the \K escape
1006 sequence, which resets the start of the match when encountered (but may
1007 be on some paths and not on others), is not supported. It causes an
1008 error if encountered.
1009
1010 6. Callouts are supported, but the value of the capture_top field is
1011 always 1, and the value of the capture_last field is always -1.
1012
1013 7. The \C escape sequence, which (in the standard algorithm) always
1014 matches a single data unit, even in UTF-8 or UTF-16 modes, is not sup-
1015 ported in these modes, because the alternative algorithm moves through
1016 the subject string one character (not data unit) at a time, for all
1017 active paths through the tree.
1018
1019 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
1020 are not supported. (*FAIL) is supported, and behaves like a failing
1021 negative assertion.
1022
1023
1024 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
1025
1026 Using the alternative matching algorithm provides the following advan-
1027 tages:
1028
1029 1. All possible matches (at a single point in the subject) are automat-
1030 ically found, and in particular, the longest match is found. To find
1031 more than one match using the standard algorithm, you have to do kludgy
1032 things with callouts.
1033
1034 2. Because the alternative algorithm scans the subject string just
1035 once, and never needs to backtrack (except for lookbehinds), it is pos-
1036 sible to pass very long subject strings to the matching function in
1037 several pieces, checking for partial matching each time. Although it is
1038 possible to do multi-segment matching using the standard algorithm by
1039 retaining partially matched substrings, it is more complicated. The
1040 pcrepartial documentation gives details of partial matching and dis-
1041 cusses multi-segment matching.
1042
1043
1044 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
1045
1046 The alternative algorithm suffers from a number of disadvantages:
1047
1048 1. It is substantially slower than the standard algorithm. This is
1049 partly because it has to search for all possible matches, but is also
1050 because it is less susceptible to optimization.
1051
1052 2. Capturing parentheses and back references are not supported.
1053
1054 3. Although atomic groups are supported, their use does not provide the
1055 performance advantage that it does for the standard algorithm.
1056
1057
1058 AUTHOR
1059
1060 Philip Hazel
1061 University Computing Service
1062 Cambridge CB2 3QH, England.
1063
1064
1065 REVISION
1066
1067 Last updated: 08 January 2012
1068 Copyright (c) 1997-2012 University of Cambridge.
1069 ------------------------------------------------------------------------------
1070
1071
1072 PCREAPI(3) PCREAPI(3)
1073
1074
1075 NAME
1076 PCRE - Perl-compatible regular expressions
1077
1078 #include <pcre.h>
1079
1080
1081 PCRE NATIVE API BASIC FUNCTIONS
1082
1083 pcre *pcre_compile(const char *pattern, int options,
1084 const char **errptr, int *erroffset,
1085 const unsigned char *tableptr);
1086
1087 pcre *pcre_compile2(const char *pattern, int options,
1088 int *errorcodeptr,
1089 const char **errptr, int *erroffset,
1090 const unsigned char *tableptr);
1091
1092 pcre_extra *pcre_study(const pcre *code, int options,
1093 const char **errptr);
1094
1095 void pcre_free_study(pcre_extra *extra);
1096
1097 int pcre_exec(const pcre *code, const pcre_extra *extra,
1098 const char *subject, int length, int startoffset,
1099 int options, int *ovector, int ovecsize);
1100
1101 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
1102 const char *subject, int length, int startoffset,
1103 int options, int *ovector, int ovecsize,
1104 int *workspace, int wscount);
1105
1106
1107 PCRE NATIVE API STRING EXTRACTION FUNCTIONS
1108
1109 int pcre_copy_named_substring(const pcre *code,
1110 const char *subject, int *ovector,
1111 int stringcount, const char *stringname,
1112 char *buffer, int buffersize);
1113
1114 int pcre_copy_substring(const char *subject, int *ovector,
1115 int stringcount, int stringnumber, char *buffer,
1116 int buffersize);
1117
1118 int pcre_get_named_substring(const pcre *code,
1119 const char *subject, int *ovector,
1120 int stringcount, const char *stringname,
1121 const char **stringptr);
1122
1123 int pcre_get_stringnumber(const pcre *code,
1124 const char *name);
1125
1126 int pcre_get_stringtable_entries(const pcre *code,
1127 const char *name, char **first, char **last);
1128
1129 int pcre_get_substring(const char *subject, int *ovector,
1130 int stringcount, int stringnumber,
1131 const char **stringptr);
1132
1133 int pcre_get_substring_list(const char *subject,
1134 int *ovector, int stringcount, const char ***listptr);
1135
1136 void pcre_free_substring(const char *stringptr);
1137
1138 void pcre_free_substring_list(const char **stringptr);
1139
1140
1141 PCRE NATIVE API AUXILIARY FUNCTIONS
1142
1143 pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
1144
1145 void pcre_jit_stack_free(pcre_jit_stack *stack);
1146
1147 void pcre_assign_jit_stack(pcre_extra *extra,
1148 pcre_jit_callback callback, void *data);
1149
1150 const unsigned char *pcre_maketables(void);
1151
1152 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1153 int what, void *where);
1154
1155 int pcre_refcount(pcre *code, int adjust);
1156
1157 int pcre_config(int what, void *where);
1158
1159 const char *pcre_version(void);
1160
1161 int pcre_pattern_to_host_byte_order(pcre *code,
1162 pcre_extra *extra, const unsigned char *tables);
1163
1164
1165 PCRE NATIVE API INDIRECTED FUNCTIONS
1166
1167 void *(*pcre_malloc)(size_t);
1168
1169 void (*pcre_free)(void *);
1170
1171 void *(*pcre_stack_malloc)(size_t);
1172
1173 void (*pcre_stack_free)(void *);
1174
1175 int (*pcre_callout)(pcre_callout_block *);
1176
1177
1178 PCRE 8-BIT AND 16-BIT LIBRARIES
1179
1180 From release 8.30, PCRE can be compiled as a library for handling
1181 16-bit character strings as well as, or instead of, the original
1182 library that handles 8-bit character strings. To avoid too much compli-
1183 cation, this document describes the 8-bit versions of the functions,
1184 with only occasional references to the 16-bit library.
1185
1186 The 16-bit functions operate in the same way as their 8-bit counter-
1187 parts; they just use different data types for their arguments and
1188 results, and their names start with pcre16_ instead of pcre_. For every
1189 option that has UTF8 in its name (for example, PCRE_UTF8), there is a
1190 corresponding 16-bit name with UTF8 replaced by UTF16. This facility is
1191 in fact just cosmetic; the 16-bit option names define the same bit val-
1192 ues.
1193
1194 References to bytes and UTF-8 in this document should be read as refer-
1195 ences to 16-bit data quantities and UTF-16 when using the 16-bit
1196 library, unless specified otherwise. More details of the specific dif-
1197 ferences for the 16-bit library are given in the pcre16 page.
1198
1199
1200 PCRE API OVERVIEW
1201
1202 PCRE has its own native API, which is described in this document. There
1203 are also some wrapper functions (for the 8-bit library only) that cor-
1204 respond to the POSIX regular expression API, but they do not give
1205 access to all the functionality. They are described in the pcreposix
1206 documentation. Both of these APIs define a set of C function calls. A
1207 C++ wrapper (again for the 8-bit library only) is also distributed with
1208 PCRE. It is documented in the pcrecpp page.
1209
1210 The native API C function prototypes are defined in the header file
1211 pcre.h, and on Unix-like systems the (8-bit) library itself is called
1212 libpcre. It can normally be accessed by adding -lpcre to the command
1213 for linking an application that uses PCRE. The header file defines the
1214 macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release
1215 numbers for the library. Applications can use these to include support
1216 for different releases of PCRE.
1217
1218 In a Windows environment, if you want to statically link an application
1219 program against a non-dll pcre.a file, you must define PCRE_STATIC
1220 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
1221 loc() and pcre_free() exported functions will be declared
1222 __declspec(dllimport), with unwanted results.
1223
1224 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
1225 pcre_exec() are used for compiling and matching regular expressions in
1226 a Perl-compatible manner. A sample program that demonstrates the sim-
1227 plest way of using them is provided in the file called pcredemo.c in
1228 the PCRE source distribution. A listing of this program is given in the
1229 pcredemo documentation, and the pcresample documentation describes how
1230 to compile and run it.
1231
1232 Just-in-time compiler support is an optional feature of PCRE that can
1233 be built in appropriate hardware environments. It greatly speeds up the
1234 matching performance of many patterns. Simple programs can easily
1235 request that it be used if available, by setting an option that is
1236 ignored when it is not relevant. More complicated programs might need
1237 to make use of the functions pcre_jit_stack_alloc(),
1238 pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control
1239 the JIT code's memory usage. These functions are discussed in the
1240 pcrejit documentation.
1241
1242 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
1243 ble, is also provided. This uses a different algorithm for the match-
1244 ing. The alternative algorithm finds all possible matches (at a given
1245 point in the subject), and scans the subject just once (unless there
1246 are lookbehind assertions). However, this algorithm does not return
1247 captured substrings. A description of the two matching algorithms and
1248 their advantages and disadvantages is given in the pcrematching docu-
1249 mentation.
1250
1251 In addition to the main compiling and matching functions, there are
1252 convenience functions for extracting captured substrings from a subject
1253 string that is matched by pcre_exec(). They are:
1254
1255 pcre_copy_substring()
1256 pcre_copy_named_substring()
1257 pcre_get_substring()
1258 pcre_get_named_substring()
1259 pcre_get_substring_list()
1260 pcre_get_stringnumber()
1261 pcre_get_stringtable_entries()
1262
1263 pcre_free_substring() and pcre_free_substring_list() are also provided,
1264 to free the memory used for extracted strings.
1265
1266 The function pcre_maketables() is used to build a set of character
1267 tables in the current locale for passing to pcre_compile(),
1268 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
1269 provided for specialist use. Most commonly, no special tables are
1270 passed, in which case internal tables that are generated when PCRE is
1271 built are used.
1272
1273 The function pcre_fullinfo() is used to find out information about a
1274 compiled pattern. The function pcre_version() returns a pointer to a
1275 string containing the version of PCRE and its date of release.
1276
1277 The function pcre_refcount() maintains a reference count in a data
1278 block containing a compiled pattern. This is provided for the benefit
1279 of object-oriented applications.
1280
1281 The global variables pcre_malloc and pcre_free initially contain the
1282 entry points of the standard malloc() and free() functions, respec-
1283 tively. PCRE calls the memory management functions via these variables,
1284 so a calling program can replace them if it wishes to intercept the
1285 calls. This should be done before calling any PCRE functions.
1286
1287 The global variables pcre_stack_malloc and pcre_stack_free are also
1288 indirections to memory management functions. These special functions
1289 are used only when PCRE is compiled to use the heap for remembering
1290 data, instead of recursive function calls, when running the pcre_exec()
1291 function. See the pcrebuild documentation for details of how to do
1292 this. It is a non-standard way of building PCRE, for use in environ-
1293 ments that have limited stacks. Because of the greater use of memory
1294 management, it runs more slowly. Separate functions are provided so
1295 that special-purpose external code can be used for this case. When
1296 used, these functions are always called in a stack-like manner (last
1297 obtained, first freed), and always for memory blocks of the same size.
1298 There is a discussion about PCRE's stack usage in the pcrestack docu-
1299 mentation.
1300
1301 The global variable pcre_callout initially contains NULL. It can be set
1302 by the caller to a "callout" function, which PCRE will then call at
1303 specified points during a matching operation. Details are given in the
1304 pcrecallout documentation.
1305
1306
1307 NEWLINES
1308
1309 PCRE supports five different conventions for indicating line breaks in
1310 strings: a single CR (carriage return) character, a single LF (line-
1311 feed) character, the two-character sequence CRLF, any of the three pre-
1312 ceding, or any Unicode newline sequence. The Unicode newline sequences
1313 are the three just mentioned, plus the single characters VT (vertical
1314 tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
1315 separator, U+2028), and PS (paragraph separator, U+2029).
1316
1317 Each of the first three conventions is used by at least one operating
1318 system as its standard newline sequence. When PCRE is built, a default
1319 can be specified. The default default is LF, which is the Unix stan-
1320 dard. When PCRE is run, the default can be overridden, either when a
1321 pattern is compiled, or when it is matched.
1322
1323 At compile time, the newline convention can be specified by the options
1324 argument of pcre_compile(), or it can be specified by special text at
1325 the start of the pattern itself; this overrides any other settings. See
1326 the pcrepattern page for details of the special character sequences.
1327
1328 In the PCRE documentation the word "newline" is used to mean "the char-
1329 acter or pair of characters that indicate a line break". The choice of
1330 newline convention affects the handling of the dot, circumflex, and
1331 dollar metacharacters, the handling of #-comments in /x mode, and, when
1332 CRLF is a recognized line ending sequence, the match position advance-
1333 ment for a non-anchored pattern. There is more detail about this in the
1334 section on pcre_exec() options below.
1335
1336 The choice of newline convention does not affect the interpretation of
1337 the \n or \r escape sequences, nor does it affect what \R matches,
1338 which is controlled in a similar way, but by separate options.
1339
1340
1341 MULTITHREADING
1342
1343 The PCRE functions can be used in multi-threading applications, with
1344 the proviso that the memory management functions pointed to by
1345 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1346 callout function pointed to by pcre_callout, are shared by all threads.
1347
1348 The compiled form of a regular expression is not altered during match-
1349 ing, so the same compiled pattern can safely be used by several threads
1350 at once.
1351
1352 If the just-in-time optimization feature is being used, it needs sepa-
1353 rate memory stack areas for each thread. See the pcrejit documentation
1354 for more details.
1355
1356
1357 SAVING PRECOMPILED PATTERNS FOR LATER USE
1358
1359 The compiled form of a regular expression can be saved and re-used at a
1360 later time, possibly by a different program, and even on a host other
1361 than the one on which it was compiled. Details are given in the
1362 pcreprecompile documentation, which includes a description of the
1363 pcre_pattern_to_host_byte_order() function. However, compiling a regu-
1364 lar expression with one version of PCRE for use with a different ver-
1365 sion is not guaranteed to work and may cause crashes.
1366
1367
1368 CHECKING BUILD-TIME OPTIONS
1369
1370 int pcre_config(int what, void *where);
1371
1372 The function pcre_config() makes it possible for a PCRE client to dis-
1373 cover which optional features have been compiled into the PCRE library.
1374 The pcrebuild documentation has more details about these optional fea-
1375 tures.
1376
1377 The first argument for pcre_config() is an integer, specifying which
1378 information is required; the second argument is a pointer to a variable
1379 into which the information is placed. The returned value is zero on
1380 success, or the negative error code PCRE_ERROR_BADOPTION if the value
1381 in the first argument is not recognized. The following information is
1382 available:
1383
1384 PCRE_CONFIG_UTF8
1385
1386 The output is an integer that is set to one if UTF-8 support is avail-
1387 able; otherwise it is set to zero. If this option is given to the
1388 16-bit version of this function, pcre16_config(), the result is
1389 PCRE_ERROR_BADOPTION.
1390
1391 PCRE_CONFIG_UTF16
1392
1393 The output is an integer that is set to one if UTF-16 support is avail-
1394 able; otherwise it is set to zero. This value should normally be given
1395 to the 16-bit version of this function, pcre16_config(). If it is given
1396 to the 8-bit version of this function, the result is PCRE_ERROR_BADOP-
1397 TION.
1398
1399 PCRE_CONFIG_UNICODE_PROPERTIES
1400
1401 The output is an integer that is set to one if support for Unicode
1402 character properties is available; otherwise it is set to zero.
1403
1404 PCRE_CONFIG_JIT
1405
1406 The output is an integer that is set to one if support for just-in-time
1407 compiling is available; otherwise it is set to zero.
1408
1409 PCRE_CONFIG_JITTARGET
1410
1411 The output is a pointer to a zero-terminated "const char *" string. If
1412 JIT support is available, the string contains the name of the architec-
1413 ture for which the JIT compiler is configured, for example "x86 32bit
1414 (little endian + unaligned)". If JIT support is not available, the
1415 result is NULL.
1416
1417 PCRE_CONFIG_NEWLINE
1418
1419 The output is an integer whose value specifies the default character
1420 sequence that is recognized as meaning "newline". The four values that
1421 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1422 and -1 for ANY. Though they are derived from ASCII, the same values
1423 are returned in EBCDIC environments. The default should normally corre-
1424 spond to the standard sequence for your operating system.
1425
1426 PCRE_CONFIG_BSR
1427
1428 The output is an integer whose value indicates what character sequences
1429 the \R escape sequence matches by default. A value of 0 means that \R
1430 matches any Unicode line ending sequence; a value of 1 means that \R
1431 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1432 tern is compiled or matched.
1433
1434 PCRE_CONFIG_LINK_SIZE
1435
1436 The output is an integer that contains the number of bytes used for
1437 internal linkage in compiled regular expressions. For the 8-bit
1438 library, the value can be 2, 3, or 4. For the 16-bit library, the value
1439 is either 2 or 4 and is still a number of bytes. The default value of 2
1440 is sufficient for all but the most massive patterns, since it allows
1441 the compiled pattern to be up to 64K in size. Larger values allow
1442 larger regular expressions to be compiled, at the expense of slower
1443 matching.
1444
1445 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1446
1447 The output is an integer that contains the threshold above which the
1448 POSIX interface uses malloc() for output vectors. Further details are
1449 given in the pcreposix documentation.
1450
1451 PCRE_CONFIG_MATCH_LIMIT
1452
1453 The output is a long integer that gives the default limit for the num-
1454 ber of internal matching function calls in a pcre_exec() execution.
1455 Further details are given with pcre_exec() below.
1456
1457 PCRE_CONFIG_MATCH_LIMIT_RECURSION
1458
1459 The output is a long integer that gives the default limit for the depth
1460 of recursion when calling the internal matching function in a
1461 pcre_exec() execution. Further details are given with pcre_exec()
1462 below.
1463
1464 PCRE_CONFIG_STACKRECURSE
1465
1466 The output is an integer that is set to one if internal recursion when
1467 running pcre_exec() is implemented by recursive function calls that use
1468 the stack to remember their state. This is the usual way that PCRE is
1469 compiled. The output is zero if PCRE was compiled to use blocks of data
1470 on the heap instead of recursive function calls. In this case,
1471 pcre_stack_malloc and pcre_stack_free are called to manage memory
1472 blocks on the heap, thus avoiding the use of the stack.
1473
1474
1475 COMPILING A PATTERN
1476
1477 pcre *pcre_compile(const char *pattern, int options,
1478 const char **errptr, int *erroffset,
1479 const unsigned char *tableptr);
1480
1481 pcre *pcre_compile2(const char *pattern, int options,
1482 int *errorcodeptr,
1483 const char **errptr, int *erroffset,
1484 const unsigned char *tableptr);
1485
1486 Either of the functions pcre_compile() or pcre_compile2() can be called
1487 to compile a pattern into an internal form. The only difference between
1488 the two interfaces is that pcre_compile2() has an additional argument,
1489 errorcodeptr, via which a numerical error code can be returned. To
1490 avoid too much repetition, we refer just to pcre_compile() below, but
1491 the information applies equally to pcre_compile2().
1492
1493 The pattern is a C string terminated by a binary zero, and is passed in
1494 the pattern argument. A pointer to a single block of memory that is
1495 obtained via pcre_malloc is returned. This contains the compiled code
1496 and related data. The pcre type is defined for the returned block; this
1497 is a typedef for a structure whose contents are not externally defined.
1498 It is up to the caller to free the memory (via pcre_free) when it is no
1499 longer required.
1500
1501 Although the compiled code of a PCRE regex is relocatable, that is, it
1502 does not depend on memory location, the complete pcre data block is not
1503 fully relocatable, because it may contain a copy of the tableptr argu-
1504 ment, which is an address (see below).
1505
1506 The options argument contains various bit settings that affect the com-
1507 pilation. It should be zero if no options are required. The available
1508 options are described below. Some of them (in particular, those that
1509 are compatible with Perl, but some others as well) can also be set and
1510 unset from within the pattern (see the detailed description in the
1511 pcrepattern documentation). For those options that can be different in
1512 different parts of the pattern, the contents of the options argument
1513 specifies their settings at the start of compilation and execution. The
1514 PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and
1515 PCRE_NO_START_OPTIMIZE options can be set at the time of matching as
1516 well as at compile time.
1517
1518 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1519 if compilation of a pattern fails, pcre_compile() returns NULL, and
1520 sets the variable pointed to by errptr to point to a textual error mes-
1521 sage. This is a static string that is part of the library. You must not
1522 try to free it. Normally, the offset from the start of the pattern to
1523 the byte that was being processed when the error was discovered is
1524 placed in the variable pointed to by erroffset, which must not be NULL
1525 (if it is, an immediate error is given). However, for an invalid UTF-8
1526 string, the offset is that of the first byte of the failing character.
1527
1528 Some errors are not detected until the whole pattern has been scanned;
1529 in these cases, the offset passed back is the length of the pattern.
1530 Note that the offset is in bytes, not characters, even in UTF-8 mode.
1531 It may sometimes point into the middle of a UTF-8 character.
1532
1533 If pcre_compile2() is used instead of pcre_compile(), and the error-
1534 codeptr argument is not NULL, a non-zero error code number is returned
1535 via this argument in the event of an error. This is in addition to the
1536 textual error message. Error codes and messages are listed below.
1537
1538 If the final argument, tableptr, is NULL, PCRE uses a default set of
1539 character tables that are built when PCRE is compiled, using the
1540 default C locale. Otherwise, tableptr must be an address that is the
1541 result of a call to pcre_maketables(). This value is stored with the
1542 compiled pattern, and used again by pcre_exec(), unless another table
1543 pointer is passed to it. For more discussion, see the section on locale
1544 support below.
1545
1546 This code fragment shows a typical straightforward call to pcre_com-
1547 pile():
1548
1549 pcre *re;
1550 const char *error;
1551 int erroffset;
1552 re = pcre_compile(
1553 "^A.*Z", /* the pattern */
1554 0, /* default options */
1555 &error, /* for error message */
1556 &erroffset, /* for error offset */
1557 NULL); /* use default character tables */
1558
1559 The following names for option bits are defined in the pcre.h header
1560 file:
1561
1562 PCRE_ANCHORED
1563
1564 If this bit is set, the pattern is forced to be "anchored", that is, it
1565 is constrained to match only at the first matching point in the string
1566 that is being searched (the "subject string"). This effect can also be
1567 achieved by appropriate constructs in the pattern itself, which is the
1568 only way to do it in Perl.
1569
1570 PCRE_AUTO_CALLOUT
1571
1572 If this bit is set, pcre_compile() automatically inserts callout items,
1573 all with number 255, before each pattern item. For discussion of the
1574 callout facility, see the pcrecallout documentation.
1575
1576 PCRE_BSR_ANYCRLF
1577 PCRE_BSR_UNICODE
1578
1579 These options (which are mutually exclusive) control what the \R escape
1580 sequence matches. The choice is either to match only CR, LF, or CRLF,
1581 or to match any Unicode newline sequence. The default is specified when
1582 PCRE is built. It can be overridden from within the pattern, or by set-
1583 ting an option when a compiled pattern is matched.
1584
1585 PCRE_CASELESS
1586
1587 If this bit is set, letters in the pattern match both upper and lower
1588 case letters. It is equivalent to Perl's /i option, and it can be
1589 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1590 always understands the concept of case for characters whose values are
1591 less than 128, so caseless matching is always possible. For characters
1592 with higher values, the concept of case is supported if PCRE is com-
1593 piled with Unicode property support, but not otherwise. If you want to
1594 use caseless matching for characters 128 and above, you must ensure
1595 that PCRE is compiled with Unicode property support as well as with
1596 UTF-8 support.
1597
1598 PCRE_DOLLAR_ENDONLY
1599
1600 If this bit is set, a dollar metacharacter in the pattern matches only
1601 at the end of the subject string. Without this option, a dollar also
1602 matches immediately before a newline at the end of the string (but not
1603 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1604 if PCRE_MULTILINE is set. There is no equivalent to this option in
1605 Perl, and no way to set it within a pattern.
1606
1607 PCRE_DOTALL
1608
1609 If this bit is set, a dot metacharacter in the pattern matches a char-
1610 acter of any value, including one that indicates a newline. However, it
1611 only ever matches one character, even if newlines are coded as CRLF.
1612 Without this option, a dot does not match when the current position is
1613 at a newline. This option is equivalent to Perl's /s option, and it can
1614 be changed within a pattern by a (?s) option setting. A negative class
1615 such as [^a] always matches newline characters, independent of the set-
1616 ting of this option.
1617
1618 PCRE_DUPNAMES
1619
1620 If this bit is set, names used to identify capturing subpatterns need
1621 not be unique. This can be helpful for certain types of pattern when it
1622 is known that only one instance of the named subpattern can ever be
1623 matched. There are more details of named subpatterns below; see also
1624 the pcrepattern documentation.
1625
1626 PCRE_EXTENDED
1627
1628 If this bit is set, white space data characters in the pattern are
1629 totally ignored except when escaped or inside a character class. White
1630 space does not include the VT character (code 11). In addition, charac-
1631 ters between an unescaped # outside a character class and the next new-
1632 line, inclusive, are also ignored. This is equivalent to Perl's /x
1633 option, and it can be changed within a pattern by a (?x) option set-
1634 ting.
1635
1636 Which characters are interpreted as newlines is controlled by the
1637 options passed to pcre_compile() or by a special sequence at the start
1638 of the pattern, as described in the section entitled "Newline conven-
1639 tions" in the pcrepattern documentation. Note that the end of this type
1640 of comment is a literal newline sequence in the pattern; escape
1641 sequences that happen to represent a newline do not count.
1642
1643 This option makes it possible to include comments inside complicated
1644 patterns. Note, however, that this applies only to data characters.
1645 White space characters may never appear within special character
1646 sequences in a pattern, for example within the sequence (?( that intro-
1647 duces a conditional subpattern.
1648
1649 PCRE_EXTRA
1650
1651 This option was invented in order to turn on additional functionality
1652 of PCRE that is incompatible with Perl, but it is currently of very
1653 little use. When set, any backslash in a pattern that is followed by a
1654 letter that has no special meaning causes an error, thus reserving
1655 these combinations for future expansion. By default, as in Perl, a
1656 backslash followed by a letter with no special meaning is treated as a
1657 literal. (Perl can, however, be persuaded to give an error for this, by
1658 running it with the -w option.) There are at present no other features
1659 controlled by this option. It can also be set by a (?X) option setting
1660 within a pattern.
1661
1662 PCRE_FIRSTLINE
1663
1664 If this option is set, an unanchored pattern is required to match
1665 before or at the first newline in the subject string, though the
1666 matched text may continue over the newline.
1667
1668 PCRE_JAVASCRIPT_COMPAT
1669
1670 If this option is set, PCRE's behaviour is changed in some ways so that
1671 it is compatible with JavaScript rather than Perl. The changes are as
1672 follows:
1673
1674 (1) A lone closing square bracket in a pattern causes a compile-time
1675 error, because this is illegal in JavaScript (by default it is treated
1676 as a data character). Thus, the pattern AB]CD becomes illegal when this
1677 option is set.
1678
1679 (2) At run time, a back reference to an unset subpattern group matches
1680 an empty string (by default this causes the current matching alterna-
1681 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1682 set (assuming it can find an "a" in the subject), whereas it fails by
1683 default, for Perl compatibility.
1684
1685 (3) \U matches an upper case "U" character; by default \U causes a com-
1686 pile time error (Perl uses \U to upper case subsequent characters).
1687
1688 (4) \u matches a lower case "u" character unless it is followed by four
1689 hexadecimal digits, in which case the hexadecimal number defines the
1690 code point to match. By default, \u causes a compile time error (Perl
1691 uses it to upper case the following character).
1692
1693 (5) \x matches a lower case "x" character unless it is followed by two
1694 hexadecimal digits, in which case the hexadecimal number defines the
1695 code point to match. By default, as in Perl, a hexadecimal number is
1696 always expected after \x, but it may have zero, one, or two digits (so,
1697 for example, \xz matches a binary zero character followed by z).
1698
1699 PCRE_MULTILINE
1700
1701 By default, PCRE treats the subject string as consisting of a single
1702 line of characters (even if it actually contains newlines). The "start
1703 of line" metacharacter (^) matches only at the start of the string,
1704 while the "end of line" metacharacter ($) matches only at the end of
1705 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1706 is set). This is the same as Perl.
1707
1708 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1709 constructs match immediately following or immediately before internal
1710 newlines in the subject string, respectively, as well as at the very
1711 start and end. This is equivalent to Perl's /m option, and it can be
1712 changed within a pattern by a (?m) option setting. If there are no new-
1713 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1714 setting PCRE_MULTILINE has no effect.
1715
1716 PCRE_NEWLINE_CR
1717 PCRE_NEWLINE_LF
1718 PCRE_NEWLINE_CRLF
1719 PCRE_NEWLINE_ANYCRLF
1720 PCRE_NEWLINE_ANY
1721
1722 These options override the default newline definition that was chosen
1723 when PCRE was built. Setting the first or the second specifies that a
1724 newline is indicated by a single character (CR or LF, respectively).
1725 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1726 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1727 that any of the three preceding sequences should be recognized. Setting
1728 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1729 recognized. The Unicode newline sequences are the three just mentioned,
1730 plus the single characters VT (vertical tab, U+000B), FF (form feed,
1731 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1732 (paragraph separator, U+2029). For the 8-bit library, the last two are
1733 recognized only in UTF-8 mode.
1734
1735 The newline setting in the options word uses three bits that are
1736 treated as a number, giving eight possibilities. Currently only six are
1737 used (default plus the five values above). This means that if you set
1738 more than one newline option, the combination may or may not be sensi-
1739 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1740 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1741 cause an error.
1742
1743 The only time that a line break in a pattern is specially recognized
1744 when compiling is when PCRE_EXTENDED is set. CR and LF are white space
1745 characters, and so are ignored in this mode. Also, an unescaped # out-
1746 side a character class indicates a comment that lasts until after the
1747 next line break sequence. In other circumstances, line break sequences
1748 in patterns are treated as literal data.
1749
1750 The newline option that is set at compile time becomes the default that
1751 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1752
1753 PCRE_NO_AUTO_CAPTURE
1754
1755 If this option is set, it disables the use of numbered capturing paren-
1756 theses in the pattern. Any opening parenthesis that is not followed by
1757 ? behaves as if it were followed by ?: but named parentheses can still
1758 be used for capturing (and they acquire numbers in the usual way).
1759 There is no equivalent of this option in Perl.
1760
1761 NO_START_OPTIMIZE
1762
1763 This is an option that acts at matching time; that is, it is really an
1764 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
1765 time, it is remembered with the compiled pattern and assumed at match-
1766 ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
1767 below.
1768
1769 PCRE_UCP
1770
1771 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
1772 \w, and some of the POSIX character classes. By default, only ASCII
1773 characters are recognized, but if PCRE_UCP is set, Unicode properties
1774 are used instead to classify characters. More details are given in the
1775 section on generic character types in the pcrepattern page. If you set
1776 PCRE_UCP, matching one of the items it affects takes much longer. The
1777 option is available only if PCRE has been compiled with Unicode prop-
1778 erty support.
1779
1780 PCRE_UNGREEDY
1781
1782 This option inverts the "greediness" of the quantifiers so that they
1783 are not greedy by default, but become greedy if followed by "?". It is
1784 not compatible with Perl. It can also be set by a (?U) option setting
1785 within the pattern.
1786
1787 PCRE_UTF8
1788
1789 This option causes PCRE to regard both the pattern and the subject as
1790 strings of UTF-8 characters instead of single-byte strings. However, it
1791 is available only when PCRE is built to include UTF support. If not,
1792 the use of this option provokes an error. Details of how this option
1793 changes the behaviour of PCRE are given in the pcreunicode page.
1794
1795 PCRE_NO_UTF8_CHECK
1796
1797 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1798 automatically checked. There is a discussion about the validity of
1799 UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is
1800 found, pcre_compile() returns an error. If you already know that your
1801 pattern is valid, and you want to skip this check for performance rea-
1802 sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the
1803 effect of passing an invalid UTF-8 string as a pattern is undefined. It
1804 may cause your program to crash. Note that this option can also be
1805 passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity
1806 checking of subject strings.
1807
1808
1809 COMPILATION ERROR CODES
1810
1811 The following table lists the error codes than may be returned by
1812 pcre_compile2(), along with the error messages that may be returned by
1813 both compiling functions. Note that error messages are always 8-bit
1814 ASCII strings, even in 16-bit mode. As PCRE has developed, some error
1815 codes have fallen out of use. To avoid confusion, they have not been
1816 re-used.
1817
1818 0 no error
1819 1 \ at end of pattern
1820 2 \c at end of pattern
1821 3 unrecognized character follows \
1822 4 numbers out of order in {} quantifier
1823 5 number too big in {} quantifier
1824 6 missing terminating ] for character class
1825 7 invalid escape sequence in character class
1826 8 range out of order in character class
1827 9 nothing to repeat
1828 10 [this code is not in use]
1829 11 internal error: unexpected repeat
1830 12 unrecognized character after (? or (?-
1831 13 POSIX named classes are supported only within a class
1832 14 missing )
1833 15 reference to non-existent subpattern
1834 16 erroffset passed as NULL
1835 17 unknown option bit(s) set
1836 18 missing ) after comment
1837 19 [this code is not in use]
1838 20 regular expression is too large
1839 21 failed to get memory
1840 22 unmatched parentheses
1841 23 internal error: code overflow
1842 24 unrecognized character after (?<
1843 25 lookbehind assertion is not fixed length
1844 26 malformed number or name after (?(
1845 27 conditional group contains more than two branches
1846 28 assertion expected after (?(
1847 29 (?R or (?[+-]digits must be followed by )
1848 30 unknown POSIX class name
1849 31 POSIX collating elements are not supported
1850 32 this version of PCRE is compiled without UTF support
1851 33 [this code is not in use]
1852 34 character value in \x{...} sequence is too large
1853 35 invalid condition (?(0)
1854 36 \C not allowed in lookbehind assertion
1855 37 PCRE does not support \L, \l, \N{name}, \U, or \u
1856 38 number after (?C is > 255
1857 39 closing ) for (?C expected
1858 40 recursive call could loop indefinitely
1859 41 unrecognized character after (?P
1860 42 syntax error in subpattern name (missing terminator)
1861 43 two named subpatterns have the same name
1862 44 invalid UTF-8 string (specifically UTF-8)
1863 45 support for \P, \p, and \X has not been compiled
1864 46 malformed \P or \p sequence
1865 47 unknown property name after \P or \p
1866 48 subpattern name is too long (maximum 32 characters)
1867 49 too many named subpatterns (maximum 10000)
1868 50 [this code is not in use]
1869 51 octal value is greater than \377 in 8-bit non-UTF-8 mode
1870 52 internal error: overran compiling workspace
1871 53 internal error: previously-checked referenced subpattern
1872 not found
1873 54 DEFINE group contains more than one branch
1874 55 repeating a DEFINE group is not allowed
1875 56 inconsistent NEWLINE options
1876 57 \g is not followed by a braced, angle-bracketed, or quoted
1877 name/number or by a plain number
1878 58 a numbered reference must not be zero
1879 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1880 60 (*VERB) not recognized
1881 61 number is too big
1882 62 subpattern name expected
1883 63 digit expected after (?+
1884 64 ] is an invalid data character in JavaScript compatibility mode
1885 65 different names for subpatterns of the same number are
1886 not allowed
1887 66 (*MARK) must have an argument
1888 67 this version of PCRE is not compiled with Unicode property
1889 support
1890 68 \c must be followed by an ASCII character
1891 69 \k is not followed by a braced, angle-bracketed, or quoted name
1892 70 internal error: unknown opcode in find_fixedlength()
1893 71 \N is not supported in a class
1894 72 too many forward references
1895 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff)
1896 74 invalid UTF-16 string (specifically UTF-16)
1897 75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN)
1898
1899 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1900 values may be used if the limits were changed when PCRE was built.
1901
1902
1903 STUDYING A PATTERN
1904
1905 pcre_extra *pcre_study(const pcre *code, int options
1906 const char **errptr);
1907
1908 If a compiled pattern is going to be used several times, it is worth
1909 spending more time analyzing it in order to speed up the time taken for
1910 matching. The function pcre_study() takes a pointer to a compiled pat-
1911 tern as its first argument. If studying the pattern produces additional
1912 information that will help speed up matching, pcre_study() returns a
1913 pointer to a pcre_extra block, in which the study_data field points to
1914 the results of the study.
1915
1916 The returned value from pcre_study() can be passed directly to
1917 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1918 tains other fields that can be set by the caller before the block is
1919 passed; these are described below in the section on matching a pattern.
1920
1921 If studying the pattern does not produce any useful information,
1922 pcre_study() returns NULL. In that circumstance, if the calling program
1923 wants to pass any of the other fields to pcre_exec() or
1924 pcre_dfa_exec(), it must set up its own pcre_extra block.
1925
1926 The second argument of pcre_study() contains option bits. There are
1927 three options:
1928
1929 PCRE_STUDY_JIT_COMPILE
1930 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE
1931 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE
1932
1933 If any of these are set, and the just-in-time compiler is available,
1934 the pattern is further compiled into machine code that executes much
1935 faster than the pcre_exec() interpretive matching function. If the
1936 just-in-time compiler is not available, these options are ignored. All
1937 other bits in the options argument must be zero.
1938
1939 JIT compilation is a heavyweight optimization. It can take some time
1940 for patterns to be analyzed, and for one-off matches and simple pat-
1941 terns the benefit of faster execution might be offset by a much slower
1942 study time. Not all patterns can be optimized by the JIT compiler. For
1943 those that cannot be handled, matching automatically falls back to the
1944 pcre_exec() interpreter. For more details, see the pcrejit documenta-
1945 tion.
1946
1947 The third argument for pcre_study() is a pointer for an error message.
1948 If studying succeeds (even if no data is returned), the variable it
1949 points to is set to NULL. Otherwise it is set to point to a textual
1950 error message. This is a static string that is part of the library. You
1951 must not try to free it. You should test the error pointer for NULL
1952 after calling pcre_study(), to be sure that it has run successfully.
1953
1954 When you are finished with a pattern, you can free the memory used for
1955 the study data by calling pcre_free_study(). This function was added to
1956 the API for release 8.20. For earlier versions, the memory could be
1957 freed with pcre_free(), just like the pattern itself. This will still
1958 work in cases where JIT optimization is not used, but it is advisable
1959 to change to the new function when convenient.
1960
1961 This is a typical way in which pcre_study() is used (except that in a
1962 real application there should be tests for errors):
1963
1964 int rc;
1965 pcre *re;
1966 pcre_extra *sd;
1967 re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
1968 sd = pcre_study(
1969 re, /* result of pcre_compile() */
1970 0, /* no options */
1971 &error); /* set to NULL or points to a message */
1972 rc = pcre_exec( /* see below for details of pcre_exec() options */
1973 re, sd, "subject", 7, 0, 0, ovector, 30);
1974 ...
1975 pcre_free_study(sd);
1976 pcre_free(re);
1977
1978 Studying a pattern does two things: first, a lower bound for the length
1979 of subject string that is needed to match the pattern is computed. This
1980 does not mean that there are any strings of that length that match, but
1981 it does guarantee that no shorter strings match. The value is used by
1982 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1983 match strings that are shorter than the lower bound. You can find out
1984 the value in a calling program via the pcre_fullinfo() function.
1985
1986 Studying a pattern is also useful for non-anchored patterns that do not
1987 have a single fixed starting character. A bitmap of possible starting
1988 bytes is created. This speeds up finding a position in the subject at
1989 which to start matching. (In 16-bit mode, the bitmap is used for 16-bit
1990 values less than 256.)
1991
1992 These two optimizations apply to both pcre_exec() and pcre_dfa_exec(),
1993 and the information is also used by the JIT compiler. The optimiza-
1994 tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option when
1995 calling pcre_exec() or pcre_dfa_exec(), but if this is done, JIT execu-
1996 tion is also disabled. You might want to do this if your pattern con-
1997 tains callouts or (*MARK) and you want to make use of these facilities
1998 in cases where matching fails. See the discussion of
1999 PCRE_NO_START_OPTIMIZE below.
2000
2001
2002 LOCALE SUPPORT
2003
2004 PCRE handles caseless matching, and determines whether characters are
2005 letters, digits, or whatever, by reference to a set of tables, indexed
2006 by character value. When running in UTF-8 mode, this applies only to
2007 characters with codes less than 128. By default, higher-valued codes
2008 never match escapes such as \w or \d, but they can be tested with \p if
2009 PCRE is built with Unicode character property support. Alternatively,
2010 the PCRE_UCP option can be set at compile time; this causes \w and
2011 friends to use Unicode property support instead of built-in tables. The
2012 use of locales with Unicode is discouraged. If you are handling charac-
2013 ters with codes greater than 128, you should either use UTF-8 and Uni-
2014 code, or use locales, but not try to mix the two.
2015
2016 PCRE contains an internal set of tables that are used when the final
2017 argument of pcre_compile() is NULL. These are sufficient for many
2018 applications. Normally, the internal tables recognize only ASCII char-
2019 acters. However, when PCRE is built, it is possible to cause the inter-
2020 nal tables to be rebuilt in the default "C" locale of the local system,
2021 which may cause them to be different.
2022
2023 The internal tables can always be overridden by tables supplied by the
2024 application that calls PCRE. These may be created in a different locale
2025 from the default. As more and more applications change to using Uni-
2026 code, the need for this locale support is expected to die away.
2027
2028 External tables are built by calling the pcre_maketables() function,
2029 which has no arguments, in the relevant locale. The result can then be
2030 passed to pcre_compile() or pcre_exec() as often as necessary. For
2031 example, to build and use tables that are appropriate for the French
2032 locale (where accented characters with values greater than 128 are
2033 treated as letters), the following code could be used:
2034
2035 setlocale(LC_CTYPE, "fr_FR");
2036 tables = pcre_maketables();
2037 re = pcre_compile(..., tables);
2038
2039 The locale name "fr_FR" is used on Linux and other Unix-like systems;
2040 if you are using Windows, the name for the French locale is "french".
2041
2042 When pcre_maketables() runs, the tables are built in memory that is
2043 obtained via pcre_malloc. It is the caller's responsibility to ensure
2044 that the memory containing the tables remains available for as long as
2045 it is needed.
2046
2047 The pointer that is passed to pcre_compile() is saved with the compiled
2048 pattern, and the same tables are used via this pointer by pcre_study()
2049 and normally also by pcre_exec(). Thus, by default, for any single pat-
2050 tern, compilation, studying and matching all happen in the same locale,
2051 but different patterns can be compiled in different locales.
2052
2053 It is possible to pass a table pointer or NULL (indicating the use of
2054 the internal tables) to pcre_exec(). Although not intended for this
2055 purpose, this facility could be used to match a pattern in a different
2056 locale from the one in which it was compiled. Passing table pointers at
2057 run time is discussed below in the section on matching a pattern.
2058
2059
2060 INFORMATION ABOUT A PATTERN
2061
2062 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
2063 int what, void *where);
2064
2065 The pcre_fullinfo() function returns information about a compiled pat-
2066 tern. It replaces the pcre_info() function, which was removed from the
2067 library at version 8.30, after more than 10 years of obsolescence.
2068
2069 The first argument for pcre_fullinfo() is a pointer to the compiled
2070 pattern. The second argument is the result of pcre_study(), or NULL if
2071 the pattern was not studied. The third argument specifies which piece
2072 of information is required, and the fourth argument is a pointer to a
2073 variable to receive the data. The yield of the function is zero for
2074 success, or one of the following negative numbers:
2075
2076 PCRE_ERROR_NULL the argument code was NULL
2077 the argument where was NULL
2078 PCRE_ERROR_BADMAGIC the "magic number" was not found
2079 PCRE_ERROR_BADENDIANNESS the pattern was compiled with different
2080 endianness
2081 PCRE_ERROR_BADOPTION the value of what was invalid
2082
2083 The "magic number" is placed at the start of each compiled pattern as
2084 an simple check against passing an arbitrary memory pointer. The endi-
2085 anness error can occur if a compiled pattern is saved and reloaded on a
2086 different host. Here is a typical call of pcre_fullinfo(), to obtain
2087 the length of the compiled pattern:
2088
2089 int rc;
2090 size_t length;
2091 rc = pcre_fullinfo(
2092 re, /* result of pcre_compile() */
2093 sd, /* result of pcre_study(), or NULL */
2094 PCRE_INFO_SIZE, /* what is required */
2095 &length); /* where to put the data */
2096
2097 The possible values for the third argument are defined in pcre.h, and
2098 are as follows:
2099
2100 PCRE_INFO_BACKREFMAX
2101
2102 Return the number of the highest back reference in the pattern. The
2103 fourth argument should point to an int variable. Zero is returned if
2104 there are no back references.
2105
2106 PCRE_INFO_CAPTURECOUNT
2107
2108 Return the number of capturing subpatterns in the pattern. The fourth
2109 argument should point to an int variable.
2110
2111 PCRE_INFO_DEFAULT_TABLES
2112
2113 Return a pointer to the internal default character tables within PCRE.
2114 The fourth argument should point to an unsigned char * variable. This
2115 information call is provided for internal use by the pcre_study() func-
2116 tion. External callers can cause PCRE to use its internal tables by
2117 passing a NULL table pointer.
2118
2119 PCRE_INFO_FIRSTBYTE
2120
2121 Return information about the first data unit of any matched string, for
2122 a non-anchored pattern. (The name of this option refers to the 8-bit
2123 library, where data units are bytes.) The fourth argument should point
2124 to an int variable.
2125
2126 If there is a fixed first value, for example, the letter "c" from a
2127 pattern such as (cat|cow|coyote), its value is returned. In the 8-bit
2128 library, the value is always less than 256; in the 16-bit library the
2129 value can be up to 0xffff.
2130
2131 If there is no fixed first value, and if either
2132
2133 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
2134 branch starts with "^", or
2135
2136 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
2137 set (if it were set, the pattern would be anchored),
2138
2139 -1 is returned, indicating that the pattern matches only at the start
2140 of a subject string or after any newline within the string. Otherwise
2141 -2 is returned. For anchored patterns, -2 is returned.
2142
2143 PCRE_INFO_FIRSTTABLE
2144
2145 If the pattern was studied, and this resulted in the construction of a
2146 256-bit table indicating a fixed set of values for the first data unit
2147 in any matching string, a pointer to the table is returned. Otherwise
2148 NULL is returned. The fourth argument should point to an unsigned char
2149 * variable.
2150
2151 PCRE_INFO_HASCRORLF
2152
2153 Return 1 if the pattern contains any explicit matches for CR or LF
2154 characters, otherwise 0. The fourth argument should point to an int
2155 variable. An explicit match is either a literal CR or LF character, or
2156 \r or \n.
2157
2158 PCRE_INFO_JCHANGED
2159
2160 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
2161 otherwise 0. The fourth argument should point to an int variable. (?J)
2162 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
2163
2164 PCRE_INFO_JIT
2165
2166 Return 1 if the pattern was studied with one of the JIT options, and
2167 just-in-time compiling was successful. The fourth argument should point
2168 to an int variable. A return value of 0 means that JIT support is not
2169 available in this version of PCRE, or that the pattern was not studied
2170 with a JIT option, or that the JIT compiler could not handle this par-
2171 ticular pattern. See the pcrejit documentation for details of what can
2172 and cannot be handled.
2173
2174 PCRE_INFO_JITSIZE
2175
2176 If the pattern was successfully studied with a JIT option, return the
2177 size of the JIT compiled code, otherwise return zero. The fourth argu-
2178 ment should point to a size_t variable.
2179
2180 PCRE_INFO_LASTLITERAL
2181
2182 Return the value of the rightmost literal data unit that must exist in
2183 any matched string, other than at its start, if such a value has been
2184 recorded. The fourth argument should point to an int variable. If there
2185 is no such value, -1 is returned. For anchored patterns, a last literal
2186 value is recorded only if it follows something of variable length. For
2187 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
2188 /^a\dz\d/ the returned value is -1.
2189
2190 PCRE_INFO_MAXLOOKBEHIND
2191
2192 Return the number of characters (NB not bytes) in the longest lookbe-
2193 hind assertion in the pattern. Note that the simple assertions \b and
2194 \B require a one-character lookbehind. This information is useful when
2195 doing multi-segment matching using the partial matching facilities.
2196
2197 PCRE_INFO_MINLENGTH
2198
2199 If the pattern was studied and a minimum length for matching subject
2200 strings was computed, its value is returned. Otherwise the returned
2201 value is -1. The value is a number of characters, which in UTF-8 mode
2202 may be different from the number of bytes. The fourth argument should
2203 point to an int variable. A non-negative value is a lower bound to the
2204 length of any matching string. There may not be any strings of that
2205 length that do actually match, but every string that does match is at
2206 least that long.
2207
2208 PCRE_INFO_NAMECOUNT
2209 PCRE_INFO_NAMEENTRYSIZE
2210 PCRE_INFO_NAMETABLE
2211
2212 PCRE supports the use of named as well as numbered capturing parenthe-
2213 ses. The names are just an additional way of identifying the parenthe-
2214 ses, which still acquire numbers. Several convenience functions such as
2215 pcre_get_named_substring() are provided for extracting captured sub-
2216 strings by name. It is also possible to extract the data directly, by
2217 first converting the name to a number in order to access the correct
2218 pointers in the output vector (described with pcre_exec() below). To do
2219 the conversion, you need to use the name-to-number map, which is
2220 described by these three values.
2221
2222 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
2223 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
2224 of each entry; both of these return an int value. The entry size
2225 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
2226 a pointer to the first entry of the table. This is a pointer to char in
2227 the 8-bit library, where the first two bytes of each entry are the num-
2228 ber of the capturing parenthesis, most significant byte first. In the
2229 16-bit library, the pointer points to 16-bit data units, the first of
2230 which contains the parenthesis number. The rest of the entry is the
2231 corresponding name, zero terminated.
2232
2233 The names are in alphabetical order. Duplicate names may appear if (?|
2234 is used to create multiple groups with the same number, as described in
2235 the section on duplicate subpattern numbers in the pcrepattern page.
2236 Duplicate names for subpatterns with different numbers are permitted
2237 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
2238 appear in the table in the order in which they were found in the pat-
2239 tern. In the absence of (?| this is the order of increasing number;
2240 when (?| is used this is not necessarily the case because later subpat-
2241 terns may have lower numbers.
2242
2243 As a simple example of the name/number table, consider the following
2244 pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is
2245 set, so white space - including newlines - is ignored):
2246
2247 (?<date> (?<year>(\d\d)?\d\d) -
2248 (?<month>\d\d) - (?<day>\d\d) )
2249
2250 There are four named subpatterns, so the table has four entries, and
2251 each entry in the table is eight bytes long. The table is as follows,
2252 with non-printing bytes shows in hexadecimal, and undefined bytes shown
2253 as ??:
2254
2255 00 01 d a t e 00 ??
2256 00 05 d a y 00 ?? ??
2257 00 04 m o n t h 00
2258 00 02 y e a r 00 ??
2259
2260 When writing code to extract data from named subpatterns using the
2261 name-to-number map, remember that the length of the entries is likely
2262 to be different for each compiled pattern.
2263
2264 PCRE_INFO_OKPARTIAL
2265
2266 Return 1 if the pattern can be used for partial matching with
2267 pcre_exec(), otherwise 0. The fourth argument should point to an int
2268 variable. From release 8.00, this always returns 1, because the
2269 restrictions that previously applied to partial matching have been
2270 lifted. The pcrepartial documentation gives details of partial match-
2271 ing.
2272
2273 PCRE_INFO_OPTIONS
2274
2275 Return a copy of the options with which the pattern was compiled. The
2276 fourth argument should point to an unsigned long int variable. These
2277 option bits are those specified in the call to pcre_compile(), modified
2278 by any top-level option settings at the start of the pattern itself. In
2279 other words, they are the options that will be in force when matching
2280 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
2281 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
2282 and PCRE_EXTENDED.
2283
2284 A pattern is automatically anchored by PCRE if all of its top-level
2285 alternatives begin with one of the following:
2286
2287 ^ unless PCRE_MULTILINE is set
2288 \A always
2289 \G always
2290 .* if PCRE_DOTALL is set and there are no back
2291 references to the subpattern in which .* appears
2292
2293 For such patterns, the PCRE_ANCHORED bit is set in the options returned
2294 by pcre_fullinfo().
2295
2296 PCRE_INFO_SIZE
2297
2298 Return the size of the compiled pattern in bytes (for both libraries).
2299 The fourth argument should point to a size_t variable. This value does
2300 not include the size of the pcre structure that is returned by
2301 pcre_compile(). The value that is passed as the argument to pcre_mal-
2302 loc() when pcre_compile() is getting memory in which to place the com-
2303 piled data is the value returned by this option plus the size of the
2304 pcre structure. Studying a compiled pattern, with or without JIT, does
2305 not alter the value returned by this option.
2306
2307 PCRE_INFO_STUDYSIZE
2308
2309 Return the size in bytes of the data block pointed to by the study_data
2310 field in a pcre_extra block. If pcre_extra is NULL, or there is no
2311 study data, zero is returned. The fourth argument should point to a
2312 size_t variable. The study_data field is set by pcre_study() to record
2313 information that will speed up matching (see the section entitled
2314 "Studying a pattern" above). The format of the study_data block is pri-
2315 vate, but its length is made available via this option so that it can
2316 be saved and restored (see the pcreprecompile documentation for
2317 details).
2318
2319
2320 REFERENCE COUNTS
2321
2322 int pcre_refcount(pcre *code, int adjust);
2323
2324 The pcre_refcount() function is used to maintain a reference count in
2325 the data block that contains a compiled pattern. It is provided for the
2326 benefit of applications that operate in an object-oriented manner,
2327 where different parts of the application may be using the same compiled
2328 pattern, but you want to free the block when they are all done.
2329
2330 When a pattern is compiled, the reference count field is initialized to
2331 zero. It is changed only by calling this function, whose action is to
2332 add the adjust value (which may be positive or negative) to it. The
2333 yield of the function is the new value. However, the value of the count
2334 is constrained to lie between 0 and 65535, inclusive. If the new value
2335 is outside these limits, it is forced to the appropriate limit value.
2336
2337 Except when it is zero, the reference count is not correctly preserved
2338 if a pattern is compiled on one host and then transferred to a host
2339 whose byte-order is different. (This seems a highly unlikely scenario.)
2340
2341
2342 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
2343
2344 int pcre_exec(const pcre *code, const pcre_extra *extra,
2345 const char *subject, int length, int startoffset,
2346 int options, int *ovector, int ovecsize);
2347
2348 The function pcre_exec() is called to match a subject string against a
2349 compiled pattern, which is passed in the code argument. If the pattern
2350 was studied, the result of the study should be passed in the extra
2351 argument. You can call pcre_exec() with the same code and extra argu-
2352 ments as many times as you like, in order to match different subject
2353 strings with the same pattern.
2354
2355 This function is the main matching facility of the library, and it
2356 operates in a Perl-like manner. For specialist use there is also an
2357 alternative matching function, which is described below in the section
2358 about the pcre_dfa_exec() function.
2359
2360 In most applications, the pattern will have been compiled (and option-
2361 ally studied) in the same process that calls pcre_exec(). However, it
2362 is possible to save compiled patterns and study data, and then use them
2363 later in different processes, possibly even on different hosts. For a
2364 discussion about this, see the pcreprecompile documentation.
2365
2366 Here is an example of a simple call to pcre_exec():
2367
2368 int rc;
2369 int ovector[30];
2370 rc = pcre_exec(
2371 re, /* result of pcre_compile() */
2372 NULL, /* we didn't study the pattern */
2373 "some string", /* the subject string */
2374 11, /* the length of the subject string */
2375 0, /* start at offset 0 in the subject */
2376 0, /* default options */
2377 ovector, /* vector of integers for substring information */
2378 30); /* number of elements (NOT size in bytes) */
2379
2380 Extra data for pcre_exec()
2381
2382 If the extra argument is not NULL, it must point to a pcre_extra data
2383 block. The pcre_study() function returns such a block (when it doesn't
2384 return NULL), but you can also create one for yourself, and pass addi-
2385 tional information in it. The pcre_extra block contains the following
2386 fields (not necessarily in this order):
2387
2388 unsigned long int flags;
2389 void *study_data;
2390 void *executable_jit;
2391 unsigned long int match_limit;
2392 unsigned long int match_limit_recursion;
2393 void *callout_data;
2394 const unsigned char *tables;
2395 unsigned char **mark;
2396
2397 In the 16-bit version of this structure, the mark field has type
2398 "PCRE_UCHAR16 **".
2399
2400 The flags field is used to specify which of the other fields are set.
2401 The flag bits are:
2402
2403 PCRE_EXTRA_CALLOUT_DATA
2404 PCRE_EXTRA_EXECUTABLE_JIT
2405 PCRE_EXTRA_MARK
2406 PCRE_EXTRA_MATCH_LIMIT
2407 PCRE_EXTRA_MATCH_LIMIT_RECURSION
2408 PCRE_EXTRA_STUDY_DATA
2409 PCRE_EXTRA_TABLES
2410
2411 Other flag bits should be set to zero. The study_data field and some-
2412 times the executable_jit field are set in the pcre_extra block that is
2413 returned by pcre_study(), together with the appropriate flag bits. You
2414 should not set these yourself, but you may add to the block by setting
2415 other fields and their corresponding flag bits.
2416
2417 The match_limit field provides a means of preventing PCRE from using up
2418 a vast amount of resources when running patterns that are not going to
2419 match, but which have a very large number of possibilities in their
2420 search trees. The classic example is a pattern that uses nested unlim-
2421 ited repeats.
2422
2423 Internally, pcre_exec() uses a function called match(), which it calls
2424 repeatedly (sometimes recursively). The limit set by match_limit is
2425 imposed on the number of times this function is called during a match,
2426 which has the effect of limiting the amount of backtracking that can
2427 take place. For patterns that are not anchored, the count restarts from
2428 zero for each position in the subject string.
2429
2430 When pcre_exec() is called with a pattern that was successfully studied
2431 with a JIT option, the way that the matching is executed is entirely
2432 different. However, there is still the possibility of runaway matching
2433 that goes on for a very long time, and so the match_limit value is also
2434 used in this case (but in a different way) to limit how long the match-
2435 ing can continue.
2436
2437 The default value for the limit can be set when PCRE is built; the
2438 default default is 10 million, which handles all but the most extreme
2439 cases. You can override the default by suppling pcre_exec() with a
2440 pcre_extra block in which match_limit is set, and
2441 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
2442 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
2443
2444 The match_limit_recursion field is similar to match_limit, but instead
2445 of limiting the total number of times that match() is called, it limits
2446 the depth of recursion. The recursion depth is a smaller number than
2447 the total number of calls, because not all calls to match() are recur-
2448 sive. This limit is of use only if it is set smaller than match_limit.
2449
2450 Limiting the recursion depth limits the amount of machine stack that
2451 can be used, or, when PCRE has been compiled to use memory on the heap
2452 instead of the stack, the amount of heap memory that can be used. This
2453 limit is not relevant, and is ignored, when matching is done using JIT
2454 compiled code.
2455
2456 The default value for match_limit_recursion can be set when PCRE is
2457 built; the default default is the same value as the default for
2458 match_limit. You can override the default by suppling pcre_exec() with
2459 a pcre_extra block in which match_limit_recursion is set, and
2460 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
2461 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
2462
2463 The callout_data field is used in conjunction with the "callout" fea-
2464 ture, and is described in the pcrecallout documentation.
2465
2466 The tables field is used to pass a character tables pointer to
2467 pcre_exec(); this overrides the value that is stored with the compiled
2468 pattern. A non-NULL value is stored with the compiled pattern only if
2469 custom tables were supplied to pcre_compile() via its tableptr argu-
2470 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
2471 PCRE's internal tables to be used. This facility is helpful when re-
2472 using patterns that have been saved after compiling with an external
2473 set of tables, because the external tables might be at a different
2474 address when pcre_exec() is called. See the pcreprecompile documenta-
2475 tion for a discussion of saving compiled patterns for later use.
2476
2477 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
2478 set to point to a suitable variable. If the pattern contains any back-
2479 tracking control verbs such as (*MARK:NAME), and the execution ends up
2480 with a name to pass back, a pointer to the name string (zero termi-
2481 nated) is placed in the variable pointed to by the mark field. The
2482 names are within the compiled pattern; if you wish to retain such a
2483 name you must copy it before freeing the memory of a compiled pattern.
2484 If there is no name to pass back, the variable pointed to by the mark
2485 field is set to NULL. For details of the backtracking control verbs,
2486 see the section entitled "Backtracking control" in the pcrepattern doc-
2487 umentation.
2488
2489 Option bits for pcre_exec()
2490
2491 The unused bits of the options argument for pcre_exec() must be zero.
2492 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2493 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
2494 PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and
2495 PCRE_PARTIAL_SOFT.
2496
2497 If the pattern was successfully studied with one of the just-in-time
2498 (JIT) compile options, the only supported options for JIT execution are
2499 PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
2500 PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an
2501 unsupported option is used, JIT execution is disabled and the normal
2502 interpretive code in pcre_exec() is run.
2503
2504 PCRE_ANCHORED
2505
2506 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2507 matching position. If a pattern was compiled with PCRE_ANCHORED, or
2508 turned out to be anchored by virtue of its contents, it cannot be made
2509 unachored at matching time.
2510
2511 PCRE_BSR_ANYCRLF
2512 PCRE_BSR_UNICODE
2513
2514 These options (which are mutually exclusive) control what the \R escape
2515 sequence matches. The choice is either to match only CR, LF, or CRLF,
2516 or to match any Unicode newline sequence. These options override the
2517 choice that was made or defaulted when the pattern was compiled.
2518
2519 PCRE_NEWLINE_CR
2520 PCRE_NEWLINE_LF
2521 PCRE_NEWLINE_CRLF
2522 PCRE_NEWLINE_ANYCRLF
2523 PCRE_NEWLINE_ANY
2524
2525 These options override the newline definition that was chosen or
2526 defaulted when the pattern was compiled. For details, see the descrip-
2527 tion of pcre_compile() above. During matching, the newline choice
2528 affects the behaviour of the dot, circumflex, and dollar metacharac-
2529 ters. It may also alter the way the match position is advanced after a
2530 match failure for an unanchored pattern.
2531
2532 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
2533 set, and a match attempt for an unanchored pattern fails when the cur-
2534 rent position is at a CRLF sequence, and the pattern contains no
2535 explicit matches for CR or LF characters, the match position is
2536 advanced by two characters instead of one, in other words, to after the
2537 CRLF.
2538
2539 The above rule is a compromise that makes the most common cases work as
2540 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2541 option is not set), it does not match the string "\r\nA" because, after
2542 failing at the start, it skips both the CR and the LF before retrying.
2543 However, the pattern [\r\n]A does match that string, because it con-
2544 tains an explicit CR or LF reference, and so advances only by one char-
2545 acter after the first failure.
2546
2547 An explicit match for CR of LF is either a literal appearance of one of
2548 those characters, or one of the \r or \n escape sequences. Implicit
2549 matches such as [^X] do not count, nor does \s (which includes CR and
2550 LF in the characters that it matches).
2551
2552 Notwithstanding the above, anomalous effects may still occur when CRLF
2553 is a valid newline sequence and explicit \r or \n escapes appear in the
2554 pattern.
2555
2556 PCRE_NOTBOL
2557
2558 This option specifies that first character of the subject string is not
2559 the beginning of a line, so the circumflex metacharacter should not
2560 match before it. Setting this without PCRE_MULTILINE (at compile time)
2561 causes circumflex never to match. This option affects only the behav-
2562 iour of the circumflex metacharacter. It does not affect \A.
2563
2564 PCRE_NOTEOL
2565
2566 This option specifies that the end of the subject string is not the end
2567 of a line, so the dollar metacharacter should not match it nor (except
2568 in multiline mode) a newline immediately before it. Setting this with-
2569 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2570 option affects only the behaviour of the dollar metacharacter. It does
2571 not affect \Z or \z.
2572
2573 PCRE_NOTEMPTY
2574
2575 An empty string is not considered to be a valid match if this option is
2576 set. If there are alternatives in the pattern, they are tried. If all
2577 the alternatives match the empty string, the entire match fails. For
2578 example, if the pattern
2579
2580 a?b?
2581
2582 is applied to a string not beginning with "a" or "b", it matches an
2583 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2584 match is not valid, so PCRE searches further into the string for occur-
2585 rences of "a" or "b".
2586
2587 PCRE_NOTEMPTY_ATSTART
2588
2589 This is like PCRE_NOTEMPTY, except that an empty string match that is
2590 not at the start of the subject is permitted. If the pattern is
2591 anchored, such a match can occur only if the pattern contains \K.
2592
2593 Perl has no direct equivalent of PCRE_NOTEMPTY or
2594 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2595 match of the empty string within its split() function, and when using
2596 the /g modifier. It is possible to emulate Perl's behaviour after
2597 matching a null string by first trying the match again at the same off-
2598 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2599 fails, by advancing the starting offset (see below) and trying an ordi-
2600 nary match again. There is some code that demonstrates how to do this
2601 in the pcredemo sample program. In the most general case, you have to
2602 check to see if the newline convention recognizes CRLF as a newline,
2603 and if so, and the current character is CR followed by LF, advance the
2604 starting offset by two characters instead of one.
2605
2606 PCRE_NO_START_OPTIMIZE
2607
2608 There are a number of optimizations that pcre_exec() uses at the start
2609 of a match, in order to speed up the process. For example, if it is
2610 known that an unanchored match must start with a specific character, it
2611 searches the subject for that character, and fails immediately if it
2612 cannot find it, without actually running the main matching function.
2613 This means that a special item such as (*COMMIT) at the start of a pat-
2614 tern is not considered until after a suitable starting point for the
2615 match has been found. When callouts or (*MARK) items are in use, these
2616 "start-up" optimizations can cause them to be skipped if the pattern is
2617 never actually used. The start-up optimizations are in effect a pre-
2618 scan of the subject that takes place before the pattern is run.
2619
2620 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
2621 possibly causing performance to suffer, but ensuring that in cases
2622 where the result is "no match", the callouts do occur, and that items
2623 such as (*COMMIT) and (*MARK) are considered at every possible starting
2624 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
2625 compile time, it cannot be unset at matching time. The use of
2626 PCRE_NO_START_OPTIMIZE disables JIT execution; when it is set, matching
2627 is always done using interpretively.
2628
2629 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
2630 operation. Consider the pattern
2631
2632 (*COMMIT)ABC
2633
2634 When this is compiled, PCRE records the fact that a match must start
2635 with the character "A". Suppose the subject string is "DEFABC". The
2636 start-up optimization scans along the subject, finds "A" and runs the
2637 first match attempt from there. The (*COMMIT) item means that the pat-
2638 tern must match the current starting position, which in this case, it
2639 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
2640 set, the initial scan along the subject string does not happen. The
2641 first match attempt is run starting from "D" and when this fails,
2642 (*COMMIT) prevents any further matches being tried, so the overall
2643 result is "no match". If the pattern is studied, more start-up opti-
2644 mizations may be used. For example, a minimum length for the subject
2645 may be recorded. Consider the pattern
2646
2647 (*MARK:A)(X|Y)
2648
2649 The minimum length for a match is one character. If the subject is
2650 "ABC", there will be attempts to match "ABC", "BC", "C", and then
2651 finally an empty string. If the pattern is studied, the final attempt
2652 does not take place, because PCRE knows that the subject is too short,
2653 and so the (*MARK) is never encountered. In this case, studying the
2654 pattern does not affect the overall match result, which is still "no
2655 match", but it does affect the auxiliary information that is returned.
2656
2657 PCRE_NO_UTF8_CHECK
2658
2659 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2660 UTF-8 string is automatically checked when pcre_exec() is subsequently
2661 called. The entire string is checked before any other processing takes
2662 place. The value of startoffset is also checked to ensure that it
2663 points to the start of a UTF-8 character. There is a discussion about
2664 the validity of UTF-8 strings in the pcreunicode page. If an invalid
2665 sequence of bytes is found, pcre_exec() returns the error
2666 PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a
2667 truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In
2668 both cases, information about the precise nature of the error may also
2669 be returned (see the descriptions of these errors in the section enti-
2670 tled Error return values from pcre_exec() below). If startoffset con-
2671 tains a value that does not point to the start of a UTF-8 character (or
2672 to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned.
2673
2674 If you already know that your subject is valid, and you want to skip
2675 these checks for performance reasons, you can set the
2676 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2677 do this for the second and subsequent calls to pcre_exec() if you are
2678 making repeated calls to find all the matches in a single subject
2679 string. However, you should be sure that the value of startoffset
2680 points to the start of a character (or the end of the subject). When
2681 PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a
2682 subject or an invalid value of startoffset is undefined. Your program
2683 may crash.
2684
2685 PCRE_PARTIAL_HARD
2686 PCRE_PARTIAL_SOFT
2687
2688 These options turn on the partial matching feature. For backwards com-
2689 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2690 match occurs if the end of the subject string is reached successfully,
2691 but there are not enough subject characters to complete the match. If
2692 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
2693 matching continues by testing any remaining alternatives. Only if no
2694 complete match can be found is PCRE_ERROR_PARTIAL returned instead of
2695 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
2696 caller is prepared to handle a partial match, but only if no complete
2697 match can be found.
2698
2699 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
2700 case, if a partial match is found, pcre_exec() immediately returns
2701 PCRE_ERROR_PARTIAL, without considering any other alternatives. In
2702 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
2703 ered to be more important that an alternative complete match.
2704
2705 In both cases, the portion of the string that was inspected when the
2706 partial match was found is set as the first matching string. There is a
2707 more detailed discussion of partial and multi-segment matching, with
2708 examples, in the pcrepartial documentation.
2709
2710 The string to be matched by pcre_exec()
2711
2712 The subject string is passed to pcre_exec() as a pointer in subject, a
2713 length in bytes in length, and a starting byte offset in startoffset.
2714 If this is negative or greater than the length of the subject,
2715 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
2716 zero, the search for a match starts at the beginning of the subject,
2717 and this is by far the most common case. In UTF-8 mode, the byte offset
2718 must point to the start of a UTF-8 character (or the end of the sub-
2719 ject). Unlike the pattern string, the subject may contain binary zero
2720 bytes.
2721
2722 A non-zero starting offset is useful when searching for another match
2723 in the same subject by calling pcre_exec() again after a previous suc-
2724 cess. Setting startoffset differs from just passing over a shortened
2725 string and setting PCRE_NOTBOL in the case of a pattern that begins
2726 with any kind of lookbehind. For example, consider the pattern
2727
2728 \Biss\B
2729
2730 which finds occurrences of "iss" in the middle of words. (\B matches
2731 only if the current position in the subject is not a word boundary.)
2732 When applied to the string "Mississipi" the first call to pcre_exec()
2733 finds the first occurrence. If pcre_exec() is called again with just
2734 the remainder of the subject, namely "issipi", it does not match,
2735 because \B is always false at the start of the subject, which is deemed
2736 to be a word boundary. However, if pcre_exec() is passed the entire
2737 string again, but with startoffset set to 4, it finds the second occur-
2738 rence of "iss" because it is able to look behind the starting point to
2739 discover that it is preceded by a letter.
2740
2741 Finding all the matches in a subject is tricky when the pattern can
2742 match an empty string. It is possible to emulate Perl's /g behaviour by
2743 first trying the match again at the same offset, with the
2744 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
2745 fails, advancing the starting offset and trying an ordinary match
2746 again. There is some code that demonstrates how to do this in the pcre-
2747 demo sample program. In the most general case, you have to check to see
2748 if the newline convention recognizes CRLF as a newline, and if so, and
2749 the current character is CR followed by LF, advance the starting offset
2750 by two characters instead of one.
2751
2752 If a non-zero starting offset is passed when the pattern is anchored,
2753 one attempt to match at the given offset is made. This can only succeed
2754 if the pattern does not require the match to be at the start of the
2755 subject.
2756
2757 How pcre_exec() returns captured substrings
2758
2759 In general, a pattern matches a certain portion of the subject, and in
2760 addition, further substrings from the subject may be picked out by
2761 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2762 this is called "capturing" in what follows, and the phrase "capturing
2763 subpattern" is used for a fragment of a pattern that picks out a sub-
2764 string. PCRE supports several other kinds of parenthesized subpattern
2765 that do not cause substrings to be captured.
2766
2767 Captured substrings are returned to the caller via a vector of integers
2768 whose address is passed in ovector. The number of elements in the vec-
2769 tor is passed in ovecsize, which must be a non-negative number. Note:
2770 this argument is NOT the size of ovector in bytes.
2771
2772 The first two-thirds of the vector is used to pass back captured sub-
2773 strings, each substring using a pair of integers. The remaining third
2774 of the vector is used as workspace by pcre_exec() while matching cap-
2775 turing subpatterns, and is not available for passing back information.
2776 The number passed in ovecsize should always be a multiple of three. If
2777 it is not, it is rounded down.
2778
2779 When a match is successful, information about captured substrings is
2780 returned in pairs of integers, starting at the beginning of ovector,
2781 and continuing up to two-thirds of its length at the most. The first
2782 element of each pair is set to the byte offset of the first character
2783 in a substring, and the second is set to the byte offset of the first
2784 character after the end of a substring. Note: these values are always
2785 byte offsets, even in UTF-8 mode. They are not character counts.
2786
2787 The first pair of integers, ovector[0] and ovector[1], identify the
2788 portion of the subject string matched by the entire pattern. The next
2789 pair is used for the first capturing subpattern, and so on. The value
2790 returned by pcre_exec() is one more than the highest numbered pair that
2791 has been set. For example, if two substrings have been captured, the
2792 returned value is 3. If there are no capturing subpatterns, the return
2793 value from a successful match is 1, indicating that just the first pair
2794 of offsets has been set.
2795
2796 If a capturing subpattern is matched repeatedly, it is the last portion
2797 of the string that it matched that is returned.
2798
2799 If the vector is too small to hold all the captured substring offsets,
2800 it is used as far as possible (up to two-thirds of its length), and the
2801 function returns a value of zero. If neither the actual string matched
2802 nor any captured substrings are of interest, pcre_exec() may be called
2803 with ovector passed as NULL and ovecsize as zero. However, if the pat-
2804 tern contains back references and the ovector is not big enough to
2805 remember the related substrings, PCRE has to get additional memory for
2806 use during matching. Thus it is usually advisable to supply an ovector
2807 of reasonable size.
2808
2809 There are some cases where zero is returned (indicating vector over-
2810 flow) when in fact the vector is exactly the right size for the final
2811 match. For example, consider the pattern
2812
2813 (a)(?:(b)c|bd)
2814
2815 If a vector of 6 elements (allowing for only 1 captured substring) is
2816 given with subject string "abd", pcre_exec() will try to set the second
2817 captured string, thereby recording a vector overflow, before failing to
2818 match "c" and backing up to try the second alternative. The zero
2819 return, however, does correctly indicate that the maximum number of
2820 slots (namely 2) have been filled. In similar cases where there is tem-
2821 porary overflow, but the final number of used slots is actually less
2822 than the maximum, a non-zero value is returned.
2823
2824 The pcre_fullinfo() function can be used to find out how many capturing
2825 subpatterns there are in a compiled pattern. The smallest size for
2826 ovector that will allow for n captured substrings, in addition to the
2827 offsets of the substring matched by the whole pattern, is (n+1)*3.
2828
2829 It is possible for capturing subpattern number n+1 to match some part
2830 of the subject when subpattern n has not been used at all. For example,
2831 if the string "abc" is matched against the pattern (a|(z))(bc) the
2832 return from the function is 4, and subpatterns 1 and 3 are matched, but
2833 2 is not. When this happens, both values in the offset pairs corre-
2834 sponding to unused subpatterns are set to -1.
2835
2836 Offset values that correspond to unused subpatterns at the end of the
2837 expression are also set to -1. For example, if the string "abc" is
2838 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2839 matched. The return from the function is 2, because the highest used
2840 capturing subpattern number is 1, and the offsets for for the second
2841 and third capturing subpatterns (assuming the vector is large enough,
2842 of course) are set to -1.
2843
2844 Note: Elements in the first two-thirds of ovector that do not corre-
2845 spond to capturing parentheses in the pattern are never changed. That
2846 is, if a pattern contains n capturing parentheses, no more than ovec-
2847 tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in
2848 the first two-thirds) retain whatever values they previously had.
2849
2850 Some convenience functions are provided for extracting the captured
2851 substrings as separate strings. These are described below.
2852
2853 Error return values from pcre_exec()
2854
2855 If pcre_exec() fails, it returns a negative number. The following are
2856 defined in the header file:
2857
2858 PCRE_ERROR_NOMATCH (-1)
2859
2860 The subject string did not match the pattern.
2861
2862 PCRE_ERROR_NULL (-2)
2863
2864 Either code or subject was passed as NULL, or ovector was NULL and
2865 ovecsize was not zero.
2866
2867 PCRE_ERROR_BADOPTION (-3)
2868
2869 An unrecognized bit was set in the options argument.
2870
2871 PCRE_ERROR_BADMAGIC (-4)
2872
2873 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2874 to catch the case when it is passed a junk pointer and to detect when a
2875 pattern that was compiled in an environment of one endianness is run in
2876 an environment with the other endianness. This is the error that PCRE
2877 gives when the magic number is not present.
2878
2879 PCRE_ERROR_UNKNOWN_OPCODE (-5)
2880
2881 While running the pattern match, an unknown item was encountered in the
2882 compiled pattern. This error could be caused by a bug in PCRE or by
2883 overwriting of the compiled pattern.
2884
2885 PCRE_ERROR_NOMEMORY (-6)
2886
2887 If a pattern contains back references, but the ovector that is passed
2888 to pcre_exec() is not big enough to remember the referenced substrings,
2889 PCRE gets a block of memory at the start of matching to use for this
2890 purpose. If the call via pcre_malloc() fails, this error is given. The
2891 memory is automatically freed at the end of matching.
2892
2893 This error is also given if pcre_stack_malloc() fails in pcre_exec().
2894 This can happen only when PCRE has been compiled with --disable-stack-
2895 for-recursion.
2896
2897 PCRE_ERROR_NOSUBSTRING (-7)
2898
2899 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2900 and pcre_get_substring_list() functions (see below). It is never
2901 returned by pcre_exec().
2902
2903 PCRE_ERROR_MATCHLIMIT (-8)
2904
2905 The backtracking limit, as specified by the match_limit field in a
2906 pcre_extra structure (or defaulted) was reached. See the description
2907 above.
2908
2909 PCRE_ERROR_CALLOUT (-9)
2910
2911 This error is never generated by pcre_exec() itself. It is provided for
2912 use by callout functions that want to yield a distinctive error code.
2913 See the pcrecallout documentation for details.
2914
2915 PCRE_ERROR_BADUTF8 (-10)
2916
2917 A string that contains an invalid UTF-8 byte sequence was passed as a
2918 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
2919 the output vector (ovecsize) is at least 2, the byte offset to the
2920 start of the the invalid UTF-8 character is placed in the first ele-
2921 ment, and a reason code is placed in the second element. The reason
2922 codes are listed in the following section. For backward compatibility,
2923 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
2924 acter at the end of the subject (reason codes 1 to 5),
2925 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
2926
2927 PCRE_ERROR_BADUTF8_OFFSET (-11)
2928
2929 The UTF-8 byte sequence that was passed as a subject was checked and
2930 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
2931 value of startoffset did not point to the beginning of a UTF-8 charac-
2932 ter or the end of the subject.
2933
2934 PCRE_ERROR_PARTIAL (-12)
2935
2936 The subject string did not match, but it did match partially. See the
2937 pcrepartial documentation for details of partial matching.
2938
2939 PCRE_ERROR_BADPARTIAL (-13)
2940
2941 This code is no longer in use. It was formerly returned when the
2942 PCRE_PARTIAL option was used with a compiled pattern containing items
2943 that were not supported for partial matching. From release 8.00
2944 onwards, there are no restrictions on partial matching.
2945
2946 PCRE_ERROR_INTERNAL (-14)
2947
2948 An unexpected internal error has occurred. This error could be caused
2949 by a bug in PCRE or by overwriting of the compiled pattern.
2950
2951 PCRE_ERROR_BADCOUNT (-15)
2952
2953 This error is given if the value of the ovecsize argument is negative.
2954
2955 PCRE_ERROR_RECURSIONLIMIT (-21)
2956
2957 The internal recursion limit, as specified by the match_limit_recursion
2958 field in a pcre_extra structure (or defaulted) was reached. See the
2959 description above.
2960
2961 PCRE_ERROR_BADNEWLINE (-23)
2962
2963 An invalid combination of PCRE_NEWLINE_xxx options was given.
2964
2965 PCRE_ERROR_BADOFFSET (-24)
2966
2967 The value of startoffset was negative or greater than the length of the
2968 subject, that is, the value in length.
2969
2970 PCRE_ERROR_SHORTUTF8 (-25)
2971
2972 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
2973 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
2974 option is set. Information about the failure is returned as for
2975 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
2976 this special error code for PCRE_PARTIAL_HARD precedes the implementa-
2977 tion of returned information; it is retained for backwards compatibil-
2978 ity.
2979
2980 PCRE_ERROR_RECURSELOOP (-26)
2981
2982 This error is returned when pcre_exec() detects a recursion loop within
2983 the pattern. Specifically, it means that either the whole pattern or a
2984 subpattern has been called recursively for the second time at the same
2985 position in the subject string. Some simple patterns that might do this
2986 are detected and faulted at compile time, but more complicated cases,
2987 in particular mutual recursions between two different subpatterns, can-
2988 not be detected until run time.
2989
2990 PCRE_ERROR_JIT_STACKLIMIT (-27)
2991
2992 This error is returned when a pattern that was successfully studied
2993 using a JIT compile option is being matched, but the memory available
2994 for the just-in-time processing stack is not large enough. See the
2995 pcrejit documentation for more details.
2996
2997 PCRE_ERROR_BADMODE (-28)
2998
2999 This error is given if a pattern that was compiled by the 8-bit library
3000 is passed to a 16-bit library function, or vice versa.
3001
3002 PCRE_ERROR_BADENDIANNESS (-29)
3003
3004 This error is given if a pattern that was compiled and saved is
3005 reloaded on a host with different endianness. The utility function
3006 pcre_pattern_to_host_byte_order() can be used to convert such a pattern
3007 so that it runs on the new host.
3008
3009 Error numbers -16 to -20, -22, and -30 are not used by pcre_exec().
3010
3011 Reason codes for invalid UTF-8 strings
3012
3013 This section applies only to the 8-bit library. The corresponding
3014 information for the 16-bit library is given in the pcre16 page.
3015
3016 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
3017 UTF8, and the size of the output vector (ovecsize) is at least 2, the
3018 offset of the start of the invalid UTF-8 character is placed in the
3019 first output vector element (ovector[0]) and a reason code is placed in
3020 the second element (ovector[1]). The reason codes are given names in
3021 the pcre.h header file:
3022
3023 PCRE_UTF8_ERR1
3024 PCRE_UTF8_ERR2
3025 PCRE_UTF8_ERR3
3026 PCRE_UTF8_ERR4
3027 PCRE_UTF8_ERR5
3028
3029 The string ends with a truncated UTF-8 character; the code specifies
3030 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
3031 characters to be no longer than 4 bytes, the encoding scheme (origi-
3032 nally defined by RFC 2279) allows for up to 6 bytes, and this is
3033 checked first; hence the possibility of 4 or 5 missing bytes.
3034
3035 PCRE_UTF8_ERR6
3036 PCRE_UTF8_ERR7
3037 PCRE_UTF8_ERR8
3038 PCRE_UTF8_ERR9
3039 PCRE_UTF8_ERR10
3040
3041 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
3042 the character do not have the binary value 0b10 (that is, either the
3043 most significant bit is 0, or the next bit is 1).
3044
3045 PCRE_UTF8_ERR11
3046 PCRE_UTF8_ERR12
3047
3048 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
3049 long; these code points are excluded by RFC 3629.
3050
3051 PCRE_UTF8_ERR13
3052
3053 A 4-byte character has a value greater than 0x10fff; these code points
3054 are excluded by RFC 3629.
3055
3056 PCRE_UTF8_ERR14
3057
3058 A 3-byte character has a value in the range 0xd800 to 0xdfff; this
3059 range of code points are reserved by RFC 3629 for use with UTF-16, and
3060 so are excluded from UTF-8.
3061
3062 PCRE_UTF8_ERR15
3063 PCRE_UTF8_ERR16
3064 PCRE_UTF8_ERR17
3065 PCRE_UTF8_ERR18
3066 PCRE_UTF8_ERR19
3067
3068 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
3069 for a value that can be represented by fewer bytes, which is invalid.
3070 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
3071 rect coding uses just one byte.
3072
3073 PCRE_UTF8_ERR20
3074
3075 The two most significant bits of the first byte of a character have the
3076 binary value 0b10 (that is, the most significant bit is 1 and the sec-
3077 ond is 0). Such a byte can only validly occur as the second or subse-
3078 quent byte of a multi-byte character.
3079
3080 PCRE_UTF8_ERR21
3081
3082 The first byte of a character has the value 0xfe or 0xff. These values
3083 can never occur in a valid UTF-8 string.
3084
3085
3086 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
3087
3088 int pcre_copy_substring(const char *subject, int *ovector,
3089 int stringcount, int stringnumber, char *buffer,
3090 int buffersize);
3091
3092 int pcre_get_substring(const char *subject, int *ovector,
3093 int stringcount, int stringnumber,
3094 const char **stringptr);
3095
3096 int pcre_get_substring_list(const char *subject,
3097 int *ovector, int stringcount, const char ***listptr);
3098
3099 Captured substrings can be accessed directly by using the offsets
3100 returned by pcre_exec() in ovector. For convenience, the functions
3101 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
3102 string_list() are provided for extracting captured substrings as new,
3103 separate, zero-terminated strings. These functions identify substrings
3104 by number. The next section describes functions for extracting named
3105 substrings.
3106
3107 A substring that contains a binary zero is correctly extracted and has
3108 a further zero added on the end, but the result is not, of course, a C
3109 string. However, you can process such a string by referring to the
3110 length that is returned by pcre_copy_substring() and pcre_get_sub-
3111 string(). Unfortunately, the interface to pcre_get_substring_list() is
3112 not adequate for handling strings containing binary zeros, because the
3113 end of the final string is not independently indicated.
3114
3115 The first three arguments are the same for all three of these func-
3116 tions: subject is the subject string that has just been successfully
3117 matched, ovector is a pointer to the vector of integer offsets that was
3118 passed to pcre_exec(), and stringcount is the number of substrings that
3119 were captured by the match, including the substring that matched the
3120 entire regular expression. This is the value returned by pcre_exec() if
3121 it is greater than zero. If pcre_exec() returned zero, indicating that
3122 it ran out of space in ovector, the value passed as stringcount should
3123 be the number of elements in the vector divided by three.
3124
3125 The functions pcre_copy_substring() and pcre_get_substring() extract a
3126 single substring, whose number is given as stringnumber. A value of
3127 zero extracts the substring that matched the entire pattern, whereas
3128 higher values extract the captured substrings. For pcre_copy_sub-
3129 string(), the string is placed in buffer, whose length is given by
3130 buffersize, while for pcre_get_substring() a new block of memory is
3131 obtained via pcre_malloc, and its address is returned via stringptr.
3132 The yield of the function is the length of the string, not including
3133 the terminating zero, or one of these error codes:
3134
3135 PCRE_ERROR_NOMEMORY (-6)
3136
3137 The buffer was too small for pcre_copy_substring(), or the attempt to
3138 get memory failed for pcre_get_substring().
3139
3140 PCRE_ERROR_NOSUBSTRING (-7)
3141
3142 There is no substring whose number is stringnumber.
3143
3144 The pcre_get_substring_list() function extracts all available sub-
3145 strings and builds a list of pointers to them. All this is done in a
3146 single block of memory that is obtained via pcre_malloc. The address of
3147 the memory block is returned via listptr, which is also the start of
3148 the list of string pointers. The end of the list is marked by a NULL
3149 pointer. The yield of the function is zero if all went well, or the
3150 error code
3151
3152 PCRE_ERROR_NOMEMORY (-6)
3153
3154 if the attempt to get the memory block failed.
3155
3156 When any of these functions encounter a substring that is unset, which
3157 can happen when capturing subpattern number n+1 matches some part of
3158 the subject, but subpattern n has not been used at all, they return an
3159 empty string. This can be distinguished from a genuine zero-length sub-
3160 string by inspecting the appropriate offset in ovector, which is nega-
3161 tive for unset substrings.
3162
3163 The two convenience functions pcre_free_substring() and pcre_free_sub-
3164 string_list() can be used to free the memory returned by a previous
3165 call of pcre_get_substring() or pcre_get_substring_list(), respec-
3166 tively. They do nothing more than call the function pointed to by
3167 pcre_free, which of course could be called directly from a C program.
3168 However, PCRE is used in some situations where it is linked via a spe-
3169 cial interface to another programming language that cannot use
3170 pcre_free directly; it is for these cases that the functions are pro-
3171 vided.
3172
3173
3174 EXTRACTING CAPTURED SUBSTRINGS BY NAME
3175
3176 int pcre_get_stringnumber(const pcre *code,
3177 const char *name);
3178
3179 int pcre_copy_named_substring(const pcre *code,
3180 const char *subject, int *ovector,
3181 int stringcount, const char *stringname,
3182 char *buffer, int buffersize);
3183
3184 int pcre_get_named_substring(const pcre *code,
3185 const char *subject, int *ovector,
3186 int stringcount, const char *stringname,
3187 const char **stringptr);
3188
3189 To extract a substring by name, you first have to find associated num-
3190 ber. For example, for this pattern
3191
3192 (a+)b(?<xxx>\d+)...
3193
3194 the number of the subpattern called "xxx" is 2. If the name is known to
3195 be unique (PCRE_DUPNAMES was not set), you can find the number from the
3196 name by calling pcre_get_stringnumber(). The first argument is the com-
3197 piled pattern, and the second is the name. The yield of the function is
3198 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
3199 subpattern of that name.
3200
3201 Given the number, you can extract the substring directly, or use one of
3202 the functions described in the previous section. For convenience, there
3203 are also two functions that do the whole job.
3204
3205 Most of the arguments of pcre_copy_named_substring() and
3206 pcre_get_named_substring() are the same as those for the similarly
3207 named functions that extract by number. As these are described in the
3208 previous section, they are not re-described here. There are just two
3209 differences:
3210
3211 First, instead of a substring number, a substring name is given. Sec-
3212 ond, there is an extra argument, given at the start, which is a pointer
3213 to the compiled pattern. This is needed in order to gain access to the
3214 name-to-number translation table.
3215
3216 These functions call pcre_get_stringnumber(), and if it succeeds, they
3217 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
3218 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
3219 behaviour may not be what you want (see the next section).
3220
3221 Warning: If the pattern uses the (?| feature to set up multiple subpat-
3222 terns with the same number, as described in the section on duplicate
3223 subpattern numbers in the pcrepattern page, you cannot use names to
3224 distinguish the different subpatterns, because names are not included
3225 in the compiled code. The matching process uses only numbers. For this
3226 reason, the use of different names for subpatterns of the same number
3227 causes an error at compile time.
3228
3229
3230 DUPLICATE SUBPATTERN NAMES
3231
3232 int pcre_get_stringtable_entries(const pcre *code,
3233 const char *name, char **first, char **last);
3234
3235 When a pattern is compiled with the PCRE_DUPNAMES option, names for
3236 subpatterns are not required to be unique. (Duplicate names are always
3237 allowed for subpatterns with the same number, created by using the (?|
3238 feature. Indeed, if such subpatterns are named, they are required to
3239 use the same names.)
3240
3241 Normally, patterns with duplicate names are such that in any one match,
3242 only one of the named subpatterns participates. An example is shown in
3243 the pcrepattern documentation.
3244
3245 When duplicates are present, pcre_copy_named_substring() and
3246 pcre_get_named_substring() return the first substring corresponding to
3247 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
3248 (-7) is returned; no data is returned. The pcre_get_stringnumber()
3249 function returns one of the numbers that are associated with the name,
3250 but it is not defined which it is.
3251
3252 If you want to get full details of all captured substrings for a given
3253 name, you must use the pcre_get_stringtable_entries() function. The
3254 first argument is the compiled pattern, and the second is the name. The
3255 third and fourth are pointers to variables which are updated by the
3256 function. After it has run, they point to the first and last entries in
3257 the name-to-number table for the given name. The function itself
3258 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
3259 there are none. The format of the table is described above in the sec-
3260 tion entitled Information about a pattern above. Given all the rele-
3261 vant entries for the name, you can extract each of their numbers, and
3262 hence the captured data, if any.
3263
3264
3265 FINDING ALL POSSIBLE MATCHES
3266
3267 The traditional matching function uses a similar algorithm to Perl,
3268 which stops when it finds the first match, starting at a given point in
3269 the subject. If you want to find all possible matches, or the longest
3270 possible match, consider using the alternative matching function (see
3271 below) instead. If you cannot use the alternative function, but still
3272 need to find all possible matches, you can kludge it up by making use
3273 of the callout facility, which is described in the pcrecallout documen-
3274 tation.
3275
3276 What you have to do is to insert a callout right at the end of the pat-
3277 tern. When your callout function is called, extract and save the cur-
3278 rent matched substring. Then return 1, which forces pcre_exec() to
3279 backtrack and try other alternatives. Ultimately, when it runs out of
3280 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
3281
3282
3283 OBTAINING AN ESTIMATE OF STACK USAGE
3284
3285 Matching certain patterns using pcre_exec() can use a lot of process
3286 stack, which in certain environments can be rather limited in size.
3287 Some users find it helpful to have an estimate of the amount of stack
3288 that is used by pcre_exec(), to help them set recursion limits, as
3289 described in the pcrestack documentation. The estimate that is output
3290 by pcretest when called with the -m and -C options is obtained by call-
3291 ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its
3292 first five arguments.
3293
3294 Normally, if its first argument is NULL, pcre_exec() immediately
3295 returns the negative error code PCRE_ERROR_NULL, but with this special
3296 combination of arguments, it returns instead a negative number whose
3297 absolute value is the approximate stack frame size in bytes. (A nega-
3298 tive number is used so that it is clear that no match has happened.)
3299 The value is approximate because in some cases, recursive calls to
3300 pcre_exec() occur when there are one or two additional variables on the
3301 stack.
3302
3303 If PCRE has been compiled to use the heap instead of the stack for
3304 recursion, the value returned is the size of each block that is
3305 obtained from the heap.
3306
3307
3308 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
3309
3310 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
3311 const char *subject, int length, int startoffset,
3312 int options, int *ovector, int ovecsize,
3313 int *workspace, int wscount);
3314
3315 The function pcre_dfa_exec() is called to match a subject string
3316 against a compiled pattern, using a matching algorithm that scans the
3317 subject string just once, and does not backtrack. This has different
3318 characteristics to the normal algorithm, and is not compatible with
3319 Perl. Some of the features of PCRE patterns are not supported. Never-
3320 theless, there are times when this kind of matching can be useful. For
3321 a discussion of the two matching algorithms, and a list of features
3322 that pcre_dfa_exec() does not support, see the pcrematching documenta-
3323 tion.
3324
3325 The arguments for the pcre_dfa_exec() function are the same as for
3326 pcre_exec(), plus two extras. The ovector argument is used in a differ-
3327 ent way, and this is described below. The other common arguments are
3328 used in the same way as for pcre_exec(), so their description is not
3329 repeated here.
3330
3331 The two additional arguments provide workspace for the function. The
3332 workspace vector should contain at least 20 elements. It is used for
3333 keeping track of multiple paths through the pattern tree. More
3334 workspace will be needed for patterns and subjects where there are a
3335 lot of potential matches.
3336
3337 Here is an example of a simple call to pcre_dfa_exec():
3338
3339 int rc;
3340 int ovector[10];
3341 int wspace[20];
3342 rc = pcre_dfa_exec(
3343 re, /* result of pcre_compile() */
3344 NULL, /* we didn't study the pattern */
3345 "some string", /* the subject string */
3346 11, /* the length of the subject string */
3347 0, /* start at offset 0 in the subject */
3348 0, /* default options */
3349 ovector, /* vector of integers for substring information */
3350 10, /* number of elements (NOT size in bytes) */
3351 wspace, /* working space vector */
3352 20); /* number of elements (NOT size in bytes) */
3353
3354 Option bits for pcre_dfa_exec()
3355
3356 The unused bits of the options argument for pcre_dfa_exec() must be
3357 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
3358 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
3359 PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF,
3360 PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR-
3361 TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
3362 four of these are exactly the same as for pcre_exec(), so their
3363 description is not repeated here.
3364
3365 PCRE_PARTIAL_HARD
3366 PCRE_PARTIAL_SOFT
3367
3368 These have the same general effect as they do for pcre_exec(), but the
3369 details are slightly different. When PCRE_PARTIAL_HARD is set for
3370 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
3371 ject is reached and there is still at least one matching possibility
3372 that requires additional characters. This happens even if some complete
3373 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
3374 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
3375 of the subject is reached, there have been no complete matches, but
3376 there is still at least one matching possibility. The portion of the
3377 string that was inspected when the longest partial match was found is
3378 set as the first matching string in both cases. There is a more
3379 detailed discussion of partial and multi-segment matching, with exam-
3380 ples, in the pcrepartial documentation.
3381
3382 PCRE_DFA_SHORTEST
3383
3384 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
3385 stop as soon as it has found one match. Because of the way the alterna-
3386 tive algorithm works, this is necessarily the shortest possible match
3387 at the first possible matching point in the subject string.
3388
3389 PCRE_DFA_RESTART
3390
3391 When pcre_dfa_exec() returns a partial match, it is possible to call it
3392 again, with additional subject characters, and have it continue with
3393 the same match. The PCRE_DFA_RESTART option requests this action; when
3394 it is set, the workspace and wscount options must reference the same
3395 vector as before because data about the match so far is left in them
3396 after a partial match. There is more discussion of this facility in the
3397 pcrepartial documentation.
3398
3399 Successful returns from pcre_dfa_exec()
3400
3401 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
3402 string in the subject. Note, however, that all the matches from one run
3403 of the function start at the same point in the subject. The shorter
3404 matches are all initial substrings of the longer matches. For example,
3405 if the pattern
3406
3407 <.*>
3408
3409 is matched against the string
3410
3411 This is <something> <something else> <something further> no more
3412
3413 the three matched strings are
3414
3415 <something>
3416 <something> <something else>
3417 <something> <something else> <something further>
3418
3419 On success, the yield of the function is a number greater than zero,
3420 which is the number of matched substrings. The substrings themselves
3421 are returned in ovector. Each string uses two elements; the first is
3422 the offset to the start, and the second is the offset to the end. In
3423 fact, all the strings have the same start offset. (Space could have
3424 been saved by giving this only once, but it was decided to retain some
3425 compatibility with the way pcre_exec() returns data, even though the
3426 meaning of the strings is different.)
3427
3428 The strings are returned in reverse order of length; that is, the long-
3429 est matching string is given first. If there were too many matches to
3430 fit into ovector, the yield of the function is zero, and the vector is
3431 filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec()
3432 can use the entire ovector for returning matched strings.
3433
3434 Error returns from pcre_dfa_exec()
3435
3436 The pcre_dfa_exec() function returns a negative number when it fails.
3437 Many of the errors are the same as for pcre_exec(), and these are
3438 described above. There are in addition the following errors that are
3439 specific to pcre_dfa_exec():
3440
3441 PCRE_ERROR_DFA_UITEM (-16)
3442
3443 This return is given if pcre_dfa_exec() encounters an item in the pat-
3444 tern that it does not support, for instance, the use of \C or a back
3445 reference.
3446
3447 PCRE_ERROR_DFA_UCOND (-17)
3448
3449 This return is given if pcre_dfa_exec() encounters a condition item
3450 that uses a back reference for the condition, or a test for recursion
3451 in a specific group. These are not supported.
3452
3453 PCRE_ERROR_DFA_UMLIMIT (-18)
3454
3455 This return is given if pcre_dfa_exec() is called with an extra block
3456 that contains a setting of the match_limit or match_limit_recursion
3457 fields. This is not supported (these fields are meaningless for DFA
3458 matching).
3459
3460 PCRE_ERROR_DFA_WSSIZE (-19)
3461
3462 This return is given if pcre_dfa_exec() runs out of space in the
3463 workspace vector.
3464
3465 PCRE_ERROR_DFA_RECURSE (-20)
3466
3467 When a recursive subpattern is processed, the matching function calls
3468 itself recursively, using private vectors for ovector and workspace.
3469 This error is given if the output vector is not large enough. This
3470 should be extremely rare, as a vector of size 1000 is used.
3471
3472 PCRE_ERROR_DFA_BADRESTART (-30)
3473
3474 When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some
3475 plausibility checks are made on the contents of the workspace, which
3476 should contain data about the previous partial match. If any of these
3477 checks fail, this error is given.
3478
3479
3480 SEE ALSO
3481
3482 pcre16(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematch-
3483 ing(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcresample(3),
3484 pcrestack(3).
3485
3486
3487 AUTHOR
3488
3489 Philip Hazel
3490 University Computing Service
3491 Cambridge CB2 3QH, England.
3492
3493
3494 REVISION
3495
3496 Last updated: 04 May 2012
3497 Copyright (c) 1997-2012 University of Cambridge.
3498 ------------------------------------------------------------------------------
3499
3500
3501 PCRECALLOUT(3) PCRECALLOUT(3)
3502
3503
3504 NAME
3505 PCRE - Perl-compatible regular expressions
3506
3507
3508 PCRE CALLOUTS
3509
3510 int (*pcre_callout)(pcre_callout_block *);
3511
3512 int (*pcre16_callout)(pcre16_callout_block *);
3513
3514 PCRE provides a feature called "callout", which is a means of temporar-
3515 ily passing control to the caller of PCRE in the middle of pattern
3516 matching. The caller of PCRE provides an external function by putting
3517 its entry point in the global variable pcre_callout (pcre16_callout for
3518 the 16-bit library). By default, this variable contains NULL, which
3519 disables all calling out.
3520
3521 Within a regular expression, (?C) indicates the points at which the
3522 external function is to be called. Different callout points can be
3523 identified by putting a number less than 256 after the letter C. The
3524 default value is zero. For example, this pattern has two callout
3525 points:
3526
3527 (?C1)abc(?C2)def
3528
3529 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled,
3530 PCRE automatically inserts callouts, all with number 255, before each
3531 item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the
3532 pattern
3533
3534 A(\d{2}|--)
3535
3536 it is processed as if it were
3537
3538 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
3539
3540 Notice that there is a callout before and after each parenthesis and
3541 alternation bar. Automatic callouts can be used for tracking the
3542 progress of pattern matching. The pcretest command has an option that
3543 sets automatic callouts; when it is used, the output indicates how the
3544 pattern is matched. This is useful information when you are trying to
3545 optimize the performance of a particular pattern.
3546
3547 The use of callouts in a pattern makes it ineligible for optimization
3548 by the just-in-time compiler. Studying such a pattern with the
3549 PCRE_STUDY_JIT_COMPILE option always fails.
3550
3551
3552 MISSING CALLOUTS
3553
3554 You should be aware that, because of optimizations in the way PCRE
3555 matches patterns by default, callouts sometimes do not happen. For
3556 example, if the pattern is
3557
3558 ab(?C4)cd
3559
3560 PCRE knows that any matching string must contain the letter "d". If the
3561 subject string is "abyz", the lack of "d" means that matching doesn't
3562 ever start, and the callout is never reached. However, with "abyd",
3563 though the result is still no match, the callout is obeyed.
3564
3565 If the pattern is studied, PCRE knows the minimum length of a matching
3566 string, and will immediately give a "no match" return without actually
3567 running a match if the subject is not long enough, or, for unanchored
3568 patterns, if it has been scanned far enough.
3569
3570 You can disable these optimizations by passing the PCRE_NO_START_OPTI-
3571 MIZE option to the matching function, or by starting the pattern with
3572 (*NO_START_OPT). This slows down the matching process, but does ensure
3573 that callouts such as the example above are obeyed.
3574
3575
3576 THE CALLOUT INTERFACE
3577
3578 During matching, when PCRE reaches a callout point, the external func-
3579 tion defined by pcre_callout or pcre16_callout is called (if it is
3580 set). This applies to both normal and DFA matching. The only argument
3581 to the callout function is a pointer to a pcre_callout or pcre16_call-
3582 out block. These structures contains the following fields:
3583
3584 int version;
3585 int callout_number;
3586 int *offset_vector;
3587 const char *subject; (8-bit version)
3588 PCRE_SPTR16 subject; (16-bit version)
3589 int subject_length;
3590 int start_match;
3591 int current_position;
3592 int capture_top;
3593 int capture_last;
3594 void *callout_data;
3595 int pattern_position;
3596 int next_item_length;
3597 const unsigned char *mark; (8-bit version)
3598 const PCRE_UCHAR16 *mark; (16-bit version)
3599
3600 The version field is an integer containing the version number of the
3601 block format. The initial version was 0; the current version is 2. The
3602 version number will change again in future if additional fields are
3603 added, but the intention is never to remove any of the existing fields.
3604
3605 The callout_number field contains the number of the callout, as com-
3606 piled into the pattern (that is, the number after ?C for manual call-
3607 outs, and 255 for automatically generated callouts).
3608
3609 The offset_vector field is a pointer to the vector of offsets that was
3610 passed by the caller to the matching function. When pcre_exec() or
3611 pcre16_exec() is used, the contents can be inspected, in order to
3612 extract substrings that have been matched so far, in the same way as
3613 for extracting substrings after a match has completed. For the DFA
3614 matching functions, this field is not useful.
3615
3616 The subject and subject_length fields contain copies of the values that
3617 were passed to the matching function.
3618
3619 The start_match field normally contains the offset within the subject
3620 at which the current match attempt started. However, if the escape
3621 sequence \K has been encountered, this value is changed to reflect the
3622 modified starting point. If the pattern is not anchored, the callout
3623 function may be called several times from the same point in the pattern
3624 for different starting points in the subject.
3625
3626 The current_position field contains the offset within the subject of
3627 the current match pointer.
3628
3629 When the pcre_exec() or pcre16_exec() is used, the capture_top field
3630 contains one more than the number of the highest numbered captured sub-
3631 string so far. If no substrings have been captured, the value of cap-
3632 ture_top is one. This is always the case when the DFA functions are
3633 used, because they do not support captured substrings.
3634
3635 The capture_last field contains the number of the most recently cap-
3636 tured substring. If no substrings have been captured, its value is -1.
3637 This is always the case for the DFA matching functions.
3638
3639 The callout_data field contains a value that is passed to a matching
3640 function specifically so that it can be passed back in callouts. It is
3641 passed in the callout_data field of a pcre_extra or pcre16_extra data
3642 structure. If no such data was passed, the value of callout_data in a
3643 callout block is NULL. There is a description of the pcre_extra struc-
3644 ture in the pcreapi documentation.
3645
3646 The pattern_position field is present from version 1 of the callout
3647 structure. It contains the offset to the next item to be matched in the
3648 pattern string.
3649
3650 The next_item_length field is present from version 1 of the callout
3651 structure. It contains the length of the next item to be matched in the
3652 pattern string. When the callout immediately precedes an alternation
3653 bar, a closing parenthesis, or the end of the pattern, the length is
3654 zero. When the callout precedes an opening parenthesis, the length is
3655 that of the entire subpattern.
3656
3657 The pattern_position and next_item_length fields are intended to help
3658 in distinguishing between different automatic callouts, which all have
3659 the same callout number. However, they are set for all callouts.
3660
3661 The mark field is present from version 2 of the callout structure. In
3662 callouts from pcre_exec() or pcre16_exec() it contains a pointer to the
3663 zero-terminated name of the most recently passed (*MARK), (*PRUNE), or
3664 (*THEN) item in the match, or NULL if no such items have been passed.
3665 Instances of (*PRUNE) or (*THEN) without a name do not obliterate a
3666 previous (*MARK). In callouts from the DFA matching functions this
3667 field always contains NULL.
3668
3669
3670 RETURN VALUES
3671
3672 The external callout function returns an integer to PCRE. If the value
3673 is zero, matching proceeds as normal. If the value is greater than
3674 zero, matching fails at the current point, but the testing of other
3675 matching possibilities goes ahead, just as if a lookahead assertion had
3676 failed. If the value is less than zero, the match is abandoned, the
3677 matching function returns the negative value.
3678
3679 Negative values should normally be chosen from the set of
3680 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
3681 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
3682 reserved for use by callout functions; it will never be used by PCRE
3683 itself.
3684
3685
3686 AUTHOR
3687
3688 Philip Hazel
3689 University Computing Service
3690 Cambridge CB2 3QH, England.
3691
3692
3693 REVISION
3694
3695 Last updated: 08 Janurary 2012
3696 Copyright (c) 1997-2012 University of Cambridge.
3697 ------------------------------------------------------------------------------
3698
3699
3700 PCRECOMPAT(3) PCRECOMPAT(3)
3701
3702
3703 NAME
3704 PCRE - Perl-compatible regular expressions
3705
3706
3707 DIFFERENCES BETWEEN PCRE AND PERL
3708
3709 This document describes the differences in the ways that PCRE and Perl
3710 handle regular expressions. The differences described here are with
3711 respect to Perl versions 5.10 and above.
3712
3713 1. PCRE has only a subset of Perl's Unicode support. Details of what it
3714 does have are given in the pcreunicode page.
3715
3716 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
3717 they do not mean what you might think. For example, (?!a){3} does not
3718 assert that the next three characters are not "a". It just asserts that
3719 the next character is not "a" three times (in principle: PCRE optimizes
3720 this to run the assertion just once). Perl allows repeat quantifiers on
3721 other assertions such as \b, but these do not seem to have any use.
3722
3723 3. Capturing subpatterns that occur inside negative lookahead asser-
3724 tions are counted, but their entries in the offsets vector are never
3725 set. Perl sets its numerical variables from any such patterns that are
3726 matched before the assertion fails to match something (thereby succeed-
3727 ing), but only if the negative lookahead assertion contains just one
3728 branch.
3729
3730 4. Though binary zero characters are supported in the subject string,
3731 they are not allowed in a pattern string because it is passed as a nor-
3732 mal C string, terminated by zero. The escape sequence \0 can be used in
3733 the pattern to represent a binary zero.
3734
3735 5. The following Perl escape sequences are not supported: \l, \u, \L,
3736 \U, and \N when followed by a character name or Unicode value. (\N on
3737 its own, matching a non-newline character, is supported.) In fact these
3738 are implemented by Perl's general string-handling and are not part of
3739 its pattern matching engine. If any of these are encountered by PCRE,
3740 an error is generated by default. However, if the PCRE_JAVASCRIPT_COM-
3741 PAT option is set, \U and \u are interpreted as JavaScript interprets
3742 them.
3743
3744 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
3745 is built with Unicode character property support. The properties that
3746 can be tested with \p and \P are limited to the general category prop-
3747 erties such as Lu and Nd, script names such as Greek or Han, and the
3748 derived properties Any and L&. PCRE does support the Cs (surrogate)
3749 property, which Perl does not; the Perl documentation says "Because
3750 Perl hides the need for the user to understand the internal representa-
3751 tion of Unicode characters, there is no need to implement the somewhat
3752 messy concept of surrogates."
3753
3754 7. PCRE implements a simpler version of \X than Perl, which changed to
3755 make \X match what Unicode calls an "extended grapheme cluster". This
3756 is more complicated than an extended Unicode sequence, which is what
3757 PCRE matches.
3758
3759 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
3760 ters in between are treated as literals. This is slightly different
3761 from Perl in that $ and @ are also handled as literals inside the
3762 quotes. In Perl, they cause variable interpolation (but of course PCRE
3763 does not have variables). Note the following examples:
3764
3765 Pattern PCRE matches Perl matches
3766
3767 \Qabc$xyz\E abc$xyz abc followed by the
3768 contents of $xyz
3769 \Qabc\$xyz\E abc\$xyz abc\$xyz
3770 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3771
3772 The \Q...\E sequence is recognized both inside and outside character
3773 classes.
3774
3775 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3776 constructions. However, there is support for recursive patterns. This
3777 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3778 "callout" feature allows an external function to be called during pat-
3779 tern matching. See the pcrecallout documentation for details.
3780
3781 10. Subpatterns that are called as subroutines (whether or not recur-
3782 sively) are always treated as atomic groups in PCRE. This is like
3783 Python, but unlike Perl. Captured values that are set outside a sub-
3784 routine call can be reference from inside in PCRE, but not in Perl.
3785 There is a discussion that explains these differences in more detail in
3786 the section on recursion differences from Perl in the pcrepattern page.
3787
3788 11. If any of the backtracking control verbs are used in an assertion
3789 or in a subpattern that is called as a subroutine (whether or not
3790 recursively), their effect is confined to that subpattern; it does not
3791 extend to the surrounding pattern. This is not always the case in Perl.
3792 In particular, if (*THEN) is present in a group that is called as a
3793 subroutine, its action is limited to that group, even if the group does
3794 not contain any | characters. There is one exception to this: the name
3795 from a *(MARK), (*PRUNE), or (*THEN) that is encountered in a success-
3796 ful positive assertion is passed back when a match succeeds (compare
3797 capturing parentheses in assertions). Note that such subpatterns are
3798 processed as anchored at the point where they are tested.
3799
3800 12. There are some differences that are concerned with the settings of
3801 captured strings when part of a pattern is repeated. For example,
3802 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3803 unset, but in PCRE it is set to "b".
3804
3805 13. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3806 pattern names is not as general as Perl's. This is a consequence of the
3807 fact the PCRE works internally just with numbers, using an external ta-
3808 ble to translate between numbers and names. In particular, a pattern
3809 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3810 the same number but different names, is not supported, and causes an
3811 error at compile time. If it were allowed, it would not be possible to
3812 distinguish which parentheses matched, because both names map to cap-
3813 turing subpattern number 1. To avoid this confusing situation, an error
3814 is given at compile time.
3815
3816 14. Perl recognizes comments in some places that PCRE does not, for
3817 example, between the ( and ? at the start of a subpattern. If the /x
3818 modifier is set, Perl allows white space between ( and ? but PCRE never
3819 does, even if the PCRE_EXTENDED option is set.
3820
3821 15. PCRE provides some extensions to the Perl regular expression facil-
3822 ities. Perl 5.10 includes new features that are not in earlier ver-
3823 sions of Perl, some of which (such as named parentheses) have been in
3824 PCRE for some time. This list is with respect to Perl 5.10:
3825
3826 (a) Although lookbehind assertions in PCRE must match fixed length
3827 strings, each alternative branch of a lookbehind assertion can match a
3828 different length of string. Perl requires them all to have the same
3829 length.
3830
3831 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3832 meta-character matches only at the very end of the string.
3833
3834 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3835 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3836 ignored. (Perl can be made to issue a warning.)
3837
3838 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3839 fiers is inverted, that is, by default they are not greedy, but if fol-
3840 lowed by a question mark they are.
3841
3842 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3843 tried only at the first matching position in the subject string.
3844
3845 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
3846 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3847 lents.
3848
3849 (g) The \R escape sequence can be restricted to match only CR, LF, or
3850 CRLF by the PCRE_BSR_ANYCRLF option.
3851
3852 (h) The callout facility is PCRE-specific.
3853
3854 (i) The partial matching facility is PCRE-specific.
3855
3856 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3857 even on different hosts that have the other endianness. However, this
3858 does not apply to optimized data created by the just-in-time compiler.
3859
3860 (k) The alternative matching functions (pcre_dfa_exec() and
3861 pcre16_dfa_exec()) match in a different way and are not Perl-compati-
3862 ble.
3863
3864 (l) PCRE recognizes some special sequences such as (*CR) at the start
3865 of a pattern that set overall options that cannot be changed within the
3866 pattern.
3867
3868
3869 AUTHOR
3870
3871 Philip Hazel
3872 University Computing Service
3873 Cambridge CB2 3QH, England.
3874
3875
3876 REVISION
3877
3878 Last updated: 01 June 2012
3879 Copyright (c) 1997-2012 University of Cambridge.
3880 ------------------------------------------------------------------------------
3881
3882
3883 PCREPATTERN(3) PCREPATTERN(3)
3884
3885
3886 NAME
3887 PCRE - Perl-compatible regular expressions
3888
3889
3890 PCRE REGULAR EXPRESSION DETAILS
3891
3892 The syntax and semantics of the regular expressions that are supported
3893 by PCRE are described in detail below. There is a quick-reference syn-
3894 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3895 semantics as closely as it can. PCRE also supports some alternative
3896 regular expression syntax (which does not conflict with the Perl syn-
3897 tax) in order to provide some compatibility with regular expressions in
3898 Python, .NET, and Oniguruma.
3899
3900 Perl's regular expressions are described in its own documentation, and
3901 regular expressions in general are covered in a number of books, some
3902 of which have copious examples. Jeffrey Friedl's "Mastering Regular
3903 Expressions", published by O'Reilly, covers regular expressions in
3904 great detail. This description of PCRE's regular expressions is
3905 intended as reference material.
3906
3907 The original operation of PCRE was on strings of one-byte characters.
3908 However, there is now also support for UTF-8 strings in the original
3909 library, and a second library that supports 16-bit and UTF-16 character
3910 strings. To use these features, PCRE must be built to include appropri-
3911 ate support. When using UTF strings you must either call the compiling
3912 function with the PCRE_UTF8 or PCRE_UTF16 option, or the pattern must
3913 start with one of these special sequences:
3914
3915 (*UTF8)
3916 (*UTF16)
3917
3918 Starting a pattern with such a sequence is equivalent to setting the
3919 relevant option. This feature is not Perl-compatible. How setting a UTF
3920 mode affects pattern matching is mentioned in several places below.
3921 There is also a summary of features in the pcreunicode page.
3922
3923 Another special sequence that may appear at the start of a pattern or
3924 in combination with (*UTF8) or (*UTF16) is:
3925
3926 (*UCP)
3927
3928 This has the same effect as setting the PCRE_UCP option: it causes
3929 sequences such as \d and \w to use Unicode properties to determine
3930 character types, instead of recognizing only characters with codes less
3931 than 128 via a lookup table.
3932
3933 If a pattern starts with (*NO_START_OPT), it has the same effect as
3934 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
3935 time. There are also some more of these special sequences that are con-
3936 cerned with the handling of newlines; they are described below.
3937
3938 The remainder of this document discusses the patterns that are sup-
3939 ported by PCRE when one its main matching functions, pcre_exec()
3940 (8-bit) or pcre16_exec() (16-bit), is used. PCRE also has alternative
3941 matching functions, pcre_dfa_exec() and pcre16_dfa_exec(), which match
3942 using a different algorithm that is not Perl-compatible. Some of the
3943 features discussed below are not available when DFA matching is used.
3944 The advantages and disadvantages of the alternative functions, and how
3945 they differ from the normal functions, are discussed in the pcrematch-
3946 ing page.
3947
3948
3949 NEWLINE CONVENTIONS
3950
3951 PCRE supports five different conventions for indicating line breaks in
3952 strings: a single CR (carriage return) character, a single LF (line-
3953 feed) character, the two-character sequence CRLF, any of the three pre-
3954 ceding, or any Unicode newline sequence. The pcreapi page has further
3955 discussion about newlines, and shows how to set the newline convention
3956 in the options arguments for the compiling and matching functions.
3957
3958 It is also possible to specify a newline convention by starting a pat-
3959 tern string with one of the following five sequences:
3960
3961 (*CR) carriage return
3962 (*LF) linefeed
3963 (*CRLF) carriage return, followed by linefeed
3964 (*ANYCRLF) any of the three above
3965 (*ANY) all Unicode newline sequences
3966
3967 These override the default and the options given to the compiling func-
3968 tion. For example, on a Unix system where LF is the default newline
3969 sequence, the pattern
3970
3971 (*CR)a.b
3972
3973 changes the convention to CR. That pattern matches "a\nb" because LF is
3974 no longer a newline. Note that these special settings, which are not
3975 Perl-compatible, are recognized only at the very start of a pattern,
3976 and that they must be in upper case. If more than one of them is
3977 present, the last one is used.
3978
3979 The newline convention affects the interpretation of the dot metachar-
3980 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3981 ever, it does not affect what the \R escape sequence matches. By
3982 default, this is any Unicode newline sequence, for Perl compatibility.
3983 However, this can be changed; see the description of \R in the section
3984 entitled "Newline sequences" below. A change of \R setting can be com-
3985 bined with a change of newline convention.
3986
3987
3988 CHARACTERS AND METACHARACTERS
3989
3990 A regular expression is a pattern that is matched against a subject
3991 string from left to right. Most characters stand for themselves in a
3992 pattern, and match the corresponding characters in the subject. As a
3993 trivial example, the pattern
3994
3995 The quick brown fox
3996
3997 matches a portion of a subject string that is identical to itself. When
3998 caseless matching is specified (the PCRE_CASELESS option), letters are
3999 matched independently of case. In a UTF mode, PCRE always understands
4000 the concept of case for characters whose values are less than 128, so
4001 caseless matching is always possible. For characters with higher val-
4002 ues, the concept of case is supported if PCRE is compiled with Unicode
4003 property support, but not otherwise. If you want to use caseless
4004 matching for characters 128 and above, you must ensure that PCRE is
4005 compiled with Unicode property support as well as with UTF support.
4006
4007 The power of regular expressions comes from the ability to include
4008 alternatives and repetitions in the pattern. These are encoded in the
4009 pattern by the use of metacharacters, which do not stand for themselves
4010 but instead are interpreted in some special way.
4011
4012 There are two different sets of metacharacters: those that are recog-
4013 nized anywhere in the pattern except within square brackets, and those
4014 that are recognized within square brackets. Outside square brackets,
4015 the metacharacters are as follows:
4016
4017 \ general escape character with several uses
4018 ^ assert start of string (or line, in multiline mode)
4019 $ assert end of string (or line, in multiline mode)
4020 . match any character except newline (by default)
4021 [ start character class definition
4022 | start of alternative branch
4023 ( start subpattern
4024 ) end subpattern
4025 ? extends the meaning of (
4026 also 0 or 1 quantifier
4027 also quantifier minimizer
4028 * 0 or more quantifier
4029 + 1 or more quantifier
4030 also "possessive quantifier"
4031 { start min/max quantifier
4032
4033 Part of a pattern that is in square brackets is called a "character
4034 class". In a character class the only metacharacters are:
4035
4036 \ general escape character
4037 ^ negate the class, but only if the first character
4038 - indicates character range
4039 [ POSIX character class (only if followed by POSIX
4040 syntax)
4041 ] terminates the character class
4042
4043 The following sections describe the use of each of the metacharacters.
4044
4045
4046 BACKSLASH
4047
4048 The backslash character has several uses. Firstly, if it is followed by
4049 a character that is not a number or a letter, it takes away any special
4050 meaning that character may have. This use of backslash as an escape
4051 character applies both inside and outside character classes.
4052
4053 For example, if you want to match a * character, you write \* in the
4054 pattern. This escaping action applies whether or not the following
4055 character would otherwise be interpreted as a metacharacter, so it is
4056 always safe to precede a non-alphanumeric with backslash to specify
4057 that it stands for itself. In particular, if you want to match a back-
4058 slash, you write \\.
4059
4060 In a UTF mode, only ASCII numbers and letters have any special meaning
4061 after a backslash. All other characters (in particular, those whose
4062 codepoints are greater than 127) are treated as literals.
4063
4064 If a pattern is compiled with the PCRE_EXTENDED option, white space in
4065 the pattern (other than in a character class) and characters between a
4066 # outside a character class and the next newline are ignored. An escap-
4067 ing backslash can be used to include a white space or # character as
4068 part of the pattern.
4069
4070 If you want to remove the special meaning from a sequence of charac-
4071 ters, you can do so by putting them between \Q and \E. This is differ-
4072 ent from Perl in that $ and @ are handled as literals in \Q...\E
4073 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
4074 tion. Note the following examples:
4075
4076 Pattern PCRE matches Perl matches
4077
4078 \Qabc$xyz\E abc$xyz abc followed by the
4079 contents of $xyz
4080 \Qabc\$xyz\E abc\$xyz abc\$xyz
4081 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
4082
4083 The \Q...\E sequence is recognized both inside and outside character
4084 classes. An isolated \E that is not preceded by \Q is ignored. If \Q
4085 is not followed by \E later in the pattern, the literal interpretation
4086 continues to the end of the pattern (that is, \E is assumed at the
4087 end). If the isolated \Q is inside a character class, this causes an
4088 error, because the character class is not terminated.
4089
4090 Non-printing characters
4091
4092 A second use of backslash provides a way of encoding non-printing char-
4093 acters in patterns in a visible manner. There is no restriction on the
4094 appearance of non-printing characters, apart from the binary zero that
4095 terminates a pattern, but when a pattern is being prepared by text
4096 editing, it is often easier to use one of the following escape
4097 sequences than the binary character it represents:
4098
4099 \a alarm, that is, the BEL character (hex 07)
4100 \cx "control-x", where x is any ASCII character
4101 \e escape (hex 1B)
4102 \f form feed (hex 0C)
4103 \n linefeed (hex 0A)
4104 \r carriage return (hex 0D)
4105 \t tab (hex 09)
4106 \ddd character with octal code ddd, or back reference
4107 \xhh character with hex code hh
4108 \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
4109 \uhhhh character with hex code hhhh (JavaScript mode only)
4110
4111 The precise effect of \cx is as follows: if x is a lower case letter,
4112 it is converted to upper case. Then bit 6 of the character (hex 40) is
4113 inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
4114 is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
4115 has a value greater than 127, a compile-time error occurs. This locks
4116 out non-ASCII characters in all modes. (When PCRE is compiled in EBCDIC
4117 mode, all byte values are valid. A lower case letter is converted to
4118 upper case, and then the 0xc0 bits are flipped.)
4119
4120 By default, after \x, from zero to two hexadecimal digits are read
4121 (letters can be in upper or lower case). Any number of hexadecimal dig-
4122 its may appear between \x{ and }, but the character code is constrained
4123 as follows:
4124
4125 8-bit non-UTF mode less than 0x100
4126 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint
4127 16-bit non-UTF mode less than 0x10000
4128 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint
4129
4130 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
4131 called "surrogate" codepoints).
4132
4133 If characters other than hexadecimal digits appear between \x{ and },
4134 or if there is no terminating }, this form of escape is not recognized.
4135 Instead, the initial \x will be interpreted as a basic hexadecimal
4136 escape, with no following digits, giving a character whose value is
4137 zero.
4138
4139 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
4140 is as just described only when it is followed by two hexadecimal dig-
4141 its. Otherwise, it matches a literal "x" character. In JavaScript
4142 mode, support for code points greater than 256 is provided by \u, which
4143 must be followed by four hexadecimal digits; otherwise it matches a
4144 literal "u" character.
4145
4146 Characters whose value is less than 256 can be defined by either of the
4147 two syntaxes for \x (or by \u in JavaScript mode). There is no differ-
4148 ence in the way they are handled. For example, \xdc is exactly the same
4149 as \x{dc} (or \u00dc in JavaScript mode).
4150
4151 After \0 up to two further octal digits are read. If there are fewer
4152 than two digits, just those that are present are used. Thus the
4153 sequence \0\x\07 specifies two binary zeros followed by a BEL character
4154 (code value 7). Make sure you supply two digits after the initial zero
4155 if the pattern character that follows is itself an octal digit.
4156
4157 The handling of a backslash followed by a digit other than 0 is compli-
4158 cated. Outside a character class, PCRE reads it and any following dig-
4159 its as a decimal number. If the number is less than 10, or if there
4160 have been at least that many previous capturing left parentheses in the
4161 expression, the entire sequence is taken as a back reference. A
4162 description of how this works is given later, following the discussion
4163 of parenthesized subpatterns.
4164
4165 Inside a character class, or if the decimal number is greater than 9
4166 and there have not been that many capturing subpatterns, PCRE re-reads
4167 up to three octal digits following the backslash, and uses them to gen-
4168 erate a data character. Any subsequent digits stand for themselves. The
4169 value of the character is constrained in the same way as characters
4170 specified in hexadecimal. For example:
4171
4172 \040 is another way of writing a space
4173 \40 is the same, provided there are fewer than 40
4174 previous capturing subpatterns
4175 \7 is always a back reference
4176 \11 might be a back reference, or another way of
4177 writing a tab
4178 \011 is always a tab
4179 \0113 is a tab followed by the character "3"
4180 \113 might be a back reference, otherwise the
4181 character with octal code 113
4182 \377 might be a back reference, otherwise
4183 the value 255 (decimal)
4184 \81 is either a back reference, or a binary zero
4185 followed by the two characters "8" and "1"
4186
4187 Note that octal values of 100 or greater must not be introduced by a
4188 leading zero, because no more than three octal digits are ever read.
4189
4190 All the sequences that define a single character value can be used both
4191 inside and outside character classes. In addition, inside a character
4192 class, \b is interpreted as the backspace character (hex 08).
4193
4194 \N is not allowed in a character class. \B, \R, and \X are not special
4195 inside a character class. Like other unrecognized escape sequences,
4196 they are treated as the literal characters "B", "R", and "X" by
4197 default, but cause an error if the PCRE_EXTRA option is set. Outside a
4198 character class, these sequences have different meanings.
4199
4200 Unsupported escape sequences
4201
4202 In Perl, the sequences \l, \L, \u, and \U are recognized by its string
4203 handler and used to modify the case of following characters. By
4204 default, PCRE does not support these escape sequences. However, if the
4205 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
4206 \u can be used to define a character by code point, as described in the
4207 previous section.
4208
4209 Absolute and relative back references
4210
4211 The sequence \g followed by an unsigned or a negative number, option-
4212 ally enclosed in braces, is an absolute or relative back reference. A
4213 named back reference can be coded as \g{name}. Back references are dis-
4214 cussed later, following the discussion of parenthesized subpatterns.
4215
4216 Absolute and relative subroutine calls
4217
4218 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
4219 name or a number enclosed either in angle brackets or single quotes, is
4220 an alternative syntax for referencing a subpattern as a "subroutine".
4221 Details are discussed later. Note that \g{...} (Perl syntax) and
4222 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
4223 reference; the latter is a subroutine call.
4224
4225 Generic character types
4226
4227 Another use of backslash is for specifying generic character types:
4228
4229 \d any decimal digit
4230 \D any character that is not a decimal digit
4231 \h any horizontal white space character
4232 \H any character that is not a horizontal white space character
4233 \s any white space character
4234 \S any character that is not a white space character
4235 \v any vertical white space character
4236 \V any character that is not a vertical white space character
4237 \w any "word" character
4238 \W any "non-word" character
4239
4240 There is also the single sequence \N, which matches a non-newline char-
4241 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
4242 not set. Perl also uses \N to match characters by name; PCRE does not
4243 support this.
4244
4245 Each pair of lower and upper case escape sequences partitions the com-
4246 plete set of characters into two disjoint sets. Any given character
4247 matches one, and only one, of each pair. The sequences can appear both
4248 inside and outside character classes. They each match one character of
4249 the appropriate type. If the current matching point is at the end of
4250 the subject string, all of them fail, because there is no character to
4251 match.
4252
4253 For compatibility with Perl, \s does not match the VT character (code
4254 11). This makes it different from the the POSIX "space" class. The \s
4255 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
4256 "use locale;" is included in a Perl script, \s may match the VT charac-
4257 ter. In PCRE, it never does.
4258
4259 A "word" character is an underscore or any character that is a letter
4260 or digit. By default, the definition of letters and digits is con-
4261 trolled by PCRE's low-valued character tables, and may vary if locale-
4262 specific matching is taking place (see "Locale support" in the pcreapi
4263 page). For example, in a French locale such as "fr_FR" in Unix-like
4264 systems, or "french" in Windows, some character codes greater than 128
4265 are used for accented letters, and these are then matched by \w. The
4266 use of locales with Unicode is discouraged.
4267
4268 By default, in a UTF mode, characters with values greater than 128
4269 never match \d, \s, or \w, and always match \D, \S, and \W. These
4270 sequences retain their original meanings from before UTF support was
4271 available, mainly for efficiency reasons. However, if PCRE is compiled
4272 with Unicode property support, and the PCRE_UCP option is set, the be-
4273 haviour is changed so that Unicode properties are used to determine
4274 character types, as follows:
4275
4276 \d any character that \p{Nd} matches (decimal digit)
4277 \s any character that \p{Z} matches, plus HT, LF, FF, CR
4278 \w any character that \p{L} or \p{N} matches, plus underscore
4279
4280 The upper case escapes match the inverse sets of characters. Note that
4281 \d matches only decimal digits, whereas \w matches any Unicode digit,
4282 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
4283 affects \b, and \B because they are defined in terms of \w and \W.
4284 Matching these sequences is noticeably slower when PCRE_UCP is set.
4285
4286 The sequences \h, \H, \v, and \V are features that were added to Perl
4287 at release 5.10. In contrast to the other sequences, which match only
4288 ASCII characters by default, these always match certain high-valued
4289 codepoints, whether or not PCRE_UCP is set. The horizontal space char-
4290 acters are:
4291
4292 U+0009 Horizontal tab
4293 U+0020 Space
4294 U+00A0 Non-break space
4295 U+1680 Ogham space mark
4296 U+180E Mongolian vowel separator
4297 U+2000 En quad
4298 U+2001 Em quad
4299 U+2002 En space
4300 U+2003 Em space
4301 U+2004 Three-per-em space
4302 U+2005 Four-per-em space
4303 U+2006 Six-per-em space
4304 U+2007 Figure space
4305 U+2008 Punctuation space
4306 U+2009 Thin space
4307 U+200A Hair space
4308 U+202F Narrow no-break space
4309 U+205F Medium mathematical space
4310 U+3000 Ideographic space
4311
4312 The vertical space characters are:
4313
4314 U+000A Linefeed
4315 U+000B Vertical tab
4316 U+000C Form feed
4317 U+000D Carriage return
4318 U+0085 Next line
4319 U+2028 Line separator
4320 U+2029 Paragraph separator
4321
4322 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than
4323 256 are relevant.
4324
4325 Newline sequences
4326
4327 Outside a character class, by default, the escape sequence \R matches
4328 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
4329 to the following:
4330
4331 (?>\r\n|\n|\x0b|\f|\r|\x85)
4332
4333 This is an example of an "atomic group", details of which are given
4334 below. This particular group matches either the two-character sequence
4335 CR followed by LF, or one of the single characters LF (linefeed,
4336 U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car-
4337 riage return, U+000D), or NEL (next line, U+0085). The two-character
4338 sequence is treated as a single unit that cannot be split.
4339
4340 In other modes, two additional characters whose codepoints are greater
4341 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
4342 rator, U+2029). Unicode character property support is not needed for
4343 these characters to be recognized.
4344
4345 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
4346 the complete set of Unicode line endings) by setting the option
4347 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
4348 (BSR is an abbrevation for "backslash R".) This can be made the default
4349 when PCRE is built; if this is the case, the other behaviour can be
4350 requested via the PCRE_BSR_UNICODE option. It is also possible to
4351 specify these settings by starting a pattern string with one of the
4352 following sequences:
4353
4354 (*BSR_ANYCRLF) CR, LF, or CRLF only
4355 (*BSR_UNICODE) any Unicode newline sequence
4356
4357 These override the default and the options given to the compiling func-
4358 tion, but they can themselves be overridden by options given to a
4359 matching function. Note that these special settings, which are not
4360 Perl-compatible, are recognized only at the very start of a pattern,
4361 and that they must be in upper case. If more than one of them is
4362 present, the last one is used. They can be combined with a change of
4363 newline convention; for example, a pattern can start with:
4364
4365 (*ANY)(*BSR_ANYCRLF)
4366
4367 They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
4368 sequences. Inside a character class, \R is treated as an unrecognized
4369 escape sequence, and so matches the letter "R" by default, but causes
4370 an error if PCRE_EXTRA is set.
4371
4372 Unicode character properties
4373
4374 When PCRE is built with Unicode character property support, three addi-
4375 tional escape sequences that match characters with specific properties
4376 are available. When in 8-bit non-UTF-8 mode, these sequences are of
4377 course limited to testing characters whose codepoints are less than
4378 256, but they do work in this mode. The extra escape sequences are:
4379
4380 \p{xx} a character with the xx property
4381 \P{xx} a character without the xx property
4382 \X an extended Unicode sequence
4383
4384 The property names represented by xx above are limited to the Unicode
4385 script names, the general category properties, "Any", which matches any
4386 character (including newline), and some special PCRE properties
4387 (described in the next section). Other Perl properties such as "InMu-
4388 sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
4389 does not match any characters, so always causes a match failure.
4390
4391 Sets of Unicode characters are defined as belonging to certain scripts.
4392 A character from one of these sets can be matched using a script name.
4393 For example:
4394
4395 \p{Greek}
4396 \P{Han}
4397
4398 Those that are not part of an identified script are lumped together as
4399 "Common". The current list of scripts is:
4400
4401 Arabic, Armenian, Avestan, Balinese, Bamum, Batak, Bengali, Bopomofo,
4402 Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Chakma,
4403 Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
4404 Devanagari, Egyptian_Hieroglyphs, Ethiopic, Georgian, Glagolitic,
4405 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
4406 gana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip-
4407 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li,
4408 Kharoshthi, Khmer, Lao, Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian,
4409 Lydian, Malayalam, Mandaic, Meetei_Mayek, Meroitic_Cursive,
4410 Meroitic_Hieroglyphs, Miao, Mongolian, Myanmar, New_Tai_Lue, Nko,
4411 Ogham, Old_Italic, Old_Persian, Old_South_Arabian, Old_Turkic,
4412 Ol_Chiki, Oriya, Osmanya, Phags_Pa, Phoenician, Rejang, Runic, Samari-
4413 tan, Saurashtra, Sharada, Shavian, Sinhala, Sora_Sompeng, Sundanese,
4414 Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet,
4415 Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Vai,
4416 Yi.
4417
4418 Each character has exactly one Unicode general category property, spec-
4419 ified by a two-letter abbreviation. For compatibility with Perl, nega-
4420 tion can be specified by including a circumflex between the opening
4421 brace and the property name. For example, \p{^Lu} is the same as
4422 \P{Lu}.
4423
4424 If only one letter is specified with \p or \P, it includes all the gen-
4425 eral category properties that start with that letter. In this case, in
4426 the absence of negation, the curly brackets in the escape sequence are
4427 optional; these two examples have the same effect:
4428
4429 \p{L}
4430 \pL
4431
4432 The following general category property codes are supported:
4433
4434 C Other
4435 Cc Control
4436 Cf Format
4437 Cn Unassigned
4438 Co Private use
4439 Cs Surrogate
4440
4441 L Letter
4442 Ll Lower case letter
4443 Lm Modifier letter
4444 Lo Other letter
4445 Lt Title case letter
4446 Lu Upper case letter
4447
4448 M Mark
4449 Mc Spacing mark
4450 Me Enclosing mark
4451 Mn Non-spacing mark
4452
4453 N Number
4454 Nd Decimal number
4455 Nl Letter number
4456 No Other number
4457
4458 P Punctuation
4459 Pc Connector punctuation
4460 Pd Dash punctuation
4461 Pe Close punctuation
4462 Pf Final punctuation
4463 Pi Initial punctuation
4464 Po Other punctuation
4465 Ps Open punctuation
4466
4467 S Symbol
4468 Sc Currency symbol
4469 Sk Modifier symbol
4470 Sm Mathematical symbol
4471 So Other symbol
4472
4473 Z Separator
4474 Zl Line separator
4475 Zp Paragraph separator
4476 Zs Space separator
4477
4478 The special property L& is also supported: it matches a character that
4479 has the Lu, Ll, or Lt property, in other words, a letter that is not
4480 classified as a modifier or "other".
4481
4482 The Cs (Surrogate) property applies only to characters in the range
4483 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
4484 so cannot be tested by PCRE, unless UTF validity checking has been
4485 turned off (see the discussion of PCRE_NO_UTF8_CHECK and
4486 PCRE_NO_UTF16_CHECK in the pcreapi page). Perl does not support the Cs
4487 property.
4488
4489 The long synonyms for property names that Perl supports (such as
4490 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
4491 any of these properties with "Is".
4492
4493 No character that is in the Unicode table has the Cn (unassigned) prop-
4494 erty. Instead, this property is assumed for any code point that is not
4495 in the Unicode table.
4496
4497 Specifying caseless matching does not affect these escape sequences.
4498 For example, \p{Lu} always matches only upper case letters.
4499
4500 The \X escape matches any number of Unicode characters that form an
4501 extended Unicode sequence. \X is equivalent to
4502
4503 (?>\PM\pM*)
4504
4505 That is, it matches a character without the "mark" property, followed
4506 by zero or more characters with the "mark" property, and treats the
4507 sequence as an atomic group (see below). Characters with the "mark"
4508 property are typically accents that affect the preceding character.
4509 None of them have codepoints less than 256, so in 8-bit non-UTF-8 mode
4510 \X matches any one character.
4511
4512 Note that recent versions of Perl have changed \X to match what Unicode
4513 calls an "extended grapheme cluster", which has a more complicated def-
4514 inition.
4515
4516 Matching characters by Unicode property is not fast, because PCRE has
4517 to search a structure that contains data for over fifteen thousand
4518 characters. That is why the traditional escape sequences such as \d and
4519 \w do not use Unicode properties in PCRE by default, though you can
4520 make them do so by setting the PCRE_UCP option or by starting the pat-
4521 tern with (*UCP).
4522
4523 PCRE's additional properties
4524
4525 As well as the standard Unicode properties described in the previous
4526 section, PCRE supports four more that make it possible to convert tra-
4527 ditional escape sequences such as \w and \s and POSIX character classes
4528 to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
4529 erties internally when PCRE_UCP is set. They are:
4530
4531 Xan Any alphanumeric character
4532 Xps Any POSIX space character
4533 Xsp Any Perl space character
4534 Xwd Any Perl "word" character
4535
4536 Xan matches characters that have either the L (letter) or the N (num-
4537 ber) property. Xps matches the characters tab, linefeed, vertical tab,
4538 form feed, or carriage return, and any other character that has the Z
4539 (separator) property. Xsp is the same as Xps, except that vertical tab
4540 is excluded. Xwd matches the same characters as Xan, plus underscore.
4541
4542 Resetting the match start
4543
4544 The escape sequence \K causes any previously matched characters not to
4545 be included in the final matched sequence. For example, the pattern:
4546
4547 foo\Kbar
4548
4549 matches "foobar", but reports that it has matched "bar". This feature
4550 is similar to a lookbehind assertion (described below). However, in
4551 this case, the part of the subject before the real match does not have
4552 to be of fixed length, as lookbehind assertions do. The use of \K does
4553 not interfere with the setting of captured substrings. For example,
4554 when the pattern
4555
4556 (foo)\Kbar
4557
4558 matches "foobar", the first substring is still set to "foo".
4559
4560 Perl documents that the use of \K within assertions is "not well
4561 defined". In PCRE, \K is acted upon when it occurs inside positive
4562 assertions, but is ignored in negative assertions.
4563
4564 Simple assertions
4565
4566 The final use of backslash is for certain simple assertions. An asser-
4567 tion specifies a condition that has to be met at a particular point in
4568 a match, without consuming any characters from the subject string. The
4569 use of subpatterns for more complicated assertions is described below.
4570 The backslashed assertions are:
4571
4572 \b matches at a word boundary
4573 \B matches when not at a word boundary
4574 \A matches at the start of the subject
4575 \Z matches at the end of the subject
4576 also matches before a newline at the end of the subject
4577 \z matches only at the end of the subject
4578 \G matches at the first matching position in the subject
4579
4580 Inside a character class, \b has a different meaning; it matches the
4581 backspace character. If any other of these assertions appears in a
4582 character class, by default it matches the corresponding literal char-
4583 acter (for example, \B matches the letter B). However, if the
4584 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
4585 ated instead.
4586
4587 A word boundary is a position in the subject string where the current
4588 character and the previous character do not both match \w or \W (i.e.
4589 one matches \w and the other matches \W), or the start or end of the
4590 string if the first or last character matches \w, respectively. In a
4591 UTF mode, the meanings of \w and \W can be changed by setting the
4592 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
4593 PCRE nor Perl has a separate "start of word" or "end of word" metase-
4594 quence. However, whatever follows \b normally determines which it is.
4595 For example, the fragment \ba matches "a" at the start of a word.
4596
4597 The \A, \Z, and \z assertions differ from the traditional circumflex
4598 and dollar (described in the next section) in that they only ever match
4599 at the very start and end of the subject string, whatever options are
4600 set. Thus, they are independent of multiline mode. These three asser-
4601 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
4602 affect only the behaviour of the circumflex and dollar metacharacters.
4603 However, if the startoffset argument of pcre_exec() is non-zero, indi-
4604 cating that matching is to start at a point other than the beginning of
4605 the subject, \A can never match. The difference between \Z and \z is
4606 that \Z matches before a newline at the end of the string as well as at
4607 the very end, whereas \z matches only at the end.
4608
4609 The \G assertion is true only when the current matching position is at
4610 the start point of the match, as specified by the startoffset argument
4611 of pcre_exec(). It differs from \A when the value of startoffset is
4612 non-zero. By calling pcre_exec() multiple times with appropriate argu-
4613 ments, you can mimic Perl's /g option, and it is in this kind of imple-
4614 mentation where \G can be useful.
4615
4616 Note, however, that PCRE's interpretation of \G, as the start of the
4617 current match, is subtly different from Perl's, which defines it as the
4618 end of the previous match. In Perl, these can be different when the
4619 previously matched string was empty. Because PCRE does just one match
4620 at a time, it cannot reproduce this behaviour.
4621
4622 If all the alternatives of a pattern begin with \G, the expression is
4623 anchored to the starting match position, and the "anchored" flag is set
4624 in the compiled regular expression.
4625
4626
4627 CIRCUMFLEX AND DOLLAR
4628
4629 Outside a character class, in the default matching mode, the circumflex
4630 character is an assertion that is true only if the current matching
4631 point is at the start of the subject string. If the startoffset argu-
4632 ment of pcre_exec() is non-zero, circumflex can never match if the
4633 PCRE_MULTILINE option is unset. Inside a character class, circumflex
4634 has an entirely different meaning (see below).
4635
4636 Circumflex need not be the first character of the pattern if a number
4637 of alternatives are involved, but it should be the first thing in each
4638 alternative in which it appears if the pattern is ever to match that
4639 branch. If all possible alternatives start with a circumflex, that is,
4640 if the pattern is constrained to match only at the start of the sub-
4641 ject, it is said to be an "anchored" pattern. (There are also other
4642 constructs that can cause a pattern to be anchored.)
4643
4644 A dollar character is an assertion that is true only if the current
4645 matching point is at the end of the subject string, or immediately
4646 before a newline at the end of the string (by default). Dollar need not
4647 be the last character of the pattern if a number of alternatives are
4648 involved, but it should be the last item in any branch in which it
4649 appears. Dollar has no special meaning in a character class.
4650
4651 The meaning of dollar can be changed so that it matches only at the
4652 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
4653 compile time. This does not affect the \Z assertion.
4654
4655 The meanings of the circumflex and dollar characters are changed if the
4656 PCRE_MULTILINE option is set. When this is the case, a circumflex
4657 matches immediately after internal newlines as well as at the start of
4658 the subject string. It does not match after a newline that ends the
4659 string. A dollar matches before any newlines in the string, as well as
4660 at the very end, when PCRE_MULTILINE is set. When newline is specified
4661 as the two-character sequence CRLF, isolated CR and LF characters do
4662 not indicate newlines.
4663
4664 For example, the pattern /^abc$/ matches the subject string "def\nabc"
4665 (where \n represents a newline) in multiline mode, but not otherwise.
4666 Consequently, patterns that are anchored in single line mode because
4667 all branches start with ^ are not anchored in multiline mode, and a
4668 match for circumflex is possible when the startoffset argument of
4669 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
4670 PCRE_MULTILINE is set.
4671
4672 Note that the sequences \A, \Z, and \z can be used to match the start
4673 and end of the subject in both modes, and if all branches of a pattern
4674 start with \A it is always anchored, whether or not PCRE_MULTILINE is
4675 set.
4676
4677
4678 FULL STOP (PERIOD, DOT) AND \N
4679
4680 Outside a character class, a dot in the pattern matches any one charac-
4681 ter in the subject string except (by default) a character that signi-
4682 fies the end of a line.
4683
4684 When a line ending is defined as a single character, dot never matches
4685 that character; when the two-character sequence CRLF is used, dot does
4686 not match CR if it is immediately followed by LF, but otherwise it
4687 matches all characters (including isolated CRs and LFs). When any Uni-
4688 code line endings are being recognized, dot does not match CR or LF or
4689 any of the other line ending characters.
4690
4691 The behaviour of dot with regard to newlines can be changed. If the
4692 PCRE_DOTALL option is set, a dot matches any one character, without
4693 exception. If the two-character sequence CRLF is present in the subject
4694 string, it takes two dots to match it.
4695
4696 The handling of dot is entirely independent of the handling of circum-
4697 flex and dollar, the only relationship being that they both involve
4698 newlines. Dot has no special meaning in a character class.
4699
4700 The escape sequence \N behaves like a dot, except that it is not
4701 affected by the PCRE_DOTALL option. In other words, it matches any
4702 character except one that signifies the end of a line. Perl also uses
4703 \N to match characters by name; PCRE does not support this.
4704
4705
4706 MATCHING A SINGLE DATA UNIT
4707
4708 Outside a character class, the escape sequence \C matches any one data
4709 unit, whether or not a UTF mode is set. In the 8-bit library, one data
4710 unit is one byte; in the 16-bit library it is a 16-bit unit. Unlike a
4711 dot, \C always matches line-ending characters. The feature is provided
4712 in Perl in order to match individual bytes in UTF-8 mode, but it is
4713 unclear how it can usefully be used. Because \C breaks up characters
4714 into individual data units, matching one unit with \C in a UTF mode
4715 means that the rest of the string may start with a malformed UTF char-
4716 acter. This has undefined results, because PCRE assumes that it is
4717 dealing with valid UTF strings (and by default it checks this at the
4718 start of processing unless the PCRE_NO_UTF8_CHECK or
4719 PCRE_NO_UTF16_CHECK option is used).
4720
4721 PCRE does not allow \C to appear in lookbehind assertions (described
4722 below) in a UTF mode, because this would make it impossible to calcu-
4723 late the length of the lookbehind.
4724
4725 In general, the \C escape sequence is best avoided. However, one way of
4726 using it that avoids the problem of malformed UTF characters is to use
4727 a lookahead to check the length of the next character, as in this pat-
4728 tern, which could be used with a UTF-8 string (ignore white space and
4729 line breaks):
4730
4731 (?| (?=[\x00-\x7f])(\C) |
4732 (?=[\x80-\x{7ff}])(\C)(\C) |
4733 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
4734 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
4735
4736 A group that starts with (?| resets the capturing parentheses numbers
4737 in each alternative (see "Duplicate Subpattern Numbers" below). The
4738 assertions at the start of each branch check the next UTF-8 character
4739 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
4740 character's individual bytes are then captured by the appropriate num-
4741 ber of groups.
4742
4743
4744 SQUARE BRACKETS AND CHARACTER CLASSES
4745
4746 An opening square bracket introduces a character class, terminated by a
4747 closing square bracket. A closing square bracket on its own is not spe-
4748 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
4749 a lone closing square bracket causes a compile-time error. If a closing
4750 square bracket is required as a member of the class, it should be the
4751 first data character in the class (after an initial circumflex, if
4752 present) or escaped with a backslash.
4753
4754 A character class matches a single character in the subject. In a UTF
4755 mode, the character may be more than one data unit long. A matched
4756 character must be in the set of characters defined by the class, unless
4757 the first character in the class definition is a circumflex, in which
4758 case the subject character must not be in the set defined by the class.
4759 If a circumflex is actually required as a member of the class, ensure
4760 it is not the first character, or escape it with a backslash.
4761
4762 For example, the character class [aeiou] matches any lower case vowel,
4763 while [^aeiou] matches any character that is not a lower case vowel.
4764 Note that a circumflex is just a convenient notation for specifying the
4765 characters that are in the class by enumerating those that are not. A
4766 class that starts with a circumflex is not an assertion; it still con-
4767 sumes a character from the subject string, and therefore it fails if
4768 the current pointer is at the end of the string.
4769
4770 In UTF-8 (UTF-16) mode, characters with values greater than 255
4771 (0xffff) can be included in a class as a literal string of data units,
4772 or by using the \x{ escaping mechanism.
4773
4774 When caseless matching is set, any letters in a class represent both
4775 their upper case and lower case versions, so for example, a caseless
4776 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
4777 match "A", whereas a caseful version would. In a UTF mode, PCRE always
4778 understands the concept of case for characters whose values are less
4779 than 128, so caseless matching is always possible. For characters with
4780 higher values, the concept of case is supported if PCRE is compiled
4781 with Unicode property support, but not otherwise. If you want to use
4782 caseless matching in a UTF mode for characters 128 and above, you must
4783 ensure that PCRE is compiled with Unicode property support as well as
4784 with UTF support.
4785
4786 Characters that might indicate line breaks are never treated in any
4787 special way when matching character classes, whatever line-ending
4788 sequence is in use, and whatever setting of the PCRE_DOTALL and
4789 PCRE_MULTILINE options is used. A class such as [^a] always matches one
4790 of these characters.
4791
4792 The minus (hyphen) character can be used to specify a range of charac-
4793 ters in a character class. For example, [d-m] matches any letter
4794 between d and m, inclusive. If a minus character is required in a
4795 class, it must be escaped with a backslash or appear in a position
4796 where it cannot be interpreted as indicating a range, typically as the
4797 first or last character in the class.
4798
4799 It is not possible to have the literal character "]" as the end charac-
4800 ter of a range. A pattern such as [W-]46] is interpreted as a class of
4801 two characters ("W" and "-") followed by a literal string "46]", so it
4802 would match "W46]" or "-46]". However, if the "]" is escaped with a
4803 backslash it is interpreted as the end of range, so [W-\]46] is inter-
4804 preted as a class containing a range followed by two other characters.
4805 The octal or hexadecimal representation of "]" can also be used to end
4806 a range.
4807
4808 Ranges operate in the collating sequence of character values. They can
4809 also be used for characters specified numerically, for example
4810 [\000-\037]. Ranges can include any characters that are valid for the
4811 current mode.
4812
4813 If a range that includes letters is used when caseless matching is set,
4814 it matches the letters in either case. For example, [W-c] is equivalent
4815 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if
4816 character tables for a French locale are in use, [\xc8-\xcb] matches
4817 accented E characters in both cases. In UTF modes, PCRE supports the
4818 concept of case for characters with values greater than 128 only when
4819 it is compiled with Unicode property support.
4820
4821 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
4822 \w, and \W may appear in a character class, and add the characters that
4823 they match to the class. For example, [\dABCDEF] matches any hexadeci-
4824 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of
4825 \d, \s, \w and their upper case partners, just as it does when they
4826 appear outside a character class, as described in the section entitled
4827 "Generic character types" above. The escape sequence \b has a different
4828 meaning inside a character class; it matches the backspace character.
4829 The sequences \B, \N, \R, and \X are not special inside a character
4830 class. Like any other unrecognized escape sequences, they are treated
4831 as the literal characters "B", "N", "R", and "X" by default, but cause
4832 an error if the PCRE_EXTRA option is set.
4833
4834 A circumflex can conveniently be used with the upper case character
4835 types to specify a more restricted set of characters than the matching
4836 lower case type. For example, the class [^\W_] matches any letter or
4837 digit, but not underscore, whereas [\w] includes underscore. A positive
4838 character class should be read as "something OR something OR ..." and a
4839 negative class as "NOT something AND NOT something AND NOT ...".
4840
4841 The only metacharacters that are recognized in character classes are
4842 backslash, hyphen (only where it can be interpreted as specifying a
4843 range), circumflex (only at the start), opening square bracket (only
4844 when it can be interpreted as introducing a POSIX class name - see the
4845 next section), and the terminating closing square bracket. However,
4846 escaping other non-alphanumeric characters does no harm.
4847
4848
4849 POSIX CHARACTER CLASSES
4850
4851 Perl supports the POSIX notation for character classes. This uses names
4852 enclosed by [: and :] within the enclosing square brackets. PCRE also
4853 supports this notation. For example,
4854
4855 [01[:alpha:]%]
4856
4857 matches "0", "1", any alphabetic character, or "%". The supported class
4858 names are:
4859
4860 alnum letters and digits
4861 alpha letters
4862 ascii character codes 0 - 127
4863 blank space or tab only
4864 cntrl control characters
4865 digit decimal digits (same as \d)
4866 graph printing characters, excluding space
4867 lower lower case letters
4868 print printing characters, including space
4869 punct printing characters, excluding letters and digits and space
4870 space white space (not quite the same as \s)
4871 upper upper case letters
4872 word "word" characters (same as \w)
4873 xdigit hexadecimal digits
4874
4875 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
4876 and space (32). Notice that this list includes the VT character (code
4877 11). This makes "space" different to \s, which does not include VT (for
4878 Perl compatibility).
4879
4880 The name "word" is a Perl extension, and "blank" is a GNU extension
4881 from Perl 5.8. Another Perl extension is negation, which is indicated
4882 by a ^ character after the colon. For example,
4883
4884 [12[:^digit:]]
4885
4886 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
4887 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4888 these are not supported, and an error is given if they are encountered.
4889
4890 By default, in UTF modes, characters with values greater than 128 do
4891 not match any of the POSIX character classes. However, if the PCRE_UCP
4892 option is passed to pcre_compile(), some of the classes are changed so
4893 that Unicode character properties are used. This is achieved by replac-
4894 ing the POSIX classes by other sequences, as follows:
4895
4896 [:alnum:] becomes \p{Xan}
4897 [:alpha:] becomes \p{L}
4898 [:blank:] becomes \h
4899 [:digit:] becomes \p{Nd}
4900 [:lower:] becomes \p{Ll}
4901 [:space:] becomes \p{Xps}
4902 [:upper:] becomes \p{Lu}
4903 [:word:] becomes \p{Xwd}
4904
4905 Negated versions, such as [:^alpha:] use \P instead of \p. The other
4906 POSIX classes are unchanged, and match only characters with code points
4907 less than 128.
4908
4909
4910 VERTICAL BAR
4911
4912 Vertical bar characters are used to separate alternative patterns. For
4913 example, the pattern
4914
4915 gilbert|sullivan
4916
4917 matches either "gilbert" or "sullivan". Any number of alternatives may
4918 appear, and an empty alternative is permitted (matching the empty
4919 string). The matching process tries each alternative in turn, from left
4920 to right, and the first one that succeeds is used. If the alternatives
4921 are within a subpattern (defined below), "succeeds" means matching the
4922 rest of the main pattern as well as the alternative in the subpattern.
4923
4924
4925 INTERNAL OPTION SETTING
4926
4927 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
4928 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
4929 within the pattern by a sequence of Perl option letters enclosed
4930 between "(?" and ")". The option letters are
4931
4932 i for PCRE_CASELESS
4933 m for PCRE_MULTILINE
4934 s for PCRE_DOTALL
4935 x for PCRE_EXTENDED
4936
4937 For example, (?im) sets caseless, multiline matching. It is also possi-
4938 ble to unset these options by preceding the letter with a hyphen, and a
4939 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
4940 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
4941 is also permitted. If a letter appears both before and after the
4942 hyphen, the option is unset.
4943
4944 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
4945 can be changed in the same way as the Perl-compatible options by using
4946 the characters J, U and X respectively.
4947
4948 When one of these option changes occurs at top level (that is, not
4949 inside subpattern parentheses), the change applies to the remainder of
4950 the pattern that follows. If the change is placed right at the start of
4951 a pattern, PCRE extracts it into the global options (and it will there-
4952 fore show up in data extracted by the pcre_fullinfo() function).
4953
4954 An option change within a subpattern (see below for a description of
4955 subpatterns) affects only that part of the subpattern that follows it,
4956 so
4957
4958 (a(?i)b)c
4959
4960 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
4961 used). By this means, options can be made to have different settings
4962 in different parts of the pattern. Any changes made in one alternative
4963 do carry on into subsequent branches within the same subpattern. For
4964 example,
4965
4966 (a(?i)b|c)
4967
4968 matches "ab", "aB", "c", and "C", even though when matching "C" the
4969 first branch is abandoned before the option setting. This is because
4970 the effects of option settings happen at compile time. There would be
4971 some very weird behaviour otherwise.
4972
4973 Note: There are other PCRE-specific options that can be set by the
4974 application when the compiling or matching functions are called. In
4975 some cases the pattern can contain special leading sequences such as
4976 (*CRLF) to override what the application has set or what has been
4977 defaulted. Details are given in the section entitled "Newline
4978 sequences" above. There are also the (*UTF8), (*UTF16), and (*UCP)
4979 leading sequences that can be used to set UTF and Unicode property
4980 modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, and
4981 the PCRE_UCP options, respectively.
4982
4983
4984 SUBPATTERNS
4985
4986 Subpatterns are delimited by parentheses (round brackets), which can be
4987 nested. Turning part of a pattern into a subpattern does two things:
4988
4989 1. It localizes a set of alternatives. For example, the pattern
4990
4991 cat(aract|erpillar|)
4992
4993 matches "cataract", "caterpillar", or "cat". Without the parentheses,
4994 it would match "cataract", "erpillar" or an empty string.
4995
4996 2. It sets up the subpattern as a capturing subpattern. This means
4997 that, when the whole pattern matches, that portion of the subject
4998 string that matched the subpattern is passed back to the caller via the
4999 ovector argument of the matching function. (This applies only to the
5000 traditional matching functions; the DFA matching functions do not sup-
5001 port capturing.)
5002
5003 Opening parentheses are counted from left to right (starting from 1) to
5004 obtain numbers for the capturing subpatterns. For example, if the
5005 string "the red king" is matched against the pattern
5006
5007 the ((red|white) (king|queen))
5008
5009 the captured substrings are "red king", "red", and "king", and are num-
5010 bered 1, 2, and 3, respectively.
5011
5012 The fact that plain parentheses fulfil two functions is not always
5013 helpful. There are often times when a grouping subpattern is required
5014 without a capturing requirement. If an opening parenthesis is followed
5015 by a question mark and a colon, the subpattern does not do any captur-
5016 ing, and is not counted when computing the number of any subsequent
5017 capturing subpatterns. For example, if the string "the white queen" is
5018 matched against the pattern
5019
5020 the ((?:red|white) (king|queen))
5021
5022 the captured substrings are "white queen" and "queen", and are numbered
5023 1 and 2. The maximum number of capturing subpatterns is 65535.
5024
5025 As a convenient shorthand, if any option settings are required at the
5026 start of a non-capturing subpattern, the option letters may appear
5027 between the "?" and the ":". Thus the two patterns
5028
5029 (?i:saturday|sunday)
5030 (?:(?i)saturday|sunday)
5031
5032 match exactly the same set of strings. Because alternative branches are
5033 tried from left to right, and options are not reset until the end of
5034 the subpattern is reached, an option setting in one branch does affect
5035 subsequent branches, so the above patterns match "SUNDAY" as well as
5036 "Saturday".
5037
5038
5039 DUPLICATE SUBPATTERN NUMBERS
5040
5041 Perl 5.10 introduced a feature whereby each alternative in a subpattern
5042 uses the same numbers for its capturing parentheses. Such a subpattern
5043 starts with (?| and is itself a non-capturing subpattern. For example,
5044 consider this pattern:
5045
5046 (?|(Sat)ur|(Sun))day
5047
5048 Because the two alternatives are inside a (?| group, both sets of cap-
5049 turing parentheses are numbered one. Thus, when the pattern matches,
5050 you can look at captured substring number one, whichever alternative
5051 matched. This construct is useful when you want to capture part, but
5052 not all, of one of a number of alternatives. Inside a (?| group, paren-
5053 theses are numbered as usual, but the number is reset at the start of
5054 each branch. The numbers of any capturing parentheses that follow the
5055 subpattern start after the highest number used in any branch. The fol-
5056 lowing example is taken from the Perl documentation. The numbers under-
5057 neath show in which buffer the captured content will be stored.
5058
5059 # before ---------------branch-reset----------- after
5060 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
5061 # 1 2 2 3 2 3 4
5062
5063 A back reference to a numbered subpattern uses the most recent value
5064 that is set for that number by any subpattern. The following pattern
5065 matches "abcabc" or "defdef":
5066
5067 /(?|(abc)|(def))\1/
5068
5069 In contrast, a subroutine call to a numbered subpattern always refers
5070 to the first one in the pattern with the given number. The following
5071 pattern matches "abcabc" or "defabc":
5072
5073 /(?|(abc)|(def))(?1)/
5074
5075 If a condition test for a subpattern's having matched refers to a non-
5076 unique number, the test is true if any of the subpatterns of that num-
5077 ber have matched.
5078
5079 An alternative approach to using this "branch reset" feature is to use
5080 duplicate named subpatterns, as described in the next section.
5081
5082
5083 NAMED SUBPATTERNS
5084
5085 Identifying capturing parentheses by number is simple, but it can be
5086 very hard to keep track of the numbers in complicated regular expres-
5087 sions. Furthermore, if an expression is modified, the numbers may
5088 change. To help with this difficulty, PCRE supports the naming of sub-
5089 patterns. This feature was not added to Perl until release 5.10. Python
5090 had the feature earlier, and PCRE introduced it at release 4.0, using
5091 the Python syntax. PCRE now supports both the Perl and the Python syn-
5092 tax. Perl allows identically numbered subpatterns to have different
5093 names, but PCRE does not.
5094
5095 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
5096 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
5097 to capturing parentheses from other parts of the pattern, such as back
5098 references, recursion, and conditions, can be made by name as well as
5099 by number.
5100
5101 Names consist of up to 32 alphanumeric characters and underscores.
5102 Named capturing parentheses are still allocated numbers as well as
5103 names, exactly as if the names were not present. The PCRE API provides
5104 function calls for extracting the name-to-number translation table from
5105 a compiled pattern. There is also a convenience function for extracting
5106 a captured substring by name.
5107
5108 By default, a name must be unique within a pattern, but it is possible
5109 to relax this constraint by setting the PCRE_DUPNAMES option at compile
5110 time. (Duplicate names are also always permitted for subpatterns with
5111 the same number, set up as described in the previous section.) Dupli-
5112 cate names can be useful for patterns where only one instance of the
5113 named parentheses can match. Suppose you want to match the name of a
5114 weekday, either as a 3-letter abbreviation or as the full name, and in
5115 both cases you want to extract the abbreviation. This pattern (ignoring
5116 the line breaks) does the job:
5117
5118 (?<DN>Mon|Fri|Sun)(?:day)?|
5119 (?<DN>Tue)(?:sday)?|
5120 (?<DN>Wed)(?:nesday)?|
5121 (?<DN>Thu)(?:rsday)?|
5122 (?<DN>Sat)(?:urday)?
5123
5124 There are five capturing substrings, but only one is ever set after a
5125 match. (An alternative way of solving this problem is to use a "branch
5126 reset" subpattern, as described in the previous section.)
5127
5128 The convenience function for extracting the data by name returns the
5129 substring for the first (and in this example, the only) subpattern of
5130 that name that matched. This saves searching to find which numbered
5131 subpattern it was.
5132
5133 If you make a back reference to a non-unique named subpattern from
5134 elsewhere in the pattern, the one that corresponds to the first occur-
5135 rence of the name is used. In the absence of duplicate numbers (see the
5136 previous section) this is the one with the lowest number. If you use a
5137 named reference in a condition test (see the section about conditions
5138 below), either to check whether a subpattern has matched, or to check
5139 for recursion, all subpatterns with the same name are tested. If the
5140 condition is true for any one of them, the overall condition is true.
5141 This is the same behaviour as testing by number. For further details of
5142 the interfaces for handling named subpatterns, see the pcreapi documen-
5143 tation.
5144
5145 Warning: You cannot use different names to distinguish between two sub-
5146 patterns with the same number because PCRE uses only the numbers when
5147 matching. For this reason, an error is given at compile time if differ-
5148 ent names are given to subpatterns with the same number. However, you
5149 can give the same name to subpatterns with the same number, even when
5150 PCRE_DUPNAMES is not set.
5151
5152
5153 REPETITION
5154
5155 Repetition is specified by quantifiers, which can follow any of the
5156 following items:
5157
5158 a literal data character
5159 the dot metacharacter
5160 the \C escape sequence
5161 the \X escape sequence
5162 the \R escape sequence
5163 an escape such as \d or \pL that matches a single character
5164 a character class
5165 a back reference (see next section)
5166 a parenthesized subpattern (including assertions)
5167 a subroutine call to a subpattern (recursive or otherwise)
5168
5169 The general repetition quantifier specifies a minimum and maximum num-
5170 ber of permitted matches, by giving the two numbers in curly brackets
5171 (braces), separated by a comma. The numbers must be less than 65536,
5172 and the first must be less than or equal to the second. For example:
5173
5174 z{2,4}
5175
5176 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
5177 special character. If the second number is omitted, but the comma is
5178 present, there is no upper limit; if the second number and the comma
5179 are both omitted, the quantifier specifies an exact number of required
5180 matches. Thus
5181
5182 [aeiou]{3,}
5183
5184 matches at least 3 successive vowels, but may match many more, while
5185
5186 \d{8}
5187
5188 matches exactly 8 digits. An opening curly bracket that appears in a
5189 position where a quantifier is not allowed, or one that does not match
5190 the syntax of a quantifier, is taken as a literal character. For exam-
5191 ple, {,6} is not a quantifier, but a literal string of four characters.
5192
5193 In UTF modes, quantifiers apply to characters rather than to individual
5194 data units. Thus, for example, \x{100}{2} matches two characters, each
5195 of which is represented by a two-byte sequence in a UTF-8 string. Simi-
5196 larly, \X{3} matches three Unicode extended sequences, each of which
5197 may be several data units long (and they may be of different lengths).
5198
5199 The quantifier {0} is permitted, causing the expression to behave as if
5200 the previous item and the quantifier were not present. This may be use-
5201 ful for subpatterns that are referenced as subroutines from elsewhere
5202 in the pattern (but see also the section entitled "Defining subpatterns
5203 for use by reference only" below). Items other than subpatterns that
5204 have a {0} quantifier are omitted from the compiled pattern.
5205
5206 For convenience, the three most common quantifiers have single-charac-
5207 ter abbreviations:
5208
5209 * is equivalent to {0,}
5210 + is equivalent to {1,}
5211 ? is equivalent to {0,1}
5212
5213 It is possible to construct infinite loops by following a subpattern
5214 that can match no characters with a quantifier that has no upper limit,
5215 for example:
5216
5217 (a?)*
5218
5219 Earlier versions of Perl and PCRE used to give an error at compile time
5220 for such patterns. However, because there are cases where this can be
5221 useful, such patterns are now accepted, but if any repetition of the
5222 subpattern does in fact match no characters, the loop is forcibly bro-
5223 ken.
5224
5225 By default, the quantifiers are "greedy", that is, they match as much
5226 as possible (up to the maximum number of permitted times), without
5227 causing the rest of the pattern to fail. The classic example of where
5228 this gives problems is in trying to match comments in C programs. These
5229 appear between /* and */ and within the comment, individual * and /
5230 characters may appear. An attempt to match C comments by applying the
5231 pattern
5232
5233 /\*.*\*/
5234
5235 to the string
5236
5237 /* first comment */ not comment /* second comment */
5238
5239 fails, because it matches the entire string owing to the greediness of
5240 the .* item.
5241
5242 However, if a quantifier is followed by a question mark, it ceases to
5243 be greedy, and instead matches the minimum number of times possible, so
5244 the pattern
5245
5246 /\*.*?\*/
5247
5248 does the right thing with the C comments. The meaning of the various
5249 quantifiers is not otherwise changed, just the preferred number of
5250 matches. Do not confuse this use of question mark with its use as a
5251 quantifier in its own right. Because it has two uses, it can sometimes
5252 appear doubled, as in
5253
5254 \d??\d
5255
5256 which matches one digit by preference, but can match two if that is the
5257 only way the rest of the pattern matches.
5258
5259 If the PCRE_UNGREEDY option is set (an option that is not available in
5260 Perl), the quantifiers are not greedy by default, but individual ones
5261 can be made greedy by following them with a question mark. In other
5262 words, it inverts the default behaviour.
5263
5264 When a parenthesized subpattern is quantified with a minimum repeat
5265 count that is greater than 1 or with a limited maximum, more memory is
5266 required for the compiled pattern, in proportion to the size of the
5267 minimum or maximum.
5268
5269 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
5270 alent to Perl's /s) is set, thus allowing the dot to match newlines,
5271 the pattern is implicitly anchored, because whatever follows will be
5272 tried against every character position in the subject string, so there
5273 is no point in retrying the overall match at any position after the
5274 first. PCRE normally treats such a pattern as though it were preceded
5275 by \A.
5276
5277 In cases where it is known that the subject string contains no new-
5278 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
5279 mization, or alternatively using ^ to indicate anchoring explicitly.
5280
5281 However, there is one situation where the optimization cannot be used.
5282 When .* is inside capturing parentheses that are the subject of a back
5283 reference elsewhere in the pattern, a match at the start may fail where
5284 a later one succeeds. Consider, for example:
5285
5286 (.*)abc\1
5287
5288 If the subject is "xyz123abc123" the match point is the fourth charac-
5289 ter. For this reason, such a pattern is not implicitly anchored.
5290
5291 When a capturing subpattern is repeated, the value captured is the sub-
5292 string that matched the final iteration. For example, after
5293
5294 (tweedle[dume]{3}\s*)+
5295
5296 has matched "tweedledum tweedledee" the value of the captured substring
5297 is "tweedledee". However, if there are nested capturing subpatterns,
5298 the corresponding captured values may have been set in previous itera-
5299 tions. For example, after
5300
5301 /(a|(b))+/
5302
5303 matches "aba" the value of the second captured substring is "b".
5304
5305
5306 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
5307
5308 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
5309 repetition, failure of what follows normally causes the repeated item
5310 to be re-evaluated to see if a different number of repeats allows the
5311 rest of the pattern to match. Sometimes it is useful to prevent this,
5312 either to change the nature of the match, or to cause it fail earlier
5313 than it otherwise might, when the author of the pattern knows there is
5314 no point in carrying on.
5315
5316 Consider, for example, the pattern \d+foo when applied to the subject
5317 line
5318
5319 123456bar
5320
5321 After matching all 6 digits and then failing to match "foo", the normal
5322 action of the matcher is to try again with only 5 digits matching the
5323 \d+ item, and then with 4, and so on, before ultimately failing.
5324 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
5325 the means for specifying that once a subpattern has matched, it is not
5326 to be re-evaluated in this way.
5327
5328 If we use atomic grouping for the previous example, the matcher gives
5329 up immediately on failing to match "foo" the first time. The notation
5330 is a kind of special parenthesis, starting with (?> as in this example:
5331
5332 (?>\d+)foo
5333
5334 This kind of parenthesis "locks up" the part of the pattern it con-
5335 tains once it has matched, and a failure further into the pattern is
5336 prevented from backtracking into it. Backtracking past it to previous
5337 items, however, works as normal.
5338
5339 An alternative description is that a subpattern of this type matches
5340 the string of characters that an identical standalone pattern would
5341 match, if anchored at the current point in the subject string.
5342
5343 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
5344 such as the above example can be thought of as a maximizing repeat that
5345 must swallow everything it can. So, while both \d+ and \d+? are pre-
5346 pared to adjust the number of digits they match in order to make the
5347 rest of the pattern match, (?>\d+) can only match an entire sequence of
5348 digits.
5349
5350 Atomic groups in general can of course contain arbitrarily complicated
5351 subpatterns, and can be nested. However, when the subpattern for an
5352 atomic group is just a single repeated item, as in the example above, a
5353 simpler notation, called a "possessive quantifier" can be used. This
5354 consists of an additional + character following a quantifier. Using
5355 this notation, the previous example can be rewritten as
5356
5357 \d++foo
5358
5359 Note that a possessive quantifier can be used with an entire group, for
5360 example:
5361
5362 (abc|xyz){2,3}+
5363
5364 Possessive quantifiers are always greedy; the setting of the
5365 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
5366 simpler forms of atomic group. However, there is no difference in the
5367 meaning of a possessive quantifier and the equivalent atomic group,
5368 though there may be a performance difference; possessive quantifiers
5369 should be slightly faster.
5370
5371 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
5372 tax. Jeffrey Friedl originated the idea (and the name) in the first
5373 edition of his book. Mike McCloskey liked it, so implemented it when he
5374 built Sun's Java package, and PCRE copied it from there. It ultimately
5375 found its way into Perl at release 5.10.
5376
5377 PCRE has an optimization that automatically "possessifies" certain sim-
5378 ple pattern constructs. For example, the sequence A+B is treated as
5379 A++B because there is no point in backtracking into a sequence of A's
5380 when B must follow.
5381
5382 When a pattern contains an unlimited repeat inside a subpattern that
5383 can itself be repeated an unlimited number of times, the use of an
5384 atomic group is the only way to avoid some failing matches taking a
5385 very long time indeed. The pattern
5386
5387 (\D+|<\d+>)*[!?]
5388
5389 matches an unlimited number of substrings that either consist of non-
5390 digits, or digits enclosed in <>, followed by either ! or ?. When it
5391 matches, it runs quickly. However, if it is applied to
5392
5393 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
5394
5395 it takes a long time before reporting failure. This is because the
5396 string can be divided between the internal \D+ repeat and the external
5397 * repeat in a large number of ways, and all have to be tried. (The
5398 example uses [!?] rather than a single character at the end, because
5399 both PCRE and Perl have an optimization that allows for fast failure
5400 when a single character is used. They remember the last single charac-
5401 ter that is required for a match, and fail early if it is not present
5402 in the string.) If the pattern is changed so that it uses an atomic
5403 group, like this:
5404
5405 ((?>\D+)|<\d+>)*[!?]
5406
5407 sequences of non-digits cannot be broken, and failure happens quickly.
5408
5409
5410 BACK REFERENCES
5411
5412 Outside a character class, a backslash followed by a digit greater than
5413 0 (and possibly further digits) is a back reference to a capturing sub-
5414 pattern earlier (that is, to its left) in the pattern, provided there
5415 have been that many previous capturing left parentheses.
5416
5417 However, if the decimal number following the backslash is less than 10,
5418 it is always taken as a back reference, and causes an error only if
5419 there are not that many capturing left parentheses in the entire pat-
5420 tern. In other words, the parentheses that are referenced need not be
5421 to the left of the reference for numbers less than 10. A "forward back
5422 reference" of this type can make sense when a repetition is involved
5423 and the subpattern to the right has participated in an earlier itera-
5424 tion.
5425
5426 It is not possible to have a numerical "forward back reference" to a
5427 subpattern whose number is 10 or more using this syntax because a
5428 sequence such as \50 is interpreted as a character defined in octal.
5429 See the subsection entitled "Non-printing characters" above for further
5430 details of the handling of digits following a backslash. There is no
5431 such problem when named parentheses are used. A back reference to any
5432 subpattern is possible using named parentheses (see below).
5433
5434 Another way of avoiding the ambiguity inherent in the use of digits
5435 following a backslash is to use the \g escape sequence. This escape
5436 must be followed by an unsigned number or a negative number, optionally
5437 enclosed in braces. These examples are all identical:
5438
5439 (ring), \1
5440 (ring), \g1
5441 (ring), \g{1}
5442
5443 An unsigned number specifies an absolute reference without the ambigu-
5444 ity that is present in the older syntax. It is also useful when literal
5445 digits follow the reference. A negative number is a relative reference.
5446 Consider this example:
5447
5448 (abc(def)ghi)\g{-1}
5449
5450 The sequence \g{-1} is a reference to the most recently started captur-
5451 ing subpattern before \g, that is, is it equivalent to \2 in this exam-
5452 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
5453 references can be helpful in long patterns, and also in patterns that
5454 are created by joining together fragments that contain references
5455 within themselves.
5456
5457 A back reference matches whatever actually matched the capturing sub-
5458 pattern in the current subject string, rather than anything matching
5459 the subpattern itself (see "Subpatterns as subroutines" below for a way
5460 of doing that). So the pattern
5461
5462 (sens|respons)e and \1ibility
5463
5464 matches "sense and sensibility" and "response and responsibility", but
5465 not "sense and responsibility". If caseful matching is in force at the
5466 time of the back reference, the case of letters is relevant. For exam-
5467 ple,
5468
5469 ((?i)rah)\s+\1
5470
5471 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
5472 original capturing subpattern is matched caselessly.
5473
5474 There are several different ways of writing back references to named
5475 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
5476 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
5477 unified back reference syntax, in which \g can be used for both numeric
5478 and named references, is also supported. We could rewrite the above
5479 example in any of the following ways:
5480
5481 (?<p1>(?i)rah)\s+\k<p1>
5482 (?'p1'(?i)rah)\s+\k{p1}
5483 (?P<p1>(?i)rah)\s+(?P=p1)
5484 (?<p1>(?i)rah)\s+\g{p1}
5485
5486 A subpattern that is referenced by name may appear in the pattern
5487 before or after the reference.
5488
5489 There may be more than one back reference to the same subpattern. If a
5490 subpattern has not actually been used in a particular match, any back
5491 references to it always fail by default. For example, the pattern
5492
5493 (a|(bc))\2
5494
5495 always fails if it starts to match "a" rather than "bc". However, if
5496 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
5497 ence to an unset value matches an empty string.
5498
5499 Because there may be many capturing parentheses in a pattern, all dig-
5500 its following a backslash are taken as part of a potential back refer-
5501 ence number. If the pattern continues with a digit character, some
5502 delimiter must be used to terminate the back reference. If the
5503 PCRE_EXTENDED option is set, this can be white space. Otherwise, the
5504 \g{ syntax or an empty comment (see "Comments" below) can be used.
5505
5506 Recursive back references
5507
5508 A back reference that occurs inside the parentheses to which it refers
5509 fails when the subpattern is first used, so, for example, (a\1) never
5510 matches. However, such references can be useful inside repeated sub-
5511 patterns. For example, the pattern
5512
5513 (a|b\1)+
5514
5515 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
5516 ation of the subpattern, the back reference matches the character
5517 string corresponding to the previous iteration. In order for this to
5518 work, the pattern must be such that the first iteration does not need
5519 to match the back reference. This can be done using alternation, as in
5520 the example above, or by a quantifier with a minimum of zero.
5521
5522 Back references of this type cause the group that they reference to be
5523 treated as an atomic group. Once the whole group has been matched, a
5524 subsequent matching failure cannot cause backtracking into the middle
5525 of the group.
5526
5527
5528 ASSERTIONS
5529
5530 An assertion is a test on the characters following or preceding the
5531 current matching point that does not actually consume any characters.
5532 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
5533 described above.
5534
5535 More complicated assertions are coded as subpatterns. There are two
5536 kinds: those that look ahead of the current position in the subject
5537 string, and those that look behind it. An assertion subpattern is
5538 matched in the normal way, except that it does not cause the current
5539 matching position to be changed.
5540
5541 Assertion subpatterns are not capturing subpatterns. If such an asser-
5542 tion contains capturing subpatterns within it, these are counted for
5543 the purposes of numbering the capturing subpatterns in the whole pat-
5544 tern. However, substring capturing is carried out only for positive
5545 assertions, because it does not make sense for negative assertions.
5546
5547 For compatibility with Perl, assertion subpatterns may be repeated;
5548 though it makes no sense to assert the same thing several times, the
5549 side effect of capturing parentheses may occasionally be useful. In
5550 practice, there only three cases:
5551
5552 (1) If the quantifier is {0}, the assertion is never obeyed during
5553 matching. However, it may contain internal capturing parenthesized
5554 groups that are called from elsewhere via the subroutine mechanism.
5555
5556 (2) If quantifier is {0,n} where n is greater than zero, it is treated
5557 as if it were {0,1}. At run time, the rest of the pattern match is
5558 tried with and without the assertion, the order depending on the greed-
5559 iness of the quantifier.
5560
5561 (3) If the minimum repetition is greater than zero, the quantifier is
5562 ignored. The assertion is obeyed just once when encountered during
5563 matching.
5564
5565 Lookahead assertions
5566
5567 Lookahead assertions start with (?= for positive assertions and (?! for
5568 negative assertions. For example,
5569
5570 \w+(?=;)
5571
5572 matches a word followed by a semicolon, but does not include the semi-
5573 colon in the match, and
5574
5575 foo(?!bar)
5576
5577 matches any occurrence of "foo" that is not followed by "bar". Note
5578 that the apparently similar pattern
5579
5580 (?!foo)bar
5581
5582 does not find an occurrence of "bar" that is preceded by something
5583 other than "foo"; it finds any occurrence of "bar" whatsoever, because
5584 the assertion (?!foo) is always true when the next three characters are
5585 "bar". A lookbehind assertion is needed to achieve the other effect.
5586
5587 If you want to force a matching failure at some point in a pattern, the
5588 most convenient way to do it is with (?!) because an empty string
5589 always matches, so an assertion that requires there not to be an empty
5590 string must always fail. The backtracking control verb (*FAIL) or (*F)
5591 is a synonym for (?!).
5592
5593 Lookbehind assertions
5594
5595 Lookbehind assertions start with (?<= for positive assertions and (?<!
5596 for negative assertions. For example,
5597
5598 (?<!foo)bar
5599
5600 does find an occurrence of "bar" that is not preceded by "foo". The
5601 contents of a lookbehind assertion are restricted such that all the
5602 strings it matches must have a fixed length. However, if there are sev-
5603 eral top-level alternatives, they do not all have to have the same
5604 fixed length. Thus
5605
5606 (?<=bullock|donkey)
5607
5608 is permitted, but
5609
5610 (?<!dogs?|cats?)
5611
5612 causes an error at compile time. Branches that match different length
5613 strings are permitted only at the top level of a lookbehind assertion.
5614 This is an extension compared with Perl, which requires all branches to
5615 match the same length of string. An assertion such as
5616
5617 (?<=ab(c|de))
5618
5619 is not permitted, because its single top-level branch can match two
5620 different lengths, but it is acceptable to PCRE if rewritten to use two
5621 top-level branches:
5622
5623 (?<=abc|abde)
5624
5625 In some cases, the escape sequence \K (see above) can be used instead
5626 of a lookbehind assertion to get round the fixed-length restriction.
5627
5628 The implementation of lookbehind assertions is, for each alternative,
5629 to temporarily move the current position back by the fixed length and
5630 then try to match. If there are insufficient characters before the cur-
5631 rent position, the assertion fails.
5632
5633 In a UTF mode, PCRE does not allow the \C escape (which matches a sin-
5634 gle data unit even in a UTF mode) to appear in lookbehind assertions,
5635 because it makes it impossible to calculate the length of the lookbe-
5636 hind. The \X and \R escapes, which can match different numbers of data
5637 units, are also not permitted.
5638
5639 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
5640 lookbehinds, as long as the subpattern matches a fixed-length string.
5641 Recursion, however, is not supported.
5642
5643 Possessive quantifiers can be used in conjunction with lookbehind
5644 assertions to specify efficient matching of fixed-length strings at the
5645 end of subject strings. Consider a simple pattern such as
5646
5647 abcd$
5648
5649 when applied to a long string that does not match. Because matching
5650 proceeds from left to right, PCRE will look for each "a" in the subject
5651 and then see if what follows matches the rest of the pattern. If the
5652 pattern is specified as
5653
5654 ^.*abcd$
5655
5656 the initial .* matches the entire string at first, but when this fails
5657 (because there is no following "a"), it backtracks to match all but the
5658 last character, then all but the last two characters, and so on. Once
5659 again the search for "a" covers the entire string, from right to left,
5660 so we are no better off. However, if the pattern is written as
5661
5662 ^.*+(?<=abcd)
5663
5664 there can be no backtracking for the .*+ item; it can match only the
5665 entire string. The subsequent lookbehind assertion does a single test
5666 on the last four characters. If it fails, the match fails immediately.
5667 For long strings, this approach makes a significant difference to the
5668 processing time.
5669
5670 Using multiple assertions
5671
5672 Several assertions (of any sort) may occur in succession. For example,
5673
5674 (?<=\d{3})(?<!999)foo
5675
5676 matches "foo" preceded by three digits that are not "999". Notice that
5677 each of the assertions is applied independently at the same point in
5678 the subject string. First there is a check that the previous three
5679 characters are all digits, and then there is a check that the same
5680 three characters are not "999". This pattern does not match "foo" pre-
5681 ceded by six characters, the first of which are digits and the last
5682 three of which are not "999". For example, it doesn't match "123abc-
5683 foo". A pattern to do that is
5684
5685 (?<=\d{3}...)(?<!999)foo
5686
5687 This time the first assertion looks at the preceding six characters,
5688 checking that the first three are digits, and then the second assertion
5689 checks that the preceding three characters are not "999".
5690
5691 Assertions can be nested in any combination. For example,
5692
5693 (?<=(?<!foo)bar)baz
5694
5695 matches an occurrence of "baz" that is preceded by "bar" which in turn
5696 is not preceded by "foo", while
5697
5698 (?<=\d{3}(?!999)...)foo
5699
5700 is another pattern that matches "foo" preceded by three digits and any
5701 three characters that are not "999".
5702
5703
5704 CONDITIONAL SUBPATTERNS
5705
5706 It is possible to cause the matching process to obey a subpattern con-
5707 ditionally or to choose between two alternative subpatterns, depending
5708 on the result of an assertion, or whether a specific capturing subpat-
5709 tern has already been matched. The two possible forms of conditional
5710 subpattern are:
5711
5712 (?(condition)yes-pattern)
5713 (?(condition)yes-pattern|no-pattern)
5714
5715 If the condition is satisfied, the yes-pattern is used; otherwise the
5716 no-pattern (if present) is used. If there are more than two alterna-
5717 tives in the subpattern, a compile-time error occurs. Each of the two
5718 alternatives may itself contain nested subpatterns of any form, includ-
5719 ing conditional subpatterns; the restriction to two alternatives
5720 applies only at the level of the condition. This pattern fragment is an
5721 example where the alternatives are complex:
5722
5723 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
5724
5725
5726 There are four kinds of condition: references to subpatterns, refer-
5727 ences to recursion, a pseudo-condition called DEFINE, and assertions.
5728
5729 Checking for a used subpattern by number
5730
5731 If the text between the parentheses consists of a sequence of digits,
5732 the condition is true if a capturing subpattern of that number has pre-
5733 viously matched. If there is more than one capturing subpattern with
5734 the same number (see the earlier section about duplicate subpattern
5735 numbers), the condition is true if any of them have matched. An alter-
5736 native notation is to precede the digits with a plus or minus sign. In
5737 this case, the subpattern number is relative rather than absolute. The
5738 most recently opened parentheses can be referenced by (?(-1), the next
5739 most recent by (?(-2), and so on. Inside loops it can also make sense
5740 to refer to subsequent groups. The next parentheses to be opened can be
5741 referenced as (?(+1), and so on. (The value zero in any of these forms
5742 is not used; it provokes a compile-time error.)
5743
5744 Consider the following pattern, which contains non-significant white
5745 space to make it more readable (assume the PCRE_EXTENDED option) and to
5746 divide it into three parts for ease of discussion:
5747
5748 ( \( )? [^()]+ (?(1) \) )
5749
5750 The first part matches an optional opening parenthesis, and if that
5751 character is present, sets it as the first captured substring. The sec-
5752 ond part matches one or more characters that are not parentheses. The
5753 third part is a conditional subpattern that tests whether or not the
5754 first set of parentheses matched. If they did, that is, if subject
5755 started with an opening parenthesis, the condition is true, and so the
5756 yes-pattern is executed and a closing parenthesis is required. Other-
5757 wise, since no-pattern is not present, the subpattern matches nothing.
5758 In other words, this pattern matches a sequence of non-parentheses,
5759 optionally enclosed in parentheses.
5760
5761 If you were embedding this pattern in a larger one, you could use a
5762 relative reference:
5763
5764 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
5765
5766 This makes the fragment independent of the parentheses in the larger
5767 pattern.
5768
5769 Checking for a used subpattern by name
5770
5771 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
5772 used subpattern by name. For compatibility with earlier versions of
5773 PCRE, which had this facility before Perl, the syntax (?(name)...) is
5774 also recognized. However, there is a possible ambiguity with this syn-
5775 tax, because subpattern names may consist entirely of digits. PCRE
5776 looks first for a named subpattern; if it cannot find one and the name
5777 consists entirely of digits, PCRE looks for a subpattern of that num-
5778 ber, which must be greater than zero. Using subpattern names that con-
5779 sist entirely of digits is not recommended.
5780
5781 Rewriting the above example to use a named subpattern gives this:
5782
5783 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
5784
5785 If the name used in a condition of this kind is a duplicate, the test
5786 is applied to all subpatterns of the same name, and is true if any one
5787 of them has matched.
5788
5789 Checking for pattern recursion
5790
5791 If the condition is the string (R), and there is no subpattern with the
5792 name R, the condition is true if a recursive call to the whole pattern
5793 or any subpattern has been made. If digits or a name preceded by amper-
5794 sand follow the letter R, for example:
5795
5796 (?(R3)...) or (?(R&name)...)
5797
5798 the condition is true if the most recent recursion is into a subpattern
5799 whose number or name is given. This condition does not check the entire
5800 recursion stack. If the name used in a condition of this kind is a
5801 duplicate, the test is applied to all subpatterns of the same name, and
5802 is true if any one of them is the most recent recursion.
5803
5804 At "top level", all these recursion test conditions are false. The
5805 syntax for recursive patterns is described below.
5806
5807 Defining subpatterns for use by reference only
5808
5809 If the condition is the string (DEFINE), and there is no subpattern
5810 with the name DEFINE, the condition is always false. In this case,
5811 there may be only one alternative in the subpattern. It is always
5812 skipped if control reaches this point in the pattern; the idea of
5813 DEFINE is that it can be used to define subroutines that can be refer-
5814 enced from elsewhere. (The use of subroutines is described below.) For
5815 example, a pattern to match an IPv4 address such as "192.168.23.245"
5816 could be written like this (ignore white space and line breaks):
5817
5818 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
5819 \b (?&byte) (\.(?&byte)){3} \b
5820
5821 The first part of the pattern is a DEFINE group inside which a another
5822 group named "byte" is defined. This matches an individual component of
5823 an IPv4 address (a number less than 256). When matching takes place,
5824 this part of the pattern is skipped because DEFINE acts like a false
5825 condition. The rest of the pattern uses references to the named group
5826 to match the four dot-separated components of an IPv4 address, insist-
5827 ing on a word boundary at each end.
5828
5829 Assertion conditions
5830
5831 If the condition is not in any of the above formats, it must be an
5832 assertion. This may be a positive or negative lookahead or lookbehind
5833 assertion. Consider this pattern, again containing non-significant
5834 white space, and with the two alternatives on the second line:
5835
5836 (?(?=[^a-z]*[a-z])
5837 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
5838
5839 The condition is a positive lookahead assertion that matches an
5840 optional sequence of non-letters followed by a letter. In other words,
5841 it tests for the presence of at least one letter in the subject. If a
5842 letter is found, the subject is matched against the first alternative;
5843 otherwise it is matched against the second. This pattern matches
5844 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
5845 letters and dd are digits.
5846
5847
5848 COMMENTS
5849
5850 There are two ways of including comments in patterns that are processed
5851 by PCRE. In both cases, the start of the comment must not be in a char-
5852 acter class, nor in the middle of any other sequence of related charac-
5853 ters such as (?: or a subpattern name or number. The characters that
5854 make up a comment play no part in the pattern matching.
5855
5856 The sequence (?# marks the start of a comment that continues up to the
5857 next closing parenthesis. Nested parentheses are not permitted. If the
5858 PCRE_EXTENDED option is set, an unescaped # character also introduces a
5859 comment, which in this case continues to immediately after the next
5860 newline character or character sequence in the pattern. Which charac-
5861 ters are interpreted as newlines is controlled by the options passed to
5862 a compiling function or by a special sequence at the start of the pat-
5863 tern, as described in the section entitled "Newline conventions" above.
5864 Note that the end of this type of comment is a literal newline sequence
5865 in the pattern; escape sequences that happen to represent a newline do
5866 not count. For example, consider this pattern when PCRE_EXTENDED is
5867 set, and the default newline convention is in force:
5868
5869 abc #comment \n still comment
5870
5871 On encountering the # character, pcre_compile() skips along, looking
5872 for a newline in the pattern. The sequence \n is still literal at this
5873 stage, so it does not terminate the comment. Only an actual character
5874 with the code value 0x0a (the default newline) does so.
5875
5876
5877 RECURSIVE PATTERNS
5878
5879 Consider the problem of matching a string in parentheses, allowing for
5880 unlimited nested parentheses. Without the use of recursion, the best
5881 that can be done is to use a pattern that matches up to some fixed
5882 depth of nesting. It is not possible to handle an arbitrary nesting
5883 depth.
5884
5885 For some time, Perl has provided a facility that allows regular expres-
5886 sions to recurse (amongst other things). It does this by interpolating
5887 Perl code in the expression at run time, and the code can refer to the
5888 expression itself. A Perl pattern using code interpolation to solve the
5889 parentheses problem can be created like this:
5890
5891 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
5892
5893 The (?p{...}) item interpolates Perl code at run time, and in this case
5894 refers recursively to the pattern in which it appears.
5895
5896 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
5897 it supports special syntax for recursion of the entire pattern, and
5898 also for individual subpattern recursion. After its introduction in
5899 PCRE and Python, this kind of recursion was subsequently introduced
5900 into Perl at release 5.10.
5901
5902 A special item that consists of (? followed by a number greater than
5903 zero and a closing parenthesis is a recursive subroutine call of the
5904 subpattern of the given number, provided that it occurs inside that
5905 subpattern. (If not, it is a non-recursive subroutine call, which is
5906 described in the next section.) The special item (?R) or (?0) is a
5907 recursive call of the entire regular expression.
5908
5909 This PCRE pattern solves the nested parentheses problem (assume the
5910 PCRE_EXTENDED option is set so that white space is ignored):
5911
5912 \( ( [^()]++ | (?R) )* \)
5913
5914 First it matches an opening parenthesis. Then it matches any number of
5915 substrings which can either be a sequence of non-parentheses, or a
5916 recursive match of the pattern itself (that is, a correctly parenthe-
5917 sized substring). Finally there is a closing parenthesis. Note the use
5918 of a possessive quantifier to avoid backtracking into sequences of non-
5919 parentheses.
5920
5921 If this were part of a larger pattern, you would not want to recurse
5922 the entire pattern, so instead you could use this:
5923
5924 ( \( ( [^()]++ | (?1) )* \) )
5925
5926 We have put the pattern into parentheses, and caused the recursion to
5927 refer to them instead of the whole pattern.
5928
5929 In a larger pattern, keeping track of parenthesis numbers can be
5930 tricky. This is made easier by the use of relative references. Instead
5931 of (?1) in the pattern above you can write (?-2) to refer to the second
5932 most recently opened parentheses preceding the recursion. In other
5933 words, a negative number counts capturing parentheses leftwards from
5934 the point at which it is encountered.
5935
5936 It is also possible to refer to subsequently opened parentheses, by
5937 writing references such as (?+2). However, these cannot be recursive
5938 because the reference is not inside the parentheses that are refer-
5939 enced. They are always non-recursive subroutine calls, as described in
5940 the next section.
5941
5942 An alternative approach is to use named parentheses instead. The Perl
5943 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
5944 supported. We could rewrite the above example as follows:
5945
5946 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
5947
5948 If there is more than one subpattern with the same name, the earliest
5949 one is used.
5950
5951 This particular example pattern that we have been looking at contains
5952 nested unlimited repeats, and so the use of a possessive quantifier for
5953 matching strings of non-parentheses is important when applying the pat-
5954 tern to strings that do not match. For example, when this pattern is
5955 applied to
5956
5957 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5958
5959 it yields "no match" quickly. However, if a possessive quantifier is
5960 not used, the match runs for a very long time indeed because there are
5961 so many different ways the + and * repeats can carve up the subject,
5962 and all have to be tested before failure can be reported.
5963
5964 At the end of a match, the values of capturing parentheses are those
5965 from the outermost level. If you want to obtain intermediate values, a
5966 callout function can be used (see below and the pcrecallout documenta-
5967 tion). If the pattern above is matched against
5968
5969 (ab(cd)ef)
5970
5971 the value for the inner capturing parentheses (numbered 2) is "ef",
5972 which is the last value taken on at the top level. If a capturing sub-
5973 pattern is not matched at the top level, its final captured value is
5974 unset, even if it was (temporarily) set at a deeper level during the
5975 matching process.
5976
5977 If there are more than 15 capturing parentheses in a pattern, PCRE has
5978 to obtain extra memory to store data during a recursion, which it does
5979 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5980 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5981
5982 Do not confuse the (?R) item with the condition (R), which tests for
5983 recursion. Consider this pattern, which matches text in angle brack-
5984 ets, allowing for arbitrary nesting. Only digits are allowed in nested
5985 brackets (that is, when recursing), whereas any characters are permit-
5986 ted at the outer level.
5987
5988 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5989
5990 In this pattern, (?(R) is the start of a conditional subpattern, with
5991 two different alternatives for the recursive and non-recursive cases.
5992 The (?R) item is the actual recursive call.
5993
5994 Differences in recursion processing between PCRE and Perl
5995
5996 Recursion processing in PCRE differs from Perl in two important ways.
5997 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
5998 always treated as an atomic group. That is, once it has matched some of
5999 the subject string, it is never re-entered, even if it contains untried
6000 alternatives and there is a subsequent matching failure. This can be
6001 illustrated by the following pattern, which purports to match a palin-
6002 dromic string that contains an odd number of characters (for example,
6003 "a", "aba", "abcba", "abcdcba"):
6004
6005 ^(.|(.)(?1)\2)$
6006
6007 The idea is that it either matches a single character, or two identical
6008 characters surrounding a sub-palindrome. In Perl, this pattern works;
6009 in PCRE it does not if the pattern is longer than three characters.
6010 Consider the subject string "abcba":
6011
6012 At the top level, the first character is matched, but as it is not at
6013 the end of the string, the first alternative fails; the second alterna-
6014 tive is taken and the recursion kicks in. The recursive call to subpat-
6015 tern 1 successfully matches the next character ("b"). (Note that the
6016 beginning and end of line te