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Revision 34 - (show annotations) (download)
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Tag code/trunk as code/tags/pcre-2.05.

1 .TH PCRE 3
2 .SH NAME
3 pcre - Perl-compatible regular expressions.
4 .SH SYNOPSIS
5 .B #include <pcre.h>
6 .PP
7 .SM
8 .br
9 .B pcre *pcre_compile(const char *\fIpattern\fR, int \fIoptions\fR,
10 .ti +5n
11 .B const char **\fIerrptr\fR, int *\fIerroffset\fR,
12 .ti +5n
13 .B const unsigned char *\fItableptr\fR);
14 .PP
15 .br
16 .B pcre_extra *pcre_study(const pcre *\fIcode\fR, int \fIoptions\fR,
17 .ti +5n
18 .B const char **\fIerrptr\fR);
19 .PP
20 .br
21 .B int pcre_exec(const pcre *\fIcode\fR, "const pcre_extra *\fIextra\fR,"
22 .ti +5n
23 .B "const char *\fIsubject\fR," int \fIlength\fR, int \fIoptions\fR,
24 .ti +5n
25 .B int *\fIovector\fR, int \fIovecsize\fR);
26 .PP
27 .br
28 .B int pcre_copy_substring(const char *\fIsubject\fR, int *\fIovector\fR,
29 .ti +5n
30 .B int \fIstringcount\fR, int \fIstringnumber\fR, char *\fIbuffer\fR,
31 .ti +5n
32 .B int \fIbuffersize\fR);
33 .PP
34 .br
35 .B int pcre_get_substring(const char *\fIsubject\fR, int *\fIovector\fR,
36 .ti +5n
37 .B int \fIstringcount\fR, int \fIstringnumber\fR,
38 .ti +5n
39 .B const char **\fIstringptr\fR);
40 .PP
41 .br
42 .B int pcre_get_substring_list(const char *\fIsubject\fR,
43 .ti +5n
44 .B int *\fIovector\fR, int \fIstringcount\fR, "const char ***\fIlistptr\fR);"
45 .PP
46 .br
47 .B const unsigned char *pcre_maketables(void);
48 .PP
49 .br
50 .B int pcre_info(const pcre *\fIcode\fR, int *\fIoptptr\fR, int
51 .B *\fIfirstcharptr\fR);
52 .PP
53 .br
54 .B char *pcre_version(void);
55 .PP
56 .br
57 .B void *(*pcre_malloc)(size_t);
58 .PP
59 .br
60 .B void (*pcre_free)(void *);
61
62
63
64 .SH DESCRIPTION
65 The PCRE library is a set of functions that implement regular expression
66 pattern matching using the same syntax and semantics as Perl 5, with just a few
67 differences (see below). The current implementation corresponds to Perl 5.005.
68
69 PCRE has its own native API, which is described in this man page. There is also
70 a set of wrapper functions that correspond to the POSIX API. See
71 \fBpcreposix (3)\fR.
72
73 The functions \fBpcre_compile()\fR, \fBpcre_study()\fR, and \fBpcre_exec()\fR
74 are used for compiling and matching regular expressions, while
75 \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
76 \fBpcre_get_substring_list()\fR are convenience functions for extracting
77 captured substrings from a matched subject string. The function
78 \fBpcre_maketables()\fR is used (optionally) to build a set of character tables
79 in the current locale for passing to \fBpcre_compile()\fR.
80
81 The function \fBpcre_info()\fR is used to find out information about a compiled
82 pattern, while the function \fBpcre_version()\fR returns a pointer to a string
83 containing the version of PCRE and its date of release.
84
85 The global variables \fBpcre_malloc\fR and \fBpcre_free\fR initially contain
86 the entry points of the standard \fBmalloc()\fR and \fBfree()\fR functions
87 respectively. PCRE calls the memory management functions via these variables,
88 so a calling program can replace them if it wishes to intercept the calls. This
89 should be done before calling any PCRE functions.
90
91
92 .SH MULTI-THREADING
93 The PCRE functions can be used in multi-threading applications, with the
94 proviso that the memory management functions pointed to by \fBpcre_malloc\fR
95 and \fBpcre_free\fR are shared by all threads.
96
97 The compiled form of a regular expression is not altered during matching, so
98 the same compiled pattern can safely be used by several threads at once.
99
100
101 .SH COMPILING A PATTERN
102 The function \fBpcre_compile()\fR is called to compile a pattern into an
103 internal form. The pattern is a C string terminated by a binary zero, and
104 is passed in the argument \fIpattern\fR. A pointer to a single block of memory
105 that is obtained via \fBpcre_malloc\fR is returned. This contains the
106 compiled code and related data. The \fBpcre\fR type is defined for this for
107 convenience, but in fact \fBpcre\fR is just a typedef for \fBvoid\fR, since the
108 contents of the block are not externally defined. It is up to the caller to
109 free the memory when it is no longer required.
110 .PP
111 The size of a compiled pattern is roughly proportional to the length of the
112 pattern string, except that each character class (other than those containing
113 just a single character, negated or not) requires 33 bytes, and repeat
114 quantifiers with a minimum greater than one or a bounded maximum cause the
115 relevant portions of the compiled pattern to be replicated.
116 .PP
117 The \fIoptions\fR argument contains independent bits that affect the
118 compilation. It should be zero if no options are required. Some of the options,
119 in particular, those that are compatible with Perl, can also be set and unset
120 from within the pattern (see the detailed description of regular expressions
121 below). For these options, the contents of the \fIoptions\fR argument specifies
122 their initial settings at the start of compilation and execution. The
123 PCRE_ANCHORED option can be set at the time of matching as well as at compile
124 time.
125 .PP
126 If \fIerrptr\fR is NULL, \fBpcre_compile()\fR returns NULL immediately.
127 Otherwise, if compilation of a pattern fails, \fBpcre_compile()\fR returns
128 NULL, and sets the variable pointed to by \fIerrptr\fR to point to a textual
129 error message. The offset from the start of the pattern to the character where
130 the error was discovered is placed in the variable pointed to by
131 \fIerroffset\fR, which must not be NULL. If it is, an immediate error is given.
132 .PP
133 If the final argument, \fItableptr\fR, is NULL, PCRE uses a default set of
134 character tables which are built when it is compiled, using the default C
135 locale. Otherwise, \fItableptr\fR must be the result of a call to
136 \fBpcre_maketables()\fR. See the section on locale support below.
137 .PP
138 The following option bits are defined in the header file:
139
140 PCRE_ANCHORED
141
142 If this bit is set, the pattern is forced to be "anchored", that is, it is
143 constrained to match only at the start of the string which is being searched
144 (the "subject string"). This effect can also be achieved by appropriate
145 constructs in the pattern itself, which is the only way to do it in Perl.
146
147 PCRE_CASELESS
148
149 If this bit is set, letters in the pattern match both upper and lower case
150 letters. It is equivalent to Perl's /i option.
151
152 PCRE_DOLLAR_ENDONLY
153
154 If this bit is set, a dollar metacharacter in the pattern matches only at the
155 end of the subject string. Without this option, a dollar also matches
156 immediately before the final character if it is a newline (but not before any
157 other newlines). The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
158 set. There is no equivalent to this option in Perl.
159
160 PCRE_DOTALL
161
162 If this bit is set, a dot metacharater in the pattern matches all characters,
163 including newlines. Without it, newlines are excluded. This option is
164 equivalent to Perl's /s option. A negative class such as [^a] always matches a
165 newline character, independent of the setting of this option.
166
167 PCRE_EXTENDED
168
169 If this bit is set, whitespace data characters in the pattern are totally
170 ignored except when escaped or inside a character class, and characters between
171 an unescaped # outside a character class and the next newline character,
172 inclusive, are also ignored. This is equivalent to Perl's /x option, and makes
173 it possible to include comments inside complicated patterns. Note, however,
174 that this applies only to data characters. Whitespace characters may never
175 appear within special character sequences in a pattern, for example within the
176 sequence (?( which introduces a conditional subpattern.
177
178 PCRE_EXTRA
179
180 This option turns on additional functionality of PCRE that is incompatible with
181 Perl. Any backslash in a pattern that is followed by a letter that has no
182 special meaning causes an error, thus reserving these combinations for future
183 expansion. By default, as in Perl, a backslash followed by a letter with no
184 special meaning is treated as a literal. There are at present no other features
185 controlled by this option.
186
187 PCRE_MULTILINE
188
189 By default, PCRE treats the subject string as consisting of a single "line" of
190 characters (even if it actually contains several newlines). The "start of line"
191 metacharacter (^) matches only at the start of the string, while the "end of
192 line" metacharacter ($) matches only at the end of the string, or before a
193 terminating newline (unless PCRE_DOLLAR_ENDONLY is set). This is the same as
194 Perl.
195
196 When PCRE_MULTILINE it is set, the "start of line" and "end of line" constructs
197 match immediately following or immediately before any newline in the subject
198 string, respectively, as well as at the very start and end. This is equivalent
199 to Perl's /m option. If there are no "\\n" characters in a subject string, or
200 no occurrences of ^ or $ in a pattern, setting PCRE_MULTILINE has no
201 effect.
202
203 PCRE_UNGREEDY
204
205 This option inverts the "greediness" of the quantifiers so that they are not
206 greedy by default, but become greedy if followed by "?". It is not compatible
207 with Perl. It can also be set by a (?U) option setting within the pattern.
208
209
210 .SH STUDYING A PATTERN
211 When a pattern is going to be used several times, it is worth spending more
212 time analyzing it in order to speed up the time taken for matching. The
213 function \fBpcre_study()\fR takes a pointer to a compiled pattern as its first
214 argument, and returns a pointer to a \fBpcre_extra\fR block (another \fBvoid\fR
215 typedef) containing additional information about the pattern; this can be
216 passed to \fBpcre_exec()\fR. If no additional information is available, NULL
217 is returned.
218
219 The second argument contains option bits. At present, no options are defined
220 for \fBpcre_study()\fR, and this argument should always be zero.
221
222 The third argument for \fBpcre_study()\fR is a pointer to an error message. If
223 studying succeeds (even if no data is returned), the variable it points to is
224 set to NULL. Otherwise it points to a textual error message.
225
226 At present, studying a pattern is useful only for non-anchored patterns that do
227 not have a single fixed starting character. A bitmap of possible starting
228 characters is created.
229
230
231 .SH LOCALE SUPPORT
232 PCRE handles caseless matching, and determines whether characters are letters,
233 digits, or whatever, by reference to a set of tables. The library contains a
234 default set of tables which is created in the default C locale when PCRE is
235 compiled. This is used when the final argument of \fBpcre_compile()\fR is NULL,
236 and is sufficient for many applications.
237
238 An alternative set of tables can, however, be supplied. Such tables are built
239 by calling the \fBpcre_maketables()\fR function, which has no arguments, in the
240 relevant locale. The result can then be passed to \fBpcre_compile()\ as often
241 as necessary. For example, to build and use tables that are appropriate for the
242 French locale (where accented characters with codes greater than 128 are
243 treated as letters), the following code could be used:
244
245 setlocale(LC_CTYPE, "fr");
246 tables = pcre_maketables();
247 re = pcre_compile(..., tables);
248
249 The tables are built in memory that is obtained via \fBpcre_malloc\fR. The
250 pointer that is passed to \fBpcre_compile\fR is saved with the compiled
251 pattern, and the same tables are used via this pointer by \fBpcre_study()\fR
252 and \fBpcre_match()\fR. Thus for any single pattern, compilation, studying and
253 matching all happen in the same locale, but different patterns can be compiled
254 in different locales. It is the caller's responsibility to ensure that the
255 memory containing the tables remains available for as long as it is needed.
256
257
258 .SH INFORMATION ABOUT A PATTERN
259 The \fBpcre_info()\fR function returns information about a compiled pattern.
260 Its yield is the number of capturing subpatterns, or one of the following
261 negative numbers:
262
263 PCRE_ERROR_NULL the argument \fIcode\fR was NULL
264 PCRE_ERROR_BADMAGIC the "magic number" was not found
265
266 If the \fIoptptr\fR argument is not NULL, a copy of the options with which the
267 pattern was compiled is placed in the integer it points to. These option bits
268 are those specified in the call to \fBpcre_compile()\fR, modified by any
269 top-level option settings within the pattern itself, and with the PCRE_ANCHORED
270 bit set if the form of the pattern implies that it can match only at the start
271 of a subject string.
272
273 If the pattern is not anchored and the \fIfirstcharptr\fR argument is not NULL,
274 it is used to pass back information about the first character of any matched
275 string. If there is a fixed first character, e.g. from a pattern such as
276 (cat|cow|coyote), then it is returned in the integer pointed to by
277 \fIfirstcharptr\fR. Otherwise, if either
278
279 (a) the pattern was compiled with the PCRE_MULTILINE option, and every branch
280 starts with "^", or
281
282 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not set
283 (if it were set, the pattern would be anchored),
284
285 then -1 is returned, indicating that the pattern matches only at the
286 start of a subject string or after any "\\n" within the string. Otherwise -2 is
287 returned.
288
289
290 .SH MATCHING A PATTERN
291 The function \fBpcre_exec()\fR is called to match a subject string against a
292 pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
293 pattern has been studied, the result of the study should be passed in the
294 \fIextra\fR argument. Otherwise this must be NULL.
295
296 The subject string is passed as a pointer in \fIsubject\fR and a length in
297 \fIlength\fR. Unlike the pattern string, it may contain binary zero characters.
298
299 The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
300 unused bits must be zero. However, if a pattern was compiled with
301 PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
302 cannot be made unachored at matching time.
303
304 There are also two further options that can be set only at matching time:
305
306 PCRE_NOTBOL
307
308 The first character of the string is not the beginning of a line, so the
309 circumflex metacharacter should not match before it. Setting this without
310 PCRE_MULTILINE (at compile time) causes circumflex never to match.
311
312 PCRE_NOTEOL
313
314 The end of the string is not the end of a line, so the dollar metacharacter
315 should not match it nor (except in multiline mode) a newline immediately before
316 it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never
317 to match.
318
319 In general, a pattern matches a certain portion of the subject, and in
320 addition, further substrings from the subject may be picked out by parts of the
321 pattern. Following the usage in Jeffrey Friedl's book, this is called
322 "capturing" in what follows, and the phrase "capturing subpattern" is used for
323 a fragment of a pattern that picks out a substring. PCRE supports several other
324 kinds of parenthesized subpattern that do not cause substrings to be captured.
325
326 Captured substrings are returned to the caller via a vector of integer offsets
327 whose address is passed in \fIovector\fR. The number of elements in the vector
328 is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass
329 back captured substrings, each substring using a pair of integers. The
330 remaining third of the vector is used as workspace by \fBpcre_exec()\fR while
331 matching capturing subpatterns, and is not available for passing back
332 information. The length passed in \fIovecsize\fR should always be a multiple of
333 three. If it is not, it is rounded down.
334
335 When a match has been successful, information about captured substrings is
336 returned in pairs of integers, starting at the beginning of \fIovector\fR, and
337 continuing up to two-thirds of its length at the most. The first element of a
338 pair is set to the offset of the first character in a substring, and the second
339 is set to the offset of the first character after the end of a substring. The
340 first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
341 subject string matched by the entire pattern. The next pair is used for the
342 first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
343 is the number of pairs that have been set. If there are no capturing
344 subpatterns, the return value from a successful match is 1, indicating that
345 just the first pair of offsets has been set.
346
347 Some convenience functions are provided for extracting the captured substrings
348 as separate strings. These are described in the following section.
349
350 It is possible for an capturing subpattern number \fIn+1\fR to match some
351 part of the subject when subpattern \fIn\fR has not been used at all. For
352 example, if the string "abc" is matched against the pattern (a|(z))(bc)
353 subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
354 values corresponding to the unused subpattern are set to -1.
355
356 If a capturing subpattern is matched repeatedly, it is the last portion of the
357 string that it matched that gets returned.
358
359 If the vector is too small to hold all the captured substrings, it is used as
360 far as possible (up to two-thirds of its length), and the function returns a
361 value of zero. In particular, if the substring offsets are not of interest,
362 \fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
363 \fIovecsize\fR as zero. However, if the pattern contains back references and
364 the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
365 to get additional memory for use during matching. Thus it is usually advisable
366 to supply an \fIovector\fR.
367
368 Note that \fBpcre_info()\fR can be used to find out how many capturing
369 subpatterns there are in a compiled pattern. The smallest size for
370 \fIovector\fR that will allow for \fIn\fR captured substrings in addition to
371 the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
372
373 If \fBpcre_exec()\fR fails, it returns a negative number. The following are
374 defined in the header file:
375
376 PCRE_ERROR_NOMATCH (-1)
377
378 The subject string did not match the pattern.
379
380 PCRE_ERROR_NULL (-2)
381
382 Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
383 NULL and \fIovecsize\fR was not zero.
384
385 PCRE_ERROR_BADOPTION (-3)
386
387 An unrecognized bit was set in the \fIoptions\fR argument.
388
389 PCRE_ERROR_BADMAGIC (-4)
390
391 PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
392 the case when it is passed a junk pointer. This is the error it gives when the
393 magic number isn't present.
394
395 PCRE_ERROR_UNKNOWN_NODE (-5)
396
397 While running the pattern match, an unknown item was encountered in the
398 compiled pattern. This error could be caused by a bug in PCRE or by overwriting
399 of the compiled pattern.
400
401 PCRE_ERROR_NOMEMORY (-6)
402
403 If a pattern contains back references, but the \fIovector\fR that is passed to
404 \fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE
405 gets a block of memory at the start of matching to use for this purpose. If the
406 call via \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
407 the end of matching.
408
409
410 .SH EXTRACTING CAPTURED SUBSTRINGS
411 Captured substrings can be accessed directly by using the offsets returned by
412 \fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
413 \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
414 \fBpcre_get_substring_list()\fR are provided for extracting captured substrings
415 as new, separate, zero-terminated strings. A substring that contains a binary
416 zero is correctly extracted and has a further zero added on the end, but the
417 result does not, of course, function as a C string.
418
419 The first three arguments are the same for all three functions: \fIsubject\fR
420 is the subject string which has just been successfully matched, \fIovector\fR
421 is a pointer to the vector of integer offsets that was passed to
422 \fBpcre_exec()\fR, and \fIstringcount\fR is the number of substrings that
423 were captured by the match, including the substring that matched the entire
424 regular expression. This is the value returned by \fBpcre_exec\fR if it
425 is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
426 ran out of space in \fIovector\fR, then the value passed as
427 \fIstringcount\fR should be the size of the vector divided by three.
428
429 The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
430 extract a single substring, whose number is given as \fIstringnumber\fR. A
431 value of zero extracts the substring that matched the entire pattern, while
432 higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
433 the string is placed in \fIbuffer\fR, whose length is given by
434 \fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is
435 obtained via \fBpcre_malloc\fR, and its address is returned via
436 \fIstringptr\fR. The yield of the function is the length of the string, not
437 including the terminating zero, or one of
438
439 PCRE_ERROR_NOMEMORY (-6)
440
441 The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get
442 memory failed for \fBpcre_get_substring()\fR.
443
444 PCRE_ERROR_NOSUBSTRING (-7)
445
446 There is no substring whose number is \fIstringnumber\fR.
447
448 The \fBpcre_get_substring_list()\fR function extracts all available substrings
449 and builds a list of pointers to them. All this is done in a single block of
450 memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
451 is returned via \fIlistptr\fR, which is also the start of the list of string
452 pointers. The end of the list is marked by a NULL pointer. The yield of the
453 function is zero if all went well, or
454
455 PCRE_ERROR_NOMEMORY (-6)
456
457 if the attempt to get the memory block failed.
458
459 When any of these functions encounter a substring that is unset, which can
460 happen when capturing subpattern number \fIn+1\fR matches some part of the
461 subject, but subpattern \fIn\fR has not been used at all, they return an empty
462 string. This can be distinguished from a genuine zero-length substring by
463 inspecting the appropriate offset in \fIovector\fR, which is negative for unset
464 substrings.
465
466
467
468 .SH LIMITATIONS
469 There are some size limitations in PCRE but it is hoped that they will never in
470 practice be relevant.
471 The maximum length of a compiled pattern is 65539 (sic) bytes.
472 All values in repeating quantifiers must be less than 65536.
473 The maximum number of capturing subpatterns is 99.
474 The maximum number of all parenthesized subpatterns, including capturing
475 subpatterns, assertions, and other types of subpattern, is 200.
476
477 The maximum length of a subject string is the largest positive number that an
478 integer variable can hold. However, PCRE uses recursion to handle subpatterns
479 and indefinite repetition. This means that the available stack space may limit
480 the size of a subject string that can be processed by certain patterns.
481
482
483 .SH DIFFERENCES FROM PERL
484 The differences described here are with respect to Perl 5.005.
485
486 1. By default, a whitespace character is any character that the C library
487 function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
488 alternative character type tables. Normally \fBisspace()\fR matches space,
489 formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
490 no longer includes vertical tab in its set of whitespace characters. The \\v
491 escape that was in the Perl documentation for a long time was never in fact
492 recognized. However, the character itself was treated as whitespace at least
493 up to 5.002. In 5.004 and 5.005 it does not match \\s.
494
495 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
496 them, but they do not mean what you might think. For example, (?!a){3} does
497 not assert that the next three characters are not "a". It just asserts that the
498 next character is not "a" three times.
499
500 3. Capturing subpatterns that occur inside negative lookahead assertions are
501 counted, but their entries in the offsets vector are never set. Perl sets its
502 numerical variables from any such patterns that are matched before the
503 assertion fails to match something (thereby succeeding), but only if the
504 negative lookahead assertion contains just one branch.
505
506 4. Though binary zero characters are supported in the subject string, they are
507 not allowed in a pattern string because it is passed as a normal C string,
508 terminated by zero. The escape sequence "\\0" can be used in the pattern to
509 represent a binary zero.
510
511 5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
512 \\E, \\Q. In fact these are implemented by Perl's general string-handling and
513 are not part of its pattern matching engine.
514
515 6. The Perl \\G assertion is not supported as it is not relevant to single
516 pattern matches.
517
518 7. Fairly obviously, PCRE does not support the (?{code}) construction.
519
520 8. There are at the time of writing some oddities in Perl 5.005_02 concerned
521 with the settings of captured strings when part of a pattern is repeated. For
522 example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
523 "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
524 the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set.
525
526 In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
527 future Perl changes to a consistent state that is different, PCRE may change to
528 follow.
529
530 9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
531 /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
532 However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
533
534 10. PCRE provides some extensions to the Perl regular expression facilities:
535
536 (a) Although lookbehind assertions must match fixed length strings, each
537 alternative branch of a lookbehind assertion can match a different length of
538 string. Perl 5.005 requires them all to have the same length.
539
540 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
541 character matches only at the very end of the string.
542
543 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
544 meaning is faulted.
545
546 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
547 inverted, that is, by default they are not greedy, but if followed by a
548 question mark they are.
549
550
551 .SH REGULAR EXPRESSION DETAILS
552 The syntax and semantics of the regular expressions supported by PCRE are
553 described below. Regular expressions are also described in the Perl
554 documentation and in a number of other books, some of which have copious
555 examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
556 O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description
557 here is intended as reference documentation.
558
559 A regular expression is a pattern that is matched against a subject string from
560 left to right. Most characters stand for themselves in a pattern, and match the
561 corresponding characters in the subject. As a trivial example, the pattern
562
563 The quick brown fox
564
565 matches a portion of a subject string that is identical to itself. The power of
566 regular expressions comes from the ability to include alternatives and
567 repetitions in the pattern. These are encoded in the pattern by the use of
568 \fImeta-characters\fR, which do not stand for themselves but instead are
569 interpreted in some special way.
570
571 There are two different sets of meta-characters: those that are recognized
572 anywhere in the pattern except within square brackets, and those that are
573 recognized in square brackets. Outside square brackets, the meta-characters are
574 as follows:
575
576 \\ general escape character with several uses
577 ^ assert start of subject (or line, in multiline mode)
578 $ assert end of subject (or line, in multiline mode)
579 . match any character except newline (by default)
580 [ start character class definition
581 | start of alternative branch
582 ( start subpattern
583 ) end subpattern
584 ? extends the meaning of (
585 also 0 or 1 quantifier
586 also quantifier minimizer
587 * 0 or more quantifier
588 + 1 or more quantifier
589 { start min/max quantifier
590
591 Part of a pattern that is in square brackets is called a "character class". In
592 a character class the only meta-characters are:
593
594 \\ general escape character
595 ^ negate the class, but only if the first character
596 - indicates character range
597 ] terminates the character class
598
599 The following sections describe the use of each of the meta-characters.
600
601
602 .SH BACKSLASH
603 The backslash character has several uses. Firstly, if it is followed by a
604 non-alphameric character, it takes away any special meaning that character may
605 have. This use of backslash as an escape character applies both inside and
606 outside character classes.
607
608 For example, if you want to match a "*" character, you write "\\*" in the
609 pattern. This applies whether or not the following character would otherwise be
610 interpreted as a meta-character, so it is always safe to precede a
611 non-alphameric with "\\" to specify that it stands for itself. In particular,
612 if you want to match a backslash, you write "\\\\".
613
614 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
615 pattern (other than in a character class) and characters between a "#" outside
616 a character class and the next newline character are ignored. An escaping
617 backslash can be used to include a whitespace or "#" character as part of the
618 pattern.
619
620 A second use of backslash provides a way of encoding non-printing characters
621 in patterns in a visible manner. There is no restriction on the appearance of
622 non-printing characters, apart from the binary zero that terminates a pattern,
623 but when a pattern is being prepared by text editing, it is usually easier to
624 use one of the following escape sequences than the binary character it
625 represents:
626
627 \\a alarm, that is, the BEL character (hex 07)
628 \\cx "control-x", where x is any character
629 \\e escape (hex 1B)
630 \\f formfeed (hex 0C)
631 \\n newline (hex 0A)
632 \\r carriage return (hex 0D)
633 \\t tab (hex 09)
634 \\xhh character with hex code hh
635 \\ddd character with octal code ddd, or backreference
636
637 The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
638 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
639 Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
640 7B.
641
642 After "\\x", up to two hexadecimal digits are read (letters can be in upper or
643 lower case).
644
645 After "\\0" up to two further octal digits are read. In both cases, if there
646 are fewer than two digits, just those that are present are used. Thus the
647 sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
648 Make sure you supply two digits after the initial zero if the character that
649 follows is itself an octal digit.
650
651 The handling of a backslash followed by a digit other than 0 is complicated.
652 Outside a character class, PCRE reads it and any following digits as a decimal
653 number. If the number is less than 10, or if there have been at least that many
654 previous capturing left parentheses in the expression, the entire sequence is
655 taken as a \fIback reference\fR. A description of how this works is given
656 later, following the discussion of parenthesized subpatterns.
657
658 Inside a character class, or if the decimal number is greater than 9 and there
659 have not been that many capturing subpatterns, PCRE re-reads up to three octal
660 digits following the backslash, and generates a single byte from the least
661 significant 8 bits of the value. Any subsequent digits stand for themselves.
662 For example:
663
664 \\040 is another way of writing a space
665 \\40 is the same, provided there are fewer than 40
666 previous capturing subpatterns
667 \\7 is always a back reference
668 \\11 might be a back reference, or another way of
669 writing a tab
670 \\011 is always a tab
671 \\0113 is a tab followed by the character "3"
672 \\113 is the character with octal code 113 (since there
673 can be no more than 99 back references)
674 \\377 is a byte consisting entirely of 1 bits
675 \\81 is either a back reference, or a binary zero
676 followed by the two characters "8" and "1"
677
678 Note that octal values of 100 or greater must not be introduced by a leading
679 zero, because no more than three octal digits are ever read.
680
681 All the sequences that define a single byte value can be used both inside and
682 outside character classes. In addition, inside a character class, the sequence
683 "\\b" is interpreted as the backspace character (hex 08). Outside a character
684 class it has a different meaning (see below).
685
686 The third use of backslash is for specifying generic character types:
687
688 \\d any decimal digit
689 \\D any character that is not a decimal digit
690 \\s any whitespace character
691 \\S any character that is not a whitespace character
692 \\w any "word" character
693 \\W any "non-word" character
694
695 Each pair of escape sequences partitions the complete set of characters into
696 two disjoint sets. Any given character matches one, and only one, of each pair.
697
698 A "word" character is any letter or digit or the underscore character, that is,
699 any character which can be part of a Perl "word". The definition of letters and
700 digits is controlled by PCRE's character tables, and may vary if locale-
701 specific matching is taking place (see "Locale support" above). For example, in
702 the "fr" (French) locale, some character codes greater than 128 are used for
703 accented letters, and these are matched by \\w.
704
705 These character type sequences can appear both inside and outside character
706 classes. They each match one character of the appropriate type. If the current
707 matching point is at the end of the subject string, all of them fail, since
708 there is no character to match.
709
710 The fourth use of backslash is for certain simple assertions. An assertion
711 specifies a condition that has to be met at a particular point in a match,
712 without consuming any characters from the subject string. The use of
713 subpatterns for more complicated assertions is described below. The backslashed
714 assertions are
715
716 \\b word boundary
717 \\B not a word boundary
718 \\A start of subject (independent of multiline mode)
719 \\Z end of subject or newline at end (independent of multiline mode)
720 \\z end of subject (independent of multiline mode)
721
722 These assertions may not appear in character classes (but note that "\\b" has a
723 different meaning, namely the backspace character, inside a character class).
724
725 A word boundary is a position in the subject string where the current character
726 and the previous character do not both match \\w or \\W (i.e. one matches
727 \\w and the other matches \\W), or the start or end of the string if the
728 first or last character matches \\w, respectively.
729
730 The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
731 dollar (described below) in that they only ever match at the very start and end
732 of the subject string, whatever options are set. They are not affected by the
733 PCRE_NOTBOL or PCRE_NOTEOL options. The difference between \\Z and \\z is that
734 \\Z matches before a newline that is the last character of the string as well
735 as at the end of the string, whereas \\z matches only at the end.
736
737
738 .SH CIRCUMFLEX AND DOLLAR
739 Outside a character class, in the default matching mode, the circumflex
740 character is an assertion which is true only if the current matching point is
741 at the start of the subject string. Inside a character class, circumflex has an
742 entirely different meaning (see below).
743
744 Circumflex need not be the first character of the pattern if a number of
745 alternatives are involved, but it should be the first thing in each alternative
746 in which it appears if the pattern is ever to match that branch. If all
747 possible alternatives start with a circumflex, that is, if the pattern is
748 constrained to match only at the start of the subject, it is said to be an
749 "anchored" pattern. (There are also other constructs that can cause a pattern
750 to be anchored.)
751
752 A dollar character is an assertion which is true only if the current matching
753 point is at the end of the subject string, or immediately before a newline
754 character that is the last character in the string (by default). Dollar need
755 not be the last character of the pattern if a number of alternatives are
756 involved, but it should be the last item in any branch in which it appears.
757 Dollar has no special meaning in a character class.
758
759 The meaning of dollar can be changed so that it matches only at the very end of
760 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
761 time. This does not affect the \\Z assertion.
762
763 The meanings of the circumflex and dollar characters are changed if the
764 PCRE_MULTILINE option is set. When this is the case, they match immediately
765 after and immediately before an internal "\\n" character, respectively, in
766 addition to matching at the start and end of the subject string. For example,
767 the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
768 but not otherwise. Consequently, patterns that are anchored in single line mode
769 because all branches start with "^" are not anchored in multiline mode. The
770 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
771
772 Note that the sequences \\A, \\Z, and \\z can be used to match the start and
773 end of the subject in both modes, and if all branches of a pattern start with
774 \\A is it always anchored, whether PCRE_MULTILINE is set or not.
775
776
777 .SH FULL STOP (PERIOD, DOT)
778 Outside a character class, a dot in the pattern matches any one character in
779 the subject, including a non-printing character, but not (by default) newline.
780 If the PCRE_DOTALL option is set, then dots match newlines as well. The
781 handling of dot is entirely independent of the handling of circumflex and
782 dollar, the only relationship being that they both involve newline characters.
783 Dot has no special meaning in a character class.
784
785
786 .SH SQUARE BRACKETS
787 An opening square bracket introduces a character class, terminated by a closing
788 square bracket. A closing square bracket on its own is not special. If a
789 closing square bracket is required as a member of the class, it should be the
790 first data character in the class (after an initial circumflex, if present) or
791 escaped with a backslash.
792
793 A character class matches a single character in the subject; the character must
794 be in the set of characters defined by the class, unless the first character in
795 the class is a circumflex, in which case the subject character must not be in
796 the set defined by the class. If a circumflex is actually required as a member
797 of the class, ensure it is not the first character, or escape it with a
798 backslash.
799
800 For example, the character class [aeiou] matches any lower case vowel, while
801 [^aeiou] matches any character that is not a lower case vowel. Note that a
802 circumflex is just a convenient notation for specifying the characters which
803 are in the class by enumerating those that are not. It is not an assertion: it
804 still consumes a character from the subject string, and fails if the current
805 pointer is at the end of the string.
806
807 When caseless matching is set, any letters in a class represent both their
808 upper case and lower case versions, so for example, a caseless [aeiou] matches
809 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
810 caseful version would.
811
812 The newline character is never treated in any special way in character classes,
813 whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
814 such as [^a] will always match a newline.
815
816 The minus (hyphen) character can be used to specify a range of characters in a
817 character class. For example, [d-m] matches any letter between d and m,
818 inclusive. If a minus character is required in a class, it must be escaped with
819 a backslash or appear in a position where it cannot be interpreted as
820 indicating a range, typically as the first or last character in the class.
821
822 It is not possible to have the literal character "]" as the end character of a
823 range. A pattern such as [W-]46] is interpreted as a class of two characters
824 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
825 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
826 the end of range, so [W-\\]46] is interpreted as a single class containing a
827 range followed by two separate characters. The octal or hexadecimal
828 representation of "]" can also be used to end a range.
829
830 Ranges operate in ASCII collating sequence. They can also be used for
831 characters specified numerically, for example [\\000-\\037]. If a range that
832 includes letters is used when caseless matching is set, it matches the letters
833 in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
834 caselessly, and if character tables for the "fr" locale are in use,
835 [\\xc8-\\xcb] matches accented E characters in both cases.
836
837 The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
838 character class, and add the characters that they match to the class. For
839 example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
840 conveniently be used with the upper case character types to specify a more
841 restricted set of characters than the matching lower case type. For example,
842 the class [^\\W_] matches any letter or digit, but not underscore.
843
844 All non-alphameric characters other than \\, -, ^ (at the start) and the
845 terminating ] are non-special in character classes, but it does no harm if they
846 are escaped.
847
848
849 .SH VERTICAL BAR
850 Vertical bar characters are used to separate alternative patterns. For example,
851 the pattern
852
853 gilbert|sullivan
854
855 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
856 and an empty alternative is permitted (matching the empty string).
857 The matching process tries each alternative in turn, from left to right,
858 and the first one that succeeds is used. If the alternatives are within a
859 subpattern (defined below), "succeeds" means matching the rest of the main
860 pattern as well as the alternative in the subpattern.
861
862
863 .SH INTERNAL OPTION SETTING
864 The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
865 can be changed from within the pattern by a sequence of Perl option letters
866 enclosed between "(?" and ")". The option letters are
867
868 i for PCRE_CASELESS
869 m for PCRE_MULTILINE
870 s for PCRE_DOTALL
871 x for PCRE_EXTENDED
872
873 For example, (?im) sets caseless, multiline matching. It is also possible to
874 unset these options by preceding the letter with a hyphen, and a combined
875 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
876 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
877 permitted. If a letter appears both before and after the hyphen, the option is
878 unset.
879
880 The scope of these option changes depends on where in the pattern the setting
881 occurs. For settings that are outside any subpattern (defined below), the
882 effect is the same as if the options were set or unset at the start of
883 matching. The following patterns all behave in exactly the same way:
884
885 (?i)abc
886 a(?i)bc
887 ab(?i)c
888 abc(?i)
889
890 which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
891 In other words, such "top level" settings apply to the whole pattern (unless
892 there are other changes inside subpatterns). If there is more than one setting
893 of the same option at top level, the rightmost setting is used.
894
895 If an option change occurs inside a subpattern, the effect is different. This
896 is a change of behaviour in Perl 5.005. An option change inside a subpattern
897 affects only that part of the subpattern that follows it, so
898
899 (a(?i)b)c
900
901 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
902 By this means, options can be made to have different settings in different
903 parts of the pattern. Any changes made in one alternative do carry on
904 into subsequent branches within the same subpattern. For example,
905
906 (a(?i)b|c)
907
908 matches "ab", "aB", "c", and "C", even though when matching "C" the first
909 branch is abandoned before the option setting. This is because the effects of
910 option settings happen at compile time. There would be some very weird
911 behaviour otherwise.
912
913 The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
914 same way as the Perl-compatible options by using the characters U and X
915 respectively. The (?X) flag setting is special in that it must always occur
916 earlier in the pattern than any of the additional features it turns on, even
917 when it is at top level. It is best put at the start.
918
919
920 .SH SUBPATTERNS
921 Subpatterns are delimited by parentheses (round brackets), which can be nested.
922 Marking part of a pattern as a subpattern does two things:
923
924 1. It localizes a set of alternatives. For example, the pattern
925
926 cat(aract|erpillar|)
927
928 matches one of the words "cat", "cataract", or "caterpillar". Without the
929 parentheses, it would match "cataract", "erpillar" or the empty string.
930
931 2. It sets up the subpattern as a capturing subpattern (as defined above).
932 When the whole pattern matches, that portion of the subject string that matched
933 the subpattern is passed back to the caller via the \fIovector\fR argument of
934 \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
935 from 1) to obtain the numbers of the capturing subpatterns.
936
937 For example, if the string "the red king" is matched against the pattern
938
939 the ((red|white) (king|queen))
940
941 the captured substrings are "red king", "red", and "king", and are numbered 1,
942 2, and 3.
943
944 The fact that plain parentheses fulfil two functions is not always helpful.
945 There are often times when a grouping subpattern is required without a
946 capturing requirement. If an opening parenthesis is followed by "?:", the
947 subpattern does not do any capturing, and is not counted when computing the
948 number of any subsequent capturing subpatterns. For example, if the string "the
949 white queen" is matched against the pattern
950
951 the ((?:red|white) (king|queen))
952
953 the captured substrings are "white queen" and "queen", and are numbered 1 and
954 2. The maximum number of captured substrings is 99, and the maximum number of
955 all subpatterns, both capturing and non-capturing, is 200.
956
957 As a convenient shorthand, if any option settings are required at the start of
958 a non-capturing subpattern, the option letters may appear between the "?" and
959 the ":". Thus the two patterns
960
961 (?i:saturday|sunday)
962 (?:(?i)saturday|sunday)
963
964 match exactly the same set of strings. Because alternative branches are tried
965 from left to right, and options are not reset until the end of the subpattern
966 is reached, an option setting in one branch does affect subsequent branches, so
967 the above patterns match "SUNDAY" as well as "Saturday".
968
969
970 .SH REPETITION
971 Repetition is specified by quantifiers, which can follow any of the following
972 items:
973
974 a single character, possibly escaped
975 the . metacharacter
976 a character class
977 a back reference (see next section)
978 a parenthesized subpattern (unless it is an assertion - see below)
979
980 The general repetition quantifier specifies a minimum and maximum number of
981 permitted matches, by giving the two numbers in curly brackets (braces),
982 separated by a comma. The numbers must be less than 65536, and the first must
983 be less than or equal to the second. For example:
984
985 z{2,4}
986
987 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
988 character. If the second number is omitted, but the comma is present, there is
989 no upper limit; if the second number and the comma are both omitted, the
990 quantifier specifies an exact number of required matches. Thus
991
992 [aeiou]{3,}
993
994 matches at least 3 successive vowels, but may match many more, while
995
996 \\d{8}
997
998 matches exactly 8 digits. An opening curly bracket that appears in a position
999 where a quantifier is not allowed, or one that does not match the syntax of a
1000 quantifier, is taken as a literal character. For example, {,6} is not a
1001 quantifier, but a literal string of four characters.
1002
1003 The quantifier {0} is permitted, causing the expression to behave as if the
1004 previous item and the quantifier were not present.
1005
1006 For convenience (and historical compatibility) the three most common
1007 quantifiers have single-character abbreviations:
1008
1009 * is equivalent to {0,}
1010 + is equivalent to {1,}
1011 ? is equivalent to {0,1}
1012
1013 It is possible to construct infinite loops by following a subpattern that can
1014 match no characters with a quantifier that has no upper limit, for example:
1015
1016 (a?)*
1017
1018 Earlier versions of Perl and PCRE used to give an error at compile time for
1019 such patterns. However, because there are cases where this can be useful, such
1020 patterns are now accepted, but if any repetition of the subpattern does in fact
1021 match no characters, the loop is forcibly broken.
1022
1023 By default, the quantifiers are "greedy", that is, they match as much as
1024 possible (up to the maximum number of permitted times), without causing the
1025 rest of the pattern to fail. The classic example of where this gives problems
1026 is in trying to match comments in C programs. These appear between the
1027 sequences /* and */ and within the sequence, individual * and / characters may
1028 appear. An attempt to match C comments by applying the pattern
1029
1030 /\\*.*\\*/
1031
1032 to the string
1033
1034 /* first command */ not comment /* second comment */
1035
1036 fails, because it matches the entire string due to the greediness of the .*
1037 item.
1038
1039 However, if a quantifier is followed by a question mark, then it ceases to be
1040 greedy, and instead matches the minimum number of times possible, so the
1041 pattern
1042
1043 /\\*.*?\\*/
1044
1045 does the right thing with the C comments. The meaning of the various
1046 quantifiers is not otherwise changed, just the preferred number of matches.
1047 Do not confuse this use of question mark with its use as a quantifier in its
1048 own right. Because it has two uses, it can sometimes appear doubled, as in
1049
1050 \\d??\\d
1051
1052 which matches one digit by preference, but can match two if that is the only
1053 way the rest of the pattern matches.
1054
1055 If the PCRE_UNGREEDY option is set (an option which is not available in Perl)
1056 then the quantifiers are not greedy by default, but individual ones can be made
1057 greedy by following them with a question mark. In other words, it inverts the
1058 default behaviour.
1059
1060 When a parenthesized subpattern is quantified with a minimum repeat count that
1061 is greater than 1 or with a limited maximum, more store is required for the
1062 compiled pattern, in proportion to the size of the minimum or maximum.
1063
1064 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1065 to Perl's /s) is set, thus allowing the . to match newlines, then the pattern
1066 is implicitly anchored, because whatever follows will be tried against every
1067 character position in the subject string, so there is no point in retrying the
1068 overall match at any position after the first. PCRE treats such a pattern as
1069 though it were preceded by \\A. In cases where it is known that the subject
1070 string contains no newlines, it is worth setting PCRE_DOTALL when the pattern
1071 begins with .* in order to obtain this optimization, or alternatively using ^
1072 to indicate anchoring explicitly.
1073
1074 When a capturing subpattern is repeated, the value captured is the substring
1075 that matched the final iteration. For example, after
1076
1077 (tweedle[dume]{3}\\s*)+
1078
1079 has matched "tweedledum tweedledee" the value of the captured substring is
1080 "tweedledee". However, if there are nested capturing subpatterns, the
1081 corresponding captured values may have been set in previous iterations. For
1082 example, after
1083
1084 /(a|(b))+/
1085
1086 matches "aba" the value of the second captured substring is "b".
1087
1088
1089 .SH BACK REFERENCES
1090 Outside a character class, a backslash followed by a digit greater than 0 (and
1091 possibly further digits) is a back reference to a capturing subpattern earlier
1092 (i.e. to its left) in the pattern, provided there have been that many previous
1093 capturing left parentheses.
1094
1095 However, if the decimal number following the backslash is less than 10, it is
1096 always taken as a back reference, and causes an error only if there are not
1097 that many capturing left parentheses in the entire pattern. In other words, the
1098 parentheses that are referenced need not be to the left of the reference for
1099 numbers less than 10. See the section entitled "Backslash" above for further
1100 details of the handling of digits following a backslash.
1101
1102 A back reference matches whatever actually matched the capturing subpattern in
1103 the current subject string, rather than anything matching the subpattern
1104 itself. So the pattern
1105
1106 (sens|respons)e and \\1ibility
1107
1108 matches "sense and sensibility" and "response and responsibility", but not
1109 "sense and responsibility". If caseful matching is in force at the time of the
1110 back reference, then the case of letters is relevant. For example,
1111
1112 ((?i)rah)\\s+\\1
1113
1114 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1115 capturing subpattern is matched caselessly.
1116
1117 There may be more than one back reference to the same subpattern. If a
1118 subpattern has not actually been used in a particular match, then any back
1119 references to it always fail. For example, the pattern
1120
1121 (a|(bc))\\2
1122
1123 always fails if it starts to match "a" rather than "bc". Because there may be
1124 up to 99 back references, all digits following the backslash are taken
1125 as part of a potential back reference number. If the pattern continues with a
1126 digit character, then some delimiter must be used to terminate the back
1127 reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1128 Otherwise an empty comment can be used.
1129
1130 A back reference that occurs inside the parentheses to which it refers fails
1131 when the subpattern is first used, so, for example, (a\\1) never matches.
1132 However, such references can be useful inside repeated subpatterns. For
1133 example, the pattern
1134
1135 (a|b\\1)+
1136
1137 matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
1138 the subpattern, the back reference matches the character string corresponding
1139 to the previous iteration. In order for this to work, the pattern must be such
1140 that the first iteration does not need to match the back reference. This can be
1141 done using alternation, as in the example above, or by a quantifier with a
1142 minimum of zero.
1143
1144
1145 .SH ASSERTIONS
1146 An assertion is a test on the characters following or preceding the current
1147 matching point that does not actually consume any characters. The simple
1148 assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
1149 complicated assertions are coded as subpatterns. There are two kinds: those
1150 that look ahead of the current position in the subject string, and those that
1151 look behind it.
1152
1153 An assertion subpattern is matched in the normal way, except that it does not
1154 cause the current matching position to be changed. Lookahead assertions start
1155 with (?= for positive assertions and (?! for negative assertions. For example,
1156
1157 \\w+(?=;)
1158
1159 matches a word followed by a semicolon, but does not include the semicolon in
1160 the match, and
1161
1162 foo(?!bar)
1163
1164 matches any occurrence of "foo" that is not followed by "bar". Note that the
1165 apparently similar pattern
1166
1167 (?!foo)bar
1168
1169 does not find an occurrence of "bar" that is preceded by something other than
1170 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1171 (?!foo) is always true when the next three characters are "bar". A
1172 lookbehind assertion is needed to achieve this effect.
1173
1174 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1175 negative assertions. For example,
1176
1177 (?<!foo)bar
1178
1179 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1180 a lookbehind assertion are restricted such that all the strings it matches must
1181 have a fixed length. However, if there are several alternatives, they do not
1182 all have to have the same fixed length. Thus
1183
1184 (?<=bullock|donkey)
1185
1186 is permitted, but
1187
1188 (?<!dogs?|cats?)
1189
1190 causes an error at compile time. Branches that match different length strings
1191 are permitted only at the top level of a lookbehind assertion. This is an
1192 extension compared with Perl 5.005, which requires all branches to match the
1193 same length of string. An assertion such as
1194
1195 (?<=ab(c|de))
1196
1197 is not permitted, because its single top-level branch can match two different
1198 lengths, but it is acceptable if rewritten to use two top-level branches:
1199
1200 (?<=abc|abde)
1201
1202 The implementation of lookbehind assertions is, for each alternative, to
1203 temporarily move the current position back by the fixed width and then try to
1204 match. If there are insufficient characters before the current position, the
1205 match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
1206 can be particularly useful for matching at the ends of strings; an example is
1207 given at the end of the section on once-only subpatterns.
1208
1209 Several assertions (of any sort) may occur in succession. For example,
1210
1211 (?<=\\d{3})(?<!999)foo
1212
1213 matches "foo" preceded by three digits that are not "999". Furthermore,
1214 assertions can be nested in any combination. For example,
1215
1216 (?<=(?<!foo)bar)baz
1217
1218 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1219 preceded by "foo".
1220
1221 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1222 because it makes no sense to assert the same thing several times. If an
1223 assertion contains capturing subpatterns within it, these are always counted
1224 for the purposes of numbering the capturing subpatterns in the whole pattern.
1225 Substring capturing is carried out for positive assertions, but it does not
1226 make sense for negative assertions.
1227
1228 Assertions count towards the maximum of 200 parenthesized subpatterns.
1229
1230
1231 .SH ONCE-ONLY SUBPATTERNS
1232 With both maximizing and minimizing repetition, failure of what follows
1233 normally causes the repeated item to be re-evaluated to see if a different
1234 number of repeats allows the rest of the pattern to match. Sometimes it is
1235 useful to prevent this, either to change the nature of the match, or to cause
1236 it fail earlier than it otherwise might, when the author of the pattern knows
1237 there is no point in carrying on.
1238
1239 Consider, for example, the pattern \\d+foo when applied to the subject line
1240
1241 123456bar
1242
1243 After matching all 6 digits and then failing to match "foo", the normal
1244 action of the matcher is to try again with only 5 digits matching the \\d+
1245 item, and then with 4, and so on, before ultimately failing. Once-only
1246 subpatterns provide the means for specifying that once a portion of the pattern
1247 has matched, it is not to be re-evaluated in this way, so the matcher would
1248 give up immediately on failing to match "foo" the first time. The notation is
1249 another kind of special parenthesis, starting with (?> as in this example:
1250
1251 (?>\\d+)bar
1252
1253 This kind of parenthesis "locks up" the part of the pattern it contains once
1254 it has matched, and a failure further into the pattern is prevented from
1255 backtracking into it. Backtracking past it to previous items, however, works as
1256 normal.
1257
1258 An alternative description is that a subpattern of this type matches the string
1259 of characters that an identical standalone pattern would match, if anchored at
1260 the current point in the subject string.
1261
1262 Once-only subpatterns are not capturing subpatterns. Simple cases such as the
1263 above example can be thought of as a maximizing repeat that must swallow
1264 everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
1265 number of digits they match in order to make the rest of the pattern match,
1266 (?>\\d+) can only match an entire sequence of digits.
1267
1268 This construction can of course contain arbitrarily complicated subpatterns,
1269 and it can be nested.
1270
1271 Once-only subpatterns can be used in conjunction with lookbehind assertions to
1272 specify efficient matching at the end of the subject string. Consider a simple
1273 pattern such as
1274
1275 abcd$
1276
1277 when applied to a long string which does not match it. Because matching
1278 proceeds from left to right, PCRE will look for each "a" in the subject and
1279 then see if what follows matches the rest of the pattern. If the pattern is
1280 specified as
1281
1282 ^.*abcd$
1283
1284 then the initial .* matches the entire string at first, but when this fails, it
1285 backtracks to match all but the last character, then all but the last two
1286 characters, and so on. Once again the search for "a" covers the entire string,
1287 from right to left, so we are no better off. However, if the pattern is written
1288 as
1289
1290 ^(?>.*)(?<=abcd)
1291
1292 then there can be no backtracking for the .* item; it can match only the entire
1293 string. The subsequent lookbehind assertion does a single test on the last four
1294 characters. If it fails, the match fails immediately. For long strings, this
1295 approach makes a significant difference to the processing time.
1296
1297
1298 .SH CONDITIONAL SUBPATTERNS
1299 It is possible to cause the matching process to obey a subpattern
1300 conditionally or to choose between two alternative subpatterns, depending on
1301 the result of an assertion, or whether a previous capturing subpattern matched
1302 or not. The two possible forms of conditional subpattern are
1303
1304 (?(condition)yes-pattern)
1305 (?(condition)yes-pattern|no-pattern)
1306
1307 If the condition is satisfied, the yes-pattern is used; otherwise the
1308 no-pattern (if present) is used. If there are more than two alternatives in the
1309 subpattern, a compile-time error occurs.
1310
1311 There are two kinds of condition. If the text between the parentheses consists
1312 of a sequence of digits, then the condition is satisfied if the capturing
1313 subpattern of that number has previously matched. Consider the following
1314 pattern, which contains non-significant white space to make it more readable
1315 (assume the PCRE_EXTENDED option) and to divide it into three parts for ease
1316 of discussion:
1317
1318 ( \\( )? [^()]+ (?(1) \\) )
1319
1320 The first part matches an optional opening parenthesis, and if that
1321 character is present, sets it as the first captured substring. The second part
1322 matches one or more characters that are not parentheses. The third part is a
1323 conditional subpattern that tests whether the first set of parentheses matched
1324 or not. If they did, that is, if subject started with an opening parenthesis,
1325 the condition is true, and so the yes-pattern is executed and a closing
1326 parenthesis is required. Otherwise, since no-pattern is not present, the
1327 subpattern matches nothing. In other words, this pattern matches a sequence of
1328 non-parentheses, optionally enclosed in parentheses.
1329
1330 If the condition is not a sequence of digits, it must be an assertion. This may
1331 be a positive or negative lookahead or lookbehind assertion. Consider this
1332 pattern, again containing non-significant white space, and with the two
1333 alternatives on the second line:
1334
1335 (?(?=[^a-z]*[a-z])
1336 \\d{2}[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} )
1337
1338 The condition is a positive lookahead assertion that matches an optional
1339 sequence of non-letters followed by a letter. In other words, it tests for the
1340 presence of at least one letter in the subject. If a letter is found, the
1341 subject is matched against the first alternative; otherwise it is matched
1342 against the second. This pattern matches strings in one of the two forms
1343 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1344
1345
1346 .SH COMMENTS
1347 The sequence (?# marks the start of a comment which continues up to the next
1348 closing parenthesis. Nested parentheses are not permitted. The characters
1349 that make up a comment play no part in the pattern matching at all.
1350
1351 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1352 character class introduces a comment that continues up to the next newline
1353 character in the pattern.
1354
1355
1356 .SH PERFORMANCE
1357 Certain items that may appear in patterns are more efficient than others. It is
1358 more efficient to use a character class like [aeiou] than a set of alternatives
1359 such as (a|e|i|o|u). In general, the simplest construction that provides the
1360 required behaviour is usually the most efficient. Jeffrey Friedl's book
1361 contains a lot of discussion about optimizing regular expressions for efficient
1362 performance.
1363
1364 When a pattern begins with .* and the PCRE_DOTALL option is set, the pattern is
1365 implicitly anchored by PCRE, since it can match only at the start of a subject
1366 string. However, if PCRE_DOTALL is not set, PCRE cannot make this optimization,
1367 because the . metacharacter does not then match a newline, and if the subject
1368 string contains newlines, the pattern may match from the character immediately
1369 following one of them instead of from the very start. For example, the pattern
1370
1371 (.*) second
1372
1373 matches the subject "first\\nand second" (where \\n stands for a newline
1374 character) with the first captured substring being "and". In order to do this,
1375 PCRE has to retry the match starting after every newline in the subject.
1376
1377 If you are using such a pattern with subject strings that do not contain
1378 newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
1379 the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
1380 having to scan along the subject looking for a newline to restart at.
1381
1382 .SH AUTHOR
1383 Philip Hazel <ph10@cam.ac.uk>
1384 .br
1385 University Computing Service,
1386 .br
1387 New Museums Site,
1388 .br
1389 Cambridge CB2 3QG, England.
1390 .br
1391 Phone: +44 1223 334714
1392
1393 Copyright (c) 1997-1999 University of Cambridge.

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