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Load pcre-2.03 into code/trunk.
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.
268
269 If the \fIfirstcharptr\fR argument is not NULL, is is used to pass back
270 information about the first character of any matched string. If there is a
271 fixed first character, e.g. from a pattern such as (cat|cow|coyote), then it is
272 returned in the integer pointed to by \fIfirstcharptr\fR. Otherwise, if the
273 pattern was compiled with the PCRE_MULTILINE option, and every branch started
274 with "^", then -1 is returned, indicating that the pattern will match at the
275 start of a subject string or after any "\\n" within the string. Otherwise -2 is
276 returned.
277
278
279 .SH MATCHING A PATTERN
280 The function \fBpcre_exec()\fR is called to match a subject string against a
281 pre-compiled pattern, which is passed in the \fIcode\fR argument. If the
282 pattern has been studied, the result of the study should be passed in the
283 \fIextra\fR argument. Otherwise this must be NULL.
284
285 The subject string is passed as a pointer in \fIsubject\fR and a length in
286 \fIlength\fR. Unlike the pattern string, it may contain binary zero characters.
287
288 The PCRE_ANCHORED option can be passed in the \fIoptions\fR argument, whose
289 unused bits must be zero. However, if a pattern was compiled with
290 PCRE_ANCHORED, or turned out to be anchored by virtue of its contents, it
291 cannot be made unachored at matching time.
292
293 There are also two further options that can be set only at matching time:
294
295 PCRE_NOTBOL
296
297 The first character of the string is not the beginning of a line, so the
298 circumflex metacharacter should not match before it. Setting this without
299 PCRE_MULTILINE (at compile time) causes circumflex never to match.
300
301 PCRE_NOTEOL
302
303 The end of the string is not the end of a line, so the dollar metacharacter
304 should not match it nor (except in multiline mode) a newline immediately before
305 it. Setting this without PCRE_MULTILINE (at compile time) causes dollar never
306 to match.
307
308 In general, a pattern matches a certain portion of the subject, and in
309 addition, further substrings from the subject may be picked out by parts of the
310 pattern. Following the usage in Jeffrey Friedl's book, this is called
311 "capturing" in what follows, and the phrase "capturing subpattern" is used for
312 a fragment of a pattern that picks out a substring. PCRE supports several other
313 kinds of parenthesized subpattern that do not cause substrings to be captured.
314
315 Captured substrings are returned to the caller via a vector of integer offsets
316 whose address is passed in \fIovector\fR. The number of elements in the vector
317 is passed in \fIovecsize\fR. The first two-thirds of the vector is used to pass
318 back captured substrings, each substring using a pair of integers. The
319 remaining third of the vector is used as workspace by \fBpcre_exec()\fR while
320 matching capturing subpatterns, and is not available for passing back
321 information. The length passed in \fIovecsize\fR should always be a multiple of
322 three. If it is not, it is rounded down.
323
324 When a match has been successful, information about captured substrings is
325 returned in pairs of integers, starting at the beginning of \fIovector\fR, and
326 continuing up to two-thirds of its length at the most. The first element of a
327 pair is set to the offset of the first character in a substring, and the second
328 is set to the offset of the first character after the end of a substring. The
329 first pair, \fIovector[0]\fR and \fIovector[1]\fR, identify the portion of the
330 subject string matched by the entire pattern. The next pair is used for the
331 first capturing subpattern, and so on. The value returned by \fBpcre_exec()\fR
332 is the number of pairs that have been set. If there are no capturing
333 subpatterns, the return value from a successful match is 1, indicating that
334 just the first pair of offsets has been set.
335
336 Some convenience functions are provided for extracting the captured substrings
337 as separate strings. These are described in the following section.
338
339 It is possible for an capturing subpattern number \fIn+1\fR to match some
340 part of the subject when subpattern \fIn\fR has not been used at all. For
341 example, if the string "abc" is matched against the pattern (a|(z))(bc)
342 subpatterns 1 and 3 are matched, but 2 is not. When this happens, both offset
343 values corresponding to the unused subpattern are set to -1.
344
345 If a capturing subpattern is matched repeatedly, it is the last portion of the
346 string that it matched that gets returned.
347
348 If the vector is too small to hold all the captured substrings, it is used as
349 far as possible (up to two-thirds of its length), and the function returns a
350 value of zero. In particular, if the substring offsets are not of interest,
351 \fBpcre_exec()\fR may be called with \fIovector\fR passed as NULL and
352 \fIovecsize\fR as zero. However, if the pattern contains back references and
353 the \fIovector\fR isn't big enough to remember the related substrings, PCRE has
354 to get additional memory for use during matching. Thus it is usually advisable
355 to supply an \fIovector\fR.
356
357 Note that \fBpcre_info()\fR can be used to find out how many capturing
358 subpatterns there are in a compiled pattern. The smallest size for
359 \fIovector\fR that will allow for \fIn\fR captured substrings in addition to
360 the offsets of the substring matched by the whole pattern is (\fIn\fR+1)*3.
361
362 If \fBpcre_exec()\fR fails, it returns a negative number. The following are
363 defined in the header file:
364
365 PCRE_ERROR_NOMATCH (-1)
366
367 The subject string did not match the pattern.
368
369 PCRE_ERROR_NULL (-2)
370
371 Either \fIcode\fR or \fIsubject\fR was passed as NULL, or \fIovector\fR was
372 NULL and \fIovecsize\fR was not zero.
373
374 PCRE_ERROR_BADOPTION (-3)
375
376 An unrecognized bit was set in the \fIoptions\fR argument.
377
378 PCRE_ERROR_BADMAGIC (-4)
379
380 PCRE stores a 4-byte "magic number" at the start of the compiled code, to catch
381 the case when it is passed a junk pointer. This is the error it gives when the
382 magic number isn't present.
383
384 PCRE_ERROR_UNKNOWN_NODE (-5)
385
386 While running the pattern match, an unknown item was encountered in the
387 compiled pattern. This error could be caused by a bug in PCRE or by overwriting
388 of the compiled pattern.
389
390 PCRE_ERROR_NOMEMORY (-6)
391
392 If a pattern contains back references, but the \fIovector\fR that is passed to
393 \fBpcre_exec()\fR is not big enough to remember the referenced substrings, PCRE
394 gets a block of memory at the start of matching to use for this purpose. If the
395 call via \fBpcre_malloc()\fR fails, this error is given. The memory is freed at
396 the end of matching.
397
398
399 .SH EXTRACTING CAPTURED SUBSTRINGS
400 Captured substrings can be accessed directly by using the offsets returned by
401 \fBpcre_exec()\fR in \fIovector\fR. For convenience, the functions
402 \fBpcre_copy_substring()\fR, \fBpcre_get_substring()\fR, and
403 \fBpcre_get_substring_list()\fR are provided for extracting captured substrings
404 as new, separate, zero-terminated strings. A substring that contains a binary
405 zero is correctly extracted and has a further zero added on the end, but the
406 result does not, of course, function as a C string.
407
408 The first three arguments are the same for all three functions: \fIsubject\fR
409 is the subject string which has just been successfully matched, \fIovector\fR
410 is a pointer to the vector of integer offsets that was passed to
411 \fBpcre_exec()\fR, and \fIstringcount\fR is the number of substrings that
412 were captured by the match, including the substring that matched the entire
413 regular expression. This is the value returned by \fBpcre_exec\fR if it
414 is greater than zero. If \fBpcre_exec()\fR returned zero, indicating that it
415 ran out of space in \fIovector\fR, then the value passed as
416 \fIstringcount\fR should be the size of the vector divided by three.
417
418 The functions \fBpcre_copy_substring()\fR and \fBpcre_get_substring()\fR
419 extract a single substring, whose number is given as \fIstringnumber\fR. A
420 value of zero extracts the substring that matched the entire pattern, while
421 higher values extract the captured substrings. For \fBpcre_copy_substring()\fR,
422 the string is placed in \fIbuffer\fR, whose length is given by
423 \fIbuffersize\fR, while for \fBpcre_get_substring()\fR a new block of store is
424 obtained via \fBpcre_malloc\fR, and its address is returned via
425 \fIstringptr\fR. The yield of the function is the length of the string, not
426 including the terminating zero, or one of
427
428 PCRE_ERROR_NOMEMORY (-6)
429
430 The buffer was too small for \fBpcre_copy_substring()\fR, or the attempt to get
431 memory failed for \fBpcre_get_substring()\fR.
432
433 PCRE_ERROR_NOSUBSTRING (-7)
434
435 There is no substring whose number is \fIstringnumber\fR.
436
437 The \fBpcre_get_substring_list()\fR function extracts all available substrings
438 and builds a list of pointers to them. All this is done in a single block of
439 memory which is obtained via \fBpcre_malloc\fR. The address of the memory block
440 is returned via \fIlistptr\fR, which is also the start of the list of string
441 pointers. The end of the list is marked by a NULL pointer. The yield of the
442 function is zero if all went well, or
443
444 PCRE_ERROR_NOMEMORY (-6)
445
446 if the attempt to get the memory block failed.
447
448 When any of these functions encounter a substring that is unset, which can
449 happen when capturing subpattern number \fIn+1\fR matches some part of the
450 subject, but subpattern \fIn\fR has not been used at all, they return an empty
451 string. This can be distinguished from a genuine zero-length substring by
452 inspecting the appropriate offset in \fIovector\fR, which is negative for unset
453 substrings.
454
455
456
457 .SH LIMITATIONS
458 There are some size limitations in PCRE but it is hoped that they will never in
459 practice be relevant.
460 The maximum length of a compiled pattern is 65539 (sic) bytes.
461 All values in repeating quantifiers must be less than 65536.
462 The maximum number of capturing subpatterns is 99.
463 The maximum number of all parenthesized subpatterns, including capturing
464 subpatterns, assertions, and other types of subpattern, is 200.
465
466 The maximum length of a subject string is the largest positive number that an
467 integer variable can hold. However, PCRE uses recursion to handle subpatterns
468 and indefinite repetition. This means that the available stack space may limit
469 the size of a subject string that can be processed by certain patterns.
470
471
472 .SH DIFFERENCES FROM PERL
473 The differences described here are with respect to Perl 5.005.
474
475 1. By default, a whitespace character is any character that the C library
476 function \fBisspace()\fR recognizes, though it is possible to compile PCRE with
477 alternative character type tables. Normally \fBisspace()\fR matches space,
478 formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
479 no longer includes vertical tab in its set of whitespace characters. The \\v
480 escape that was in the Perl documentation for a long time was never in fact
481 recognized. However, the character itself was treated as whitespace at least
482 up to 5.002. In 5.004 and 5.005 it does not match \\s.
483
484 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
485 them, but they do not mean what you might think. For example, (?!a){3} does
486 not assert that the next three characters are not "a". It just asserts that the
487 next character is not "a" three times.
488
489 3. Capturing subpatterns that occur inside negative lookahead assertions are
490 counted, but their entries in the offsets vector are never set. Perl sets its
491 numerical variables from any such patterns that are matched before the
492 assertion fails to match something (thereby succeeding), but only if the
493 negative lookahead assertion contains just one branch.
494
495 4. Though binary zero characters are supported in the subject string, they are
496 not allowed in a pattern string because it is passed as a normal C string,
497 terminated by zero. The escape sequence "\\0" can be used in the pattern to
498 represent a binary zero.
499
500 5. The following Perl escape sequences are not supported: \\l, \\u, \\L, \\U,
501 \\E, \\Q. In fact these are implemented by Perl's general string-handling and
502 are not part of its pattern matching engine.
503
504 6. The Perl \\G assertion is not supported as it is not relevant to single
505 pattern matches.
506
507 7. Fairly obviously, PCRE does not support the (?{code}) construction.
508
509 8. There are at the time of writing some oddities in Perl 5.005_02 concerned
510 with the settings of captured strings when part of a pattern is repeated. For
511 example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
512 "b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
513 the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set.
514
515 In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
516 future Perl changes to a consistent state that is different, PCRE may change to
517 follow.
518
519 9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
520 /^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
521 However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.
522
523 10. PCRE provides some extensions to the Perl regular expression facilities:
524
525 (a) Although lookbehind assertions must match fixed length strings, each
526 alternative branch of a lookbehind assertion can match a different length of
527 string. Perl 5.005 requires them all to have the same length.
528
529 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
530 character matches only at the very end of the string.
531
532 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
533 meaning is faulted.
534
535 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
536 inverted, that is, by default they are not greedy, but if followed by a
537 question mark they are.
538
539
540 .SH REGULAR EXPRESSION DETAILS
541 The syntax and semantics of the regular expressions supported by PCRE are
542 described below. Regular expressions are also described in the Perl
543 documentation and in a number of other books, some of which have copious
544 examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
545 O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description
546 here is intended as reference documentation.
547
548 A regular expression is a pattern that is matched against a subject string from
549 left to right. Most characters stand for themselves in a pattern, and match the
550 corresponding characters in the subject. As a trivial example, the pattern
551
552 The quick brown fox
553
554 matches a portion of a subject string that is identical to itself. The power of
555 regular expressions comes from the ability to include alternatives and
556 repetitions in the pattern. These are encoded in the pattern by the use of
557 \fImeta-characters\fR, which do not stand for themselves but instead are
558 interpreted in some special way.
559
560 There are two different sets of meta-characters: those that are recognized
561 anywhere in the pattern except within square brackets, and those that are
562 recognized in square brackets. Outside square brackets, the meta-characters are
563 as follows:
564
565 \\ general escape character with several uses
566 ^ assert start of subject (or line, in multiline mode)
567 $ assert end of subject (or line, in multiline mode)
568 . match any character except newline (by default)
569 [ start character class definition
570 | start of alternative branch
571 ( start subpattern
572 ) end subpattern
573 ? extends the meaning of (
574 also 0 or 1 quantifier
575 also quantifier minimizer
576 * 0 or more quantifier
577 + 1 or more quantifier
578 { start min/max quantifier
579
580 Part of a pattern that is in square brackets is called a "character class". In
581 a character class the only meta-characters are:
582
583 \\ general escape character
584 ^ negate the class, but only if the first character
585 - indicates character range
586 ] terminates the character class
587
588 The following sections describe the use of each of the meta-characters.
589
590
591 .SH BACKSLASH
592 The backslash character has several uses. Firstly, if it is followed by a
593 non-alphameric character, it takes away any special meaning that character may
594 have. This use of backslash as an escape character applies both inside and
595 outside character classes.
596
597 For example, if you want to match a "*" character, you write "\\*" in the
598 pattern. This applies whether or not the following character would otherwise be
599 interpreted as a meta-character, so it is always safe to precede a
600 non-alphameric with "\\" to specify that it stands for itself. In particular,
601 if you want to match a backslash, you write "\\\\".
602
603 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
604 pattern (other than in a character class) and characters between a "#" outside
605 a character class and the next newline character are ignored. An escaping
606 backslash can be used to include a whitespace or "#" character as part of the
607 pattern.
608
609 A second use of backslash provides a way of encoding non-printing characters
610 in patterns in a visible manner. There is no restriction on the appearance of
611 non-printing characters, apart from the binary zero that terminates a pattern,
612 but when a pattern is being prepared by text editing, it is usually easier to
613 use one of the following escape sequences than the binary character it
614 represents:
615
616 \\a alarm, that is, the BEL character (hex 07)
617 \\cx "control-x", where x is any character
618 \\e escape (hex 1B)
619 \\f formfeed (hex 0C)
620 \\n newline (hex 0A)
621 \\r carriage return (hex 0D)
622 \\t tab (hex 09)
623 \\xhh character with hex code hh
624 \\ddd character with octal code ddd, or backreference
625
626 The precise effect of "\\cx" is as follows: if "x" is a lower case letter, it
627 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
628 Thus "\\cz" becomes hex 1A, but "\\c{" becomes hex 3B, while "\\c;" becomes hex
629 7B.
630
631 After "\\x", up to two hexadecimal digits are read (letters can be in upper or
632 lower case).
633
634 After "\\0" up to two further octal digits are read. In both cases, if there
635 are fewer than two digits, just those that are present are used. Thus the
636 sequence "\\0\\x\\07" specifies two binary zeros followed by a BEL character.
637 Make sure you supply two digits after the initial zero if the character that
638 follows is itself an octal digit.
639
640 The handling of a backslash followed by a digit other than 0 is complicated.
641 Outside a character class, PCRE reads it and any following digits as a decimal
642 number. If the number is less than 10, or if there have been at least that many
643 previous capturing left parentheses in the expression, the entire sequence is
644 taken as a \fIback reference\fR. A description of how this works is given
645 later, following the discussion of parenthesized subpatterns.
646
647 Inside a character class, or if the decimal number is greater than 9 and there
648 have not been that many capturing subpatterns, PCRE re-reads up to three octal
649 digits following the backslash, and generates a single byte from the least
650 significant 8 bits of the value. Any subsequent digits stand for themselves.
651 For example:
652
653 \\040 is another way of writing a space
654 \\40 is the same, provided there are fewer than 40
655 previous capturing subpatterns
656 \\7 is always a back reference
657 \\11 might be a back reference, or another way of
658 writing a tab
659 \\011 is always a tab
660 \\0113 is a tab followed by the character "3"
661 \\113 is the character with octal code 113 (since there
662 can be no more than 99 back references)
663 \\377 is a byte consisting entirely of 1 bits
664 \\81 is either a back reference, or a binary zero
665 followed by the two characters "8" and "1"
666
667 Note that octal values of 100 or greater must not be introduced by a leading
668 zero, because no more than three octal digits are ever read.
669
670 All the sequences that define a single byte value can be used both inside and
671 outside character classes. In addition, inside a character class, the sequence
672 "\\b" is interpreted as the backspace character (hex 08). Outside a character
673 class it has a different meaning (see below).
674
675 The third use of backslash is for specifying generic character types:
676
677 \\d any decimal digit
678 \\D any character that is not a decimal digit
679 \\s any whitespace character
680 \\S any character that is not a whitespace character
681 \\w any "word" character
682 \\W any "non-word" character
683
684 Each pair of escape sequences partitions the complete set of characters into
685 two disjoint sets. Any given character matches one, and only one, of each pair.
686
687 A "word" character is any letter or digit or the underscore character, that is,
688 any character which can be part of a Perl "word". The definition of letters and
689 digits is controlled by PCRE's character tables, and may vary if locale-
690 specific matching is taking place (see "Locale support" above). For example, in
691 the "fr" (French) locale, some character codes greater than 128 are used for
692 accented letters, and these are matched by \\w.
693
694 These character type sequences can appear both inside and outside character
695 classes. They each match one character of the appropriate type. If the current
696 matching point is at the end of the subject string, all of them fail, since
697 there is no character to match.
698
699 The fourth use of backslash is for certain simple assertions. An assertion
700 specifies a condition that has to be met at a particular point in a match,
701 without consuming any characters from the subject string. The use of
702 subpatterns for more complicated assertions is described below. The backslashed
703 assertions are
704
705 \\b word boundary
706 \\B not a word boundary
707 \\A start of subject (independent of multiline mode)
708 \\Z end of subject or newline at end (independent of multiline mode)
709 \\z end of subject (independent of multiline mode)
710
711 These assertions may not appear in character classes (but note that "\\b" has a
712 different meaning, namely the backspace character, inside a character class).
713
714 A word boundary is a position in the subject string where the current character
715 and the previous character do not both match \\w or \\W (i.e. one matches
716 \\w and the other matches \\W), or the start or end of the string if the
717 first or last character matches \\w, respectively.
718
719 The \\A, \\Z, and \\z assertions differ from the traditional circumflex and
720 dollar (described below) in that they only ever match at the very start and end
721 of the subject string, whatever options are set. They are not affected by the
722 PCRE_NOTBOL or PCRE_NOTEOL options. The difference between \\Z and \\z is that
723 \\Z matches before a newline that is the last character of the string as well
724 as at the end of the string, whereas \\z matches only at the end.
725
726
727 .SH CIRCUMFLEX AND DOLLAR
728 Outside a character class, in the default matching mode, the circumflex
729 character is an assertion which is true only if the current matching point is
730 at the start of the subject string. Inside a character class, circumflex has an
731 entirely different meaning (see below).
732
733 Circumflex need not be the first character of the pattern if a number of
734 alternatives are involved, but it should be the first thing in each alternative
735 in which it appears if the pattern is ever to match that branch. If all
736 possible alternatives start with a circumflex, that is, if the pattern is
737 constrained to match only at the start of the subject, it is said to be an
738 "anchored" pattern. (There are also other constructs that can cause a pattern
739 to be anchored.)
740
741 A dollar character is an assertion which is true only if the current matching
742 point is at the end of the subject string, or immediately before a newline
743 character that is the last character in the string (by default). Dollar need
744 not be the last character of the pattern if a number of alternatives are
745 involved, but it should be the last item in any branch in which it appears.
746 Dollar has no special meaning in a character class.
747
748 The meaning of dollar can be changed so that it matches only at the very end of
749 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
750 time. This does not affect the \\Z assertion.
751
752 The meanings of the circumflex and dollar characters are changed if the
753 PCRE_MULTILINE option is set. When this is the case, they match immediately
754 after and immediately before an internal "\\n" character, respectively, in
755 addition to matching at the start and end of the subject string. For example,
756 the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,
757 but not otherwise. Consequently, patterns that are anchored in single line mode
758 because all branches start with "^" are not anchored in multiline mode. The
759 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
760
761 Note that the sequences \\A, \\Z, and \\z can be used to match the start and
762 end of the subject in both modes, and if all branches of a pattern start with
763 \\A is it always anchored, whether PCRE_MULTILINE is set or not.
764
765
766 .SH FULL STOP (PERIOD, DOT)
767 Outside a character class, a dot in the pattern matches any one character in
768 the subject, including a non-printing character, but not (by default) newline.
769 If the PCRE_DOTALL option is set, then dots match newlines as well. The
770 handling of dot is entirely independent of the handling of circumflex and
771 dollar, the only relationship being that they both involve newline characters.
772 Dot has no special meaning in a character class.
773
774
775 .SH SQUARE BRACKETS
776 An opening square bracket introduces a character class, terminated by a closing
777 square bracket. A closing square bracket on its own is not special. If a
778 closing square bracket is required as a member of the class, it should be the
779 first data character in the class (after an initial circumflex, if present) or
780 escaped with a backslash.
781
782 A character class matches a single character in the subject; the character must
783 be in the set of characters defined by the class, unless the first character in
784 the class is a circumflex, in which case the subject character must not be in
785 the set defined by the class. If a circumflex is actually required as a member
786 of the class, ensure it is not the first character, or escape it with a
787 backslash.
788
789 For example, the character class [aeiou] matches any lower case vowel, while
790 [^aeiou] matches any character that is not a lower case vowel. Note that a
791 circumflex is just a convenient notation for specifying the characters which
792 are in the class by enumerating those that are not. It is not an assertion: it
793 still consumes a character from the subject string, and fails if the current
794 pointer is at the end of the string.
795
796 When caseless matching is set, any letters in a class represent both their
797 upper case and lower case versions, so for example, a caseless [aeiou] matches
798 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
799 caseful version would.
800
801 The newline character is never treated in any special way in character classes,
802 whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
803 such as [^a] will always match a newline.
804
805 The minus (hyphen) character can be used to specify a range of characters in a
806 character class. For example, [d-m] matches any letter between d and m,
807 inclusive. If a minus character is required in a class, it must be escaped with
808 a backslash or appear in a position where it cannot be interpreted as
809 indicating a range, typically as the first or last character in the class.
810
811 It is not possible to have the literal character "]" as the end character of a
812 range. A pattern such as [W-]46] is interpreted as a class of two characters
813 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
814 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
815 the end of range, so [W-\\]46] is interpreted as a single class containing a
816 range followed by two separate characters. The octal or hexadecimal
817 representation of "]" can also be used to end a range.
818
819 Ranges operate in ASCII collating sequence. They can also be used for
820 characters specified numerically, for example [\\000-\\037]. If a range that
821 includes letters is used when caseless matching is set, it matches the letters
822 in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched
823 caselessly, and if character tables for the "fr" locale are in use,
824 [\\xc8-\\xcb] matches accented E characters in both cases.
825
826 The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a
827 character class, and add the characters that they match to the class. For
828 example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can
829 conveniently be used with the upper case character types to specify a more
830 restricted set of characters than the matching lower case type. For example,
831 the class [^\\W_] matches any letter or digit, but not underscore.
832
833 All non-alphameric characters other than \\, -, ^ (at the start) and the
834 terminating ] are non-special in character classes, but it does no harm if they
835 are escaped.
836
837
838 .SH VERTICAL BAR
839 Vertical bar characters are used to separate alternative patterns. For example,
840 the pattern
841
842 gilbert|sullivan
843
844 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
845 and an empty alternative is permitted (matching the empty string).
846 The matching process tries each alternative in turn, from left to right,
847 and the first one that succeeds is used. If the alternatives are within a
848 subpattern (defined below), "succeeds" means matching the rest of the main
849 pattern as well as the alternative in the subpattern.
850
851
852 .SH INTERNAL OPTION SETTING
853 The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
854 can be changed from within the pattern by a sequence of Perl option letters
855 enclosed between "(?" and ")". The option letters are
856
857 i for PCRE_CASELESS
858 m for PCRE_MULTILINE
859 s for PCRE_DOTALL
860 x for PCRE_EXTENDED
861
862 For example, (?im) sets caseless, multiline matching. It is also possible to
863 unset these options by preceding the letter with a hyphen, and a combined
864 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
865 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
866 permitted. If a letter appears both before and after the hyphen, the option is
867 unset.
868
869 The scope of these option changes depends on where in the pattern the setting
870 occurs. For settings that are outside any subpattern (defined below), the
871 effect is the same as if the options were set or unset at the start of
872 matching. The following patterns all behave in exactly the same way:
873
874 (?i)abc
875 a(?i)bc
876 ab(?i)c
877 abc(?i)
878
879 which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
880 In other words, such "top level" settings apply to the whole pattern (unless
881 there are other changes inside subpatterns). If there is more than one setting
882 of the same option at top level, the rightmost setting is used.
883
884 If an option change occurs inside a subpattern, the effect is different. This
885 is a change of behaviour in Perl 5.005. An option change inside a subpattern
886 affects only that part of the subpattern that follows it, so
887
888 (a(?i)b)c
889
890 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
891 By this means, options can be made to have different settings in different
892 parts of the pattern. Any changes made in one alternative do carry on
893 into subsequent branches within the same subpattern. For example,
894
895 (a(?i)b|c)
896
897 matches "ab", "aB", "c", and "C", even though when matching "C" the first
898 branch is abandoned before the option setting. This is because the effects of
899 option settings happen at compile time. There would be some very weird
900 behaviour otherwise.
901
902 The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
903 same way as the Perl-compatible options by using the characters U and X
904 respectively. The (?X) flag setting is special in that it must always occur
905 earlier in the pattern than any of the additional features it turns on, even
906 when it is at top level. It is best put at the start.
907
908
909 .SH SUBPATTERNS
910 Subpatterns are delimited by parentheses (round brackets), which can be nested.
911 Marking part of a pattern as a subpattern does two things:
912
913 1. It localizes a set of alternatives. For example, the pattern
914
915 cat(aract|erpillar|)
916
917 matches one of the words "cat", "cataract", or "caterpillar". Without the
918 parentheses, it would match "cataract", "erpillar" or the empty string.
919
920 2. It sets up the subpattern as a capturing subpattern (as defined above).
921 When the whole pattern matches, that portion of the subject string that matched
922 the subpattern is passed back to the caller via the \fIovector\fR argument of
923 \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting
924 from 1) to obtain the numbers of the capturing subpatterns.
925
926 For example, if the string "the red king" is matched against the pattern
927
928 the ((red|white) (king|queen))
929
930 the captured substrings are "red king", "red", and "king", and are numbered 1,
931 2, and 3.
932
933 The fact that plain parentheses fulfil two functions is not always helpful.
934 There are often times when a grouping subpattern is required without a
935 capturing requirement. If an opening parenthesis is followed by "?:", the
936 subpattern does not do any capturing, and is not counted when computing the
937 number of any subsequent capturing subpatterns. For example, if the string "the
938 white queen" is matched against the pattern
939
940 the ((?:red|white) (king|queen))
941
942 the captured substrings are "white queen" and "queen", and are numbered 1 and
943 2. The maximum number of captured substrings is 99, and the maximum number of
944 all subpatterns, both capturing and non-capturing, is 200.
945
946 As a convenient shorthand, if any option settings are required at the start of
947 a non-capturing subpattern, the option letters may appear between the "?" and
948 the ":". Thus the two patterns
949
950 (?i:saturday|sunday)
951 (?:(?i)saturday|sunday)
952
953 match exactly the same set of strings. Because alternative branches are tried
954 from left to right, and options are not reset until the end of the subpattern
955 is reached, an option setting in one branch does affect subsequent branches, so
956 the above patterns match "SUNDAY" as well as "Saturday".
957
958
959 .SH REPETITION
960 Repetition is specified by quantifiers, which can follow any of the following
961 items:
962
963 a single character, possibly escaped
964 the . metacharacter
965 a character class
966 a back reference (see next section)
967 a parenthesized subpattern (unless it is an assertion - see below)
968
969 The general repetition quantifier specifies a minimum and maximum number of
970 permitted matches, by giving the two numbers in curly brackets (braces),
971 separated by a comma. The numbers must be less than 65536, and the first must
972 be less than or equal to the second. For example:
973
974 z{2,4}
975
976 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
977 character. If the second number is omitted, but the comma is present, there is
978 no upper limit; if the second number and the comma are both omitted, the
979 quantifier specifies an exact number of required matches. Thus
980
981 [aeiou]{3,}
982
983 matches at least 3 successive vowels, but may match many more, while
984
985 \\d{8}
986
987 matches exactly 8 digits. An opening curly bracket that appears in a position
988 where a quantifier is not allowed, or one that does not match the syntax of a
989 quantifier, is taken as a literal character. For example, {,6} is not a
990 quantifier, but a literal string of four characters.
991
992 The quantifier {0} is permitted, causing the expression to behave as if the
993 previous item and the quantifier were not present.
994
995 For convenience (and historical compatibility) the three most common
996 quantifiers have single-character abbreviations:
997
998 * is equivalent to {0,}
999 + is equivalent to {1,}
1000 ? is equivalent to {0,1}
1001
1002 It is possible to construct infinite loops by following a subpattern that can
1003 match no characters with a quantifier that has no upper limit, for example:
1004
1005 (a?)*
1006
1007 Earlier versions of Perl and PCRE used to give an error at compile time for
1008 such patterns. However, because there are cases where this can be useful, such
1009 patterns are now accepted, but if any repetition of the subpattern does in fact
1010 match no characters, the loop is forcibly broken.
1011
1012 By default, the quantifiers are "greedy", that is, they match as much as
1013 possible (up to the maximum number of permitted times), without causing the
1014 rest of the pattern to fail. The classic example of where this gives problems
1015 is in trying to match comments in C programs. These appear between the
1016 sequences /* and */ and within the sequence, individual * and / characters may
1017 appear. An attempt to match C comments by applying the pattern
1018
1019 /\\*.*\\*/
1020
1021 to the string
1022
1023 /* first command */ not comment /* second comment */
1024
1025 fails, because it matches the entire string due to the greediness of the .*
1026 item.
1027
1028 However, if a quantifier is followed by a question mark, then it ceases to be
1029 greedy, and instead matches the minimum number of times possible, so the
1030 pattern
1031
1032 /\\*.*?\\*/
1033
1034 does the right thing with the C comments. The meaning of the various
1035 quantifiers is not otherwise changed, just the preferred number of matches.
1036 Do not confuse this use of question mark with its use as a quantifier in its
1037 own right. Because it has two uses, it can sometimes appear doubled, as in
1038
1039 \\d??\\d
1040
1041 which matches one digit by preference, but can match two if that is the only
1042 way the rest of the pattern matches.
1043
1044 If the PCRE_UNGREEDY option is set (an option which is not available in Perl)
1045 then the quantifiers are not greedy by default, but individual ones can be made
1046 greedy by following them with a question mark. In other words, it inverts the
1047 default behaviour.
1048
1049 When a parenthesized subpattern is quantified with a minimum repeat count that
1050 is greater than 1 or with a limited maximum, more store is required for the
1051 compiled pattern, in proportion to the size of the minimum or maximum.
1052
1053 If a pattern starts with .* then it is implicitly anchored, since whatever
1054 follows will be tried against every character position in the subject string.
1055 PCRE treats this as though it were preceded by \\A.
1056
1057 When a capturing subpattern is repeated, the value captured is the substring
1058 that matched the final iteration. For example, after
1059
1060 (tweedle[dume]{3}\\s*)+
1061
1062 has matched "tweedledum tweedledee" the value of the captured substring is
1063 "tweedledee". However, if there are nested capturing subpatterns, the
1064 corresponding captured values may have been set in previous iterations. For
1065 example, after
1066
1067 /(a|(b))+/
1068
1069 matches "aba" the value of the second captured substring is "b".
1070
1071
1072 .SH BACK REFERENCES
1073 Outside a character class, a backslash followed by a digit greater than 0 (and
1074 possibly further digits) is a back reference to a capturing subpattern earlier
1075 (i.e. to its left) in the pattern, provided there have been that many previous
1076 capturing left parentheses.
1077
1078 However, if the decimal number following the backslash is less than 10, it is
1079 always taken as a back reference, and causes an error only if there are not
1080 that many capturing left parentheses in the entire pattern. In other words, the
1081 parentheses that are referenced need not be to the left of the reference for
1082 numbers less than 10. See the section entitled "Backslash" above for further
1083 details of the handling of digits following a backslash.
1084
1085 A back reference matches whatever actually matched the capturing subpattern in
1086 the current subject string, rather than anything matching the subpattern
1087 itself. So the pattern
1088
1089 (sens|respons)e and \\1ibility
1090
1091 matches "sense and sensibility" and "response and responsibility", but not
1092 "sense and responsibility". If caseful matching is in force at the time of the
1093 back reference, then the case of letters is relevant. For example,
1094
1095 ((?i)rah)\\s+\\1
1096
1097 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1098 capturing subpattern is matched caselessly.
1099
1100 There may be more than one back reference to the same subpattern. If a
1101 subpattern has not actually been used in a particular match, then any back
1102 references to it always fail. For example, the pattern
1103
1104 (a|(bc))\\2
1105
1106 always fails if it starts to match "a" rather than "bc". Because there may be
1107 up to 99 back references, all digits following the backslash are taken
1108 as part of a potential back reference number. If the pattern continues with a
1109 digit character, then some delimiter must be used to terminate the back
1110 reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1111 Otherwise an empty comment can be used.
1112
1113 A back reference that occurs inside the parentheses to which it refers fails
1114 when the subpattern is first used, so, for example, (a\\1) never matches.
1115 However, such references can be useful inside repeated subpatterns. For
1116 example, the pattern
1117
1118 (a|b\\1)+
1119
1120 matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
1121 the subpattern, the back reference matches the character string corresponding
1122 to the previous iteration. In order for this to work, the pattern must be such
1123 that the first iteration does not need to match the back reference. This can be
1124 done using alternation, as in the example above, or by a quantifier with a
1125 minimum of zero.
1126
1127
1128 .SH ASSERTIONS
1129 An assertion is a test on the characters following or preceding the current
1130 matching point that does not actually consume any characters. The simple
1131 assertions coded as \\b, \\B, \\A, \\Z, \\z, ^ and $ are described above. More
1132 complicated assertions are coded as subpatterns. There are two kinds: those
1133 that look ahead of the current position in the subject string, and those that
1134 look behind it.
1135
1136 An assertion subpattern is matched in the normal way, except that it does not
1137 cause the current matching position to be changed. Lookahead assertions start
1138 with (?= for positive assertions and (?! for negative assertions. For example,
1139
1140 \\w+(?=;)
1141
1142 matches a word followed by a semicolon, but does not include the semicolon in
1143 the match, and
1144
1145 foo(?!bar)
1146
1147 matches any occurrence of "foo" that is not followed by "bar". Note that the
1148 apparently similar pattern
1149
1150 (?!foo)bar
1151
1152 does not find an occurrence of "bar" that is preceded by something other than
1153 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1154 (?!foo) is always true when the next three characters are "bar". A
1155 lookbehind assertion is needed to achieve this effect.
1156
1157 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1158 negative assertions. For example,
1159
1160 (?<!foo)bar
1161
1162 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1163 a lookbehind assertion are restricted such that all the strings it matches must
1164 have a fixed length. However, if there are several alternatives, they do not
1165 all have to have the same fixed length. Thus
1166
1167 (?<=bullock|donkey)
1168
1169 is permitted, but
1170
1171 (?<!dogs?|cats?)
1172
1173 causes an error at compile time. Branches that match different length strings
1174 are permitted only at the top level of a lookbehind assertion. This is an
1175 extension compared with Perl 5.005, which requires all branches to match the
1176 same length of string. An assertion such as
1177
1178 (?<=ab(c|de))
1179
1180 is not permitted, because its single top-level branch can match two different
1181 lengths, but it is acceptable if rewritten to use two top-level branches:
1182
1183 (?<=abc|abde)
1184
1185 The implementation of lookbehind assertions is, for each alternative, to
1186 temporarily move the current position back by the fixed width and then try to
1187 match. If there are insufficient characters before the current position, the
1188 match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
1189 can be particularly useful for matching at the ends of strings; an example is
1190 given at the end of the section on once-only subpatterns.
1191
1192 Several assertions (of any sort) may occur in succession. For example,
1193
1194 (?<=\\d{3})(?<!999)foo
1195
1196 matches "foo" preceded by three digits that are not "999". Furthermore,
1197 assertions can be nested in any combination. For example,
1198
1199 (?<=(?<!foo)bar)baz
1200
1201 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1202 preceded by "foo".
1203
1204 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1205 because it makes no sense to assert the same thing several times. If an
1206 assertion contains capturing subpatterns within it, these are always counted
1207 for the purposes of numbering the capturing subpatterns in the whole pattern.
1208 Substring capturing is carried out for positive assertions, but it does not
1209 make sense for negative assertions.
1210
1211 Assertions count towards the maximum of 200 parenthesized subpatterns.
1212
1213
1214 .SH ONCE-ONLY SUBPATTERNS
1215 With both maximizing and minimizing repetition, failure of what follows
1216 normally causes the repeated item to be re-evaluated to see if a different
1217 number of repeats allows the rest of the pattern to match. Sometimes it is
1218 useful to prevent this, either to change the nature of the match, or to cause
1219 it fail earlier than it otherwise might, when the author of the pattern knows
1220 there is no point in carrying on.
1221
1222 Consider, for example, the pattern \\d+foo when applied to the subject line
1223
1224 123456bar
1225
1226 After matching all 6 digits and then failing to match "foo", the normal
1227 action of the matcher is to try again with only 5 digits matching the \\d+
1228 item, and then with 4, and so on, before ultimately failing. Once-only
1229 subpatterns provide the means for specifying that once a portion of the pattern
1230 has matched, it is not to be re-evaluated in this way, so the matcher would
1231 give up immediately on failing to match "foo" the first time. The notation is
1232 another kind of special parenthesis, starting with (?> as in this example:
1233
1234 (?>\\d+)bar
1235
1236 This kind of parenthesis "locks up" the part of the pattern it contains once
1237 it has matched, and a failure further into the pattern is prevented from
1238 backtracking into it. Backtracking past it to previous items, however, works as
1239 normal.
1240
1241 An alternative description is that a subpattern of this type matches the string
1242 of characters that an identical standalone pattern would match, if anchored at
1243 the current point in the subject string.
1244
1245 Once-only subpatterns are not capturing subpatterns. Simple cases such as the
1246 above example can be thought of as a maximizing repeat that must swallow
1247 everything it can. So, while both \\d+ and \\d+? are prepared to adjust the
1248 number of digits they match in order to make the rest of the pattern match,
1249 (?>\\d+) can only match an entire sequence of digits.
1250
1251 This construction can of course contain arbitrarily complicated subpatterns,
1252 and it can be nested.
1253
1254 Once-only subpatterns can be used in conjunction with lookbehind assertions to
1255 specify efficient matching at the end of the subject string. Consider a simple
1256 pattern such as
1257
1258 abcd$
1259
1260 when applied to a long string which does not match it. Because matching
1261 proceeds from left to right, PCRE will look for each "a" in the subject and
1262 then see if what follows matches the rest of the pattern. If the pattern is
1263 specified as
1264
1265 .*abcd$
1266
1267 then the initial .* matches the entire string at first, but when this fails, it
1268 backtracks to match all but the last character, then all but the last two
1269 characters, and so on. Once again the search for "a" covers the entire string,
1270 from right to left, so we are no better off. However, if the pattern is written
1271 as
1272
1273 (?>.*)(?<=abcd)
1274
1275 then there can be no backtracking for the .* item; it can match only the entire
1276 string. The subsequent lookbehind assertion does a single test on the last four
1277 characters. If it fails, the match fails immediately. For long strings, this
1278 approach makes a significant difference to the processing time.
1279
1280
1281 .SH CONDITIONAL SUBPATTERNS
1282 It is possible to cause the matching process to obey a subpattern
1283 conditionally or to choose between two alternative subpatterns, depending on
1284 the result of an assertion, or whether a previous capturing subpattern matched
1285 or not. The two possible forms of conditional subpattern are
1286
1287 (?(condition)yes-pattern)
1288 (?(condition)yes-pattern|no-pattern)
1289
1290 If the condition is satisfied, the yes-pattern is used; otherwise the
1291 no-pattern (if present) is used. If there are more than two alternatives in the
1292 subpattern, a compile-time error occurs.
1293
1294 There are two kinds of condition. If the text between the parentheses consists
1295 of a sequence of digits, then the condition is satisfied if the capturing
1296 subpattern of that number has previously matched. Consider the following
1297 pattern, which contains non-significant white space to make it more readable
1298 (assume the PCRE_EXTENDED option) and to divide it into three parts for ease
1299 of discussion:
1300
1301 ( \\( )? [^()]+ (?(1) \\) )
1302
1303 The first part matches an optional opening parenthesis, and if that
1304 character is present, sets it as the first captured substring. The second part
1305 matches one or more characters that are not parentheses. The third part is a
1306 conditional subpattern that tests whether the first set of parentheses matched
1307 or not. If they did, that is, if subject started with an opening parenthesis,
1308 the condition is true, and so the yes-pattern is executed and a closing
1309 parenthesis is required. Otherwise, since no-pattern is not present, the
1310 subpattern matches nothing. In other words, this pattern matches a sequence of
1311 non-parentheses, optionally enclosed in parentheses.
1312
1313 If the condition is not a sequence of digits, it must be an assertion. This may
1314 be a positive or negative lookahead or lookbehind assertion. Consider this
1315 pattern, again containing non-significant white space, and with the two
1316 alternatives on the second line:
1317
1318 (?(?=[^a-z]*[a-z])
1319 \\d{2}[a-z]{3}-\\d{2} | \\d{2}-\\d{2}-\\d{2} )
1320
1321 The condition is a positive lookahead assertion that matches an optional
1322 sequence of non-letters followed by a letter. In other words, it tests for the
1323 presence of at least one letter in the subject. If a letter is found, the
1324 subject is matched against the first alternative; otherwise it is matched
1325 against the second. This pattern matches strings in one of the two forms
1326 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1327
1328
1329 .SH COMMENTS
1330 The sequence (?# marks the start of a comment which continues up to the next
1331 closing parenthesis. Nested parentheses are not permitted. The characters
1332 that make up a comment play no part in the pattern matching at all.
1333
1334 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1335 character class introduces a comment that continues up to the next newline
1336 character in the pattern.
1337
1338
1339 .SH PERFORMANCE
1340 Certain items that may appear in patterns are more efficient than others. It is
1341 more efficient to use a character class like [aeiou] than a set of alternatives
1342 such as (a|e|i|o|u). In general, the simplest construction that provides the
1343 required behaviour is usually the most efficient. Jeffrey Friedl's book
1344 contains a lot of discussion about optimizing regular expressions for efficient
1345 performance.
1346
1347
1348 .SH AUTHOR
1349 Philip Hazel <ph10@cam.ac.uk>
1350 .br
1351 University Computing Service,
1352 .br
1353 New Museums Site,
1354 .br
1355 Cambridge CB2 3QG, England.
1356 .br
1357 Phone: +44 1223 334714
1358
1359 Copyright (c) 1997-1999 University of Cambridge.

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