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Documentation for JIT support.
3 PCRE - Perl-compatible regular expressions
5 .rs
6 .sp
7 The syntax and semantics of the regular expressions that are supported by PCRE
8 are described in detail below. There is a quick-reference syntax summary in the
9 .\" HREF
10 \fBpcresyntax\fP
11 .\"
12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13 also supports some alternative regular expression syntax (which does not
14 conflict with the Perl syntax) in order to provide some compatibility with
15 regular expressions in Python, .NET, and Oniguruma.
16 .P
17 Perl's regular expressions are described in its own documentation, and
18 regular expressions in general are covered in a number of books, some of which
19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20 published by O'Reilly, covers regular expressions in great detail. This
21 description of PCRE's regular expressions is intended as reference material.
22 .P
23 The original operation of PCRE was on strings of one-byte characters. However,
24 there is now also support for UTF-8 character strings. To use this,
25 PCRE must be built to include UTF-8 support, and you must call
26 \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27 is also a special sequence that can be given at the start of a pattern:
28 .sp
29 (*UTF8)
30 .sp
31 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32 option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33 pattern matching is mentioned in several places below. There is also a summary
34 of UTF-8 features in the
35 .\" HREF
36 \fBpcreunicode\fP
37 .\"
38 page.
39 .P
40 Another special sequence that may appear at the start of a pattern or in
41 combination with (*UTF8) is:
42 .sp
43 (*UCP)
44 .sp
45 This has the same effect as setting the PCRE_UCP option: it causes sequences
46 such as \ed and \ew to use Unicode properties to determine character types,
47 instead of recognizing only characters with codes less than 128 via a lookup
48 table.
49 .P
50 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52 also some more of these special sequences that are concerned with the handling
53 of newlines; they are described below.
54 .P
55 The remainder of this document discusses the patterns that are supported by
56 PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57 From release 6.0, PCRE offers a second matching function,
58 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
59 Perl-compatible. Some of the features discussed below are not available when
60 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61 alternative function, and how it differs from the normal function, are
62 discussed in the
63 .\" HREF
64 \fBpcrematching\fP
65 .\"
66 page.
67 .
68 .
69 .\" HTML <a name="newlines"></a>
71 .rs
72 .sp
73 PCRE supports five different conventions for indicating line breaks in
74 strings: a single CR (carriage return) character, a single LF (linefeed)
75 character, the two-character sequence CRLF, any of the three preceding, or any
76 Unicode newline sequence. The
77 .\" HREF
78 \fBpcreapi\fP
79 .\"
80 page has
81 .\" HTML <a href="pcreapi.html#newlines">
82 .\" </a>
83 further discussion
84 .\"
85 about newlines, and shows how to set the newline convention in the
86 \fIoptions\fP arguments for the compiling and matching functions.
87 .P
88 It is also possible to specify a newline convention by starting a pattern
89 string with one of the following five sequences:
90 .sp
91 (*CR) carriage return
92 (*LF) linefeed
93 (*CRLF) carriage return, followed by linefeed
94 (*ANYCRLF) any of the three above
95 (*ANY) all Unicode newline sequences
96 .sp
97 These override the default and the options given to \fBpcre_compile()\fP or
98 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99 newline sequence, the pattern
100 .sp
101 (*CR)a.b
102 .sp
103 changes the convention to CR. That pattern matches "a\enb" because LF is no
104 longer a newline. Note that these special settings, which are not
105 Perl-compatible, are recognized only at the very start of a pattern, and that
106 they must be in upper case. If more than one of them is present, the last one
107 is used.
108 .P
109 The newline convention affects the interpretation of the dot metacharacter when
110 PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111 affect what the \eR escape sequence matches. By default, this is any Unicode
112 newline sequence, for Perl compatibility. However, this can be changed; see the
113 description of \eR in the section entitled
114 .\" HTML <a href="#newlineseq">
115 .\" </a>
116 "Newline sequences"
117 .\"
118 below. A change of \eR setting can be combined with a change of newline
119 convention.
120 .
121 .
123 .rs
124 .sp
125 A regular expression is a pattern that is matched against a subject string from
126 left to right. Most characters stand for themselves in a pattern, and match the
127 corresponding characters in the subject. As a trivial example, the pattern
128 .sp
129 The quick brown fox
130 .sp
131 matches a portion of a subject string that is identical to itself. When
132 caseless matching is specified (the PCRE_CASELESS option), letters are matched
133 independently of case. In UTF-8 mode, PCRE always understands the concept of
134 case for characters whose values are less than 128, so caseless matching is
135 always possible. For characters with higher values, the concept of case is
136 supported if PCRE is compiled with Unicode property support, but not otherwise.
137 If you want to use caseless matching for characters 128 and above, you must
138 ensure that PCRE is compiled with Unicode property support as well as with
139 UTF-8 support.
140 .P
141 The power of regular expressions comes from the ability to include alternatives
142 and repetitions in the pattern. These are encoded in the pattern by the use of
143 \fImetacharacters\fP, which do not stand for themselves but instead are
144 interpreted in some special way.
145 .P
146 There are two different sets of metacharacters: those that are recognized
147 anywhere in the pattern except within square brackets, and those that are
148 recognized within square brackets. Outside square brackets, the metacharacters
149 are as follows:
150 .sp
151 \e general escape character with several uses
152 ^ assert start of string (or line, in multiline mode)
153 $ assert end of string (or line, in multiline mode)
154 . match any character except newline (by default)
155 [ start character class definition
156 | start of alternative branch
157 ( start subpattern
158 ) end subpattern
159 ? extends the meaning of (
160 also 0 or 1 quantifier
161 also quantifier minimizer
162 * 0 or more quantifier
163 + 1 or more quantifier
164 also "possessive quantifier"
165 { start min/max quantifier
166 .sp
167 Part of a pattern that is in square brackets is called a "character class". In
168 a character class the only metacharacters are:
169 .sp
170 \e general escape character
171 ^ negate the class, but only if the first character
172 - indicates character range
173 .\" JOIN
174 [ POSIX character class (only if followed by POSIX
175 syntax)
176 ] terminates the character class
177 .sp
178 The following sections describe the use of each of the metacharacters.
179 .
180 .
182 .rs
183 .sp
184 The backslash character has several uses. Firstly, if it is followed by a
185 character that is not a number or a letter, it takes away any special meaning
186 that character may have. This use of backslash as an escape character applies
187 both inside and outside character classes.
188 .P
189 For example, if you want to match a * character, you write \e* in the pattern.
190 This escaping action applies whether or not the following character would
191 otherwise be interpreted as a metacharacter, so it is always safe to precede a
192 non-alphanumeric with backslash to specify that it stands for itself. In
193 particular, if you want to match a backslash, you write \e\e.
194 .P
195 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196 backslash. All other characters (in particular, those whose codepoints are
197 greater than 127) are treated as literals.
198 .P
199 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200 pattern (other than in a character class) and characters between a # outside
201 a character class and the next newline are ignored. An escaping backslash can
202 be used to include a whitespace or # character as part of the pattern.
203 .P
204 If you want to remove the special meaning from a sequence of characters, you
205 can do so by putting them between \eQ and \eE. This is different from Perl in
206 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
207 Perl, $ and @ cause variable interpolation. Note the following examples:
208 .sp
209 Pattern PCRE matches Perl matches
210 .sp
211 .\" JOIN
212 \eQabc$xyz\eE abc$xyz abc followed by the
213 contents of $xyz
214 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
215 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
216 .sp
217 The \eQ...\eE sequence is recognized both inside and outside character classes.
218 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219 by \eE later in the pattern, the literal interpretation continues to the end of
220 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221 a character class, this causes an error, because the character class is not
222 terminated.
223 .
224 .
225 .\" HTML <a name="digitsafterbackslash"></a>
226 .SS "Non-printing characters"
227 .rs
228 .sp
229 A second use of backslash provides a way of encoding non-printing characters
230 in patterns in a visible manner. There is no restriction on the appearance of
231 non-printing characters, apart from the binary zero that terminates a pattern,
232 but when a pattern is being prepared by text editing, it is often easier to use
233 one of the following escape sequences than the binary character it represents:
234 .sp
235 \ea alarm, that is, the BEL character (hex 07)
236 \ecx "control-x", where x is any ASCII character
237 \ee escape (hex 1B)
238 \ef formfeed (hex 0C)
239 \en linefeed (hex 0A)
240 \er carriage return (hex 0D)
241 \et tab (hex 09)
242 \eddd character with octal code ddd, or back reference
243 \exhh character with hex code hh
244 \ex{hhh..} character with hex code hhh..
245 .sp
246 The precise effect of \ecx is as follows: if x is a lower case letter, it
247 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
248 Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
249 \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
250 than 127, a compile-time error occurs. This locks out non-ASCII characters in
251 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
252 values are valid. A lower case letter is converted to upper case, and then the
253 0xc0 bits are flipped.)
254 .P
255 After \ex, from zero to two hexadecimal digits are read (letters can be in
256 upper or lower case). Any number of hexadecimal digits may appear between \ex{
257 and }, but the value of the character code must be less than 256 in non-UTF-8
258 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
259 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
260 point, which is 10FFFF.
261 .P
262 If characters other than hexadecimal digits appear between \ex{ and }, or if
263 there is no terminating }, this form of escape is not recognized. Instead, the
264 initial \ex will be interpreted as a basic hexadecimal escape, with no
265 following digits, giving a character whose value is zero.
266 .P
267 Characters whose value is less than 256 can be defined by either of the two
268 syntaxes for \ex. There is no difference in the way they are handled. For
269 example, \exdc is exactly the same as \ex{dc}.
270 .P
271 After \e0 up to two further octal digits are read. If there are fewer than two
272 digits, just those that are present are used. Thus the sequence \e0\ex\e07
273 specifies two binary zeros followed by a BEL character (code value 7). Make
274 sure you supply two digits after the initial zero if the pattern character that
275 follows is itself an octal digit.
276 .P
277 The handling of a backslash followed by a digit other than 0 is complicated.
278 Outside a character class, PCRE reads it and any following digits as a decimal
279 number. If the number is less than 10, or if there have been at least that many
280 previous capturing left parentheses in the expression, the entire sequence is
281 taken as a \fIback reference\fP. A description of how this works is given
282 .\" HTML <a href="#backreferences">
283 .\" </a>
284 later,
285 .\"
286 following the discussion of
287 .\" HTML <a href="#subpattern">
288 .\" </a>
289 parenthesized subpatterns.
290 .\"
291 .P
292 Inside a character class, or if the decimal number is greater than 9 and there
293 have not been that many capturing subpatterns, PCRE re-reads up to three octal
294 digits following the backslash, and uses them to generate a data character. Any
295 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
296 character specified in octal must be less than \e400. In UTF-8 mode, values up
297 to \e777 are permitted. For example:
298 .sp
299 \e040 is another way of writing a space
300 .\" JOIN
301 \e40 is the same, provided there are fewer than 40
302 previous capturing subpatterns
303 \e7 is always a back reference
304 .\" JOIN
305 \e11 might be a back reference, or another way of
306 writing a tab
307 \e011 is always a tab
308 \e0113 is a tab followed by the character "3"
309 .\" JOIN
310 \e113 might be a back reference, otherwise the
311 character with octal code 113
312 .\" JOIN
313 \e377 might be a back reference, otherwise
314 the byte consisting entirely of 1 bits
315 .\" JOIN
316 \e81 is either a back reference, or a binary zero
317 followed by the two characters "8" and "1"
318 .sp
319 Note that octal values of 100 or greater must not be introduced by a leading
320 zero, because no more than three octal digits are ever read.
321 .P
322 All the sequences that define a single character value can be used both inside
323 and outside character classes. In addition, inside a character class, the
324 sequence \eb is interpreted as the backspace character (hex 08). The sequences
325 \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
326 unrecognized escape sequences, they are treated as the literal characters "B",
327 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
328 set. Outside a character class, these sequences have different meanings.
329 .
330 .
331 .SS "Absolute and relative back references"
332 .rs
333 .sp
334 The sequence \eg followed by an unsigned or a negative number, optionally
335 enclosed in braces, is an absolute or relative back reference. A named back
336 reference can be coded as \eg{name}. Back references are discussed
337 .\" HTML <a href="#backreferences">
338 .\" </a>
339 later,
340 .\"
341 following the discussion of
342 .\" HTML <a href="#subpattern">
343 .\" </a>
344 parenthesized subpatterns.
345 .\"
346 .
347 .
348 .SS "Absolute and relative subroutine calls"
349 .rs
350 .sp
351 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
352 a number enclosed either in angle brackets or single quotes, is an alternative
353 syntax for referencing a subpattern as a "subroutine". Details are discussed
354 .\" HTML <a href="#onigurumasubroutines">
355 .\" </a>
356 later.
357 .\"
358 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
359 synonymous. The former is a back reference; the latter is a
360 .\" HTML <a href="#subpatternsassubroutines">
361 .\" </a>
362 subroutine
363 .\"
364 call.
365 .
366 .
367 .\" HTML <a name="genericchartypes"></a>
368 .SS "Generic character types"
369 .rs
370 .sp
371 Another use of backslash is for specifying generic character types:
372 .sp
373 \ed any decimal digit
374 \eD any character that is not a decimal digit
375 \eh any horizontal whitespace character
376 \eH any character that is not a horizontal whitespace character
377 \es any whitespace character
378 \eS any character that is not a whitespace character
379 \ev any vertical whitespace character
380 \eV any character that is not a vertical whitespace character
381 \ew any "word" character
382 \eW any "non-word" character
383 .sp
384 There is also the single sequence \eN, which matches a non-newline character.
385 This is the same as
386 .\" HTML <a href="#fullstopdot">
387 .\" </a>
388 the "." metacharacter
389 .\"
390 when PCRE_DOTALL is not set.
391 .P
392 Each pair of lower and upper case escape sequences partitions the complete set
393 of characters into two disjoint sets. Any given character matches one, and only
394 one, of each pair. The sequences can appear both inside and outside character
395 classes. They each match one character of the appropriate type. If the current
396 matching point is at the end of the subject string, all of them fail, because
397 there is no character to match.
398 .P
399 For compatibility with Perl, \es does not match the VT character (code 11).
400 This makes it different from the the POSIX "space" class. The \es characters
401 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
402 included in a Perl script, \es may match the VT character. In PCRE, it never
403 does.
404 .P
405 A "word" character is an underscore or any character that is a letter or digit.
406 By default, the definition of letters and digits is controlled by PCRE's
407 low-valued character tables, and may vary if locale-specific matching is taking
408 place (see
409 .\" HTML <a href="pcreapi.html#localesupport">
410 .\" </a>
411 "Locale support"
412 .\"
413 in the
414 .\" HREF
415 \fBpcreapi\fP
416 .\"
417 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
418 or "french" in Windows, some character codes greater than 128 are used for
419 accented letters, and these are then matched by \ew. The use of locales with
420 Unicode is discouraged.
421 .P
422 By default, in UTF-8 mode, characters with values greater than 128 never match
423 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
424 their original meanings from before UTF-8 support was available, mainly for
425 efficiency reasons. However, if PCRE is compiled with Unicode property support,
426 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
427 properties are used to determine character types, as follows:
428 .sp
429 \ed any character that \ep{Nd} matches (decimal digit)
430 \es any character that \ep{Z} matches, plus HT, LF, FF, CR
431 \ew any character that \ep{L} or \ep{N} matches, plus underscore
432 .sp
433 The upper case escapes match the inverse sets of characters. Note that \ed
434 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
435 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
436 \eB because they are defined in terms of \ew and \eW. Matching these sequences
437 is noticeably slower when PCRE_UCP is set.
438 .P
439 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
440 release 5.10. In contrast to the other sequences, which match only ASCII
441 characters by default, these always match certain high-valued codepoints in
442 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
443 are:
444 .sp
445 U+0009 Horizontal tab
446 U+0020 Space
447 U+00A0 Non-break space
448 U+1680 Ogham space mark
449 U+180E Mongolian vowel separator
450 U+2000 En quad
451 U+2001 Em quad
452 U+2002 En space
453 U+2003 Em space
454 U+2004 Three-per-em space
455 U+2005 Four-per-em space
456 U+2006 Six-per-em space
457 U+2007 Figure space
458 U+2008 Punctuation space
459 U+2009 Thin space
460 U+200A Hair space
461 U+202F Narrow no-break space
462 U+205F Medium mathematical space
463 U+3000 Ideographic space
464 .sp
465 The vertical space characters are:
466 .sp
467 U+000A Linefeed
468 U+000B Vertical tab
469 U+000C Formfeed
470 U+000D Carriage return
471 U+0085 Next line
472 U+2028 Line separator
473 U+2029 Paragraph separator
474 .
475 .
476 .\" HTML <a name="newlineseq"></a>
477 .SS "Newline sequences"
478 .rs
479 .sp
480 Outside a character class, by default, the escape sequence \eR matches any
481 Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
482 .sp
483 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
484 .sp
485 This is an example of an "atomic group", details of which are given
486 .\" HTML <a href="#atomicgroup">
487 .\" </a>
488 below.
489 .\"
490 This particular group matches either the two-character sequence CR followed by
491 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
492 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
493 line, U+0085). The two-character sequence is treated as a single unit that
494 cannot be split.
495 .P
496 In UTF-8 mode, two additional characters whose codepoints are greater than 255
497 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
498 Unicode character property support is not needed for these characters to be
499 recognized.
500 .P
501 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
502 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
503 either at compile time or when the pattern is matched. (BSR is an abbrevation
504 for "backslash R".) This can be made the default when PCRE is built; if this is
505 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
506 It is also possible to specify these settings by starting a pattern string with
507 one of the following sequences:
508 .sp
509 (*BSR_ANYCRLF) CR, LF, or CRLF only
510 (*BSR_UNICODE) any Unicode newline sequence
511 .sp
512 These override the default and the options given to \fBpcre_compile()\fP or
513 \fBpcre_compile2()\fP, but they can be overridden by options given to
514 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
515 which are not Perl-compatible, are recognized only at the very start of a
516 pattern, and that they must be in upper case. If more than one of them is
517 present, the last one is used. They can be combined with a change of newline
518 convention; for example, a pattern can start with:
519 .sp
521 .sp
522 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
523 a character class, \eR is treated as an unrecognized escape sequence, and so
524 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
525 .
526 .
527 .\" HTML <a name="uniextseq"></a>
528 .SS Unicode character properties
529 .rs
530 .sp
531 When PCRE is built with Unicode character property support, three additional
532 escape sequences that match characters with specific properties are available.
533 When not in UTF-8 mode, these sequences are of course limited to testing
534 characters whose codepoints are less than 256, but they do work in this mode.
535 The extra escape sequences are:
536 .sp
537 \ep{\fIxx\fP} a character with the \fIxx\fP property
538 \eP{\fIxx\fP} a character without the \fIxx\fP property
539 \eX an extended Unicode sequence
540 .sp
541 The property names represented by \fIxx\fP above are limited to the Unicode
542 script names, the general category properties, "Any", which matches any
543 character (including newline), and some special PCRE properties (described
544 in the
545 .\" HTML <a href="#extraprops">
546 .\" </a>
547 next section).
548 .\"
549 Other Perl properties such as "InMusicalSymbols" are not currently supported by
550 PCRE. Note that \eP{Any} does not match any characters, so always causes a
551 match failure.
552 .P
553 Sets of Unicode characters are defined as belonging to certain scripts. A
554 character from one of these sets can be matched using a script name. For
555 example:
556 .sp
557 \ep{Greek}
558 \eP{Han}
559 .sp
560 Those that are not part of an identified script are lumped together as
561 "Common". The current list of scripts is:
562 .P
563 Arabic,
564 Armenian,
565 Avestan,
566 Balinese,
567 Bamum,
568 Bengali,
569 Bopomofo,
570 Braille,
571 Buginese,
572 Buhid,
573 Canadian_Aboriginal,
574 Carian,
575 Cham,
576 Cherokee,
577 Common,
578 Coptic,
579 Cuneiform,
580 Cypriot,
581 Cyrillic,
582 Deseret,
583 Devanagari,
584 Egyptian_Hieroglyphs,
585 Ethiopic,
586 Georgian,
587 Glagolitic,
588 Gothic,
589 Greek,
590 Gujarati,
591 Gurmukhi,
592 Han,
593 Hangul,
594 Hanunoo,
595 Hebrew,
596 Hiragana,
597 Imperial_Aramaic,
598 Inherited,
599 Inscriptional_Pahlavi,
600 Inscriptional_Parthian,
601 Javanese,
602 Kaithi,
603 Kannada,
604 Katakana,
605 Kayah_Li,
606 Kharoshthi,
607 Khmer,
608 Lao,
609 Latin,
610 Lepcha,
611 Limbu,
612 Linear_B,
613 Lisu,
614 Lycian,
615 Lydian,
616 Malayalam,
617 Meetei_Mayek,
618 Mongolian,
619 Myanmar,
620 New_Tai_Lue,
621 Nko,
622 Ogham,
623 Old_Italic,
624 Old_Persian,
625 Old_South_Arabian,
626 Old_Turkic,
627 Ol_Chiki,
628 Oriya,
629 Osmanya,
630 Phags_Pa,
631 Phoenician,
632 Rejang,
633 Runic,
634 Samaritan,
635 Saurashtra,
636 Shavian,
637 Sinhala,
638 Sundanese,
639 Syloti_Nagri,
640 Syriac,
641 Tagalog,
642 Tagbanwa,
643 Tai_Le,
644 Tai_Tham,
645 Tai_Viet,
646 Tamil,
647 Telugu,
648 Thaana,
649 Thai,
650 Tibetan,
651 Tifinagh,
652 Ugaritic,
653 Vai,
654 Yi.
655 .P
656 Each character has exactly one Unicode general category property, specified by
657 a two-letter abbreviation. For compatibility with Perl, negation can be
658 specified by including a circumflex between the opening brace and the property
659 name. For example, \ep{^Lu} is the same as \eP{Lu}.
660 .P
661 If only one letter is specified with \ep or \eP, it includes all the general
662 category properties that start with that letter. In this case, in the absence
663 of negation, the curly brackets in the escape sequence are optional; these two
664 examples have the same effect:
665 .sp
666 \ep{L}
667 \epL
668 .sp
669 The following general category property codes are supported:
670 .sp
671 C Other
672 Cc Control
673 Cf Format
674 Cn Unassigned
675 Co Private use
676 Cs Surrogate
677 .sp
678 L Letter
679 Ll Lower case letter
680 Lm Modifier letter
681 Lo Other letter
682 Lt Title case letter
683 Lu Upper case letter
684 .sp
685 M Mark
686 Mc Spacing mark
687 Me Enclosing mark
688 Mn Non-spacing mark
689 .sp
690 N Number
691 Nd Decimal number
692 Nl Letter number
693 No Other number
694 .sp
695 P Punctuation
696 Pc Connector punctuation
697 Pd Dash punctuation
698 Pe Close punctuation
699 Pf Final punctuation
700 Pi Initial punctuation
701 Po Other punctuation
702 Ps Open punctuation
703 .sp
704 S Symbol
705 Sc Currency symbol
706 Sk Modifier symbol
707 Sm Mathematical symbol
708 So Other symbol
709 .sp
710 Z Separator
711 Zl Line separator
712 Zp Paragraph separator
713 Zs Space separator
714 .sp
715 The special property L& is also supported: it matches a character that has
716 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
717 a modifier or "other".
718 .P
719 The Cs (Surrogate) property applies only to characters in the range U+D800 to
720 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
721 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
722 (see the discussion of PCRE_NO_UTF8_CHECK in the
723 .\" HREF
724 \fBpcreapi\fP
725 .\"
726 page). Perl does not support the Cs property.
727 .P
728 The long synonyms for property names that Perl supports (such as \ep{Letter})
729 are not supported by PCRE, nor is it permitted to prefix any of these
730 properties with "Is".
731 .P
732 No character that is in the Unicode table has the Cn (unassigned) property.
733 Instead, this property is assumed for any code point that is not in the
734 Unicode table.
735 .P
736 Specifying caseless matching does not affect these escape sequences. For
737 example, \ep{Lu} always matches only upper case letters.
738 .P
739 The \eX escape matches any number of Unicode characters that form an extended
740 Unicode sequence. \eX is equivalent to
741 .sp
742 (?>\ePM\epM*)
743 .sp
744 That is, it matches a character without the "mark" property, followed by zero
745 or more characters with the "mark" property, and treats the sequence as an
746 atomic group
747 .\" HTML <a href="#atomicgroup">
748 .\" </a>
749 (see below).
750 .\"
751 Characters with the "mark" property are typically accents that affect the
752 preceding character. None of them have codepoints less than 256, so in
753 non-UTF-8 mode \eX matches any one character.
754 .P
755 Note that recent versions of Perl have changed \eX to match what Unicode calls
756 an "extended grapheme cluster", which has a more complicated definition.
757 .P
758 Matching characters by Unicode property is not fast, because PCRE has to search
759 a structure that contains data for over fifteen thousand characters. That is
760 why the traditional escape sequences such as \ed and \ew do not use Unicode
761 properties in PCRE by default, though you can make them do so by setting the
762 PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
763 (*UCP).
764 .
765 .
766 .\" HTML <a name="extraprops"></a>
767 .SS PCRE's additional properties
768 .rs
769 .sp
770 As well as the standard Unicode properties described in the previous
771 section, PCRE supports four more that make it possible to convert traditional
772 escape sequences such as \ew and \es and POSIX character classes to use Unicode
773 properties. PCRE uses these non-standard, non-Perl properties internally when
774 PCRE_UCP is set. They are:
775 .sp
776 Xan Any alphanumeric character
777 Xps Any POSIX space character
778 Xsp Any Perl space character
779 Xwd Any Perl "word" character
780 .sp
781 Xan matches characters that have either the L (letter) or the N (number)
782 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
783 carriage return, and any other character that has the Z (separator) property.
784 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
785 same characters as Xan, plus underscore.
786 .
787 .
788 .\" HTML <a name="resetmatchstart"></a>
789 .SS "Resetting the match start"
790 .rs
791 .sp
792 The escape sequence \eK causes any previously matched characters not to be
793 included in the final matched sequence. For example, the pattern:
794 .sp
795 foo\eKbar
796 .sp
797 matches "foobar", but reports that it has matched "bar". This feature is
798 similar to a lookbehind assertion
799 .\" HTML <a href="#lookbehind">
800 .\" </a>
801 (described below).
802 .\"
803 However, in this case, the part of the subject before the real match does not
804 have to be of fixed length, as lookbehind assertions do. The use of \eK does
805 not interfere with the setting of
806 .\" HTML <a href="#subpattern">
807 .\" </a>
808 captured substrings.
809 .\"
810 For example, when the pattern
811 .sp
812 (foo)\eKbar
813 .sp
814 matches "foobar", the first substring is still set to "foo".
815 .P
816 Perl documents that the use of \eK within assertions is "not well defined". In
817 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
818 ignored in negative assertions.
819 .
820 .
821 .\" HTML <a name="smallassertions"></a>
822 .SS "Simple assertions"
823 .rs
824 .sp
825 The final use of backslash is for certain simple assertions. An assertion
826 specifies a condition that has to be met at a particular point in a match,
827 without consuming any characters from the subject string. The use of
828 subpatterns for more complicated assertions is described
829 .\" HTML <a href="#bigassertions">
830 .\" </a>
831 below.
832 .\"
833 The backslashed assertions are:
834 .sp
835 \eb matches at a word boundary
836 \eB matches when not at a word boundary
837 \eA matches at the start of the subject
838 \eZ matches at the end of the subject
839 also matches before a newline at the end of the subject
840 \ez matches only at the end of the subject
841 \eG matches at the first matching position in the subject
842 .sp
843 Inside a character class, \eb has a different meaning; it matches the backspace
844 character. If any other of these assertions appears in a character class, by
845 default it matches the corresponding literal character (for example, \eB
846 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
847 escape sequence" error is generated instead.
848 .P
849 A word boundary is a position in the subject string where the current character
850 and the previous character do not both match \ew or \eW (i.e. one matches
851 \ew and the other matches \eW), or the start or end of the string if the
852 first or last character matches \ew, respectively. In UTF-8 mode, the meanings
853 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
854 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
855 of word" or "end of word" metasequence. However, whatever follows \eb normally
856 determines which it is. For example, the fragment \eba matches "a" at the start
857 of a word.
858 .P
859 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
860 dollar (described in the next section) in that they only ever match at the very
861 start and end of the subject string, whatever options are set. Thus, they are
862 independent of multiline mode. These three assertions are not affected by the
863 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
864 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
865 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
866 at a point other than the beginning of the subject, \eA can never match. The
867 difference between \eZ and \ez is that \eZ matches before a newline at the end
868 of the string as well as at the very end, whereas \ez matches only at the end.
869 .P
870 The \eG assertion is true only when the current matching position is at the
871 start point of the match, as specified by the \fIstartoffset\fP argument of
872 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
873 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
874 arguments, you can mimic Perl's /g option, and it is in this kind of
875 implementation where \eG can be useful.
876 .P
877 Note, however, that PCRE's interpretation of \eG, as the start of the current
878 match, is subtly different from Perl's, which defines it as the end of the
879 previous match. In Perl, these can be different when the previously matched
880 string was empty. Because PCRE does just one match at a time, it cannot
881 reproduce this behaviour.
882 .P
883 If all the alternatives of a pattern begin with \eG, the expression is anchored
884 to the starting match position, and the "anchored" flag is set in the compiled
885 regular expression.
886 .
887 .
889 .rs
890 .sp
891 Outside a character class, in the default matching mode, the circumflex
892 character is an assertion that is true only if the current matching point is
893 at the start of the subject string. If the \fIstartoffset\fP argument of
894 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
895 option is unset. Inside a character class, circumflex has an entirely different
896 meaning
897 .\" HTML <a href="#characterclass">
898 .\" </a>
899 (see below).
900 .\"
901 .P
902 Circumflex need not be the first character of the pattern if a number of
903 alternatives are involved, but it should be the first thing in each alternative
904 in which it appears if the pattern is ever to match that branch. If all
905 possible alternatives start with a circumflex, that is, if the pattern is
906 constrained to match only at the start of the subject, it is said to be an
907 "anchored" pattern. (There are also other constructs that can cause a pattern
908 to be anchored.)
909 .P
910 A dollar character is an assertion that is true only if the current matching
911 point is at the end of the subject string, or immediately before a newline
912 at the end of the string (by default). Dollar need not be the last character of
913 the pattern if a number of alternatives are involved, but it should be the last
914 item in any branch in which it appears. Dollar has no special meaning in a
915 character class.
916 .P
917 The meaning of dollar can be changed so that it matches only at the very end of
918 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
919 does not affect the \eZ assertion.
920 .P
921 The meanings of the circumflex and dollar characters are changed if the
922 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
923 immediately after internal newlines as well as at the start of the subject
924 string. It does not match after a newline that ends the string. A dollar
925 matches before any newlines in the string, as well as at the very end, when
926 PCRE_MULTILINE is set. When newline is specified as the two-character
927 sequence CRLF, isolated CR and LF characters do not indicate newlines.
928 .P
929 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
930 \en represents a newline) in multiline mode, but not otherwise. Consequently,
931 patterns that are anchored in single line mode because all branches start with
932 ^ are not anchored in multiline mode, and a match for circumflex is possible
933 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
934 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
935 .P
936 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
937 end of the subject in both modes, and if all branches of a pattern start with
938 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
939 .
940 .
941 .\" HTML <a name="fullstopdot"></a>
943 .rs
944 .sp
945 Outside a character class, a dot in the pattern matches any one character in
946 the subject string except (by default) a character that signifies the end of a
947 line. In UTF-8 mode, the matched character may be more than one byte long.
948 .P
949 When a line ending is defined as a single character, dot never matches that
950 character; when the two-character sequence CRLF is used, dot does not match CR
951 if it is immediately followed by LF, but otherwise it matches all characters
952 (including isolated CRs and LFs). When any Unicode line endings are being
953 recognized, dot does not match CR or LF or any of the other line ending
954 characters.
955 .P
956 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
957 option is set, a dot matches any one character, without exception. If the
958 two-character sequence CRLF is present in the subject string, it takes two dots
959 to match it.
960 .P
961 The handling of dot is entirely independent of the handling of circumflex and
962 dollar, the only relationship being that they both involve newlines. Dot has no
963 special meaning in a character class.
964 .P
965 The escape sequence \eN behaves like a dot, except that it is not affected by
966 the PCRE_DOTALL option. In other words, it matches any character except one
967 that signifies the end of a line.
968 .
969 .
971 .rs
972 .sp
973 Outside a character class, the escape sequence \eC matches any one byte, both
974 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
975 characters. The feature is provided in Perl in order to match individual bytes
976 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
977 rest of the string may start with a malformed UTF-8 character. For this reason,
978 the \eC escape sequence is best avoided.
979 .P
980 PCRE does not allow \eC to appear in lookbehind assertions
981 .\" HTML <a href="#lookbehind">
982 .\" </a>
983 (described below),
984 .\"
985 because in UTF-8 mode this would make it impossible to calculate the length of
986 the lookbehind.
987 .
988 .
989 .\" HTML <a name="characterclass"></a>
991 .rs
992 .sp
993 An opening square bracket introduces a character class, terminated by a closing
994 square bracket. A closing square bracket on its own is not special by default.
995 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
996 bracket causes a compile-time error. If a closing square bracket is required as
997 a member of the class, it should be the first data character in the class
998 (after an initial circumflex, if present) or escaped with a backslash.
999 .P
1000 A character class matches a single character in the subject. In UTF-8 mode, the
1001 character may be more than one byte long. A matched character must be in the
1002 set of characters defined by the class, unless the first character in the class
1003 definition is a circumflex, in which case the subject character must not be in
1004 the set defined by the class. If a circumflex is actually required as a member
1005 of the class, ensure it is not the first character, or escape it with a
1006 backslash.
1007 .P
1008 For example, the character class [aeiou] matches any lower case vowel, while
1009 [^aeiou] matches any character that is not a lower case vowel. Note that a
1010 circumflex is just a convenient notation for specifying the characters that
1011 are in the class by enumerating those that are not. A class that starts with a
1012 circumflex is not an assertion; it still consumes a character from the subject
1013 string, and therefore it fails if the current pointer is at the end of the
1014 string.
1015 .P
1016 In UTF-8 mode, characters with values greater than 255 can be included in a
1017 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
1018 .P
1019 When caseless matching is set, any letters in a class represent both their
1020 upper case and lower case versions, so for example, a caseless [aeiou] matches
1021 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1022 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1023 case for characters whose values are less than 128, so caseless matching is
1024 always possible. For characters with higher values, the concept of case is
1025 supported if PCRE is compiled with Unicode property support, but not otherwise.
1026 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1027 you must ensure that PCRE is compiled with Unicode property support as well as
1028 with UTF-8 support.
1029 .P
1030 Characters that might indicate line breaks are never treated in any special way
1031 when matching character classes, whatever line-ending sequence is in use, and
1032 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1033 such as [^a] always matches one of these characters.
1034 .P
1035 The minus (hyphen) character can be used to specify a range of characters in a
1036 character class. For example, [d-m] matches any letter between d and m,
1037 inclusive. If a minus character is required in a class, it must be escaped with
1038 a backslash or appear in a position where it cannot be interpreted as
1039 indicating a range, typically as the first or last character in the class.
1040 .P
1041 It is not possible to have the literal character "]" as the end character of a
1042 range. A pattern such as [W-]46] is interpreted as a class of two characters
1043 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1044 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1045 the end of range, so [W-\e]46] is interpreted as a class containing a range
1046 followed by two other characters. The octal or hexadecimal representation of
1047 "]" can also be used to end a range.
1048 .P
1049 Ranges operate in the collating sequence of character values. They can also be
1050 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1051 mode, ranges can include characters whose values are greater than 255, for
1052 example [\ex{100}-\ex{2ff}].
1053 .P
1054 If a range that includes letters is used when caseless matching is set, it
1055 matches the letters in either case. For example, [W-c] is equivalent to
1056 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1057 tables for a French locale are in use, [\exc8-\excb] matches accented E
1058 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1059 characters with values greater than 128 only when it is compiled with Unicode
1060 property support.
1061 .P
1062 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1063 \eV, \ew, and \eW may appear in a character class, and add the characters that
1064 they match to the class. For example, [\edABCDEF] matches any hexadecimal
1065 digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1066 and their upper case partners, just as it does when they appear outside a
1067 character class, as described in the section entitled
1068 .\" HTML <a href="#genericchartypes">
1069 .\" </a>
1070 "Generic character types"
1071 .\"
1072 above. The escape sequence \eb has a different meaning inside a character
1073 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1074 are not special inside a character class. Like any other unrecognized escape
1075 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1076 default, but cause an error if the PCRE_EXTRA option is set.
1077 .P
1078 A circumflex can conveniently be used with the upper case character types to
1079 specify a more restricted set of characters than the matching lower case type.
1080 For example, the class [^\eW_] matches any letter or digit, but not underscore,
1081 whereas [\ew] includes underscore. A positive character class should be read as
1082 "something OR something OR ..." and a negative class as "NOT something AND NOT
1083 something AND NOT ...".
1084 .P
1085 The only metacharacters that are recognized in character classes are backslash,
1086 hyphen (only where it can be interpreted as specifying a range), circumflex
1087 (only at the start), opening square bracket (only when it can be interpreted as
1088 introducing a POSIX class name - see the next section), and the terminating
1089 closing square bracket. However, escaping other non-alphanumeric characters
1090 does no harm.
1091 .
1092 .
1094 .rs
1095 .sp
1096 Perl supports the POSIX notation for character classes. This uses names
1097 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1098 this notation. For example,
1099 .sp
1100 [01[:alpha:]%]
1101 .sp
1102 matches "0", "1", any alphabetic character, or "%". The supported class names
1103 are:
1104 .sp
1105 alnum letters and digits
1106 alpha letters
1107 ascii character codes 0 - 127
1108 blank space or tab only
1109 cntrl control characters
1110 digit decimal digits (same as \ed)
1111 graph printing characters, excluding space
1112 lower lower case letters
1113 print printing characters, including space
1114 punct printing characters, excluding letters and digits and space
1115 space white space (not quite the same as \es)
1116 upper upper case letters
1117 word "word" characters (same as \ew)
1118 xdigit hexadecimal digits
1119 .sp
1120 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1121 space (32). Notice that this list includes the VT character (code 11). This
1122 makes "space" different to \es, which does not include VT (for Perl
1123 compatibility).
1124 .P
1125 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1126 5.8. Another Perl extension is negation, which is indicated by a ^ character
1127 after the colon. For example,
1128 .sp
1129 [12[:^digit:]]
1130 .sp
1131 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1132 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1133 supported, and an error is given if they are encountered.
1134 .P
1135 By default, in UTF-8 mode, characters with values greater than 128 do not match
1136 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1137 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1138 character properties are used. This is achieved by replacing the POSIX classes
1139 by other sequences, as follows:
1140 .sp
1141 [:alnum:] becomes \ep{Xan}
1142 [:alpha:] becomes \ep{L}
1143 [:blank:] becomes \eh
1144 [:digit:] becomes \ep{Nd}
1145 [:lower:] becomes \ep{Ll}
1146 [:space:] becomes \ep{Xps}
1147 [:upper:] becomes \ep{Lu}
1148 [:word:] becomes \ep{Xwd}
1149 .sp
1150 Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1151 classes are unchanged, and match only characters with code points less than
1152 128.
1153 .
1154 .
1156 .rs
1157 .sp
1158 Vertical bar characters are used to separate alternative patterns. For example,
1159 the pattern
1160 .sp
1161 gilbert|sullivan
1162 .sp
1163 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1164 and an empty alternative is permitted (matching the empty string). The matching
1165 process tries each alternative in turn, from left to right, and the first one
1166 that succeeds is used. If the alternatives are within a subpattern
1167 .\" HTML <a href="#subpattern">
1168 .\" </a>
1169 (defined below),
1170 .\"
1171 "succeeds" means matching the rest of the main pattern as well as the
1172 alternative in the subpattern.
1173 .
1174 .
1176 .rs
1177 .sp
1178 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1179 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1180 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1181 The option letters are
1182 .sp
1183 i for PCRE_CASELESS
1185 s for PCRE_DOTALL
1186 x for PCRE_EXTENDED
1187 .sp
1188 For example, (?im) sets caseless, multiline matching. It is also possible to
1189 unset these options by preceding the letter with a hyphen, and a combined
1190 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1191 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1192 permitted. If a letter appears both before and after the hyphen, the option is
1193 unset.
1194 .P
1195 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1196 changed in the same way as the Perl-compatible options by using the characters
1197 J, U and X respectively.
1198 .P
1199 When one of these option changes occurs at top level (that is, not inside
1200 subpattern parentheses), the change applies to the remainder of the pattern
1201 that follows. If the change is placed right at the start of a pattern, PCRE
1202 extracts it into the global options (and it will therefore show up in data
1203 extracted by the \fBpcre_fullinfo()\fP function).
1204 .P
1205 An option change within a subpattern (see below for a description of
1206 subpatterns) affects only that part of the subpattern that follows it, so
1207 .sp
1208 (a(?i)b)c
1209 .sp
1210 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1211 By this means, options can be made to have different settings in different
1212 parts of the pattern. Any changes made in one alternative do carry on
1213 into subsequent branches within the same subpattern. For example,
1214 .sp
1215 (a(?i)b|c)
1216 .sp
1217 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1218 branch is abandoned before the option setting. This is because the effects of
1219 option settings happen at compile time. There would be some very weird
1220 behaviour otherwise.
1221 .P
1222 \fBNote:\fP There are other PCRE-specific options that can be set by the
1223 application when the compile or match functions are called. In some cases the
1224 pattern can contain special leading sequences such as (*CRLF) to override what
1225 the application has set or what has been defaulted. Details are given in the
1226 section entitled
1227 .\" HTML <a href="#newlineseq">
1228 .\" </a>
1229 "Newline sequences"
1230 .\"
1231 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1232 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1233 PCRE_UTF8 and the PCRE_UCP options, respectively.
1234 .
1235 .
1236 .\" HTML <a name="subpattern"></a>
1238 .rs
1239 .sp
1240 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1241 Turning part of a pattern into a subpattern does two things:
1242 .sp
1243 1. It localizes a set of alternatives. For example, the pattern
1244 .sp
1245 cat(aract|erpillar|)
1246 .sp
1247 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1248 match "cataract", "erpillar" or an empty string.
1249 .sp
1250 2. It sets up the subpattern as a capturing subpattern. This means that, when
1251 the whole pattern matches, that portion of the subject string that matched the
1252 subpattern is passed back to the caller via the \fIovector\fP argument of
1253 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1254 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1255 string "the red king" is matched against the pattern
1256 .sp
1257 the ((red|white) (king|queen))
1258 .sp
1259 the captured substrings are "red king", "red", and "king", and are numbered 1,
1260 2, and 3, respectively.
1261 .P
1262 The fact that plain parentheses fulfil two functions is not always helpful.
1263 There are often times when a grouping subpattern is required without a
1264 capturing requirement. If an opening parenthesis is followed by a question mark
1265 and a colon, the subpattern does not do any capturing, and is not counted when
1266 computing the number of any subsequent capturing subpatterns. For example, if
1267 the string "the white queen" is matched against the pattern
1268 .sp
1269 the ((?:red|white) (king|queen))
1270 .sp
1271 the captured substrings are "white queen" and "queen", and are numbered 1 and
1272 2. The maximum number of capturing subpatterns is 65535.
1273 .P
1274 As a convenient shorthand, if any option settings are required at the start of
1275 a non-capturing subpattern, the option letters may appear between the "?" and
1276 the ":". Thus the two patterns
1277 .sp
1278 (?i:saturday|sunday)
1279 (?:(?i)saturday|sunday)
1280 .sp
1281 match exactly the same set of strings. Because alternative branches are tried
1282 from left to right, and options are not reset until the end of the subpattern
1283 is reached, an option setting in one branch does affect subsequent branches, so
1284 the above patterns match "SUNDAY" as well as "Saturday".
1285 .
1286 .
1287 .\" HTML <a name="dupsubpatternnumber"></a>
1289 .rs
1290 .sp
1291 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1292 the same numbers for its capturing parentheses. Such a subpattern starts with
1293 (?| and is itself a non-capturing subpattern. For example, consider this
1294 pattern:
1295 .sp
1296 (?|(Sat)ur|(Sun))day
1297 .sp
1298 Because the two alternatives are inside a (?| group, both sets of capturing
1299 parentheses are numbered one. Thus, when the pattern matches, you can look
1300 at captured substring number one, whichever alternative matched. This construct
1301 is useful when you want to capture part, but not all, of one of a number of
1302 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1303 number is reset at the start of each branch. The numbers of any capturing
1304 parentheses that follow the subpattern start after the highest number used in
1305 any branch. The following example is taken from the Perl documentation. The
1306 numbers underneath show in which buffer the captured content will be stored.
1307 .sp
1308 # before ---------------branch-reset----------- after
1309 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1310 # 1 2 2 3 2 3 4
1311 .sp
1312 A back reference to a numbered subpattern uses the most recent value that is
1313 set for that number by any subpattern. The following pattern matches "abcabc"
1314 or "defdef":
1315 .sp
1316 /(?|(abc)|(def))\e1/
1317 .sp
1318 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1319 refers to the first one in the pattern with the given number. The following
1320 pattern matches "abcabc" or "defabc":
1321 .sp
1322 /(?|(abc)|(def))(?1)/
1323 .sp
1324 If a
1325 .\" HTML <a href="#conditions">
1326 .\" </a>
1327 condition test
1328 .\"
1329 for a subpattern's having matched refers to a non-unique number, the test is
1330 true if any of the subpatterns of that number have matched.
1331 .P
1332 An alternative approach to using this "branch reset" feature is to use
1333 duplicate named subpatterns, as described in the next section.
1334 .
1335 .
1337 .rs
1338 .sp
1339 Identifying capturing parentheses by number is simple, but it can be very hard
1340 to keep track of the numbers in complicated regular expressions. Furthermore,
1341 if an expression is modified, the numbers may change. To help with this
1342 difficulty, PCRE supports the naming of subpatterns. This feature was not
1343 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1344 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1345 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1346 have different names, but PCRE does not.
1347 .P
1348 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1349 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1350 parentheses from other parts of the pattern, such as
1351 .\" HTML <a href="#backreferences">
1352 .\" </a>
1353 back references,
1354 .\"
1355 .\" HTML <a href="#recursion">
1356 .\" </a>
1357 recursion,
1358 .\"
1359 and
1360 .\" HTML <a href="#conditions">
1361 .\" </a>
1362 conditions,
1363 .\"
1364 can be made by name as well as by number.
1365 .P
1366 Names consist of up to 32 alphanumeric characters and underscores. Named
1367 capturing parentheses are still allocated numbers as well as names, exactly as
1368 if the names were not present. The PCRE API provides function calls for
1369 extracting the name-to-number translation table from a compiled pattern. There
1370 is also a convenience function for extracting a captured substring by name.
1371 .P
1372 By default, a name must be unique within a pattern, but it is possible to relax
1373 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1374 names are also always permitted for subpatterns with the same number, set up as
1375 described in the previous section.) Duplicate names can be useful for patterns
1376 where only one instance of the named parentheses can match. Suppose you want to
1377 match the name of a weekday, either as a 3-letter abbreviation or as the full
1378 name, and in both cases you want to extract the abbreviation. This pattern
1379 (ignoring the line breaks) does the job:
1380 .sp
1381 (?<DN>Mon|Fri|Sun)(?:day)?|
1382 (?<DN>Tue)(?:sday)?|
1383 (?<DN>Wed)(?:nesday)?|
1384 (?<DN>Thu)(?:rsday)?|
1385 (?<DN>Sat)(?:urday)?
1386 .sp
1387 There are five capturing substrings, but only one is ever set after a match.
1388 (An alternative way of solving this problem is to use a "branch reset"
1389 subpattern, as described in the previous section.)
1390 .P
1391 The convenience function for extracting the data by name returns the substring
1392 for the first (and in this example, the only) subpattern of that name that
1393 matched. This saves searching to find which numbered subpattern it was.
1394 .P
1395 If you make a back reference to a non-unique named subpattern from elsewhere in
1396 the pattern, the one that corresponds to the first occurrence of the name is
1397 used. In the absence of duplicate numbers (see the previous section) this is
1398 the one with the lowest number. If you use a named reference in a condition
1399 test (see the
1400 .\"
1401 .\" HTML <a href="#conditions">
1402 .\" </a>
1403 section about conditions
1404 .\"
1405 below), either to check whether a subpattern has matched, or to check for
1406 recursion, all subpatterns with the same name are tested. If the condition is
1407 true for any one of them, the overall condition is true. This is the same
1408 behaviour as testing by number. For further details of the interfaces for
1409 handling named subpatterns, see the
1410 .\" HREF
1411 \fBpcreapi\fP
1412 .\"
1413 documentation.
1414 .P
1415 \fBWarning:\fP You cannot use different names to distinguish between two
1416 subpatterns with the same number because PCRE uses only the numbers when
1417 matching. For this reason, an error is given at compile time if different names
1418 are given to subpatterns with the same number. However, you can give the same
1419 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1420 .
1421 .
1423 .rs
1424 .sp
1425 Repetition is specified by quantifiers, which can follow any of the following
1426 items:
1427 .sp
1428 a literal data character
1429 the dot metacharacter
1430 the \eC escape sequence
1431 the \eX escape sequence (in UTF-8 mode with Unicode properties)
1432 the \eR escape sequence
1433 an escape such as \ed or \epL that matches a single character
1434 a character class
1435 a back reference (see next section)
1436 a parenthesized subpattern (including assertions)
1437 a recursive or "subroutine" call to a subpattern
1438 .sp
1439 The general repetition quantifier specifies a minimum and maximum number of
1440 permitted matches, by giving the two numbers in curly brackets (braces),
1441 separated by a comma. The numbers must be less than 65536, and the first must
1442 be less than or equal to the second. For example:
1443 .sp
1444 z{2,4}
1445 .sp
1446 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1447 character. If the second number is omitted, but the comma is present, there is
1448 no upper limit; if the second number and the comma are both omitted, the
1449 quantifier specifies an exact number of required matches. Thus
1450 .sp
1451 [aeiou]{3,}
1452 .sp
1453 matches at least 3 successive vowels, but may match many more, while
1454 .sp
1455 \ed{8}
1456 .sp
1457 matches exactly 8 digits. An opening curly bracket that appears in a position
1458 where a quantifier is not allowed, or one that does not match the syntax of a
1459 quantifier, is taken as a literal character. For example, {,6} is not a
1460 quantifier, but a literal string of four characters.
1461 .P
1462 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1463 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1464 which is represented by a two-byte sequence. Similarly, when Unicode property
1465 support is available, \eX{3} matches three Unicode extended sequences, each of
1466 which may be several bytes long (and they may be of different lengths).
1467 .P
1468 The quantifier {0} is permitted, causing the expression to behave as if the
1469 previous item and the quantifier were not present. This may be useful for
1470 subpatterns that are referenced as
1471 .\" HTML <a href="#subpatternsassubroutines">
1472 .\" </a>
1473 subroutines
1474 .\"
1475 from elsewhere in the pattern (but see also the section entitled
1476 .\" HTML <a href="#subdefine">
1477 .\" </a>
1478 "Defining subpatterns for use by reference only"
1479 .\"
1480 below). Items other than subpatterns that have a {0} quantifier are omitted
1481 from the compiled pattern.
1482 .P
1483 For convenience, the three most common quantifiers have single-character
1484 abbreviations:
1485 .sp
1486 * is equivalent to {0,}
1487 + is equivalent to {1,}
1488 ? is equivalent to {0,1}
1489 .sp
1490 It is possible to construct infinite loops by following a subpattern that can
1491 match no characters with a quantifier that has no upper limit, for example:
1492 .sp
1493 (a?)*
1494 .sp
1495 Earlier versions of Perl and PCRE used to give an error at compile time for
1496 such patterns. However, because there are cases where this can be useful, such
1497 patterns are now accepted, but if any repetition of the subpattern does in fact
1498 match no characters, the loop is forcibly broken.
1499 .P
1500 By default, the quantifiers are "greedy", that is, they match as much as
1501 possible (up to the maximum number of permitted times), without causing the
1502 rest of the pattern to fail. The classic example of where this gives problems
1503 is in trying to match comments in C programs. These appear between /* and */
1504 and within the comment, individual * and / characters may appear. An attempt to
1505 match C comments by applying the pattern
1506 .sp
1507 /\e*.*\e*/
1508 .sp
1509 to the string
1510 .sp
1511 /* first comment */ not comment /* second comment */
1512 .sp
1513 fails, because it matches the entire string owing to the greediness of the .*
1514 item.
1515 .P
1516 However, if a quantifier is followed by a question mark, it ceases to be
1517 greedy, and instead matches the minimum number of times possible, so the
1518 pattern
1519 .sp
1520 /\e*.*?\e*/
1521 .sp
1522 does the right thing with the C comments. The meaning of the various
1523 quantifiers is not otherwise changed, just the preferred number of matches.
1524 Do not confuse this use of question mark with its use as a quantifier in its
1525 own right. Because it has two uses, it can sometimes appear doubled, as in
1526 .sp
1527 \ed??\ed
1528 .sp
1529 which matches one digit by preference, but can match two if that is the only
1530 way the rest of the pattern matches.
1531 .P
1532 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1533 the quantifiers are not greedy by default, but individual ones can be made
1534 greedy by following them with a question mark. In other words, it inverts the
1535 default behaviour.
1536 .P
1537 When a parenthesized subpattern is quantified with a minimum repeat count that
1538 is greater than 1 or with a limited maximum, more memory is required for the
1539 compiled pattern, in proportion to the size of the minimum or maximum.
1540 .P
1541 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1542 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1543 implicitly anchored, because whatever follows will be tried against every
1544 character position in the subject string, so there is no point in retrying the
1545 overall match at any position after the first. PCRE normally treats such a
1546 pattern as though it were preceded by \eA.
1547 .P
1548 In cases where it is known that the subject string contains no newlines, it is
1549 worth setting PCRE_DOTALL in order to obtain this optimization, or
1550 alternatively using ^ to indicate anchoring explicitly.
1551 .P
1552 However, there is one situation where the optimization cannot be used. When .*
1553 is inside capturing parentheses that are the subject of a back reference
1554 elsewhere in the pattern, a match at the start may fail where a later one
1555 succeeds. Consider, for example:
1556 .sp
1557 (.*)abc\e1
1558 .sp
1559 If the subject is "xyz123abc123" the match point is the fourth character. For
1560 this reason, such a pattern is not implicitly anchored.
1561 .P
1562 When a capturing subpattern is repeated, the value captured is the substring
1563 that matched the final iteration. For example, after
1564 .sp
1565 (tweedle[dume]{3}\es*)+
1566 .sp
1567 has matched "tweedledum tweedledee" the value of the captured substring is
1568 "tweedledee". However, if there are nested capturing subpatterns, the
1569 corresponding captured values may have been set in previous iterations. For
1570 example, after
1571 .sp
1572 /(a|(b))+/
1573 .sp
1574 matches "aba" the value of the second captured substring is "b".
1575 .
1576 .
1577 .\" HTML <a name="atomicgroup"></a>
1579 .rs
1580 .sp
1581 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1582 repetition, failure of what follows normally causes the repeated item to be
1583 re-evaluated to see if a different number of repeats allows the rest of the
1584 pattern to match. Sometimes it is useful to prevent this, either to change the
1585 nature of the match, or to cause it fail earlier than it otherwise might, when
1586 the author of the pattern knows there is no point in carrying on.
1587 .P
1588 Consider, for example, the pattern \ed+foo when applied to the subject line
1589 .sp
1590 123456bar
1591 .sp
1592 After matching all 6 digits and then failing to match "foo", the normal
1593 action of the matcher is to try again with only 5 digits matching the \ed+
1594 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1595 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1596 that once a subpattern has matched, it is not to be re-evaluated in this way.
1597 .P
1598 If we use atomic grouping for the previous example, the matcher gives up
1599 immediately on failing to match "foo" the first time. The notation is a kind of
1600 special parenthesis, starting with (?> as in this example:
1601 .sp
1602 (?>\ed+)foo
1603 .sp
1604 This kind of parenthesis "locks up" the part of the pattern it contains once
1605 it has matched, and a failure further into the pattern is prevented from
1606 backtracking into it. Backtracking past it to previous items, however, works as
1607 normal.
1608 .P
1609 An alternative description is that a subpattern of this type matches the string
1610 of characters that an identical standalone pattern would match, if anchored at
1611 the current point in the subject string.
1612 .P
1613 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1614 the above example can be thought of as a maximizing repeat that must swallow
1615 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1616 number of digits they match in order to make the rest of the pattern match,
1617 (?>\ed+) can only match an entire sequence of digits.
1618 .P
1619 Atomic groups in general can of course contain arbitrarily complicated
1620 subpatterns, and can be nested. However, when the subpattern for an atomic
1621 group is just a single repeated item, as in the example above, a simpler
1622 notation, called a "possessive quantifier" can be used. This consists of an
1623 additional + character following a quantifier. Using this notation, the
1624 previous example can be rewritten as
1625 .sp
1626 \ed++foo
1627 .sp
1628 Note that a possessive quantifier can be used with an entire group, for
1629 example:
1630 .sp
1631 (abc|xyz){2,3}+
1632 .sp
1633 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1634 option is ignored. They are a convenient notation for the simpler forms of
1635 atomic group. However, there is no difference in the meaning of a possessive
1636 quantifier and the equivalent atomic group, though there may be a performance
1637 difference; possessive quantifiers should be slightly faster.
1638 .P
1639 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1640 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1641 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1642 package, and PCRE copied it from there. It ultimately found its way into Perl
1643 at release 5.10.
1644 .P
1645 PCRE has an optimization that automatically "possessifies" certain simple
1646 pattern constructs. For example, the sequence A+B is treated as A++B because
1647 there is no point in backtracking into a sequence of A's when B must follow.
1648 .P
1649 When a pattern contains an unlimited repeat inside a subpattern that can itself
1650 be repeated an unlimited number of times, the use of an atomic group is the
1651 only way to avoid some failing matches taking a very long time indeed. The
1652 pattern
1653 .sp
1654 (\eD+|<\ed+>)*[!?]
1655 .sp
1656 matches an unlimited number of substrings that either consist of non-digits, or
1657 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1658 quickly. However, if it is applied to
1659 .sp
1660 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1661 .sp
1662 it takes a long time before reporting failure. This is because the string can
1663 be divided between the internal \eD+ repeat and the external * repeat in a
1664 large number of ways, and all have to be tried. (The example uses [!?] rather
1665 than a single character at the end, because both PCRE and Perl have an
1666 optimization that allows for fast failure when a single character is used. They
1667 remember the last single character that is required for a match, and fail early
1668 if it is not present in the string.) If the pattern is changed so that it uses
1669 an atomic group, like this:
1670 .sp
1671 ((?>\eD+)|<\ed+>)*[!?]
1672 .sp
1673 sequences of non-digits cannot be broken, and failure happens quickly.
1674 .
1675 .
1676 .\" HTML <a name="backreferences"></a>
1678 .rs
1679 .sp
1680 Outside a character class, a backslash followed by a digit greater than 0 (and
1681 possibly further digits) is a back reference to a capturing subpattern earlier
1682 (that is, to its left) in the pattern, provided there have been that many
1683 previous capturing left parentheses.
1684 .P
1685 However, if the decimal number following the backslash is less than 10, it is
1686 always taken as a back reference, and causes an error only if there are not
1687 that many capturing left parentheses in the entire pattern. In other words, the
1688 parentheses that are referenced need not be to the left of the reference for
1689 numbers less than 10. A "forward back reference" of this type can make sense
1690 when a repetition is involved and the subpattern to the right has participated
1691 in an earlier iteration.
1692 .P
1693 It is not possible to have a numerical "forward back reference" to a subpattern
1694 whose number is 10 or more using this syntax because a sequence such as \e50 is
1695 interpreted as a character defined in octal. See the subsection entitled
1696 "Non-printing characters"
1697 .\" HTML <a href="#digitsafterbackslash">
1698 .\" </a>
1699 above
1700 .\"
1701 for further details of the handling of digits following a backslash. There is
1702 no such problem when named parentheses are used. A back reference to any
1703 subpattern is possible using named parentheses (see below).
1704 .P
1705 Another way of avoiding the ambiguity inherent in the use of digits following a
1706 backslash is to use the \eg escape sequence. This escape must be followed by an
1707 unsigned number or a negative number, optionally enclosed in braces. These
1708 examples are all identical:
1709 .sp
1710 (ring), \e1
1711 (ring), \eg1
1712 (ring), \eg{1}
1713 .sp
1714 An unsigned number specifies an absolute reference without the ambiguity that
1715 is present in the older syntax. It is also useful when literal digits follow
1716 the reference. A negative number is a relative reference. Consider this
1717 example:
1718 .sp
1719 (abc(def)ghi)\eg{-1}
1720 .sp
1721 The sequence \eg{-1} is a reference to the most recently started capturing
1722 subpattern before \eg, that is, is it equivalent to \e2 in this example.
1723 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1724 can be helpful in long patterns, and also in patterns that are created by
1725 joining together fragments that contain references within themselves.
1726 .P
1727 A back reference matches whatever actually matched the capturing subpattern in
1728 the current subject string, rather than anything matching the subpattern
1729 itself (see
1730 .\" HTML <a href="#subpatternsassubroutines">
1731 .\" </a>
1732 "Subpatterns as subroutines"
1733 .\"
1734 below for a way of doing that). So the pattern
1735 .sp
1736 (sens|respons)e and \e1ibility
1737 .sp
1738 matches "sense and sensibility" and "response and responsibility", but not
1739 "sense and responsibility". If caseful matching is in force at the time of the
1740 back reference, the case of letters is relevant. For example,
1741 .sp
1742 ((?i)rah)\es+\e1
1743 .sp
1744 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1745 capturing subpattern is matched caselessly.
1746 .P
1747 There are several different ways of writing back references to named
1748 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1749 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1750 back reference syntax, in which \eg can be used for both numeric and named
1751 references, is also supported. We could rewrite the above example in any of
1752 the following ways:
1753 .sp
1754 (?<p1>(?i)rah)\es+\ek<p1>
1755 (?'p1'(?i)rah)\es+\ek{p1}
1756 (?P<p1>(?i)rah)\es+(?P=p1)
1757 (?<p1>(?i)rah)\es+\eg{p1}
1758 .sp
1759 A subpattern that is referenced by name may appear in the pattern before or
1760 after the reference.
1761 .P
1762 There may be more than one back reference to the same subpattern. If a
1763 subpattern has not actually been used in a particular match, any back
1764 references to it always fail by default. For example, the pattern
1765 .sp
1766 (a|(bc))\e2
1767 .sp
1768 always fails if it starts to match "a" rather than "bc". However, if the
1769 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1770 unset value matches an empty string.
1771 .P
1772 Because there may be many capturing parentheses in a pattern, all digits
1773 following a backslash are taken as part of a potential back reference number.
1774 If the pattern continues with a digit character, some delimiter must be used to
1775 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1776 whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1777 .\" HTML <a href="#comments">
1778 .\" </a>
1779 "Comments"
1780 .\"
1781 below) can be used.
1782 .
1783 .SS "Recursive back references"
1784 .rs
1785 .sp
1786 A back reference that occurs inside the parentheses to which it refers fails
1787 when the subpattern is first used, so, for example, (a\e1) never matches.
1788 However, such references can be useful inside repeated subpatterns. For
1789 example, the pattern
1790 .sp
1791 (a|b\e1)+
1792 .sp
1793 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1794 the subpattern, the back reference matches the character string corresponding
1795 to the previous iteration. In order for this to work, the pattern must be such
1796 that the first iteration does not need to match the back reference. This can be
1797 done using alternation, as in the example above, or by a quantifier with a
1798 minimum of zero.
1799 .P
1800 Back references of this type cause the group that they reference to be treated
1801 as an
1802 .\" HTML <a href="#atomicgroup">
1803 .\" </a>
1804 atomic group.
1805 .\"
1806 Once the whole group has been matched, a subsequent matching failure cannot
1807 cause backtracking into the middle of the group.
1808 .
1809 .
1810 .\" HTML <a name="bigassertions"></a>
1812 .rs
1813 .sp
1814 An assertion is a test on the characters following or preceding the current
1815 matching point that does not actually consume any characters. The simple
1816 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1817 .\" HTML <a href="#smallassertions">
1818 .\" </a>
1819 above.
1820 .\"
1821 .P
1822 More complicated assertions are coded as subpatterns. There are two kinds:
1823 those that look ahead of the current position in the subject string, and those
1824 that look behind it. An assertion subpattern is matched in the normal way,
1825 except that it does not cause the current matching position to be changed.
1826 .P
1827 Assertion subpatterns are not capturing subpatterns. If such an assertion
1828 contains capturing subpatterns within it, these are counted for the purposes of
1829 numbering the capturing subpatterns in the whole pattern. However, substring
1830 capturing is carried out only for positive assertions, because it does not make
1831 sense for negative assertions.
1832 .P
1833 For compatibility with Perl, assertion subpatterns may be repeated; though
1834 it makes no sense to assert the same thing several times, the side effect of
1835 capturing parentheses may occasionally be useful. In practice, there only three
1836 cases:
1837 .sp
1838 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1839 However, it may contain internal capturing parenthesized groups that are called
1840 from elsewhere via the
1841 .\" HTML <a href="#subpatternsassubroutines">
1842 .\" </a>
1843 subroutine mechanism.
1844 .\"
1845 .sp
1846 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1847 were {0,1}. At run time, the rest of the pattern match is tried with and
1848 without the assertion, the order depending on the greediness of the quantifier.
1849 .sp
1850 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1851 The assertion is obeyed just once when encountered during matching.
1852 .
1853 .
1854 .SS "Lookahead assertions"
1855 .rs
1856 .sp
1857 Lookahead assertions start with (?= for positive assertions and (?! for
1858 negative assertions. For example,
1859 .sp
1860 \ew+(?=;)
1861 .sp
1862 matches a word followed by a semicolon, but does not include the semicolon in
1863 the match, and
1864 .sp
1865 foo(?!bar)
1866 .sp
1867 matches any occurrence of "foo" that is not followed by "bar". Note that the
1868 apparently similar pattern
1869 .sp
1870 (?!foo)bar
1871 .sp
1872 does not find an occurrence of "bar" that is preceded by something other than
1873 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1874 (?!foo) is always true when the next three characters are "bar". A
1875 lookbehind assertion is needed to achieve the other effect.
1876 .P
1877 If you want to force a matching failure at some point in a pattern, the most
1878 convenient way to do it is with (?!) because an empty string always matches, so
1879 an assertion that requires there not to be an empty string must always fail.
1880 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1881 .
1882 .
1883 .\" HTML <a name="lookbehind"></a>
1884 .SS "Lookbehind assertions"
1885 .rs
1886 .sp
1887 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1888 negative assertions. For example,
1889 .sp
1890 (?<!foo)bar
1891 .sp
1892 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1893 a lookbehind assertion are restricted such that all the strings it matches must
1894 have a fixed length. However, if there are several top-level alternatives, they
1895 do not all have to have the same fixed length. Thus
1896 .sp
1897 (?<=bullock|donkey)
1898 .sp
1899 is permitted, but
1900 .sp
1901 (?<!dogs?|cats?)
1902 .sp
1903 causes an error at compile time. Branches that match different length strings
1904 are permitted only at the top level of a lookbehind assertion. This is an
1905 extension compared with Perl, which requires all branches to match the same
1906 length of string. An assertion such as
1907 .sp
1908 (?<=ab(c|de))
1909 .sp
1910 is not permitted, because its single top-level branch can match two different
1911 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1912 branches:
1913 .sp
1914 (?<=abc|abde)
1915 .sp
1916 In some cases, the escape sequence \eK
1917 .\" HTML <a href="#resetmatchstart">
1918 .\" </a>
1919 (see above)
1920 .\"
1921 can be used instead of a lookbehind assertion to get round the fixed-length
1922 restriction.
1923 .P
1924 The implementation of lookbehind assertions is, for each alternative, to
1925 temporarily move the current position back by the fixed length and then try to
1926 match. If there are insufficient characters before the current position, the
1927 assertion fails.
1928 .P
1929 PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1930 to appear in lookbehind assertions, because it makes it impossible to calculate
1931 the length of the lookbehind. The \eX and \eR escapes, which can match
1932 different numbers of bytes, are also not permitted.
1933 .P
1934 .\" HTML <a href="#subpatternsassubroutines">
1935 .\" </a>
1936 "Subroutine"
1937 .\"
1938 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1939 as the subpattern matches a fixed-length string.
1940 .\" HTML <a href="#recursion">
1941 .\" </a>
1942 Recursion,
1943 .\"
1944 however, is not supported.
1945 .P
1946 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1947 specify efficient matching of fixed-length strings at the end of subject
1948 strings. Consider a simple pattern such as
1949 .sp
1950 abcd$
1951 .sp
1952 when applied to a long string that does not match. Because matching proceeds
1953 from left to right, PCRE will look for each "a" in the subject and then see if
1954 what follows matches the rest of the pattern. If the pattern is specified as
1955 .sp
1956 ^.*abcd$
1957 .sp
1958 the initial .* matches the entire string at first, but when this fails (because
1959 there is no following "a"), it backtracks to match all but the last character,
1960 then all but the last two characters, and so on. Once again the search for "a"
1961 covers the entire string, from right to left, so we are no better off. However,
1962 if the pattern is written as
1963 .sp
1964 ^.*+(?<=abcd)
1965 .sp
1966 there can be no backtracking for the .*+ item; it can match only the entire
1967 string. The subsequent lookbehind assertion does a single test on the last four
1968 characters. If it fails, the match fails immediately. For long strings, this
1969 approach makes a significant difference to the processing time.
1970 .
1971 .
1972 .SS "Using multiple assertions"
1973 .rs
1974 .sp
1975 Several assertions (of any sort) may occur in succession. For example,
1976 .sp
1977 (?<=\ed{3})(?<!999)foo
1978 .sp
1979 matches "foo" preceded by three digits that are not "999". Notice that each of
1980 the assertions is applied independently at the same point in the subject
1981 string. First there is a check that the previous three characters are all
1982 digits, and then there is a check that the same three characters are not "999".
1983 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1984 of which are digits and the last three of which are not "999". For example, it
1985 doesn't match "123abcfoo". A pattern to do that is
1986 .sp
1987 (?<=\ed{3}...)(?<!999)foo
1988 .sp
1989 This time the first assertion looks at the preceding six characters, checking
1990 that the first three are digits, and then the second assertion checks that the
1991 preceding three characters are not "999".
1992 .P
1993 Assertions can be nested in any combination. For example,
1994 .sp
1995 (?<=(?<!foo)bar)baz
1996 .sp
1997 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1998 preceded by "foo", while
1999 .sp
2000 (?<=\ed{3}(?!999)...)foo
2001 .sp
2002 is another pattern that matches "foo" preceded by three digits and any three
2003 characters that are not "999".
2004 .
2005 .
2006 .\" HTML <a name="conditions"></a>
2008 .rs
2009 .sp
2010 It is possible to cause the matching process to obey a subpattern
2011 conditionally or to choose between two alternative subpatterns, depending on
2012 the result of an assertion, or whether a specific capturing subpattern has
2013 already been matched. The two possible forms of conditional subpattern are:
2014 .sp
2015 (?(condition)yes-pattern)
2016 (?(condition)yes-pattern|no-pattern)
2017 .sp
2018 If the condition is satisfied, the yes-pattern is used; otherwise the
2019 no-pattern (if present) is used. If there are more than two alternatives in the
2020 subpattern, a compile-time error occurs. Each of the two alternatives may
2021 itself contain nested subpatterns of any form, including conditional
2022 subpatterns; the restriction to two alternatives applies only at the level of
2023 the condition. This pattern fragment is an example where the alternatives are
2024 complex:
2025 .sp
2026 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2027 .sp
2028 .P
2029 There are four kinds of condition: references to subpatterns, references to
2030 recursion, a pseudo-condition called DEFINE, and assertions.
2031 .
2032 .SS "Checking for a used subpattern by number"
2033 .rs
2034 .sp
2035 If the text between the parentheses consists of a sequence of digits, the
2036 condition is true if a capturing subpattern of that number has previously
2037 matched. If there is more than one capturing subpattern with the same number
2038 (see the earlier
2039 .\"
2040 .\" HTML <a href="#recursion">
2041 .\" </a>
2042 section about duplicate subpattern numbers),
2043 .\"
2044 the condition is true if any of them have matched. An alternative notation is
2045 to precede the digits with a plus or minus sign. In this case, the subpattern
2046 number is relative rather than absolute. The most recently opened parentheses
2047 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2048 loops it can also make sense to refer to subsequent groups. The next
2049 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2050 zero in any of these forms is not used; it provokes a compile-time error.)
2051 .P
2052 Consider the following pattern, which contains non-significant white space to
2053 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2054 three parts for ease of discussion:
2055 .sp
2056 ( \e( )? [^()]+ (?(1) \e) )
2057 .sp
2058 The first part matches an optional opening parenthesis, and if that
2059 character is present, sets it as the first captured substring. The second part
2060 matches one or more characters that are not parentheses. The third part is a
2061 conditional subpattern that tests whether or not the first set of parentheses
2062 matched. If they did, that is, if subject started with an opening parenthesis,
2063 the condition is true, and so the yes-pattern is executed and a closing
2064 parenthesis is required. Otherwise, since no-pattern is not present, the
2065 subpattern matches nothing. In other words, this pattern matches a sequence of
2066 non-parentheses, optionally enclosed in parentheses.
2067 .P
2068 If you were embedding this pattern in a larger one, you could use a relative
2069 reference:
2070 .sp
2071 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2072 .sp
2073 This makes the fragment independent of the parentheses in the larger pattern.
2074 .
2075 .SS "Checking for a used subpattern by name"
2076 .rs
2077 .sp
2078 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2079 subpattern by name. For compatibility with earlier versions of PCRE, which had
2080 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2081 there is a possible ambiguity with this syntax, because subpattern names may
2082 consist entirely of digits. PCRE looks first for a named subpattern; if it
2083 cannot find one and the name consists entirely of digits, PCRE looks for a
2084 subpattern of that number, which must be greater than zero. Using subpattern
2085 names that consist entirely of digits is not recommended.
2086 .P
2087 Rewriting the above example to use a named subpattern gives this:
2088 .sp
2089 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2090 .sp
2091 If the name used in a condition of this kind is a duplicate, the test is
2092 applied to all subpatterns of the same name, and is true if any one of them has
2093 matched.
2094 .
2095 .SS "Checking for pattern recursion"
2096 .rs
2097 .sp
2098 If the condition is the string (R), and there is no subpattern with the name R,
2099 the condition is true if a recursive call to the whole pattern or any
2100 subpattern has been made. If digits or a name preceded by ampersand follow the
2101 letter R, for example:
2102 .sp
2103 (?(R3)...) or (?(R&name)...)
2104 .sp
2105 the condition is true if the most recent recursion is into a subpattern whose
2106 number or name is given. This condition does not check the entire recursion
2107 stack. If the name used in a condition of this kind is a duplicate, the test is
2108 applied to all subpatterns of the same name, and is true if any one of them is
2109 the most recent recursion.
2110 .P
2111 At "top level", all these recursion test conditions are false.
2112 .\" HTML <a href="#recursion">
2113 .\" </a>
2114 The syntax for recursive patterns
2115 .\"
2116 is described below.
2117 .
2118 .\" HTML <a name="subdefine"></a>
2119 .SS "Defining subpatterns for use by reference only"
2120 .rs
2121 .sp
2122 If the condition is the string (DEFINE), and there is no subpattern with the
2123 name DEFINE, the condition is always false. In this case, there may be only one
2124 alternative in the subpattern. It is always skipped if control reaches this
2125 point in the pattern; the idea of DEFINE is that it can be used to define
2126 "subroutines" that can be referenced from elsewhere. (The use of
2127 .\" HTML <a href="#subpatternsassubroutines">
2128 .\" </a>
2129 "subroutines"
2130 .\"
2131 is described below.) For example, a pattern to match an IPv4 address such as
2132 "" could be written like this (ignore whitespace and line
2133 breaks):
2134 .sp
2135 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2136 \eb (?&byte) (\e.(?&byte)){3} \eb
2137 .sp
2138 The first part of the pattern is a DEFINE group inside which a another group
2139 named "byte" is defined. This matches an individual component of an IPv4
2140 address (a number less than 256). When matching takes place, this part of the
2141 pattern is skipped because DEFINE acts like a false condition. The rest of the
2142 pattern uses references to the named group to match the four dot-separated
2143 components of an IPv4 address, insisting on a word boundary at each end.
2144 .
2145 .SS "Assertion conditions"
2146 .rs
2147 .sp
2148 If the condition is not in any of the above formats, it must be an assertion.
2149 This may be a positive or negative lookahead or lookbehind assertion. Consider
2150 this pattern, again containing non-significant white space, and with the two
2151 alternatives on the second line:
2152 .sp
2153 (?(?=[^a-z]*[a-z])
2154 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2155 .sp
2156 The condition is a positive lookahead assertion that matches an optional
2157 sequence of non-letters followed by a letter. In other words, it tests for the
2158 presence of at least one letter in the subject. If a letter is found, the
2159 subject is matched against the first alternative; otherwise it is matched
2160 against the second. This pattern matches strings in one of the two forms
2161 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2162 .
2163 .
2164 .\" HTML <a name="comments"></a>
2166 .rs
2167 .sp
2168 There are two ways of including comments in patterns that are processed by
2169 PCRE. In both cases, the start of the comment must not be in a character class,
2170 nor in the middle of any other sequence of related characters such as (?: or a
2171 subpattern name or number. The characters that make up a comment play no part
2172 in the pattern matching.
2173 .P
2174 The sequence (?# marks the start of a comment that continues up to the next
2175 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2176 option is set, an unescaped # character also introduces a comment, which in
2177 this case continues to immediately after the next newline character or
2178 character sequence in the pattern. Which characters are interpreted as newlines
2179 is controlled by the options passed to \fBpcre_compile()\fP or by a special
2180 sequence at the start of the pattern, as described in the section entitled
2181 .\" HTML <a href="#newlines">
2182 .\" </a>
2183 "Newline conventions"
2184 .\"
2185 above. Note that the end of this type of comment is a literal newline sequence
2186 in the pattern; escape sequences that happen to represent a newline do not
2187 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2188 default newline convention is in force:
2189 .sp
2190 abc #comment \en still comment
2191 .sp
2192 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2193 a newline in the pattern. The sequence \en is still literal at this stage, so
2194 it does not terminate the comment. Only an actual character with the code value
2195 0x0a (the default newline) does so.
2196 .
2197 .
2198 .\" HTML <a name="recursion"></a>
2200 .rs
2201 .sp
2202 Consider the problem of matching a string in parentheses, allowing for
2203 unlimited nested parentheses. Without the use of recursion, the best that can
2204 be done is to use a pattern that matches up to some fixed depth of nesting. It
2205 is not possible to handle an arbitrary nesting depth.
2206 .P
2207 For some time, Perl has provided a facility that allows regular expressions to
2208 recurse (amongst other things). It does this by interpolating Perl code in the
2209 expression at run time, and the code can refer to the expression itself. A Perl
2210 pattern using code interpolation to solve the parentheses problem can be
2211 created like this:
2212 .sp
2213 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2214 .sp
2215 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2216 recursively to the pattern in which it appears.
2217 .P
2218 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2219 supports special syntax for recursion of the entire pattern, and also for
2220 individual subpattern recursion. After its introduction in PCRE and Python,
2221 this kind of recursion was subsequently introduced into Perl at release 5.10.
2222 .P
2223 A special item that consists of (? followed by a number greater than zero and a
2224 closing parenthesis is a recursive call of the subpattern of the given number,
2225 provided that it occurs inside that subpattern. (If not, it is a
2226 .\" HTML <a href="#subpatternsassubroutines">
2227 .\" </a>
2228 "subroutine"
2229 .\"
2230 call, which is described in the next section.) The special item (?R) or (?0) is
2231 a recursive call of the entire regular expression.
2232 .P
2233 This PCRE pattern solves the nested parentheses problem (assume the
2234 PCRE_EXTENDED option is set so that white space is ignored):
2235 .sp
2236 \e( ( [^()]++ | (?R) )* \e)
2237 .sp
2238 First it matches an opening parenthesis. Then it matches any number of
2239 substrings which can either be a sequence of non-parentheses, or a recursive
2240 match of the pattern itself (that is, a correctly parenthesized substring).
2241 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2242 to avoid backtracking into sequences of non-parentheses.
2243 .P
2244 If this were part of a larger pattern, you would not want to recurse the entire
2245 pattern, so instead you could use this:
2246 .sp
2247 ( \e( ( [^()]++ | (?1) )* \e) )
2248 .sp
2249 We have put the pattern into parentheses, and caused the recursion to refer to
2250 them instead of the whole pattern.
2251 .P
2252 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2253 is made easier by the use of relative references. Instead of (?1) in the
2254 pattern above you can write (?-2) to refer to the second most recently opened
2255 parentheses preceding the recursion. In other words, a negative number counts
2256 capturing parentheses leftwards from the point at which it is encountered.
2257 .P
2258 It is also possible to refer to subsequently opened parentheses, by writing
2259 references such as (?+2). However, these cannot be recursive because the
2260 reference is not inside the parentheses that are referenced. They are always
2261 .\" HTML <a href="#subpatternsassubroutines">
2262 .\" </a>
2263 "subroutine"
2264 .\"
2265 calls, as described in the next section.
2266 .P
2267 An alternative approach is to use named parentheses instead. The Perl syntax
2268 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2269 could rewrite the above example as follows:
2270 .sp
2271 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2272 .sp
2273 If there is more than one subpattern with the same name, the earliest one is
2274 used.
2275 .P
2276 This particular example pattern that we have been looking at contains nested
2277 unlimited repeats, and so the use of a possessive quantifier for matching
2278 strings of non-parentheses is important when applying the pattern to strings
2279 that do not match. For example, when this pattern is applied to
2280 .sp
2281 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2282 .sp
2283 it yields "no match" quickly. However, if a possessive quantifier is not used,
2284 the match runs for a very long time indeed because there are so many different
2285 ways the + and * repeats can carve up the subject, and all have to be tested
2286 before failure can be reported.
2287 .P
2288 At the end of a match, the values of capturing parentheses are those from
2289 the outermost level. If you want to obtain intermediate values, a callout
2290 function can be used (see below and the
2291 .\" HREF
2292 \fBpcrecallout\fP
2293 .\"
2294 documentation). If the pattern above is matched against
2295 .sp
2296 (ab(cd)ef)
2297 .sp
2298 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2299 the last value taken on at the top level. If a capturing subpattern is not
2300 matched at the top level, its final value is unset, even if it is (temporarily)
2301 set at a deeper level.
2302 .P
2303 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2304 obtain extra memory to store data during a recursion, which it does by using
2305 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2306 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2307 .P
2308 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2309 Consider this pattern, which matches text in angle brackets, allowing for
2310 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2311 recursing), whereas any characters are permitted at the outer level.
2312 .sp
2313 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2314 .sp
2315 In this pattern, (?(R) is the start of a conditional subpattern, with two
2316 different alternatives for the recursive and non-recursive cases. The (?R) item
2317 is the actual recursive call.
2318 .
2319 .
2320 .\" HTML <a name="recursiondifference"></a>
2321 .SS "Recursion difference from Perl"
2322 .rs
2323 .sp
2324 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2325 treated as an atomic group. That is, once it has matched some of the subject
2326 string, it is never re-entered, even if it contains untried alternatives and
2327 there is a subsequent matching failure. This can be illustrated by the
2328 following pattern, which purports to match a palindromic string that contains
2329 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2330 .sp
2331 ^(.|(.)(?1)\e2)$
2332 .sp
2333 The idea is that it either matches a single character, or two identical
2334 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2335 it does not if the pattern is longer than three characters. Consider the
2336 subject string "abcba":
2337 .P
2338 At the top level, the first character is matched, but as it is not at the end
2339 of the string, the first alternative fails; the second alternative is taken
2340 and the recursion kicks in. The recursive call to subpattern 1 successfully
2341 matches the next character ("b"). (Note that the beginning and end of line
2342 tests are not part of the recursion).
2343 .P
2344 Back at the top level, the next character ("c") is compared with what
2345 subpattern 2 matched, which was "a". This fails. Because the recursion is
2346 treated as an atomic group, there are now no backtracking points, and so the
2347 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2348 try the second alternative.) However, if the pattern is written with the
2349 alternatives in the other order, things are different:
2350 .sp
2351 ^((.)(?1)\e2|.)$
2352 .sp
2353 This time, the recursing alternative is tried first, and continues to recurse
2354 until it runs out of characters, at which point the recursion fails. But this
2355 time we do have another alternative to try at the higher level. That is the big
2356 difference: in the previous case the remaining alternative is at a deeper
2357 recursion level, which PCRE cannot use.
2358 .P
2359 To change the pattern so that it matches all palindromic strings, not just
2360 those with an odd number of characters, it is tempting to change the pattern to
2361 this:
2362 .sp
2363 ^((.)(?1)\e2|.?)$
2364 .sp
2365 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2366 deeper recursion has matched a single character, it cannot be entered again in
2367 order to match an empty string. The solution is to separate the two cases, and
2368 write out the odd and even cases as alternatives at the higher level:
2369 .sp
2370 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2371 .sp
2372 If you want to match typical palindromic phrases, the pattern has to ignore all
2373 non-word characters, which can be done like this:
2374 .sp
2375 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2376 .sp
2377 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2378 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2379 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2380 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2381 more) to match typical phrases, and Perl takes so long that you think it has
2382 gone into a loop.
2383 .P
2384 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2385 string does not start with a palindrome that is shorter than the entire string.
2386 For example, although "abcba" is correctly matched, if the subject is "ababa",
2387 PCRE finds the palindrome "aba" at the start, then fails at top level because
2388 the end of the string does not follow. Once again, it cannot jump back into the
2389 recursion to try other alternatives, so the entire match fails.
2390 .
2391 .
2392 .\" HTML <a name="subpatternsassubroutines"></a>
2394 .rs
2395 .sp
2396 If the syntax for a recursive subpattern reference (either by number or by
2397 name) is used outside the parentheses to which it refers, it operates like a
2398 subroutine in a programming language. The "called" subpattern may be defined
2399 before or after the reference. A numbered reference can be absolute or
2400 relative, as in these examples:
2401 .sp
2402 (...(absolute)...)...(?2)...
2403 (...(relative)...)...(?-1)...
2404 (...(?+1)...(relative)...
2405 .sp
2406 An earlier example pointed out that the pattern
2407 .sp
2408 (sens|respons)e and \e1ibility
2409 .sp
2410 matches "sense and sensibility" and "response and responsibility", but not
2411 "sense and responsibility". If instead the pattern
2412 .sp
2413 (sens|respons)e and (?1)ibility
2414 .sp
2415 is used, it does match "sense and responsibility" as well as the other two
2416 strings. Another example is given in the discussion of DEFINE above.
2417 .P
2418 Like recursive subpatterns, a subroutine call is always treated as an atomic
2419 group. That is, once it has matched some of the subject string, it is never
2420 re-entered, even if it contains untried alternatives and there is a subsequent
2421 matching failure. Any capturing parentheses that are set during the subroutine
2422 call revert to their previous values afterwards.
2423 .P
2424 When a subpattern is used as a subroutine, processing options such as
2425 case-independence are fixed when the subpattern is defined. They cannot be
2426 changed for different calls. For example, consider this pattern:
2427 .sp
2428 (abc)(?i:(?-1))
2429 .sp
2430 It matches "abcabc". It does not match "abcABC" because the change of
2431 processing option does not affect the called subpattern.
2432 .
2433 .
2434 .\" HTML <a name="onigurumasubroutines"></a>
2436 .rs
2437 .sp
2438 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2439 a number enclosed either in angle brackets or single quotes, is an alternative
2440 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2441 are two of the examples used above, rewritten using this syntax:
2442 .sp
2443 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2444 (sens|respons)e and \eg'1'ibility
2445 .sp
2446 PCRE supports an extension to Oniguruma: if a number is preceded by a
2447 plus or a minus sign it is taken as a relative reference. For example:
2448 .sp
2449 (abc)(?i:\eg<-1>)
2450 .sp
2451 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2452 synonymous. The former is a back reference; the latter is a subroutine call.
2453 .
2454 .
2456 .rs
2457 .sp
2458 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2459 code to be obeyed in the middle of matching a regular expression. This makes it
2460 possible, amongst other things, to extract different substrings that match the
2461 same pair of parentheses when there is a repetition.
2462 .P
2463 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2464 code. The feature is called "callout". The caller of PCRE provides an external
2465 function by putting its entry point in the global variable \fIpcre_callout\fP.
2466 By default, this variable contains NULL, which disables all calling out.
2467 .P
2468 Within a regular expression, (?C) indicates the points at which the external
2469 function is to be called. If you want to identify different callout points, you
2470 can put a number less than 256 after the letter C. The default value is zero.
2471 For example, this pattern has two callout points:
2472 .sp
2473 (?C1)abc(?C2)def
2474 .sp
2475 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2476 automatically installed before each item in the pattern. They are all numbered
2477 255.
2478 .P
2479 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2480 set), the external function is called. It is provided with the number of the
2481 callout, the position in the pattern, and, optionally, one item of data
2482 originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2483 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2484 description of the interface to the callout function is given in the
2485 .\" HREF
2486 \fBpcrecallout\fP
2487 .\"
2488 documentation.
2489 .
2490 .
2491 .\" HTML <a name="backtrackcontrol"></a>
2493 .rs
2494 .sp
2495 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2496 are described in the Perl documentation as "experimental and subject to change
2497 or removal in a future version of Perl". It goes on to say: "Their usage in
2498 production code should be noted to avoid problems during upgrades." The same
2499 remarks apply to the PCRE features described in this section.
2500 .P
2501 Since these verbs are specifically related to backtracking, most of them can be
2502 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2503 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2504 failing negative assertion, they cause an error if encountered by
2505 \fBpcre_dfa_exec()\fP.
2506 .P
2507 If any of these verbs are used in an assertion or subroutine subpattern
2508 (including recursive subpatterns), their effect is confined to that subpattern;
2509 it does not extend to the surrounding pattern, with one exception: a *MARK that
2510 is encountered in a positive assertion \fIis\fP passed back (compare capturing
2511 parentheses in assertions). Note that such subpatterns are processed as
2512 anchored at the point where they are tested.
2513 .P
2514 The new verbs make use of what was previously invalid syntax: an opening
2515 parenthesis followed by an asterisk. They are generally of the form
2516 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2517 depending on whether or not an argument is present. An name is a sequence of
2518 letters, digits, and underscores. If the name is empty, that is, if the closing
2519 parenthesis immediately follows the colon, the effect is as if the colon were
2520 not there. Any number of these verbs may occur in a pattern.
2521 .P
2522 PCRE contains some optimizations that are used to speed up matching by running
2523 some checks at the start of each match attempt. For example, it may know the
2524 minimum length of matching subject, or that a particular character must be
2525 present. When one of these optimizations suppresses the running of a match, any
2526 included backtracking verbs will not, of course, be processed. You can suppress
2527 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2528 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2529 pattern with (*NO_START_OPT).
2530 .
2531 .
2532 .SS "Verbs that act immediately"
2533 .rs
2534 .sp
2535 The following verbs act as soon as they are encountered. They may not be
2536 followed by a name.
2537 .sp
2538 (*ACCEPT)
2539 .sp
2540 This verb causes the match to end successfully, skipping the remainder of the
2541 pattern. When inside a recursion, only the innermost pattern is ended
2542 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2543 captured. (This feature was added to PCRE at release 8.00.) For example:
2544 .sp
2545 A((?:A|B(*ACCEPT)|C)D)
2546 .sp
2547 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2548 the outer parentheses.
2549 .sp
2550 (*FAIL) or (*F)
2551 .sp
2552 This verb causes the match to fail, forcing backtracking to occur. It is
2553 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2554 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2555 Perl features that are not present in PCRE. The nearest equivalent is the
2556 callout feature, as for example in this pattern:
2557 .sp
2558 a+(?C)(*FAIL)
2559 .sp
2560 A match with the string "aaaa" always fails, but the callout is taken before
2561 each backtrack happens (in this example, 10 times).
2562 .
2563 .
2564 .SS "Recording which path was taken"
2565 .rs
2566 .sp
2567 There is one verb whose main purpose is to track how a match was arrived at,
2568 though it also has a secondary use in conjunction with advancing the match
2569 starting point (see (*SKIP) below).
2570 .sp
2571 (*MARK:NAME) or (*:NAME)
2572 .sp
2573 A name is always required with this verb. There may be as many instances of
2574 (*MARK) as you like in a pattern, and their names do not have to be unique.
2575 .P
2576 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2577 to the caller via the \fIpcre_extra\fP data structure, as described in the
2578 .\" HTML <a href="pcreapi.html#extradata">
2579 .\" </a>
2580 section on \fIpcre_extra\fP
2581 .\"
2582 in the
2583 .\" HREF
2584 \fBpcreapi\fP
2585 .\"
2586 documentation. No data is returned for a partial match. Here is an example of
2587 \fBpcretest\fP output, where the /K modifier requests the retrieval and
2588 outputting of (*MARK) data:
2589 .sp
2590 /X(*MARK:A)Y|X(*MARK:B)Z/K
2591 XY
2592 0: XY
2593 MK: A
2594 XZ
2595 0: XZ
2596 MK: B
2597 .sp
2598 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2599 indicates which of the two alternatives matched. This is a more efficient way
2600 of obtaining this information than putting each alternative in its own
2601 capturing parentheses.
2602 .P
2603 If (*MARK) is encountered in a positive assertion, its name is recorded and
2604 passed back if it is the last-encountered. This does not happen for negative
2605 assetions.
2606 .P
2607 A name may also be returned after a failed match if the final path through the
2608 pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2609 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2610 starting point for matching is advanced, the final check is often with an empty
2611 string, causing a failure before (*MARK) is reached. For example:
2612 .sp
2613 /X(*MARK:A)Y|X(*MARK:B)Z/K
2614 XP
2615 No match
2616 .sp
2617 There are three potential starting points for this match (starting with X,
2618 starting with P, and with an empty string). If the pattern is anchored, the
2619 result is different:
2620 .sp
2621 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2622 XP
2623 No match, mark = B
2624 .sp
2625 PCRE's start-of-match optimizations can also interfere with this. For example,
2626 if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2627 subject length for a match, a shorter subject will not be scanned at all.
2628 .P
2629 Note that similar anomalies (though different in detail) exist in Perl, no
2630 doubt for the same reasons. The use of (*MARK) data after a failed match of an
2631 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2632 .
2633 .
2634 .SS "Verbs that act after backtracking"
2635 .rs
2636 .sp
2637 The following verbs do nothing when they are encountered. Matching continues
2638 with what follows, but if there is no subsequent match, causing a backtrack to
2639 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2640 the verb. However, when one of these verbs appears inside an atomic group, its
2641 effect is confined to that group, because once the group has been matched,
2642 there is never any backtracking into it. In this situation, backtracking can
2643 "jump back" to the left of the entire atomic group. (Remember also, as stated
2644 above, that this localization also applies in subroutine calls and assertions.)
2645 .P
2646 These verbs differ in exactly what kind of failure occurs when backtracking
2647 reaches them.
2648 .sp
2649 (*COMMIT)
2650 .sp
2651 This verb, which may not be followed by a name, causes the whole match to fail
2652 outright if the rest of the pattern does not match. Even if the pattern is
2653 unanchored, no further attempts to find a match by advancing the starting point
2654 take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2655 finding a match at the current starting point, or not at all. For example:
2656 .sp
2657 a+(*COMMIT)b
2658 .sp
2659 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2660 dynamic anchor, or "I've started, so I must finish." The name of the most
2661 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2662 match failure.
2663 .P
2664 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2665 unless PCRE's start-of-match optimizations are turned off, as shown in this
2666 \fBpcretest\fP example:
2667 .sp
2668 /(*COMMIT)abc/
2669 xyzabc
2670 0: abc
2671 xyzabc\eY
2672 No match
2673 .sp
2674 PCRE knows that any match must start with "a", so the optimization skips along
2675 the subject to "a" before running the first match attempt, which succeeds. When
2676 the optimization is disabled by the \eY escape in the second subject, the match
2677 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2678 starting points.
2679 .sp
2680 (*PRUNE) or (*PRUNE:NAME)
2681 .sp
2682 This verb causes the match to fail at the current starting position in the
2683 subject if the rest of the pattern does not match. If the pattern is
2684 unanchored, the normal "bumpalong" advance to the next starting character then
2685 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2686 reached, or when matching to the right of (*PRUNE), but if there is no match to
2687 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2688 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2689 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2690 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2691 match fails completely; the name is passed back if this is the final attempt.
2692 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2693 pattern (*PRUNE) has the same effect as (*COMMIT).
2694 .sp
2695 (*SKIP)
2696 .sp
2697 This verb, when given without a name, is like (*PRUNE), except that if the
2698 pattern is unanchored, the "bumpalong" advance is not to the next character,
2699 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2700 signifies that whatever text was matched leading up to it cannot be part of a
2701 successful match. Consider:
2702 .sp
2703 a+(*SKIP)b
2704 .sp
2705 If the subject is "aaaac...", after the first match attempt fails (starting at
2706 the first character in the string), the starting point skips on to start the
2707 next attempt at "c". Note that a possessive quantifer does not have the same
2708 effect as this example; although it would suppress backtracking during the
2709 first match attempt, the second attempt would start at the second character
2710 instead of skipping on to "c".
2711 .sp
2712 (*SKIP:NAME)
2713 .sp
2714 When (*SKIP) has an associated name, its behaviour is modified. If the
2715 following pattern fails to match, the previous path through the pattern is
2716 searched for the most recent (*MARK) that has the same name. If one is found,
2717 the "bumpalong" advance is to the subject position that corresponds to that
2718 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2719 matching name is found, normal "bumpalong" of one character happens (the
2720 (*SKIP) is ignored).
2721 .sp
2722 (*THEN) or (*THEN:NAME)
2723 .sp
2724 This verb causes a skip to the next alternation in the innermost enclosing
2725 group if the rest of the pattern does not match. That is, it cancels pending
2726 backtracking, but only within the current alternation. Its name comes from the
2727 observation that it can be used for a pattern-based if-then-else block:
2728 .sp
2729 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2730 .sp
2731 If the COND1 pattern matches, FOO is tried (and possibly further items after
2732 the end of the group if FOO succeeds); on failure the matcher skips to the
2733 second alternative and tries COND2, without backtracking into COND1. The
2734 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2735 overall match fails. If (*THEN) is not directly inside an alternation, it acts
2736 like (*PRUNE).
2737 .
2738 .P
2739 The above verbs provide four different "strengths" of control when subsequent
2740 matching fails. (*THEN) is the weakest, carrying on the match at the next
2741 alternation. (*PRUNE) comes next, failing the match at the current starting
2742 position, but allowing an advance to the next character (for an unanchored
2743 pattern). (*SKIP) is similar, except that the advance may be more than one
2744 character. (*COMMIT) is the strongest, causing the entire match to fail.
2745 .P
2746 If more than one is present in a pattern, the "stongest" one wins. For example,
2747 consider this pattern, where A, B, etc. are complex pattern fragments:
2748 .sp
2749 (A(*COMMIT)B(*THEN)C|D)
2750 .sp
2751 Once A has matched, PCRE is committed to this match, at the current starting
2752 position. If subsequently B matches, but C does not, the normal (*THEN) action
2753 of trying the next alternation (that is, D) does not happen because (*COMMIT)
2754 overrides.
2755 .
2756 .
2757 .SH "SEE ALSO"
2758 .rs
2759 .sp
2760 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2761 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2762 .
2763 .
2765 .rs
2766 .sp
2767 .nf
2768 Philip Hazel
2769 University Computing Service
2770 Cambridge CB2 3QH, England.
2771 .fi
2772 .
2773 .
2775 .rs
2776 .sp
2777 .nf
2778 Last updated: 24 August 2011
2779 Copyright (c) 1997-2011 University of Cambridge.
2780 .fi


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