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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 .\" HTML <a href="pcre.html#utf8support">
36 .\" </a>
37 section on UTF-8 support
38 .\"
39 in the main
40 .\" HREF
41 \fBpcre\fP
42 .\"
43 page.
44 .P
45 Another special sequence that may appear at the start of a pattern or in
46 combination with (*UTF8) is:
47 .sp
48 (*UCP)
49 .sp
50 This has the same effect as setting the PCRE_UCP option: it causes sequences
51 such as \ed and \ew to use Unicode properties to determine character types,
52 instead of recognizing only characters with codes less than 128 via a lookup
53 table.
54 .P
55 If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56 PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57 also some more of these special sequences that are concerned with the handling
58 of newlines; they are described below.
59 .P
60 The remainder of this document discusses the patterns that are supported by
61 PCRE when its main matching function, \fBpcre_exec()\fP, is used.
62 From release 6.0, PCRE offers a second matching function,
63 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
64 Perl-compatible. Some of the features discussed below are not available when
65 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
66 alternative function, and how it differs from the normal function, are
67 discussed in the
68 .\" HREF
69 \fBpcrematching\fP
70 .\"
71 page.
72 .
73 .
74 .\" HTML <a name="newlines"></a>
76 .rs
77 .sp
78 PCRE supports five different conventions for indicating line breaks in
79 strings: a single CR (carriage return) character, a single LF (linefeed)
80 character, the two-character sequence CRLF, any of the three preceding, or any
81 Unicode newline sequence. The
82 .\" HREF
83 \fBpcreapi\fP
84 .\"
85 page has
86 .\" HTML <a href="pcreapi.html#newlines">
87 .\" </a>
88 further discussion
89 .\"
90 about newlines, and shows how to set the newline convention in the
91 \fIoptions\fP arguments for the compiling and matching functions.
92 .P
93 It is also possible to specify a newline convention by starting a pattern
94 string with one of the following five sequences:
95 .sp
96 (*CR) carriage return
97 (*LF) linefeed
98 (*CRLF) carriage return, followed by linefeed
99 (*ANYCRLF) any of the three above
100 (*ANY) all Unicode newline sequences
101 .sp
102 These override the default and the options given to \fBpcre_compile()\fP or
103 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
104 newline sequence, the pattern
105 .sp
106 (*CR)a.b
107 .sp
108 changes the convention to CR. That pattern matches "a\enb" because LF is no
109 longer a newline. Note that these special settings, which are not
110 Perl-compatible, are recognized only at the very start of a pattern, and that
111 they must be in upper case. If more than one of them is present, the last one
112 is used.
113 .P
114 The newline convention affects the interpretation of the dot metacharacter when
115 PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
116 affect what the \eR escape sequence matches. By default, this is any Unicode
117 newline sequence, for Perl compatibility. However, this can be changed; see the
118 description of \eR in the section entitled
119 .\" HTML <a href="#newlineseq">
120 .\" </a>
121 "Newline sequences"
122 .\"
123 below. A change of \eR setting can be combined with a change of newline
124 convention.
125 .
126 .
128 .rs
129 .sp
130 A regular expression is a pattern that is matched against a subject string from
131 left to right. Most characters stand for themselves in a pattern, and match the
132 corresponding characters in the subject. As a trivial example, the pattern
133 .sp
134 The quick brown fox
135 .sp
136 matches a portion of a subject string that is identical to itself. When
137 caseless matching is specified (the PCRE_CASELESS option), letters are matched
138 independently of case. In UTF-8 mode, PCRE always understands the concept of
139 case for characters whose values are less than 128, so caseless matching is
140 always possible. For characters with higher values, the concept of case is
141 supported if PCRE is compiled with Unicode property support, but not otherwise.
142 If you want to use caseless matching for characters 128 and above, you must
143 ensure that PCRE is compiled with Unicode property support as well as with
144 UTF-8 support.
145 .P
146 The power of regular expressions comes from the ability to include alternatives
147 and repetitions in the pattern. These are encoded in the pattern by the use of
148 \fImetacharacters\fP, which do not stand for themselves but instead are
149 interpreted in some special way.
150 .P
151 There are two different sets of metacharacters: those that are recognized
152 anywhere in the pattern except within square brackets, and those that are
153 recognized within square brackets. Outside square brackets, the metacharacters
154 are as follows:
155 .sp
156 \e general escape character with several uses
157 ^ assert start of string (or line, in multiline mode)
158 $ assert end of string (or line, in multiline mode)
159 . match any character except newline (by default)
160 [ start character class definition
161 | start of alternative branch
162 ( start subpattern
163 ) end subpattern
164 ? extends the meaning of (
165 also 0 or 1 quantifier
166 also quantifier minimizer
167 * 0 or more quantifier
168 + 1 or more quantifier
169 also "possessive quantifier"
170 { start min/max quantifier
171 .sp
172 Part of a pattern that is in square brackets is called a "character class". In
173 a character class the only metacharacters are:
174 .sp
175 \e general escape character
176 ^ negate the class, but only if the first character
177 - indicates character range
178 .\" JOIN
179 [ POSIX character class (only if followed by POSIX
180 syntax)
181 ] terminates the character class
182 .sp
183 The following sections describe the use of each of the metacharacters.
184 .
185 .
187 .rs
188 .sp
189 The backslash character has several uses. Firstly, if it is followed by a
190 character that is not a number or a letter, it takes away any special meaning
191 that character may have. This use of backslash as an escape character applies
192 both inside and outside character classes.
193 .P
194 For example, if you want to match a * character, you write \e* in the pattern.
195 This escaping action applies whether or not the following character would
196 otherwise be interpreted as a metacharacter, so it is always safe to precede a
197 non-alphanumeric with backslash to specify that it stands for itself. In
198 particular, if you want to match a backslash, you write \e\e.
199 .P
200 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
201 backslash. All other characters (in particular, those whose codepoints are
202 greater than 127) are treated as literals.
203 .P
204 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
205 pattern (other than in a character class) and characters between a # outside
206 a character class and the next newline are ignored. An escaping backslash can
207 be used to include a whitespace or # character as part of the pattern.
208 .P
209 If you want to remove the special meaning from a sequence of characters, you
210 can do so by putting them between \eQ and \eE. This is different from Perl in
211 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
212 Perl, $ and @ cause variable interpolation. Note the following examples:
213 .sp
214 Pattern PCRE matches Perl matches
215 .sp
216 .\" JOIN
217 \eQabc$xyz\eE abc$xyz abc followed by the
218 contents of $xyz
219 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
220 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
221 .sp
222 The \eQ...\eE sequence is recognized both inside and outside character classes.
223 An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
224 by \eE later in the pattern, the literal interpretation continues to the end of
225 the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
226 a character class, this causes an error, because the character class is not
227 terminated.
228 .
229 .
230 .\" HTML <a name="digitsafterbackslash"></a>
231 .SS "Non-printing characters"
232 .rs
233 .sp
234 A second use of backslash provides a way of encoding non-printing characters
235 in patterns in a visible manner. There is no restriction on the appearance of
236 non-printing characters, apart from the binary zero that terminates a pattern,
237 but when a pattern is being prepared by text editing, it is often easier to use
238 one of the following escape sequences than the binary character it represents:
239 .sp
240 \ea alarm, that is, the BEL character (hex 07)
241 \ecx "control-x", where x is any ASCII character
242 \ee escape (hex 1B)
243 \ef formfeed (hex 0C)
244 \en linefeed (hex 0A)
245 \er carriage return (hex 0D)
246 \et tab (hex 09)
247 \eddd character with octal code ddd, or back reference
248 \exhh character with hex code hh
249 \ex{hhh..} character with hex code hhh..
250 .sp
251 The precise effect of \ecx is as follows: if x is a lower case letter, it
252 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
253 Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
254 \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
255 than 127, a compile-time error occurs. This locks out non-ASCII characters in
256 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
257 values are valid. A lower case letter is converted to upper case, and then the
258 0xc0 bits are flipped.)
259 .P
260 After \ex, from zero to two hexadecimal digits are read (letters can be in
261 upper or lower case). Any number of hexadecimal digits may appear between \ex{
262 and }, but the value of the character code must be less than 256 in non-UTF-8
263 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
264 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
265 point, which is 10FFFF.
266 .P
267 If characters other than hexadecimal digits appear between \ex{ and }, or if
268 there is no terminating }, this form of escape is not recognized. Instead, the
269 initial \ex will be interpreted as a basic hexadecimal escape, with no
270 following digits, giving a character whose value is zero.
271 .P
272 Characters whose value is less than 256 can be defined by either of the two
273 syntaxes for \ex. There is no difference in the way they are handled. For
274 example, \exdc is exactly the same as \ex{dc}.
275 .P
276 After \e0 up to two further octal digits are read. If there are fewer than two
277 digits, just those that are present are used. Thus the sequence \e0\ex\e07
278 specifies two binary zeros followed by a BEL character (code value 7). Make
279 sure you supply two digits after the initial zero if the pattern character that
280 follows is itself an octal digit.
281 .P
282 The handling of a backslash followed by a digit other than 0 is complicated.
283 Outside a character class, PCRE reads it and any following digits as a decimal
284 number. If the number is less than 10, or if there have been at least that many
285 previous capturing left parentheses in the expression, the entire sequence is
286 taken as a \fIback reference\fP. A description of how this works is given
287 .\" HTML <a href="#backreferences">
288 .\" </a>
289 later,
290 .\"
291 following the discussion of
292 .\" HTML <a href="#subpattern">
293 .\" </a>
294 parenthesized subpatterns.
295 .\"
296 .P
297 Inside a character class, or if the decimal number is greater than 9 and there
298 have not been that many capturing subpatterns, PCRE re-reads up to three octal
299 digits following the backslash, and uses them to generate a data character. Any
300 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
301 character specified in octal must be less than \e400. In UTF-8 mode, values up
302 to \e777 are permitted. For example:
303 .sp
304 \e040 is another way of writing a space
305 .\" JOIN
306 \e40 is the same, provided there are fewer than 40
307 previous capturing subpatterns
308 \e7 is always a back reference
309 .\" JOIN
310 \e11 might be a back reference, or another way of
311 writing a tab
312 \e011 is always a tab
313 \e0113 is a tab followed by the character "3"
314 .\" JOIN
315 \e113 might be a back reference, otherwise the
316 character with octal code 113
317 .\" JOIN
318 \e377 might be a back reference, otherwise
319 the byte consisting entirely of 1 bits
320 .\" JOIN
321 \e81 is either a back reference, or a binary zero
322 followed by the two characters "8" and "1"
323 .sp
324 Note that octal values of 100 or greater must not be introduced by a leading
325 zero, because no more than three octal digits are ever read.
326 .P
327 All the sequences that define a single character value can be used both inside
328 and outside character classes. In addition, inside a character class, the
329 sequence \eb is interpreted as the backspace character (hex 08). The sequences
330 \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
331 unrecognized escape sequences, they are treated as the literal characters "B",
332 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
333 set. Outside a character class, these sequences have different meanings.
334 .
335 .
336 .SS "Absolute and relative back references"
337 .rs
338 .sp
339 The sequence \eg followed by an unsigned or a negative number, optionally
340 enclosed in braces, is an absolute or relative back reference. A named back
341 reference can be coded as \eg{name}. Back references are discussed
342 .\" HTML <a href="#backreferences">
343 .\" </a>
344 later,
345 .\"
346 following the discussion of
347 .\" HTML <a href="#subpattern">
348 .\" </a>
349 parenthesized subpatterns.
350 .\"
351 .
352 .
353 .SS "Absolute and relative subroutine calls"
354 .rs
355 .sp
356 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
357 a number enclosed either in angle brackets or single quotes, is an alternative
358 syntax for referencing a subpattern as a "subroutine". Details are discussed
359 .\" HTML <a href="#onigurumasubroutines">
360 .\" </a>
361 later.
362 .\"
363 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
364 synonymous. The former is a back reference; the latter is a
365 .\" HTML <a href="#subpatternsassubroutines">
366 .\" </a>
367 subroutine
368 .\"
369 call.
370 .
371 .
372 .\" HTML <a name="genericchartypes"></a>
373 .SS "Generic character types"
374 .rs
375 .sp
376 Another use of backslash is for specifying generic character types:
377 .sp
378 \ed any decimal digit
379 \eD any character that is not a decimal digit
380 \eh any horizontal whitespace character
381 \eH any character that is not a horizontal whitespace character
382 \es any whitespace character
383 \eS any character that is not a whitespace character
384 \ev any vertical whitespace character
385 \eV any character that is not a vertical whitespace character
386 \ew any "word" character
387 \eW any "non-word" character
388 .sp
389 There is also the single sequence \eN, which matches a non-newline character.
390 This is the same as
391 .\" HTML <a href="#fullstopdot">
392 .\" </a>
393 the "." metacharacter
394 .\"
395 when PCRE_DOTALL is not set.
396 .P
397 Each pair of lower and upper case escape sequences partitions the complete set
398 of characters into two disjoint sets. Any given character matches one, and only
399 one, of each pair. The sequences can appear both inside and outside character
400 classes. They each match one character of the appropriate type. If the current
401 matching point is at the end of the subject string, all of them fail, because
402 there is no character to match.
403 .P
404 For compatibility with Perl, \es does not match the VT character (code 11).
405 This makes it different from the the POSIX "space" class. The \es characters
406 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
407 included in a Perl script, \es may match the VT character. In PCRE, it never
408 does.
409 .P
410 A "word" character is an underscore or any character that is a letter or digit.
411 By default, the definition of letters and digits is controlled by PCRE's
412 low-valued character tables, and may vary if locale-specific matching is taking
413 place (see
414 .\" HTML <a href="pcreapi.html#localesupport">
415 .\" </a>
416 "Locale support"
417 .\"
418 in the
419 .\" HREF
420 \fBpcreapi\fP
421 .\"
422 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
423 or "french" in Windows, some character codes greater than 128 are used for
424 accented letters, and these are then matched by \ew. The use of locales with
425 Unicode is discouraged.
426 .P
427 By default, in UTF-8 mode, characters with values greater than 128 never match
428 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
429 their original meanings from before UTF-8 support was available, mainly for
430 efficiency reasons. However, if PCRE is compiled with Unicode property support,
431 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
432 properties are used to determine character types, as follows:
433 .sp
434 \ed any character that \ep{Nd} matches (decimal digit)
435 \es any character that \ep{Z} matches, plus HT, LF, FF, CR
436 \ew any character that \ep{L} or \ep{N} matches, plus underscore
437 .sp
438 The upper case escapes match the inverse sets of characters. Note that \ed
439 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
440 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
441 \eB because they are defined in terms of \ew and \eW. Matching these sequences
442 is noticeably slower when PCRE_UCP is set.
443 .P
444 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
445 release 5.10. In contrast to the other sequences, which match only ASCII
446 characters by default, these always match certain high-valued codepoints in
447 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
448 are:
449 .sp
450 U+0009 Horizontal tab
451 U+0020 Space
452 U+00A0 Non-break space
453 U+1680 Ogham space mark
454 U+180E Mongolian vowel separator
455 U+2000 En quad
456 U+2001 Em quad
457 U+2002 En space
458 U+2003 Em space
459 U+2004 Three-per-em space
460 U+2005 Four-per-em space
461 U+2006 Six-per-em space
462 U+2007 Figure space
463 U+2008 Punctuation space
464 U+2009 Thin space
465 U+200A Hair space
466 U+202F Narrow no-break space
467 U+205F Medium mathematical space
468 U+3000 Ideographic space
469 .sp
470 The vertical space characters are:
471 .sp
472 U+000A Linefeed
473 U+000B Vertical tab
474 U+000C Formfeed
475 U+000D Carriage return
476 U+0085 Next line
477 U+2028 Line separator
478 U+2029 Paragraph separator
479 .
480 .
481 .\" HTML <a name="newlineseq"></a>
482 .SS "Newline sequences"
483 .rs
484 .sp
485 Outside a character class, by default, the escape sequence \eR matches any
486 Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
487 .sp
488 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
489 .sp
490 This is an example of an "atomic group", details of which are given
491 .\" HTML <a href="#atomicgroup">
492 .\" </a>
493 below.
494 .\"
495 This particular group matches either the two-character sequence CR followed by
496 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
497 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
498 line, U+0085). The two-character sequence is treated as a single unit that
499 cannot be split.
500 .P
501 In UTF-8 mode, two additional characters whose codepoints are greater than 255
502 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
503 Unicode character property support is not needed for these characters to be
504 recognized.
505 .P
506 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
507 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
508 either at compile time or when the pattern is matched. (BSR is an abbrevation
509 for "backslash R".) This can be made the default when PCRE is built; if this is
510 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
511 It is also possible to specify these settings by starting a pattern string with
512 one of the following sequences:
513 .sp
514 (*BSR_ANYCRLF) CR, LF, or CRLF only
515 (*BSR_UNICODE) any Unicode newline sequence
516 .sp
517 These override the default and the options given to \fBpcre_compile()\fP or
518 \fBpcre_compile2()\fP, but they can be overridden by options given to
519 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
520 which are not Perl-compatible, are recognized only at the very start of a
521 pattern, and that they must be in upper case. If more than one of them is
522 present, the last one is used. They can be combined with a change of newline
523 convention; for example, a pattern can start with:
524 .sp
526 .sp
527 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
528 a character class, \eR is treated as an unrecognized escape sequence, and so
529 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
530 .
531 .
532 .\" HTML <a name="uniextseq"></a>
533 .SS Unicode character properties
534 .rs
535 .sp
536 When PCRE is built with Unicode character property support, three additional
537 escape sequences that match characters with specific properties are available.
538 When not in UTF-8 mode, these sequences are of course limited to testing
539 characters whose codepoints are less than 256, but they do work in this mode.
540 The extra escape sequences are:
541 .sp
542 \ep{\fIxx\fP} a character with the \fIxx\fP property
543 \eP{\fIxx\fP} a character without the \fIxx\fP property
544 \eX an extended Unicode sequence
545 .sp
546 The property names represented by \fIxx\fP above are limited to the Unicode
547 script names, the general category properties, "Any", which matches any
548 character (including newline), and some special PCRE properties (described
549 in the
550 .\" HTML <a href="#extraprops">
551 .\" </a>
552 next section).
553 .\"
554 Other Perl properties such as "InMusicalSymbols" are not currently supported by
555 PCRE. Note that \eP{Any} does not match any characters, so always causes a
556 match failure.
557 .P
558 Sets of Unicode characters are defined as belonging to certain scripts. A
559 character from one of these sets can be matched using a script name. For
560 example:
561 .sp
562 \ep{Greek}
563 \eP{Han}
564 .sp
565 Those that are not part of an identified script are lumped together as
566 "Common". The current list of scripts is:
567 .P
568 Arabic,
569 Armenian,
570 Avestan,
571 Balinese,
572 Bamum,
573 Bengali,
574 Bopomofo,
575 Braille,
576 Buginese,
577 Buhid,
578 Canadian_Aboriginal,
579 Carian,
580 Cham,
581 Cherokee,
582 Common,
583 Coptic,
584 Cuneiform,
585 Cypriot,
586 Cyrillic,
587 Deseret,
588 Devanagari,
589 Egyptian_Hieroglyphs,
590 Ethiopic,
591 Georgian,
592 Glagolitic,
593 Gothic,
594 Greek,
595 Gujarati,
596 Gurmukhi,
597 Han,
598 Hangul,
599 Hanunoo,
600 Hebrew,
601 Hiragana,
602 Imperial_Aramaic,
603 Inherited,
604 Inscriptional_Pahlavi,
605 Inscriptional_Parthian,
606 Javanese,
607 Kaithi,
608 Kannada,
609 Katakana,
610 Kayah_Li,
611 Kharoshthi,
612 Khmer,
613 Lao,
614 Latin,
615 Lepcha,
616 Limbu,
617 Linear_B,
618 Lisu,
619 Lycian,
620 Lydian,
621 Malayalam,
622 Meetei_Mayek,
623 Mongolian,
624 Myanmar,
625 New_Tai_Lue,
626 Nko,
627 Ogham,
628 Old_Italic,
629 Old_Persian,
630 Old_South_Arabian,
631 Old_Turkic,
632 Ol_Chiki,
633 Oriya,
634 Osmanya,
635 Phags_Pa,
636 Phoenician,
637 Rejang,
638 Runic,
639 Samaritan,
640 Saurashtra,
641 Shavian,
642 Sinhala,
643 Sundanese,
644 Syloti_Nagri,
645 Syriac,
646 Tagalog,
647 Tagbanwa,
648 Tai_Le,
649 Tai_Tham,
650 Tai_Viet,
651 Tamil,
652 Telugu,
653 Thaana,
654 Thai,
655 Tibetan,
656 Tifinagh,
657 Ugaritic,
658 Vai,
659 Yi.
660 .P
661 Each character has exactly one Unicode general category property, specified by
662 a two-letter abbreviation. For compatibility with Perl, negation can be
663 specified by including a circumflex between the opening brace and the property
664 name. For example, \ep{^Lu} is the same as \eP{Lu}.
665 .P
666 If only one letter is specified with \ep or \eP, it includes all the general
667 category properties that start with that letter. In this case, in the absence
668 of negation, the curly brackets in the escape sequence are optional; these two
669 examples have the same effect:
670 .sp
671 \ep{L}
672 \epL
673 .sp
674 The following general category property codes are supported:
675 .sp
676 C Other
677 Cc Control
678 Cf Format
679 Cn Unassigned
680 Co Private use
681 Cs Surrogate
682 .sp
683 L Letter
684 Ll Lower case letter
685 Lm Modifier letter
686 Lo Other letter
687 Lt Title case letter
688 Lu Upper case letter
689 .sp
690 M Mark
691 Mc Spacing mark
692 Me Enclosing mark
693 Mn Non-spacing mark
694 .sp
695 N Number
696 Nd Decimal number
697 Nl Letter number
698 No Other number
699 .sp
700 P Punctuation
701 Pc Connector punctuation
702 Pd Dash punctuation
703 Pe Close punctuation
704 Pf Final punctuation
705 Pi Initial punctuation
706 Po Other punctuation
707 Ps Open punctuation
708 .sp
709 S Symbol
710 Sc Currency symbol
711 Sk Modifier symbol
712 Sm Mathematical symbol
713 So Other symbol
714 .sp
715 Z Separator
716 Zl Line separator
717 Zp Paragraph separator
718 Zs Space separator
719 .sp
720 The special property L& is also supported: it matches a character that has
721 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
722 a modifier or "other".
723 .P
724 The Cs (Surrogate) property applies only to characters in the range U+D800 to
725 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
726 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
727 (see the discussion of PCRE_NO_UTF8_CHECK in the
728 .\" HREF
729 \fBpcreapi\fP
730 .\"
731 page). Perl does not support the Cs property.
732 .P
733 The long synonyms for property names that Perl supports (such as \ep{Letter})
734 are not supported by PCRE, nor is it permitted to prefix any of these
735 properties with "Is".
736 .P
737 No character that is in the Unicode table has the Cn (unassigned) property.
738 Instead, this property is assumed for any code point that is not in the
739 Unicode table.
740 .P
741 Specifying caseless matching does not affect these escape sequences. For
742 example, \ep{Lu} always matches only upper case letters.
743 .P
744 The \eX escape matches any number of Unicode characters that form an extended
745 Unicode sequence. \eX is equivalent to
746 .sp
747 (?>\ePM\epM*)
748 .sp
749 That is, it matches a character without the "mark" property, followed by zero
750 or more characters with the "mark" property, and treats the sequence as an
751 atomic group
752 .\" HTML <a href="#atomicgroup">
753 .\" </a>
754 (see below).
755 .\"
756 Characters with the "mark" property are typically accents that affect the
757 preceding character. None of them have codepoints less than 256, so in
758 non-UTF-8 mode \eX matches any one character.
759 .P
760 Note that recent versions of Perl have changed \eX to match what Unicode calls
761 an "extended grapheme cluster", which has a more complicated definition.
762 .P
763 Matching characters by Unicode property is not fast, because PCRE has to search
764 a structure that contains data for over fifteen thousand characters. That is
765 why the traditional escape sequences such as \ed and \ew do not use Unicode
766 properties in PCRE by default, though you can make them do so by setting the
767 PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
768 (*UCP).
769 .
770 .
771 .\" HTML <a name="extraprops"></a>
772 .SS PCRE's additional properties
773 .rs
774 .sp
775 As well as the standard Unicode properties described in the previous
776 section, PCRE supports four more that make it possible to convert traditional
777 escape sequences such as \ew and \es and POSIX character classes to use Unicode
778 properties. PCRE uses these non-standard, non-Perl properties internally when
779 PCRE_UCP is set. They are:
780 .sp
781 Xan Any alphanumeric character
782 Xps Any POSIX space character
783 Xsp Any Perl space character
784 Xwd Any Perl "word" character
785 .sp
786 Xan matches characters that have either the L (letter) or the N (number)
787 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
788 carriage return, and any other character that has the Z (separator) property.
789 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
790 same characters as Xan, plus underscore.
791 .
792 .
793 .\" HTML <a name="resetmatchstart"></a>
794 .SS "Resetting the match start"
795 .rs
796 .sp
797 The escape sequence \eK causes any previously matched characters not to be
798 included in the final matched sequence. For example, the pattern:
799 .sp
800 foo\eKbar
801 .sp
802 matches "foobar", but reports that it has matched "bar". This feature is
803 similar to a lookbehind assertion
804 .\" HTML <a href="#lookbehind">
805 .\" </a>
806 (described below).
807 .\"
808 However, in this case, the part of the subject before the real match does not
809 have to be of fixed length, as lookbehind assertions do. The use of \eK does
810 not interfere with the setting of
811 .\" HTML <a href="#subpattern">
812 .\" </a>
813 captured substrings.
814 .\"
815 For example, when the pattern
816 .sp
817 (foo)\eKbar
818 .sp
819 matches "foobar", the first substring is still set to "foo".
820 .P
821 Perl documents that the use of \eK within assertions is "not well defined". In
822 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
823 ignored in negative assertions.
824 .
825 .
826 .\" HTML <a name="smallassertions"></a>
827 .SS "Simple assertions"
828 .rs
829 .sp
830 The final use of backslash is for certain simple assertions. An assertion
831 specifies a condition that has to be met at a particular point in a match,
832 without consuming any characters from the subject string. The use of
833 subpatterns for more complicated assertions is described
834 .\" HTML <a href="#bigassertions">
835 .\" </a>
836 below.
837 .\"
838 The backslashed assertions are:
839 .sp
840 \eb matches at a word boundary
841 \eB matches when not at a word boundary
842 \eA matches at the start of the subject
843 \eZ matches at the end of the subject
844 also matches before a newline at the end of the subject
845 \ez matches only at the end of the subject
846 \eG matches at the first matching position in the subject
847 .sp
848 Inside a character class, \eb has a different meaning; it matches the backspace
849 character. If any other of these assertions appears in a character class, by
850 default it matches the corresponding literal character (for example, \eB
851 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
852 escape sequence" error is generated instead.
853 .P
854 A word boundary is a position in the subject string where the current character
855 and the previous character do not both match \ew or \eW (i.e. one matches
856 \ew and the other matches \eW), or the start or end of the string if the
857 first or last character matches \ew, respectively. In UTF-8 mode, the meanings
858 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
859 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
860 of word" or "end of word" metasequence. However, whatever follows \eb normally
861 determines which it is. For example, the fragment \eba matches "a" at the start
862 of a word.
863 .P
864 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
865 dollar (described in the next section) in that they only ever match at the very
866 start and end of the subject string, whatever options are set. Thus, they are
867 independent of multiline mode. These three assertions are not affected by the
868 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
869 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
870 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
871 at a point other than the beginning of the subject, \eA can never match. The
872 difference between \eZ and \ez is that \eZ matches before a newline at the end
873 of the string as well as at the very end, whereas \ez matches only at the end.
874 .P
875 The \eG assertion is true only when the current matching position is at the
876 start point of the match, as specified by the \fIstartoffset\fP argument of
877 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
878 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
879 arguments, you can mimic Perl's /g option, and it is in this kind of
880 implementation where \eG can be useful.
881 .P
882 Note, however, that PCRE's interpretation of \eG, as the start of the current
883 match, is subtly different from Perl's, which defines it as the end of the
884 previous match. In Perl, these can be different when the previously matched
885 string was empty. Because PCRE does just one match at a time, it cannot
886 reproduce this behaviour.
887 .P
888 If all the alternatives of a pattern begin with \eG, the expression is anchored
889 to the starting match position, and the "anchored" flag is set in the compiled
890 regular expression.
891 .
892 .
894 .rs
895 .sp
896 Outside a character class, in the default matching mode, the circumflex
897 character is an assertion that is true only if the current matching point is
898 at the start of the subject string. If the \fIstartoffset\fP argument of
899 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
900 option is unset. Inside a character class, circumflex has an entirely different
901 meaning
902 .\" HTML <a href="#characterclass">
903 .\" </a>
904 (see below).
905 .\"
906 .P
907 Circumflex need not be the first character of the pattern if a number of
908 alternatives are involved, but it should be the first thing in each alternative
909 in which it appears if the pattern is ever to match that branch. If all
910 possible alternatives start with a circumflex, that is, if the pattern is
911 constrained to match only at the start of the subject, it is said to be an
912 "anchored" pattern. (There are also other constructs that can cause a pattern
913 to be anchored.)
914 .P
915 A dollar character is an assertion that is true only if the current matching
916 point is at the end of the subject string, or immediately before a newline
917 at the end of the string (by default). Dollar need not be the last character of
918 the pattern if a number of alternatives are involved, but it should be the last
919 item in any branch in which it appears. Dollar has no special meaning in a
920 character class.
921 .P
922 The meaning of dollar can be changed so that it matches only at the very end of
923 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
924 does not affect the \eZ assertion.
925 .P
926 The meanings of the circumflex and dollar characters are changed if the
927 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
928 immediately after internal newlines as well as at the start of the subject
929 string. It does not match after a newline that ends the string. A dollar
930 matches before any newlines in the string, as well as at the very end, when
931 PCRE_MULTILINE is set. When newline is specified as the two-character
932 sequence CRLF, isolated CR and LF characters do not indicate newlines.
933 .P
934 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
935 \en represents a newline) in multiline mode, but not otherwise. Consequently,
936 patterns that are anchored in single line mode because all branches start with
937 ^ are not anchored in multiline mode, and a match for circumflex is possible
938 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
939 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
940 .P
941 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
942 end of the subject in both modes, and if all branches of a pattern start with
943 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
944 .
945 .
946 .\" HTML <a name="fullstopdot"></a>
948 .rs
949 .sp
950 Outside a character class, a dot in the pattern matches any one character in
951 the subject string except (by default) a character that signifies the end of a
952 line. In UTF-8 mode, the matched character may be more than one byte long.
953 .P
954 When a line ending is defined as a single character, dot never matches that
955 character; when the two-character sequence CRLF is used, dot does not match CR
956 if it is immediately followed by LF, but otherwise it matches all characters
957 (including isolated CRs and LFs). When any Unicode line endings are being
958 recognized, dot does not match CR or LF or any of the other line ending
959 characters.
960 .P
961 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
962 option is set, a dot matches any one character, without exception. If the
963 two-character sequence CRLF is present in the subject string, it takes two dots
964 to match it.
965 .P
966 The handling of dot is entirely independent of the handling of circumflex and
967 dollar, the only relationship being that they both involve newlines. Dot has no
968 special meaning in a character class.
969 .P
970 The escape sequence \eN behaves like a dot, except that it is not affected by
971 the PCRE_DOTALL option. In other words, it matches any character except one
972 that signifies the end of a line.
973 .
974 .
976 .rs
977 .sp
978 Outside a character class, the escape sequence \eC matches any one byte, both
979 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
980 characters. The feature is provided in Perl in order to match individual bytes
981 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
982 rest of the string may start with a malformed UTF-8 character. For this reason,
983 the \eC escape sequence is best avoided.
984 .P
985 PCRE does not allow \eC to appear in lookbehind assertions
986 .\" HTML <a href="#lookbehind">
987 .\" </a>
988 (described below),
989 .\"
990 because in UTF-8 mode this would make it impossible to calculate the length of
991 the lookbehind.
992 .
993 .
994 .\" HTML <a name="characterclass"></a>
996 .rs
997 .sp
998 An opening square bracket introduces a character class, terminated by a closing
999 square bracket. A closing square bracket on its own is not special by default.
1000 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1001 bracket causes a compile-time error. If a closing square bracket is required as
1002 a member of the class, it should be the first data character in the class
1003 (after an initial circumflex, if present) or escaped with a backslash.
1004 .P
1005 A character class matches a single character in the subject. In UTF-8 mode, the
1006 character may be more than one byte long. A matched character must be in the
1007 set of characters defined by the class, unless the first character in the class
1008 definition is a circumflex, in which case the subject character must not be in
1009 the set defined by the class. If a circumflex is actually required as a member
1010 of the class, ensure it is not the first character, or escape it with a
1011 backslash.
1012 .P
1013 For example, the character class [aeiou] matches any lower case vowel, while
1014 [^aeiou] matches any character that is not a lower case vowel. Note that a
1015 circumflex is just a convenient notation for specifying the characters that
1016 are in the class by enumerating those that are not. A class that starts with a
1017 circumflex is not an assertion; it still consumes a character from the subject
1018 string, and therefore it fails if the current pointer is at the end of the
1019 string.
1020 .P
1021 In UTF-8 mode, characters with values greater than 255 can be included in a
1022 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
1023 .P
1024 When caseless matching is set, any letters in a class represent both their
1025 upper case and lower case versions, so for example, a caseless [aeiou] matches
1026 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1027 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1028 case for characters whose values are less than 128, so caseless matching is
1029 always possible. For characters with higher values, the concept of case is
1030 supported if PCRE is compiled with Unicode property support, but not otherwise.
1031 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1032 you must ensure that PCRE is compiled with Unicode property support as well as
1033 with UTF-8 support.
1034 .P
1035 Characters that might indicate line breaks are never treated in any special way
1036 when matching character classes, whatever line-ending sequence is in use, and
1037 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1038 such as [^a] always matches one of these characters.
1039 .P
1040 The minus (hyphen) character can be used to specify a range of characters in a
1041 character class. For example, [d-m] matches any letter between d and m,
1042 inclusive. If a minus character is required in a class, it must be escaped with
1043 a backslash or appear in a position where it cannot be interpreted as
1044 indicating a range, typically as the first or last character in the class.
1045 .P
1046 It is not possible to have the literal character "]" as the end character of a
1047 range. A pattern such as [W-]46] is interpreted as a class of two characters
1048 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1049 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1050 the end of range, so [W-\e]46] is interpreted as a class containing a range
1051 followed by two other characters. The octal or hexadecimal representation of
1052 "]" can also be used to end a range.
1053 .P
1054 Ranges operate in the collating sequence of character values. They can also be
1055 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1056 mode, ranges can include characters whose values are greater than 255, for
1057 example [\ex{100}-\ex{2ff}].
1058 .P
1059 If a range that includes letters is used when caseless matching is set, it
1060 matches the letters in either case. For example, [W-c] is equivalent to
1061 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1062 tables for a French locale are in use, [\exc8-\excb] matches accented E
1063 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1064 characters with values greater than 128 only when it is compiled with Unicode
1065 property support.
1066 .P
1067 The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1068 \eV, \ew, and \eW may appear in a character class, and add the characters that
1069 they match to the class. For example, [\edABCDEF] matches any hexadecimal
1070 digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1071 and their upper case partners, just as it does when they appear outside a
1072 character class, as described in the section entitled
1073 .\" HTML <a href="#genericchartypes">
1074 .\" </a>
1075 "Generic character types"
1076 .\"
1077 above. The escape sequence \eb has a different meaning inside a character
1078 class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1079 are not special inside a character class. Like any other unrecognized escape
1080 sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1081 default, but cause an error if the PCRE_EXTRA option is set.
1082 .P
1083 A circumflex can conveniently be used with the upper case character types to
1084 specify a more restricted set of characters than the matching lower case type.
1085 For example, the class [^\eW_] matches any letter or digit, but not underscore,
1086 whereas [\ew] includes underscore. A positive character class should be read as
1087 "something OR something OR ..." and a negative class as "NOT something AND NOT
1088 something AND NOT ...".
1089 .P
1090 The only metacharacters that are recognized in character classes are backslash,
1091 hyphen (only where it can be interpreted as specifying a range), circumflex
1092 (only at the start), opening square bracket (only when it can be interpreted as
1093 introducing a POSIX class name - see the next section), and the terminating
1094 closing square bracket. However, escaping other non-alphanumeric characters
1095 does no harm.
1096 .
1097 .
1099 .rs
1100 .sp
1101 Perl supports the POSIX notation for character classes. This uses names
1102 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1103 this notation. For example,
1104 .sp
1105 [01[:alpha:]%]
1106 .sp
1107 matches "0", "1", any alphabetic character, or "%". The supported class names
1108 are:
1109 .sp
1110 alnum letters and digits
1111 alpha letters
1112 ascii character codes 0 - 127
1113 blank space or tab only
1114 cntrl control characters
1115 digit decimal digits (same as \ed)
1116 graph printing characters, excluding space
1117 lower lower case letters
1118 print printing characters, including space
1119 punct printing characters, excluding letters and digits and space
1120 space white space (not quite the same as \es)
1121 upper upper case letters
1122 word "word" characters (same as \ew)
1123 xdigit hexadecimal digits
1124 .sp
1125 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1126 space (32). Notice that this list includes the VT character (code 11). This
1127 makes "space" different to \es, which does not include VT (for Perl
1128 compatibility).
1129 .P
1130 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1131 5.8. Another Perl extension is negation, which is indicated by a ^ character
1132 after the colon. For example,
1133 .sp
1134 [12[:^digit:]]
1135 .sp
1136 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1137 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1138 supported, and an error is given if they are encountered.
1139 .P
1140 By default, in UTF-8 mode, characters with values greater than 128 do not match
1141 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1142 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1143 character properties are used. This is achieved by replacing the POSIX classes
1144 by other sequences, as follows:
1145 .sp
1146 [:alnum:] becomes \ep{Xan}
1147 [:alpha:] becomes \ep{L}
1148 [:blank:] becomes \eh
1149 [:digit:] becomes \ep{Nd}
1150 [:lower:] becomes \ep{Ll}
1151 [:space:] becomes \ep{Xps}
1152 [:upper:] becomes \ep{Lu}
1153 [:word:] becomes \ep{Xwd}
1154 .sp
1155 Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1156 classes are unchanged, and match only characters with code points less than
1157 128.
1158 .
1159 .
1161 .rs
1162 .sp
1163 Vertical bar characters are used to separate alternative patterns. For example,
1164 the pattern
1165 .sp
1166 gilbert|sullivan
1167 .sp
1168 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1169 and an empty alternative is permitted (matching the empty string). The matching
1170 process tries each alternative in turn, from left to right, and the first one
1171 that succeeds is used. If the alternatives are within a subpattern
1172 .\" HTML <a href="#subpattern">
1173 .\" </a>
1174 (defined below),
1175 .\"
1176 "succeeds" means matching the rest of the main pattern as well as the
1177 alternative in the subpattern.
1178 .
1179 .
1181 .rs
1182 .sp
1183 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1184 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1185 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1186 The option letters are
1187 .sp
1188 i for PCRE_CASELESS
1190 s for PCRE_DOTALL
1191 x for PCRE_EXTENDED
1192 .sp
1193 For example, (?im) sets caseless, multiline matching. It is also possible to
1194 unset these options by preceding the letter with a hyphen, and a combined
1195 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1196 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1197 permitted. If a letter appears both before and after the hyphen, the option is
1198 unset.
1199 .P
1200 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1201 changed in the same way as the Perl-compatible options by using the characters
1202 J, U and X respectively.
1203 .P
1204 When one of these option changes occurs at top level (that is, not inside
1205 subpattern parentheses), the change applies to the remainder of the pattern
1206 that follows. If the change is placed right at the start of a pattern, PCRE
1207 extracts it into the global options (and it will therefore show up in data
1208 extracted by the \fBpcre_fullinfo()\fP function).
1209 .P
1210 An option change within a subpattern (see below for a description of
1211 subpatterns) affects only that part of the subpattern that follows it, so
1212 .sp
1213 (a(?i)b)c
1214 .sp
1215 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1216 By this means, options can be made to have different settings in different
1217 parts of the pattern. Any changes made in one alternative do carry on
1218 into subsequent branches within the same subpattern. For example,
1219 .sp
1220 (a(?i)b|c)
1221 .sp
1222 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1223 branch is abandoned before the option setting. This is because the effects of
1224 option settings happen at compile time. There would be some very weird
1225 behaviour otherwise.
1226 .P
1227 \fBNote:\fP There are other PCRE-specific options that can be set by the
1228 application when the compile or match functions are called. In some cases the
1229 pattern can contain special leading sequences such as (*CRLF) to override what
1230 the application has set or what has been defaulted. Details are given in the
1231 section entitled
1232 .\" HTML <a href="#newlineseq">
1233 .\" </a>
1234 "Newline sequences"
1235 .\"
1236 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1237 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1238 PCRE_UTF8 and the PCRE_UCP options, respectively.
1239 .
1240 .
1241 .\" HTML <a name="subpattern"></a>
1243 .rs
1244 .sp
1245 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1246 Turning part of a pattern into a subpattern does two things:
1247 .sp
1248 1. It localizes a set of alternatives. For example, the pattern
1249 .sp
1250 cat(aract|erpillar|)
1251 .sp
1252 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1253 match "cataract", "erpillar" or an empty string.
1254 .sp
1255 2. It sets up the subpattern as a capturing subpattern. This means that, when
1256 the whole pattern matches, that portion of the subject string that matched the
1257 subpattern is passed back to the caller via the \fIovector\fP argument of
1258 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1259 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1260 string "the red king" is matched against the pattern
1261 .sp
1262 the ((red|white) (king|queen))
1263 .sp
1264 the captured substrings are "red king", "red", and "king", and are numbered 1,
1265 2, and 3, respectively.
1266 .P
1267 The fact that plain parentheses fulfil two functions is not always helpful.
1268 There are often times when a grouping subpattern is required without a
1269 capturing requirement. If an opening parenthesis is followed by a question mark
1270 and a colon, the subpattern does not do any capturing, and is not counted when
1271 computing the number of any subsequent capturing subpatterns. For example, if
1272 the string "the white queen" is matched against the pattern
1273 .sp
1274 the ((?:red|white) (king|queen))
1275 .sp
1276 the captured substrings are "white queen" and "queen", and are numbered 1 and
1277 2. The maximum number of capturing subpatterns is 65535.
1278 .P
1279 As a convenient shorthand, if any option settings are required at the start of
1280 a non-capturing subpattern, the option letters may appear between the "?" and
1281 the ":". Thus the two patterns
1282 .sp
1283 (?i:saturday|sunday)
1284 (?:(?i)saturday|sunday)
1285 .sp
1286 match exactly the same set of strings. Because alternative branches are tried
1287 from left to right, and options are not reset until the end of the subpattern
1288 is reached, an option setting in one branch does affect subsequent branches, so
1289 the above patterns match "SUNDAY" as well as "Saturday".
1290 .
1291 .
1292 .\" HTML <a name="dupsubpatternnumber"></a>
1294 .rs
1295 .sp
1296 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1297 the same numbers for its capturing parentheses. Such a subpattern starts with
1298 (?| and is itself a non-capturing subpattern. For example, consider this
1299 pattern:
1300 .sp
1301 (?|(Sat)ur|(Sun))day
1302 .sp
1303 Because the two alternatives are inside a (?| group, both sets of capturing
1304 parentheses are numbered one. Thus, when the pattern matches, you can look
1305 at captured substring number one, whichever alternative matched. This construct
1306 is useful when you want to capture part, but not all, of one of a number of
1307 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1308 number is reset at the start of each branch. The numbers of any capturing
1309 parentheses that follow the subpattern start after the highest number used in
1310 any branch. The following example is taken from the Perl documentation. The
1311 numbers underneath show in which buffer the captured content will be stored.
1312 .sp
1313 # before ---------------branch-reset----------- after
1314 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1315 # 1 2 2 3 2 3 4
1316 .sp
1317 A back reference to a numbered subpattern uses the most recent value that is
1318 set for that number by any subpattern. The following pattern matches "abcabc"
1319 or "defdef":
1320 .sp
1321 /(?|(abc)|(def))\e1/
1322 .sp
1323 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1324 refers to the first one in the pattern with the given number. The following
1325 pattern matches "abcabc" or "defabc":
1326 .sp
1327 /(?|(abc)|(def))(?1)/
1328 .sp
1329 If a
1330 .\" HTML <a href="#conditions">
1331 .\" </a>
1332 condition test
1333 .\"
1334 for a subpattern's having matched refers to a non-unique number, the test is
1335 true if any of the subpatterns of that number have matched.
1336 .P
1337 An alternative approach to using this "branch reset" feature is to use
1338 duplicate named subpatterns, as described in the next section.
1339 .
1340 .
1342 .rs
1343 .sp
1344 Identifying capturing parentheses by number is simple, but it can be very hard
1345 to keep track of the numbers in complicated regular expressions. Furthermore,
1346 if an expression is modified, the numbers may change. To help with this
1347 difficulty, PCRE supports the naming of subpatterns. This feature was not
1348 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1349 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1350 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1351 have different names, but PCRE does not.
1352 .P
1353 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1354 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1355 parentheses from other parts of the pattern, such as
1356 .\" HTML <a href="#backreferences">
1357 .\" </a>
1358 back references,
1359 .\"
1360 .\" HTML <a href="#recursion">
1361 .\" </a>
1362 recursion,
1363 .\"
1364 and
1365 .\" HTML <a href="#conditions">
1366 .\" </a>
1367 conditions,
1368 .\"
1369 can be made by name as well as by number.
1370 .P
1371 Names consist of up to 32 alphanumeric characters and underscores. Named
1372 capturing parentheses are still allocated numbers as well as names, exactly as
1373 if the names were not present. The PCRE API provides function calls for
1374 extracting the name-to-number translation table from a compiled pattern. There
1375 is also a convenience function for extracting a captured substring by name.
1376 .P
1377 By default, a name must be unique within a pattern, but it is possible to relax
1378 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1379 names are also always permitted for subpatterns with the same number, set up as
1380 described in the previous section.) Duplicate names can be useful for patterns
1381 where only one instance of the named parentheses can match. Suppose you want to
1382 match the name of a weekday, either as a 3-letter abbreviation or as the full
1383 name, and in both cases you want to extract the abbreviation. This pattern
1384 (ignoring the line breaks) does the job:
1385 .sp
1386 (?<DN>Mon|Fri|Sun)(?:day)?|
1387 (?<DN>Tue)(?:sday)?|
1388 (?<DN>Wed)(?:nesday)?|
1389 (?<DN>Thu)(?:rsday)?|
1390 (?<DN>Sat)(?:urday)?
1391 .sp
1392 There are five capturing substrings, but only one is ever set after a match.
1393 (An alternative way of solving this problem is to use a "branch reset"
1394 subpattern, as described in the previous section.)
1395 .P
1396 The convenience function for extracting the data by name returns the substring
1397 for the first (and in this example, the only) subpattern of that name that
1398 matched. This saves searching to find which numbered subpattern it was.
1399 .P
1400 If you make a back reference to a non-unique named subpattern from elsewhere in
1401 the pattern, the one that corresponds to the first occurrence of the name is
1402 used. In the absence of duplicate numbers (see the previous section) this is
1403 the one with the lowest number. If you use a named reference in a condition
1404 test (see the
1405 .\"
1406 .\" HTML <a href="#conditions">
1407 .\" </a>
1408 section about conditions
1409 .\"
1410 below), either to check whether a subpattern has matched, or to check for
1411 recursion, all subpatterns with the same name are tested. If the condition is
1412 true for any one of them, the overall condition is true. This is the same
1413 behaviour as testing by number. For further details of the interfaces for
1414 handling named subpatterns, see the
1415 .\" HREF
1416 \fBpcreapi\fP
1417 .\"
1418 documentation.
1419 .P
1420 \fBWarning:\fP You cannot use different names to distinguish between two
1421 subpatterns with the same number because PCRE uses only the numbers when
1422 matching. For this reason, an error is given at compile time if different names
1423 are given to subpatterns with the same number. However, you can give the same
1424 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1425 .
1426 .
1428 .rs
1429 .sp
1430 Repetition is specified by quantifiers, which can follow any of the following
1431 items:
1432 .sp
1433 a literal data character
1434 the dot metacharacter
1435 the \eC escape sequence
1436 the \eX escape sequence (in UTF-8 mode with Unicode properties)
1437 the \eR escape sequence
1438 an escape such as \ed or \epL that matches a single character
1439 a character class
1440 a back reference (see next section)
1441 a parenthesized subpattern (including assertions)
1442 a recursive or "subroutine" call to a subpattern
1443 .sp
1444 The general repetition quantifier specifies a minimum and maximum number of
1445 permitted matches, by giving the two numbers in curly brackets (braces),
1446 separated by a comma. The numbers must be less than 65536, and the first must
1447 be less than or equal to the second. For example:
1448 .sp
1449 z{2,4}
1450 .sp
1451 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1452 character. If the second number is omitted, but the comma is present, there is
1453 no upper limit; if the second number and the comma are both omitted, the
1454 quantifier specifies an exact number of required matches. Thus
1455 .sp
1456 [aeiou]{3,}
1457 .sp
1458 matches at least 3 successive vowels, but may match many more, while
1459 .sp
1460 \ed{8}
1461 .sp
1462 matches exactly 8 digits. An opening curly bracket that appears in a position
1463 where a quantifier is not allowed, or one that does not match the syntax of a
1464 quantifier, is taken as a literal character. For example, {,6} is not a
1465 quantifier, but a literal string of four characters.
1466 .P
1467 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1468 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1469 which is represented by a two-byte sequence. Similarly, when Unicode property
1470 support is available, \eX{3} matches three Unicode extended sequences, each of
1471 which may be several bytes long (and they may be of different lengths).
1472 .P
1473 The quantifier {0} is permitted, causing the expression to behave as if the
1474 previous item and the quantifier were not present. This may be useful for
1475 subpatterns that are referenced as
1476 .\" HTML <a href="#subpatternsassubroutines">
1477 .\" </a>
1478 subroutines
1479 .\"
1480 from elsewhere in the pattern (but see also the section entitled
1481 .\" HTML <a href="#subdefine">
1482 .\" </a>
1483 "Defining subpatterns for use by reference only"
1484 .\"
1485 below). Items other than subpatterns that have a {0} quantifier are omitted
1486 from the compiled pattern.
1487 .P
1488 For convenience, the three most common quantifiers have single-character
1489 abbreviations:
1490 .sp
1491 * is equivalent to {0,}
1492 + is equivalent to {1,}
1493 ? is equivalent to {0,1}
1494 .sp
1495 It is possible to construct infinite loops by following a subpattern that can
1496 match no characters with a quantifier that has no upper limit, for example:
1497 .sp
1498 (a?)*
1499 .sp
1500 Earlier versions of Perl and PCRE used to give an error at compile time for
1501 such patterns. However, because there are cases where this can be useful, such
1502 patterns are now accepted, but if any repetition of the subpattern does in fact
1503 match no characters, the loop is forcibly broken.
1504 .P
1505 By default, the quantifiers are "greedy", that is, they match as much as
1506 possible (up to the maximum number of permitted times), without causing the
1507 rest of the pattern to fail. The classic example of where this gives problems
1508 is in trying to match comments in C programs. These appear between /* and */
1509 and within the comment, individual * and / characters may appear. An attempt to
1510 match C comments by applying the pattern
1511 .sp
1512 /\e*.*\e*/
1513 .sp
1514 to the string
1515 .sp
1516 /* first comment */ not comment /* second comment */
1517 .sp
1518 fails, because it matches the entire string owing to the greediness of the .*
1519 item.
1520 .P
1521 However, if a quantifier is followed by a question mark, it ceases to be
1522 greedy, and instead matches the minimum number of times possible, so the
1523 pattern
1524 .sp
1525 /\e*.*?\e*/
1526 .sp
1527 does the right thing with the C comments. The meaning of the various
1528 quantifiers is not otherwise changed, just the preferred number of matches.
1529 Do not confuse this use of question mark with its use as a quantifier in its
1530 own right. Because it has two uses, it can sometimes appear doubled, as in
1531 .sp
1532 \ed??\ed
1533 .sp
1534 which matches one digit by preference, but can match two if that is the only
1535 way the rest of the pattern matches.
1536 .P
1537 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1538 the quantifiers are not greedy by default, but individual ones can be made
1539 greedy by following them with a question mark. In other words, it inverts the
1540 default behaviour.
1541 .P
1542 When a parenthesized subpattern is quantified with a minimum repeat count that
1543 is greater than 1 or with a limited maximum, more memory is required for the
1544 compiled pattern, in proportion to the size of the minimum or maximum.
1545 .P
1546 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1547 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1548 implicitly anchored, because whatever follows will be tried against every
1549 character position in the subject string, so there is no point in retrying the
1550 overall match at any position after the first. PCRE normally treats such a
1551 pattern as though it were preceded by \eA.
1552 .P
1553 In cases where it is known that the subject string contains no newlines, it is
1554 worth setting PCRE_DOTALL in order to obtain this optimization, or
1555 alternatively using ^ to indicate anchoring explicitly.
1556 .P
1557 However, there is one situation where the optimization cannot be used. When .*
1558 is inside capturing parentheses that are the subject of a back reference
1559 elsewhere in the pattern, a match at the start may fail where a later one
1560 succeeds. Consider, for example:
1561 .sp
1562 (.*)abc\e1
1563 .sp
1564 If the subject is "xyz123abc123" the match point is the fourth character. For
1565 this reason, such a pattern is not implicitly anchored.
1566 .P
1567 When a capturing subpattern is repeated, the value captured is the substring
1568 that matched the final iteration. For example, after
1569 .sp
1570 (tweedle[dume]{3}\es*)+
1571 .sp
1572 has matched "tweedledum tweedledee" the value of the captured substring is
1573 "tweedledee". However, if there are nested capturing subpatterns, the
1574 corresponding captured values may have been set in previous iterations. For
1575 example, after
1576 .sp
1577 /(a|(b))+/
1578 .sp
1579 matches "aba" the value of the second captured substring is "b".
1580 .
1581 .
1582 .\" HTML <a name="atomicgroup"></a>
1584 .rs
1585 .sp
1586 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1587 repetition, failure of what follows normally causes the repeated item to be
1588 re-evaluated to see if a different number of repeats allows the rest of the
1589 pattern to match. Sometimes it is useful to prevent this, either to change the
1590 nature of the match, or to cause it fail earlier than it otherwise might, when
1591 the author of the pattern knows there is no point in carrying on.
1592 .P
1593 Consider, for example, the pattern \ed+foo when applied to the subject line
1594 .sp
1595 123456bar
1596 .sp
1597 After matching all 6 digits and then failing to match "foo", the normal
1598 action of the matcher is to try again with only 5 digits matching the \ed+
1599 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1600 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1601 that once a subpattern has matched, it is not to be re-evaluated in this way.
1602 .P
1603 If we use atomic grouping for the previous example, the matcher gives up
1604 immediately on failing to match "foo" the first time. The notation is a kind of
1605 special parenthesis, starting with (?> as in this example:
1606 .sp
1607 (?>\ed+)foo
1608 .sp
1609 This kind of parenthesis "locks up" the part of the pattern it contains once
1610 it has matched, and a failure further into the pattern is prevented from
1611 backtracking into it. Backtracking past it to previous items, however, works as
1612 normal.
1613 .P
1614 An alternative description is that a subpattern of this type matches the string
1615 of characters that an identical standalone pattern would match, if anchored at
1616 the current point in the subject string.
1617 .P
1618 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1619 the above example can be thought of as a maximizing repeat that must swallow
1620 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1621 number of digits they match in order to make the rest of the pattern match,
1622 (?>\ed+) can only match an entire sequence of digits.
1623 .P
1624 Atomic groups in general can of course contain arbitrarily complicated
1625 subpatterns, and can be nested. However, when the subpattern for an atomic
1626 group is just a single repeated item, as in the example above, a simpler
1627 notation, called a "possessive quantifier" can be used. This consists of an
1628 additional + character following a quantifier. Using this notation, the
1629 previous example can be rewritten as
1630 .sp
1631 \ed++foo
1632 .sp
1633 Note that a possessive quantifier can be used with an entire group, for
1634 example:
1635 .sp
1636 (abc|xyz){2,3}+
1637 .sp
1638 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1639 option is ignored. They are a convenient notation for the simpler forms of
1640 atomic group. However, there is no difference in the meaning of a possessive
1641 quantifier and the equivalent atomic group, though there may be a performance
1642 difference; possessive quantifiers should be slightly faster.
1643 .P
1644 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1645 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1646 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1647 package, and PCRE copied it from there. It ultimately found its way into Perl
1648 at release 5.10.
1649 .P
1650 PCRE has an optimization that automatically "possessifies" certain simple
1651 pattern constructs. For example, the sequence A+B is treated as A++B because
1652 there is no point in backtracking into a sequence of A's when B must follow.
1653 .P
1654 When a pattern contains an unlimited repeat inside a subpattern that can itself
1655 be repeated an unlimited number of times, the use of an atomic group is the
1656 only way to avoid some failing matches taking a very long time indeed. The
1657 pattern
1658 .sp
1659 (\eD+|<\ed+>)*[!?]
1660 .sp
1661 matches an unlimited number of substrings that either consist of non-digits, or
1662 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1663 quickly. However, if it is applied to
1664 .sp
1665 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1666 .sp
1667 it takes a long time before reporting failure. This is because the string can
1668 be divided between the internal \eD+ repeat and the external * repeat in a
1669 large number of ways, and all have to be tried. (The example uses [!?] rather
1670 than a single character at the end, because both PCRE and Perl have an
1671 optimization that allows for fast failure when a single character is used. They
1672 remember the last single character that is required for a match, and fail early
1673 if it is not present in the string.) If the pattern is changed so that it uses
1674 an atomic group, like this:
1675 .sp
1676 ((?>\eD+)|<\ed+>)*[!?]
1677 .sp
1678 sequences of non-digits cannot be broken, and failure happens quickly.
1679 .
1680 .
1681 .\" HTML <a name="backreferences"></a>
1683 .rs
1684 .sp
1685 Outside a character class, a backslash followed by a digit greater than 0 (and
1686 possibly further digits) is a back reference to a capturing subpattern earlier
1687 (that is, to its left) in the pattern, provided there have been that many
1688 previous capturing left parentheses.
1689 .P
1690 However, if the decimal number following the backslash is less than 10, it is
1691 always taken as a back reference, and causes an error only if there are not
1692 that many capturing left parentheses in the entire pattern. In other words, the
1693 parentheses that are referenced need not be to the left of the reference for
1694 numbers less than 10. A "forward back reference" of this type can make sense
1695 when a repetition is involved and the subpattern to the right has participated
1696 in an earlier iteration.
1697 .P
1698 It is not possible to have a numerical "forward back reference" to a subpattern
1699 whose number is 10 or more using this syntax because a sequence such as \e50 is
1700 interpreted as a character defined in octal. See the subsection entitled
1701 "Non-printing characters"
1702 .\" HTML <a href="#digitsafterbackslash">
1703 .\" </a>
1704 above
1705 .\"
1706 for further details of the handling of digits following a backslash. There is
1707 no such problem when named parentheses are used. A back reference to any
1708 subpattern is possible using named parentheses (see below).
1709 .P
1710 Another way of avoiding the ambiguity inherent in the use of digits following a
1711 backslash is to use the \eg escape sequence. This escape must be followed by an
1712 unsigned number or a negative number, optionally enclosed in braces. These
1713 examples are all identical:
1714 .sp
1715 (ring), \e1
1716 (ring), \eg1
1717 (ring), \eg{1}
1718 .sp
1719 An unsigned number specifies an absolute reference without the ambiguity that
1720 is present in the older syntax. It is also useful when literal digits follow
1721 the reference. A negative number is a relative reference. Consider this
1722 example:
1723 .sp
1724 (abc(def)ghi)\eg{-1}
1725 .sp
1726 The sequence \eg{-1} is a reference to the most recently started capturing
1727 subpattern before \eg, that is, is it equivalent to \e2 in this example.
1728 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1729 can be helpful in long patterns, and also in patterns that are created by
1730 joining together fragments that contain references within themselves.
1731 .P
1732 A back reference matches whatever actually matched the capturing subpattern in
1733 the current subject string, rather than anything matching the subpattern
1734 itself (see
1735 .\" HTML <a href="#subpatternsassubroutines">
1736 .\" </a>
1737 "Subpatterns as subroutines"
1738 .\"
1739 below for a way of doing that). So the pattern
1740 .sp
1741 (sens|respons)e and \e1ibility
1742 .sp
1743 matches "sense and sensibility" and "response and responsibility", but not
1744 "sense and responsibility". If caseful matching is in force at the time of the
1745 back reference, the case of letters is relevant. For example,
1746 .sp
1747 ((?i)rah)\es+\e1
1748 .sp
1749 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1750 capturing subpattern is matched caselessly.
1751 .P
1752 There are several different ways of writing back references to named
1753 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1754 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1755 back reference syntax, in which \eg can be used for both numeric and named
1756 references, is also supported. We could rewrite the above example in any of
1757 the following ways:
1758 .sp
1759 (?<p1>(?i)rah)\es+\ek<p1>
1760 (?'p1'(?i)rah)\es+\ek{p1}
1761 (?P<p1>(?i)rah)\es+(?P=p1)
1762 (?<p1>(?i)rah)\es+\eg{p1}
1763 .sp
1764 A subpattern that is referenced by name may appear in the pattern before or
1765 after the reference.
1766 .P
1767 There may be more than one back reference to the same subpattern. If a
1768 subpattern has not actually been used in a particular match, any back
1769 references to it always fail by default. For example, the pattern
1770 .sp
1771 (a|(bc))\e2
1772 .sp
1773 always fails if it starts to match "a" rather than "bc". However, if the
1774 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1775 unset value matches an empty string.
1776 .P
1777 Because there may be many capturing parentheses in a pattern, all digits
1778 following a backslash are taken as part of a potential back reference number.
1779 If the pattern continues with a digit character, some delimiter must be used to
1780 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1781 whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1782 .\" HTML <a href="#comments">
1783 .\" </a>
1784 "Comments"
1785 .\"
1786 below) can be used.
1787 .
1788 .SS "Recursive back references"
1789 .rs
1790 .sp
1791 A back reference that occurs inside the parentheses to which it refers fails
1792 when the subpattern is first used, so, for example, (a\e1) never matches.
1793 However, such references can be useful inside repeated subpatterns. For
1794 example, the pattern
1795 .sp
1796 (a|b\e1)+
1797 .sp
1798 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1799 the subpattern, the back reference matches the character string corresponding
1800 to the previous iteration. In order for this to work, the pattern must be such
1801 that the first iteration does not need to match the back reference. This can be
1802 done using alternation, as in the example above, or by a quantifier with a
1803 minimum of zero.
1804 .P
1805 Back references of this type cause the group that they reference to be treated
1806 as an
1807 .\" HTML <a href="#atomicgroup">
1808 .\" </a>
1809 atomic group.
1810 .\"
1811 Once the whole group has been matched, a subsequent matching failure cannot
1812 cause backtracking into the middle of the group.
1813 .
1814 .
1815 .\" HTML <a name="bigassertions"></a>
1817 .rs
1818 .sp
1819 An assertion is a test on the characters following or preceding the current
1820 matching point that does not actually consume any characters. The simple
1821 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1822 .\" HTML <a href="#smallassertions">
1823 .\" </a>
1824 above.
1825 .\"
1826 .P
1827 More complicated assertions are coded as subpatterns. There are two kinds:
1828 those that look ahead of the current position in the subject string, and those
1829 that look behind it. An assertion subpattern is matched in the normal way,
1830 except that it does not cause the current matching position to be changed.
1831 .P
1832 Assertion subpatterns are not capturing subpatterns. If such an assertion
1833 contains capturing subpatterns within it, these are counted for the purposes of
1834 numbering the capturing subpatterns in the whole pattern. However, substring
1835 capturing is carried out only for positive assertions, because it does not make
1836 sense for negative assertions.
1837 .P
1838 For compatibility with Perl, assertion subpatterns may be repeated; though
1839 it makes no sense to assert the same thing several times, the side effect of
1840 capturing parentheses may occasionally be useful. In practice, there only three
1841 cases:
1842 .sp
1843 (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1844 However, it may contain internal capturing parenthesized groups that are called
1845 from elsewhere via the
1846 .\" HTML <a href="#subpatternsassubroutines">
1847 .\" </a>
1848 subroutine mechanism.
1849 .\"
1850 .sp
1851 (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1852 were {0,1}. At run time, the rest of the pattern match is tried with and
1853 without the assertion, the order depending on the greediness of the quantifier.
1854 .sp
1855 (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1856 The assertion is obeyed just once when encountered during matching.
1857 .
1858 .
1859 .SS "Lookahead assertions"
1860 .rs
1861 .sp
1862 Lookahead assertions start with (?= for positive assertions and (?! for
1863 negative assertions. For example,
1864 .sp
1865 \ew+(?=;)
1866 .sp
1867 matches a word followed by a semicolon, but does not include the semicolon in
1868 the match, and
1869 .sp
1870 foo(?!bar)
1871 .sp
1872 matches any occurrence of "foo" that is not followed by "bar". Note that the
1873 apparently similar pattern
1874 .sp
1875 (?!foo)bar
1876 .sp
1877 does not find an occurrence of "bar" that is preceded by something other than
1878 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1879 (?!foo) is always true when the next three characters are "bar". A
1880 lookbehind assertion is needed to achieve the other effect.
1881 .P
1882 If you want to force a matching failure at some point in a pattern, the most
1883 convenient way to do it is with (?!) because an empty string always matches, so
1884 an assertion that requires there not to be an empty string must always fail.
1885 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1886 .
1887 .
1888 .\" HTML <a name="lookbehind"></a>
1889 .SS "Lookbehind assertions"
1890 .rs
1891 .sp
1892 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1893 negative assertions. For example,
1894 .sp
1895 (?<!foo)bar
1896 .sp
1897 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1898 a lookbehind assertion are restricted such that all the strings it matches must
1899 have a fixed length. However, if there are several top-level alternatives, they
1900 do not all have to have the same fixed length. Thus
1901 .sp
1902 (?<=bullock|donkey)
1903 .sp
1904 is permitted, but
1905 .sp
1906 (?<!dogs?|cats?)
1907 .sp
1908 causes an error at compile time. Branches that match different length strings
1909 are permitted only at the top level of a lookbehind assertion. This is an
1910 extension compared with Perl, which requires all branches to match the same
1911 length of string. An assertion such as
1912 .sp
1913 (?<=ab(c|de))
1914 .sp
1915 is not permitted, because its single top-level branch can match two different
1916 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1917 branches:
1918 .sp
1919 (?<=abc|abde)
1920 .sp
1921 In some cases, the escape sequence \eK
1922 .\" HTML <a href="#resetmatchstart">
1923 .\" </a>
1924 (see above)
1925 .\"
1926 can be used instead of a lookbehind assertion to get round the fixed-length
1927 restriction.
1928 .P
1929 The implementation of lookbehind assertions is, for each alternative, to
1930 temporarily move the current position back by the fixed length and then try to
1931 match. If there are insufficient characters before the current position, the
1932 assertion fails.
1933 .P
1934 PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1935 to appear in lookbehind assertions, because it makes it impossible to calculate
1936 the length of the lookbehind. The \eX and \eR escapes, which can match
1937 different numbers of bytes, are also not permitted.
1938 .P
1939 .\" HTML <a href="#subpatternsassubroutines">
1940 .\" </a>
1941 "Subroutine"
1942 .\"
1943 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1944 as the subpattern matches a fixed-length string.
1945 .\" HTML <a href="#recursion">
1946 .\" </a>
1947 Recursion,
1948 .\"
1949 however, is not supported.
1950 .P
1951 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1952 specify efficient matching of fixed-length strings at the end of subject
1953 strings. Consider a simple pattern such as
1954 .sp
1955 abcd$
1956 .sp
1957 when applied to a long string that does not match. Because matching proceeds
1958 from left to right, PCRE will look for each "a" in the subject and then see if
1959 what follows matches the rest of the pattern. If the pattern is specified as
1960 .sp
1961 ^.*abcd$
1962 .sp
1963 the initial .* matches the entire string at first, but when this fails (because
1964 there is no following "a"), it backtracks to match all but the last character,
1965 then all but the last two characters, and so on. Once again the search for "a"
1966 covers the entire string, from right to left, so we are no better off. However,
1967 if the pattern is written as
1968 .sp
1969 ^.*+(?<=abcd)
1970 .sp
1971 there can be no backtracking for the .*+ item; it can match only the entire
1972 string. The subsequent lookbehind assertion does a single test on the last four
1973 characters. If it fails, the match fails immediately. For long strings, this
1974 approach makes a significant difference to the processing time.
1975 .
1976 .
1977 .SS "Using multiple assertions"
1978 .rs
1979 .sp
1980 Several assertions (of any sort) may occur in succession. For example,
1981 .sp
1982 (?<=\ed{3})(?<!999)foo
1983 .sp
1984 matches "foo" preceded by three digits that are not "999". Notice that each of
1985 the assertions is applied independently at the same point in the subject
1986 string. First there is a check that the previous three characters are all
1987 digits, and then there is a check that the same three characters are not "999".
1988 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1989 of which are digits and the last three of which are not "999". For example, it
1990 doesn't match "123abcfoo". A pattern to do that is
1991 .sp
1992 (?<=\ed{3}...)(?<!999)foo
1993 .sp
1994 This time the first assertion looks at the preceding six characters, checking
1995 that the first three are digits, and then the second assertion checks that the
1996 preceding three characters are not "999".
1997 .P
1998 Assertions can be nested in any combination. For example,
1999 .sp
2000 (?<=(?<!foo)bar)baz
2001 .sp
2002 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
2003 preceded by "foo", while
2004 .sp
2005 (?<=\ed{3}(?!999)...)foo
2006 .sp
2007 is another pattern that matches "foo" preceded by three digits and any three
2008 characters that are not "999".
2009 .
2010 .
2011 .\" HTML <a name="conditions"></a>
2013 .rs
2014 .sp
2015 It is possible to cause the matching process to obey a subpattern
2016 conditionally or to choose between two alternative subpatterns, depending on
2017 the result of an assertion, or whether a specific capturing subpattern has
2018 already been matched. The two possible forms of conditional subpattern are:
2019 .sp
2020 (?(condition)yes-pattern)
2021 (?(condition)yes-pattern|no-pattern)
2022 .sp
2023 If the condition is satisfied, the yes-pattern is used; otherwise the
2024 no-pattern (if present) is used. If there are more than two alternatives in the
2025 subpattern, a compile-time error occurs. Each of the two alternatives may
2026 itself contain nested subpatterns of any form, including conditional
2027 subpatterns; the restriction to two alternatives applies only at the level of
2028 the condition. This pattern fragment is an example where the alternatives are
2029 complex:
2030 .sp
2031 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2032 .sp
2033 .P
2034 There are four kinds of condition: references to subpatterns, references to
2035 recursion, a pseudo-condition called DEFINE, and assertions.
2036 .
2037 .SS "Checking for a used subpattern by number"
2038 .rs
2039 .sp
2040 If the text between the parentheses consists of a sequence of digits, the
2041 condition is true if a capturing subpattern of that number has previously
2042 matched. If there is more than one capturing subpattern with the same number
2043 (see the earlier
2044 .\"
2045 .\" HTML <a href="#recursion">
2046 .\" </a>
2047 section about duplicate subpattern numbers),
2048 .\"
2049 the condition is true if any of them have matched. An alternative notation is
2050 to precede the digits with a plus or minus sign. In this case, the subpattern
2051 number is relative rather than absolute. The most recently opened parentheses
2052 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2053 loops it can also make sense to refer to subsequent groups. The next
2054 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2055 zero in any of these forms is not used; it provokes a compile-time error.)
2056 .P
2057 Consider the following pattern, which contains non-significant white space to
2058 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2059 three parts for ease of discussion:
2060 .sp
2061 ( \e( )? [^()]+ (?(1) \e) )
2062 .sp
2063 The first part matches an optional opening parenthesis, and if that
2064 character is present, sets it as the first captured substring. The second part
2065 matches one or more characters that are not parentheses. The third part is a
2066 conditional subpattern that tests whether or not the first set of parentheses
2067 matched. If they did, that is, if subject started with an opening parenthesis,
2068 the condition is true, and so the yes-pattern is executed and a closing
2069 parenthesis is required. Otherwise, since no-pattern is not present, the
2070 subpattern matches nothing. In other words, this pattern matches a sequence of
2071 non-parentheses, optionally enclosed in parentheses.
2072 .P
2073 If you were embedding this pattern in a larger one, you could use a relative
2074 reference:
2075 .sp
2076 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2077 .sp
2078 This makes the fragment independent of the parentheses in the larger pattern.
2079 .
2080 .SS "Checking for a used subpattern by name"
2081 .rs
2082 .sp
2083 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2084 subpattern by name. For compatibility with earlier versions of PCRE, which had
2085 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2086 there is a possible ambiguity with this syntax, because subpattern names may
2087 consist entirely of digits. PCRE looks first for a named subpattern; if it
2088 cannot find one and the name consists entirely of digits, PCRE looks for a
2089 subpattern of that number, which must be greater than zero. Using subpattern
2090 names that consist entirely of digits is not recommended.
2091 .P
2092 Rewriting the above example to use a named subpattern gives this:
2093 .sp
2094 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2095 .sp
2096 If the name used in a condition of this kind is a duplicate, the test is
2097 applied to all subpatterns of the same name, and is true if any one of them has
2098 matched.
2099 .
2100 .SS "Checking for pattern recursion"
2101 .rs
2102 .sp
2103 If the condition is the string (R), and there is no subpattern with the name R,
2104 the condition is true if a recursive call to the whole pattern or any
2105 subpattern has been made. If digits or a name preceded by ampersand follow the
2106 letter R, for example:
2107 .sp
2108 (?(R3)...) or (?(R&name)...)
2109 .sp
2110 the condition is true if the most recent recursion is into a subpattern whose
2111 number or name is given. This condition does not check the entire recursion
2112 stack. If the name used in a condition of this kind is a duplicate, the test is
2113 applied to all subpatterns of the same name, and is true if any one of them is
2114 the most recent recursion.
2115 .P
2116 At "top level", all these recursion test conditions are false.
2117 .\" HTML <a href="#recursion">
2118 .\" </a>
2119 The syntax for recursive patterns
2120 .\"
2121 is described below.
2122 .
2123 .\" HTML <a name="subdefine"></a>
2124 .SS "Defining subpatterns for use by reference only"
2125 .rs
2126 .sp
2127 If the condition is the string (DEFINE), and there is no subpattern with the
2128 name DEFINE, the condition is always false. In this case, there may be only one
2129 alternative in the subpattern. It is always skipped if control reaches this
2130 point in the pattern; the idea of DEFINE is that it can be used to define
2131 "subroutines" that can be referenced from elsewhere. (The use of
2132 .\" HTML <a href="#subpatternsassubroutines">
2133 .\" </a>
2134 "subroutines"
2135 .\"
2136 is described below.) For example, a pattern to match an IPv4 address such as
2137 "" could be written like this (ignore whitespace and line
2138 breaks):
2139 .sp
2140 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2141 \eb (?&byte) (\e.(?&byte)){3} \eb
2142 .sp
2143 The first part of the pattern is a DEFINE group inside which a another group
2144 named "byte" is defined. This matches an individual component of an IPv4
2145 address (a number less than 256). When matching takes place, this part of the
2146 pattern is skipped because DEFINE acts like a false condition. The rest of the
2147 pattern uses references to the named group to match the four dot-separated
2148 components of an IPv4 address, insisting on a word boundary at each end.
2149 .
2150 .SS "Assertion conditions"
2151 .rs
2152 .sp
2153 If the condition is not in any of the above formats, it must be an assertion.
2154 This may be a positive or negative lookahead or lookbehind assertion. Consider
2155 this pattern, again containing non-significant white space, and with the two
2156 alternatives on the second line:
2157 .sp
2158 (?(?=[^a-z]*[a-z])
2159 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2160 .sp
2161 The condition is a positive lookahead assertion that matches an optional
2162 sequence of non-letters followed by a letter. In other words, it tests for the
2163 presence of at least one letter in the subject. If a letter is found, the
2164 subject is matched against the first alternative; otherwise it is matched
2165 against the second. This pattern matches strings in one of the two forms
2166 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2167 .
2168 .
2169 .\" HTML <a name="comments"></a>
2171 .rs
2172 .sp
2173 There are two ways of including comments in patterns that are processed by
2174 PCRE. In both cases, the start of the comment must not be in a character class,
2175 nor in the middle of any other sequence of related characters such as (?: or a
2176 subpattern name or number. The characters that make up a comment play no part
2177 in the pattern matching.
2178 .P
2179 The sequence (?# marks the start of a comment that continues up to the next
2180 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2181 option is set, an unescaped # character also introduces a comment, which in
2182 this case continues to immediately after the next newline character or
2183 character sequence in the pattern. Which characters are interpreted as newlines
2184 is controlled by the options passed to \fBpcre_compile()\fP or by a special
2185 sequence at the start of the pattern, as described in the section entitled
2186 .\" HTML <a href="#newlines">
2187 .\" </a>
2188 "Newline conventions"
2189 .\"
2190 above. Note that the end of this type of comment is a literal newline sequence
2191 in the pattern; escape sequences that happen to represent a newline do not
2192 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2193 default newline convention is in force:
2194 .sp
2195 abc #comment \en still comment
2196 .sp
2197 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2198 a newline in the pattern. The sequence \en is still literal at this stage, so
2199 it does not terminate the comment. Only an actual character with the code value
2200 0x0a (the default newline) does so.
2201 .
2202 .
2203 .\" HTML <a name="recursion"></a>
2205 .rs
2206 .sp
2207 Consider the problem of matching a string in parentheses, allowing for
2208 unlimited nested parentheses. Without the use of recursion, the best that can
2209 be done is to use a pattern that matches up to some fixed depth of nesting. It
2210 is not possible to handle an arbitrary nesting depth.
2211 .P
2212 For some time, Perl has provided a facility that allows regular expressions to
2213 recurse (amongst other things). It does this by interpolating Perl code in the
2214 expression at run time, and the code can refer to the expression itself. A Perl
2215 pattern using code interpolation to solve the parentheses problem can be
2216 created like this:
2217 .sp
2218 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2219 .sp
2220 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2221 recursively to the pattern in which it appears.
2222 .P
2223 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2224 supports special syntax for recursion of the entire pattern, and also for
2225 individual subpattern recursion. After its introduction in PCRE and Python,
2226 this kind of recursion was subsequently introduced into Perl at release 5.10.
2227 .P
2228 A special item that consists of (? followed by a number greater than zero and a
2229 closing parenthesis is a recursive call of the subpattern of the given number,
2230 provided that it occurs inside that subpattern. (If not, it is a
2231 .\" HTML <a href="#subpatternsassubroutines">
2232 .\" </a>
2233 "subroutine"
2234 .\"
2235 call, which is described in the next section.) The special item (?R) or (?0) is
2236 a recursive call of the entire regular expression.
2237 .P
2238 This PCRE pattern solves the nested parentheses problem (assume the
2239 PCRE_EXTENDED option is set so that white space is ignored):
2240 .sp
2241 \e( ( [^()]++ | (?R) )* \e)
2242 .sp
2243 First it matches an opening parenthesis. Then it matches any number of
2244 substrings which can either be a sequence of non-parentheses, or a recursive
2245 match of the pattern itself (that is, a correctly parenthesized substring).
2246 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2247 to avoid backtracking into sequences of non-parentheses.
2248 .P
2249 If this were part of a larger pattern, you would not want to recurse the entire
2250 pattern, so instead you could use this:
2251 .sp
2252 ( \e( ( [^()]++ | (?1) )* \e) )
2253 .sp
2254 We have put the pattern into parentheses, and caused the recursion to refer to
2255 them instead of the whole pattern.
2256 .P
2257 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2258 is made easier by the use of relative references. Instead of (?1) in the
2259 pattern above you can write (?-2) to refer to the second most recently opened
2260 parentheses preceding the recursion. In other words, a negative number counts
2261 capturing parentheses leftwards from the point at which it is encountered.
2262 .P
2263 It is also possible to refer to subsequently opened parentheses, by writing
2264 references such as (?+2). However, these cannot be recursive because the
2265 reference is not inside the parentheses that are referenced. They are always
2266 .\" HTML <a href="#subpatternsassubroutines">
2267 .\" </a>
2268 "subroutine"
2269 .\"
2270 calls, as described in the next section.
2271 .P
2272 An alternative approach is to use named parentheses instead. The Perl syntax
2273 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2274 could rewrite the above example as follows:
2275 .sp
2276 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2277 .sp
2278 If there is more than one subpattern with the same name, the earliest one is
2279 used.
2280 .P
2281 This particular example pattern that we have been looking at contains nested
2282 unlimited repeats, and so the use of a possessive quantifier for matching
2283 strings of non-parentheses is important when applying the pattern to strings
2284 that do not match. For example, when this pattern is applied to
2285 .sp
2286 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2287 .sp
2288 it yields "no match" quickly. However, if a possessive quantifier is not used,
2289 the match runs for a very long time indeed because there are so many different
2290 ways the + and * repeats can carve up the subject, and all have to be tested
2291 before failure can be reported.
2292 .P
2293 At the end of a match, the values of capturing parentheses are those from
2294 the outermost level. If you want to obtain intermediate values, a callout
2295 function can be used (see below and the
2296 .\" HREF
2297 \fBpcrecallout\fP
2298 .\"
2299 documentation). If the pattern above is matched against
2300 .sp
2301 (ab(cd)ef)
2302 .sp
2303 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2304 the last value taken on at the top level. If a capturing subpattern is not
2305 matched at the top level, its final value is unset, even if it is (temporarily)
2306 set at a deeper level.
2307 .P
2308 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2309 obtain extra memory to store data during a recursion, which it does by using
2310 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2311 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2312 .P
2313 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2314 Consider this pattern, which matches text in angle brackets, allowing for
2315 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2316 recursing), whereas any characters are permitted at the outer level.
2317 .sp
2318 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2319 .sp
2320 In this pattern, (?(R) is the start of a conditional subpattern, with two
2321 different alternatives for the recursive and non-recursive cases. The (?R) item
2322 is the actual recursive call.
2323 .
2324 .
2325 .\" HTML <a name="recursiondifference"></a>
2326 .SS "Recursion difference from Perl"
2327 .rs
2328 .sp
2329 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2330 treated as an atomic group. That is, once it has matched some of the subject
2331 string, it is never re-entered, even if it contains untried alternatives and
2332 there is a subsequent matching failure. This can be illustrated by the
2333 following pattern, which purports to match a palindromic string that contains
2334 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2335 .sp
2336 ^(.|(.)(?1)\e2)$
2337 .sp
2338 The idea is that it either matches a single character, or two identical
2339 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2340 it does not if the pattern is longer than three characters. Consider the
2341 subject string "abcba":
2342 .P
2343 At the top level, the first character is matched, but as it is not at the end
2344 of the string, the first alternative fails; the second alternative is taken
2345 and the recursion kicks in. The recursive call to subpattern 1 successfully
2346 matches the next character ("b"). (Note that the beginning and end of line
2347 tests are not part of the recursion).
2348 .P
2349 Back at the top level, the next character ("c") is compared with what
2350 subpattern 2 matched, which was "a". This fails. Because the recursion is
2351 treated as an atomic group, there are now no backtracking points, and so the
2352 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2353 try the second alternative.) However, if the pattern is written with the
2354 alternatives in the other order, things are different:
2355 .sp
2356 ^((.)(?1)\e2|.)$
2357 .sp
2358 This time, the recursing alternative is tried first, and continues to recurse
2359 until it runs out of characters, at which point the recursion fails. But this
2360 time we do have another alternative to try at the higher level. That is the big
2361 difference: in the previous case the remaining alternative is at a deeper
2362 recursion level, which PCRE cannot use.
2363 .P
2364 To change the pattern so that it matches all palindromic strings, not just
2365 those with an odd number of characters, it is tempting to change the pattern to
2366 this:
2367 .sp
2368 ^((.)(?1)\e2|.?)$
2369 .sp
2370 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2371 deeper recursion has matched a single character, it cannot be entered again in
2372 order to match an empty string. The solution is to separate the two cases, and
2373 write out the odd and even cases as alternatives at the higher level:
2374 .sp
2375 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2376 .sp
2377 If you want to match typical palindromic phrases, the pattern has to ignore all
2378 non-word characters, which can be done like this:
2379 .sp
2380 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2381 .sp
2382 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2383 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2384 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2385 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2386 more) to match typical phrases, and Perl takes so long that you think it has
2387 gone into a loop.
2388 .P
2389 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2390 string does not start with a palindrome that is shorter than the entire string.
2391 For example, although "abcba" is correctly matched, if the subject is "ababa",
2392 PCRE finds the palindrome "aba" at the start, then fails at top level because
2393 the end of the string does not follow. Once again, it cannot jump back into the
2394 recursion to try other alternatives, so the entire match fails.
2395 .
2396 .
2397 .\" HTML <a name="subpatternsassubroutines"></a>
2399 .rs
2400 .sp
2401 If the syntax for a recursive subpattern reference (either by number or by
2402 name) is used outside the parentheses to which it refers, it operates like a
2403 subroutine in a programming language. The "called" subpattern may be defined
2404 before or after the reference. A numbered reference can be absolute or
2405 relative, as in these examples:
2406 .sp
2407 (...(absolute)...)...(?2)...
2408 (...(relative)...)...(?-1)...
2409 (...(?+1)...(relative)...
2410 .sp
2411 An earlier example pointed out that the pattern
2412 .sp
2413 (sens|respons)e and \e1ibility
2414 .sp
2415 matches "sense and sensibility" and "response and responsibility", but not
2416 "sense and responsibility". If instead the pattern
2417 .sp
2418 (sens|respons)e and (?1)ibility
2419 .sp
2420 is used, it does match "sense and responsibility" as well as the other two
2421 strings. Another example is given in the discussion of DEFINE above.
2422 .P
2423 Like recursive subpatterns, a subroutine call is always treated as an atomic
2424 group. That is, once it has matched some of the subject string, it is never
2425 re-entered, even if it contains untried alternatives and there is a subsequent
2426 matching failure. Any capturing parentheses that are set during the subroutine
2427 call revert to their previous values afterwards.
2428 .P
2429 When a subpattern is used as a subroutine, processing options such as
2430 case-independence are fixed when the subpattern is defined. They cannot be
2431 changed for different calls. For example, consider this pattern:
2432 .sp
2433 (abc)(?i:(?-1))
2434 .sp
2435 It matches "abcabc". It does not match "abcABC" because the change of
2436 processing option does not affect the called subpattern.
2437 .
2438 .
2439 .\" HTML <a name="onigurumasubroutines"></a>
2441 .rs
2442 .sp
2443 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2444 a number enclosed either in angle brackets or single quotes, is an alternative
2445 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2446 are two of the examples used above, rewritten using this syntax:
2447 .sp
2448 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2449 (sens|respons)e and \eg'1'ibility
2450 .sp
2451 PCRE supports an extension to Oniguruma: if a number is preceded by a
2452 plus or a minus sign it is taken as a relative reference. For example:
2453 .sp
2454 (abc)(?i:\eg<-1>)
2455 .sp
2456 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2457 synonymous. The former is a back reference; the latter is a subroutine call.
2458 .
2459 .
2461 .rs
2462 .sp
2463 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2464 code to be obeyed in the middle of matching a regular expression. This makes it
2465 possible, amongst other things, to extract different substrings that match the
2466 same pair of parentheses when there is a repetition.
2467 .P
2468 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2469 code. The feature is called "callout". The caller of PCRE provides an external
2470 function by putting its entry point in the global variable \fIpcre_callout\fP.
2471 By default, this variable contains NULL, which disables all calling out.
2472 .P
2473 Within a regular expression, (?C) indicates the points at which the external
2474 function is to be called. If you want to identify different callout points, you
2475 can put a number less than 256 after the letter C. The default value is zero.
2476 For example, this pattern has two callout points:
2477 .sp
2478 (?C1)abc(?C2)def
2479 .sp
2480 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2481 automatically installed before each item in the pattern. They are all numbered
2482 255.
2483 .P
2484 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2485 set), the external function is called. It is provided with the number of the
2486 callout, the position in the pattern, and, optionally, one item of data
2487 originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2488 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2489 description of the interface to the callout function is given in the
2490 .\" HREF
2491 \fBpcrecallout\fP
2492 .\"
2493 documentation.
2494 .
2495 .
2496 .\" HTML <a name="backtrackcontrol"></a>
2498 .rs
2499 .sp
2500 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2501 are described in the Perl documentation as "experimental and subject to change
2502 or removal in a future version of Perl". It goes on to say: "Their usage in
2503 production code should be noted to avoid problems during upgrades." The same
2504 remarks apply to the PCRE features described in this section.
2505 .P
2506 Since these verbs are specifically related to backtracking, most of them can be
2507 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2508 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2509 failing negative assertion, they cause an error if encountered by
2510 \fBpcre_dfa_exec()\fP.
2511 .P
2512 If any of these verbs are used in an assertion or subroutine subpattern
2513 (including recursive subpatterns), their effect is confined to that subpattern;
2514 it does not extend to the surrounding pattern, with one exception: a *MARK that
2515 is encountered in a positive assertion \fIis\fP passed back (compare capturing
2516 parentheses in assertions). Note that such subpatterns are processed as
2517 anchored at the point where they are tested.
2518 .P
2519 The new verbs make use of what was previously invalid syntax: an opening
2520 parenthesis followed by an asterisk. They are generally of the form
2521 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2522 depending on whether or not an argument is present. An name is a sequence of
2523 letters, digits, and underscores. If the name is empty, that is, if the closing
2524 parenthesis immediately follows the colon, the effect is as if the colon were
2525 not there. Any number of these verbs may occur in a pattern.
2526 .P
2527 PCRE contains some optimizations that are used to speed up matching by running
2528 some checks at the start of each match attempt. For example, it may know the
2529 minimum length of matching subject, or that a particular character must be
2530 present. When one of these optimizations suppresses the running of a match, any
2531 included backtracking verbs will not, of course, be processed. You can suppress
2532 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2533 when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2534 pattern with (*NO_START_OPT).
2535 .
2536 .
2537 .SS "Verbs that act immediately"
2538 .rs
2539 .sp
2540 The following verbs act as soon as they are encountered. They may not be
2541 followed by a name.
2542 .sp
2543 (*ACCEPT)
2544 .sp
2545 This verb causes the match to end successfully, skipping the remainder of the
2546 pattern. When inside a recursion, only the innermost pattern is ended
2547 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2548 captured. (This feature was added to PCRE at release 8.00.) For example:
2549 .sp
2550 A((?:A|B(*ACCEPT)|C)D)
2551 .sp
2552 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2553 the outer parentheses.
2554 .sp
2555 (*FAIL) or (*F)
2556 .sp
2557 This verb causes the match to fail, forcing backtracking to occur. It is
2558 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2559 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2560 Perl features that are not present in PCRE. The nearest equivalent is the
2561 callout feature, as for example in this pattern:
2562 .sp
2563 a+(?C)(*FAIL)
2564 .sp
2565 A match with the string "aaaa" always fails, but the callout is taken before
2566 each backtrack happens (in this example, 10 times).
2567 .
2568 .
2569 .SS "Recording which path was taken"
2570 .rs
2571 .sp
2572 There is one verb whose main purpose is to track how a match was arrived at,
2573 though it also has a secondary use in conjunction with advancing the match
2574 starting point (see (*SKIP) below).
2575 .sp
2576 (*MARK:NAME) or (*:NAME)
2577 .sp
2578 A name is always required with this verb. There may be as many instances of
2579 (*MARK) as you like in a pattern, and their names do not have to be unique.
2580 .P
2581 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2582 to the caller via the \fIpcre_extra\fP data structure, as described in the
2583 .\" HTML <a href="pcreapi.html#extradata">
2584 .\" </a>
2585 section on \fIpcre_extra\fP
2586 .\"
2587 in the
2588 .\" HREF
2589 \fBpcreapi\fP
2590 .\"
2591 documentation. No data is returned for a partial match. Here is an example of
2592 \fBpcretest\fP output, where the /K modifier requests the retrieval and
2593 outputting of (*MARK) data:
2594 .sp
2595 /X(*MARK:A)Y|X(*MARK:B)Z/K
2596 XY
2597 0: XY
2598 MK: A
2599 XZ
2600 0: XZ
2601 MK: B
2602 .sp
2603 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2604 indicates which of the two alternatives matched. This is a more efficient way
2605 of obtaining this information than putting each alternative in its own
2606 capturing parentheses.
2607 .P
2608 If (*MARK) is encountered in a positive assertion, its name is recorded and
2609 passed back if it is the last-encountered. This does not happen for negative
2610 assetions.
2611 .P
2612 A name may also be returned after a failed match if the final path through the
2613 pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2614 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2615 starting point for matching is advanced, the final check is often with an empty
2616 string, causing a failure before (*MARK) is reached. For example:
2617 .sp
2618 /X(*MARK:A)Y|X(*MARK:B)Z/K
2619 XP
2620 No match
2621 .sp
2622 There are three potential starting points for this match (starting with X,
2623 starting with P, and with an empty string). If the pattern is anchored, the
2624 result is different:
2625 .sp
2626 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2627 XP
2628 No match, mark = B
2629 .sp
2630 PCRE's start-of-match optimizations can also interfere with this. For example,
2631 if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2632 subject length for a match, a shorter subject will not be scanned at all.
2633 .P
2634 Note that similar anomalies (though different in detail) exist in Perl, no
2635 doubt for the same reasons. The use of (*MARK) data after a failed match of an
2636 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2637 .
2638 .
2639 .SS "Verbs that act after backtracking"
2640 .rs
2641 .sp
2642 The following verbs do nothing when they are encountered. Matching continues
2643 with what follows, but if there is no subsequent match, causing a backtrack to
2644 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2645 the verb. However, when one of these verbs appears inside an atomic group, its
2646 effect is confined to that group, because once the group has been matched,
2647 there is never any backtracking into it. In this situation, backtracking can
2648 "jump back" to the left of the entire atomic group. (Remember also, as stated
2649 above, that this localization also applies in subroutine calls and assertions.)
2650 .P
2651 These verbs differ in exactly what kind of failure occurs when backtracking
2652 reaches them.
2653 .sp
2654 (*COMMIT)
2655 .sp
2656 This verb, which may not be followed by a name, causes the whole match to fail
2657 outright if the rest of the pattern does not match. Even if the pattern is
2658 unanchored, no further attempts to find a match by advancing the starting point
2659 take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2660 finding a match at the current starting point, or not at all. For example:
2661 .sp
2662 a+(*COMMIT)b
2663 .sp
2664 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2665 dynamic anchor, or "I've started, so I must finish." The name of the most
2666 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2667 match failure.
2668 .P
2669 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2670 unless PCRE's start-of-match optimizations are turned off, as shown in this
2671 \fBpcretest\fP example:
2672 .sp
2673 /(*COMMIT)abc/
2674 xyzabc
2675 0: abc
2676 xyzabc\eY
2677 No match
2678 .sp
2679 PCRE knows that any match must start with "a", so the optimization skips along
2680 the subject to "a" before running the first match attempt, which succeeds. When
2681 the optimization is disabled by the \eY escape in the second subject, the match
2682 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2683 starting points.
2684 .sp
2685 (*PRUNE) or (*PRUNE:NAME)
2686 .sp
2687 This verb causes the match to fail at the current starting position in the
2688 subject if the rest of the pattern does not match. If the pattern is
2689 unanchored, the normal "bumpalong" advance to the next starting character then
2690 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2691 reached, or when matching to the right of (*PRUNE), but if there is no match to
2692 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2693 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2694 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2695 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2696 match fails completely; the name is passed back if this is the final attempt.
2697 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2698 pattern (*PRUNE) has the same effect as (*COMMIT).
2699 .sp
2700 (*SKIP)
2701 .sp
2702 This verb, when given without a name, is like (*PRUNE), except that if the
2703 pattern is unanchored, the "bumpalong" advance is not to the next character,
2704 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2705 signifies that whatever text was matched leading up to it cannot be part of a
2706 successful match. Consider:
2707 .sp
2708 a+(*SKIP)b
2709 .sp
2710 If the subject is "aaaac...", after the first match attempt fails (starting at
2711 the first character in the string), the starting point skips on to start the
2712 next attempt at "c". Note that a possessive quantifer does not have the same
2713 effect as this example; although it would suppress backtracking during the
2714 first match attempt, the second attempt would start at the second character
2715 instead of skipping on to "c".
2716 .sp
2717 (*SKIP:NAME)
2718 .sp
2719 When (*SKIP) has an associated name, its behaviour is modified. If the
2720 following pattern fails to match, the previous path through the pattern is
2721 searched for the most recent (*MARK) that has the same name. If one is found,
2722 the "bumpalong" advance is to the subject position that corresponds to that
2723 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2724 matching name is found, normal "bumpalong" of one character happens (the
2725 (*SKIP) is ignored).
2726 .sp
2727 (*THEN) or (*THEN:NAME)
2728 .sp
2729 This verb causes a skip to the next alternation in the innermost enclosing
2730 group if the rest of the pattern does not match. That is, it cancels pending
2731 backtracking, but only within the current alternation. Its name comes from the
2732 observation that it can be used for a pattern-based if-then-else block:
2733 .sp
2734 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2735 .sp
2736 If the COND1 pattern matches, FOO is tried (and possibly further items after
2737 the end of the group if FOO succeeds); on failure the matcher skips to the
2738 second alternative and tries COND2, without backtracking into COND1. The
2739 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2740 overall match fails. If (*THEN) is not directly inside an alternation, it acts
2741 like (*PRUNE).
2742 .
2743 .P
2744 The above verbs provide four different "strengths" of control when subsequent
2745 matching fails. (*THEN) is the weakest, carrying on the match at the next
2746 alternation. (*PRUNE) comes next, failing the match at the current starting
2747 position, but allowing an advance to the next character (for an unanchored
2748 pattern). (*SKIP) is similar, except that the advance may be more than one
2749 character. (*COMMIT) is the strongest, causing the entire match to fail.
2750 .P
2751 If more than one is present in a pattern, the "stongest" one wins. For example,
2752 consider this pattern, where A, B, etc. are complex pattern fragments:
2753 .sp
2754 (A(*COMMIT)B(*THEN)C|D)
2755 .sp
2756 Once A has matched, PCRE is committed to this match, at the current starting
2757 position. If subsequently B matches, but C does not, the normal (*THEN) action
2758 of trying the next alternation (that is, D) does not happen because (*COMMIT)
2759 overrides.
2760 .
2761 .
2762 .SH "SEE ALSO"
2763 .rs
2764 .sp
2765 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2766 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2767 .
2768 .
2770 .rs
2771 .sp
2772 .nf
2773 Philip Hazel
2774 University Computing Service
2775 Cambridge CB2 3QH, England.
2776 .fi
2777 .
2778 .
2780 .rs
2781 .sp
2782 .nf
2783 Last updated: 24 July 2011
2784 Copyright (c) 1997-2011 University of Cambridge.
2785 .fi


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