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1 nigel 79 .TH PCREPATTERN 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 nigel 63 .rs
6     .sp
7 ph10 208 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 ph10 333 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 ph10 208 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 nigel 75 .P
23     The original operation of PCRE was on strings of one-byte characters. However,
24 ph10 461 there is now also support for UTF-8 character strings. To use this,
25 ph10 456 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 ph10 412 .sp
29     (*UTF8)
30 ph10 416 .sp
31 ph10 412 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 nigel 63 .\" HTML <a href="pcre.html#utf8support">
36     .\" </a>
37     section on UTF-8 support
38     .\"
39     in the main
40     .\" HREF
41 nigel 75 \fBpcre\fP
42 nigel 63 .\"
43     page.
44 nigel 75 .P
45 nigel 77 The remainder of this document discusses the patterns that are supported by
46     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47     From release 6.0, PCRE offers a second matching function,
48     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49 ph10 172 Perl-compatible. Some of the features discussed below are not available when
50 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51     alternative function, and how it differs from the normal function, are
52     discussed in the
53 nigel 77 .\" HREF
54     \fBpcrematching\fP
55     .\"
56     page.
57 nigel 93 .
58     .
59 ph10 227 .SH "NEWLINE CONVENTIONS"
60     .rs
61     .sp
62     PCRE supports five different conventions for indicating line breaks in
63     strings: a single CR (carriage return) character, a single LF (linefeed)
64     character, the two-character sequence CRLF, any of the three preceding, or any
65     Unicode newline sequence. The
66     .\" HREF
67     \fBpcreapi\fP
68     .\"
69     page has
70     .\" HTML <a href="pcreapi.html#newlines">
71     .\" </a>
72     further discussion
73     .\"
74     about newlines, and shows how to set the newline convention in the
75     \fIoptions\fP arguments for the compiling and matching functions.
76     .P
77     It is also possible to specify a newline convention by starting a pattern
78     string with one of the following five sequences:
79     .sp
80     (*CR) carriage return
81     (*LF) linefeed
82     (*CRLF) carriage return, followed by linefeed
83     (*ANYCRLF) any of the three above
84     (*ANY) all Unicode newline sequences
85     .sp
86 ph10 461 These override the default and the options given to \fBpcre_compile()\fP or
87 ph10 456 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88     newline sequence, the pattern
89 ph10 227 .sp
90     (*CR)a.b
91     .sp
92     changes the convention to CR. That pattern matches "a\enb" because LF is no
93     longer a newline. Note that these special settings, which are not
94     Perl-compatible, are recognized only at the very start of a pattern, and that
95 ph10 231 they must be in upper case. If more than one of them is present, the last one
96     is used.
97     .P
98     The newline convention does not affect what the \eR escape sequence matches. By
99     default, this is any Unicode newline sequence, for Perl compatibility. However,
100     this can be changed; see the description of \eR in the section entitled
101     .\" HTML <a href="#newlineseq">
102     .\" </a>
103     "Newline sequences"
104     .\"
105 ph10 247 below. A change of \eR setting can be combined with a change of newline
106 ph10 246 convention.
107 ph10 227 .
108     .
109 nigel 93 .SH "CHARACTERS AND METACHARACTERS"
110     .rs
111     .sp
112 nigel 63 A regular expression is a pattern that is matched against a subject string from
113     left to right. Most characters stand for themselves in a pattern, and match the
114     corresponding characters in the subject. As a trivial example, the pattern
115 nigel 75 .sp
116 nigel 63 The quick brown fox
117 nigel 75 .sp
118 nigel 77 matches a portion of a subject string that is identical to itself. When
119     caseless matching is specified (the PCRE_CASELESS option), letters are matched
120     independently of case. In UTF-8 mode, PCRE always understands the concept of
121     case for characters whose values are less than 128, so caseless matching is
122     always possible. For characters with higher values, the concept of case is
123     supported if PCRE is compiled with Unicode property support, but not otherwise.
124     If you want to use caseless matching for characters 128 and above, you must
125     ensure that PCRE is compiled with Unicode property support as well as with
126     UTF-8 support.
127     .P
128     The power of regular expressions comes from the ability to include alternatives
129     and repetitions in the pattern. These are encoded in the pattern by the use of
130 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
131 nigel 63 interpreted in some special way.
132 nigel 75 .P
133     There are two different sets of metacharacters: those that are recognized
134 nigel 63 anywhere in the pattern except within square brackets, and those that are
135 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
136     are as follows:
137 nigel 75 .sp
138     \e general escape character with several uses
139 nigel 63 ^ assert start of string (or line, in multiline mode)
140     $ assert end of string (or line, in multiline mode)
141     . match any character except newline (by default)
142     [ start character class definition
143     | start of alternative branch
144     ( start subpattern
145     ) end subpattern
146     ? extends the meaning of (
147     also 0 or 1 quantifier
148     also quantifier minimizer
149     * 0 or more quantifier
150     + 1 or more quantifier
151     also "possessive quantifier"
152     { start min/max quantifier
153 nigel 75 .sp
154 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
155 nigel 75 a character class the only metacharacters are:
156     .sp
157     \e general escape character
158 nigel 63 ^ negate the class, but only if the first character
159     - indicates character range
160 nigel 75 .\" JOIN
161 nigel 63 [ POSIX character class (only if followed by POSIX
162     syntax)
163     ] terminates the character class
164 nigel 75 .sp
165     The following sections describe the use of each of the metacharacters.
166     .
167 nigel 93 .
168 nigel 63 .SH BACKSLASH
169     .rs
170     .sp
171     The backslash character has several uses. Firstly, if it is followed by a
172 nigel 91 non-alphanumeric character, it takes away any special meaning that character
173     may have. This use of backslash as an escape character applies both inside and
174 nigel 63 outside character classes.
175 nigel 75 .P
176     For example, if you want to match a * character, you write \e* in the pattern.
177 nigel 63 This escaping action applies whether or not the following character would
178 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
179     non-alphanumeric with backslash to specify that it stands for itself. In
180     particular, if you want to match a backslash, you write \e\e.
181     .P
182 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
183     pattern (other than in a character class) and characters between a # outside
184 nigel 91 a character class and the next newline are ignored. An escaping backslash can
185     be used to include a whitespace or # character as part of the pattern.
186 nigel 75 .P
187 nigel 63 If you want to remove the special meaning from a sequence of characters, you
188 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
189     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
190 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
191 nigel 75 .sp
192 nigel 63 Pattern PCRE matches Perl matches
193 nigel 75 .sp
194     .\" JOIN
195     \eQabc$xyz\eE abc$xyz abc followed by the
196 nigel 63 contents of $xyz
197 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
198     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
199     .sp
200     The \eQ...\eE sequence is recognized both inside and outside character classes.
201     .
202     .
203     .\" HTML <a name="digitsafterbackslash"></a>
204     .SS "Non-printing characters"
205     .rs
206     .sp
207 nigel 63 A second use of backslash provides a way of encoding non-printing characters
208     in patterns in a visible manner. There is no restriction on the appearance of
209     non-printing characters, apart from the binary zero that terminates a pattern,
210 ph10 456 but when a pattern is being prepared by text editing, it is often easier to use
211     one of the following escape sequences than the binary character it represents:
212 nigel 75 .sp
213     \ea alarm, that is, the BEL character (hex 07)
214     \ecx "control-x", where x is any character
215     \ee escape (hex 1B)
216     \ef formfeed (hex 0C)
217 ph10 227 \en linefeed (hex 0A)
218 nigel 75 \er carriage return (hex 0D)
219     \et tab (hex 09)
220 ph10 488 \eddd character with octal code ddd, or back reference
221 nigel 75 \exhh character with hex code hh
222 nigel 87 \ex{hhh..} character with hex code hhh..
223 nigel 75 .sp
224     The precise effect of \ecx is as follows: if x is a lower case letter, it
225 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
226 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
227 nigel 63 7B.
228 nigel 75 .P
229     After \ex, from zero to two hexadecimal digits are read (letters can be in
230 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
231     and }, but the value of the character code must be less than 256 in non-UTF-8
232 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234     point, which is 10FFFF.
235 nigel 75 .P
236 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if
237     there is no terminating }, this form of escape is not recognized. Instead, the
238     initial \ex will be interpreted as a basic hexadecimal escape, with no
239     following digits, giving a character whose value is zero.
240     .P
241 nigel 63 Characters whose value is less than 256 can be defined by either of the two
242 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
243     example, \exdc is exactly the same as \ex{dc}.
244 nigel 75 .P
245 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
246     digits, just those that are present are used. Thus the sequence \e0\ex\e07
247     specifies two binary zeros followed by a BEL character (code value 7). Make
248     sure you supply two digits after the initial zero if the pattern character that
249     follows is itself an octal digit.
250 nigel 75 .P
251 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
252     Outside a character class, PCRE reads it and any following digits as a decimal
253     number. If the number is less than 10, or if there have been at least that many
254     previous capturing left parentheses in the expression, the entire sequence is
255 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
256     .\" HTML <a href="#backreferences">
257     .\" </a>
258     later,
259     .\"
260     following the discussion of
261     .\" HTML <a href="#subpattern">
262     .\" </a>
263     parenthesized subpatterns.
264     .\"
265     .P
266 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
267     have not been that many capturing subpatterns, PCRE re-reads up to three octal
268 nigel 93 digits following the backslash, and uses them to generate a data character. Any
269 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
270     character specified in octal must be less than \e400. In UTF-8 mode, values up
271     to \e777 are permitted. For example:
272 nigel 75 .sp
273     \e040 is another way of writing a space
274     .\" JOIN
275     \e40 is the same, provided there are fewer than 40
276 nigel 63 previous capturing subpatterns
277 nigel 75 \e7 is always a back reference
278     .\" JOIN
279     \e11 might be a back reference, or another way of
280 nigel 63 writing a tab
281 nigel 75 \e011 is always a tab
282     \e0113 is a tab followed by the character "3"
283     .\" JOIN
284     \e113 might be a back reference, otherwise the
285 nigel 63 character with octal code 113
286 nigel 75 .\" JOIN
287     \e377 might be a back reference, otherwise
288 nigel 63 the byte consisting entirely of 1 bits
289 nigel 75 .\" JOIN
290     \e81 is either a back reference, or a binary zero
291 nigel 63 followed by the two characters "8" and "1"
292 nigel 75 .sp
293 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
294     zero, because no more than three octal digits are ever read.
295 nigel 75 .P
296 nigel 91 All the sequences that define a single character value can be used both inside
297     and outside character classes. In addition, inside a character class, the
298     sequence \eb is interpreted as the backspace character (hex 08), and the
299 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X",
300     respectively. Outside a character class, these sequences have different
301     meanings
302 nigel 75 .\" HTML <a href="#uniextseq">
303     .\" </a>
304     (see below).
305     .\"
306     .
307     .
308 nigel 93 .SS "Absolute and relative back references"
309     .rs
310     .sp
311 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
312     enclosed in braces, is an absolute or relative back reference. A named back
313     reference can be coded as \eg{name}. Back references are discussed
314 nigel 93 .\" HTML <a href="#backreferences">
315     .\" </a>
316     later,
317     .\"
318     following the discussion of
319     .\" HTML <a href="#subpattern">
320     .\" </a>
321     parenthesized subpatterns.
322     .\"
323     .
324     .
325 ph10 333 .SS "Absolute and relative subroutine calls"
326     .rs
327     .sp
328 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329     a number enclosed either in angle brackets or single quotes, is an alternative
330     syntax for referencing a subpattern as a "subroutine". Details are discussed
331 ph10 333 .\" HTML <a href="#onigurumasubroutines">
332     .\" </a>
333     later.
334     .\"
335 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336 ph10 461 synonymous. The former is a back reference; the latter is a
337 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
338     .\" </a>
339     subroutine
340     .\"
341     call.
342 ph10 333 .
343     .
344 nigel 75 .SS "Generic character types"
345     .rs
346     .sp
347 nigel 93 Another use of backslash is for specifying generic character types. The
348 nigel 75 following are always recognized:
349     .sp
350 ph10 182 \ed any decimal digit
351 nigel 75 \eD any character that is not a decimal digit
352 ph10 178 \eh any horizontal whitespace character
353 ph10 182 \eH any character that is not a horizontal whitespace character
354 nigel 75 \es any whitespace character
355     \eS any character that is not a whitespace character
356 ph10 178 \ev any vertical whitespace character
357 ph10 182 \eV any character that is not a vertical whitespace character
358 nigel 75 \ew any "word" character
359     \eW any "non-word" character
360     .sp
361 nigel 63 Each pair of escape sequences partitions the complete set of characters into
362     two disjoint sets. Any given character matches one, and only one, of each pair.
363 nigel 75 .P
364     These character type sequences can appear both inside and outside character
365     classes. They each match one character of the appropriate type. If the current
366     matching point is at the end of the subject string, all of them fail, since
367     there is no character to match.
368     .P
369     For compatibility with Perl, \es does not match the VT character (code 11).
370     This makes it different from the the POSIX "space" class. The \es characters
371 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
373 ph10 178 does.
374 nigel 75 .P
375 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377 ph10 182 character property support is available. These sequences retain their original
378     meanings from before UTF-8 support was available, mainly for efficiency
379 ph10 406 reasons. Note that this also affects \eb, because it is defined in terms of \ew
380 ph10 394 and \eW.
381 ph10 178 .P
382 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384     The horizontal space characters are:
385     .sp
386     U+0009 Horizontal tab
387     U+0020 Space
388     U+00A0 Non-break space
389     U+1680 Ogham space mark
390     U+180E Mongolian vowel separator
391     U+2000 En quad
392     U+2001 Em quad
393     U+2002 En space
394     U+2003 Em space
395     U+2004 Three-per-em space
396     U+2005 Four-per-em space
397     U+2006 Six-per-em space
398     U+2007 Figure space
399     U+2008 Punctuation space
400     U+2009 Thin space
401     U+200A Hair space
402     U+202F Narrow no-break space
403     U+205F Medium mathematical space
404     U+3000 Ideographic space
405     .sp
406     The vertical space characters are:
407     .sp
408     U+000A Linefeed
409     U+000B Vertical tab
410     U+000C Formfeed
411     U+000D Carriage return
412     U+0085 Next line
413     U+2028 Line separator
414     U+2029 Paragraph separator
415     .P
416 nigel 75 A "word" character is an underscore or any character less than 256 that is a
417     letter or digit. The definition of letters and digits is controlled by PCRE's
418     low-valued character tables, and may vary if locale-specific matching is taking
419     place (see
420 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
421     .\" </a>
422     "Locale support"
423     .\"
424     in the
425     .\" HREF
426 nigel 75 \fBpcreapi\fP
427 nigel 63 .\"
428 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
429     or "french" in Windows, some character codes greater than 128 are used for
430 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
431     is discouraged.
432 nigel 75 .
433     .
434 ph10 231 .\" HTML <a name="newlineseq"></a>
435 nigel 93 .SS "Newline sequences"
436     .rs
437     .sp
438 ph10 231 Outside a character class, by default, the escape sequence \eR matches any
439     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440     equivalent to the following:
441 nigel 93 .sp
442     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443     .sp
444     This is an example of an "atomic group", details of which are given
445     .\" HTML <a href="#atomicgroup">
446     .\" </a>
447     below.
448     .\"
449     This particular group matches either the two-character sequence CR followed by
450     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
451     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
452     line, U+0085). The two-character sequence is treated as a single unit that
453     cannot be split.
454     .P
455     In UTF-8 mode, two additional characters whose codepoints are greater than 255
456     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
457     Unicode character property support is not needed for these characters to be
458     recognized.
459     .P
460 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
463 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is
464     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465     It is also possible to specify these settings by starting a pattern string with
466     one of the following sequences:
467 ph10 231 .sp
468     (*BSR_ANYCRLF) CR, LF, or CRLF only
469     (*BSR_UNICODE) any Unicode newline sequence
470     .sp
471 ph10 461 These override the default and the options given to \fBpcre_compile()\fP or
472 ph10 456 \fBpcre_compile2()\fP, but they can be overridden by options given to
473     \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474     which are not Perl-compatible, are recognized only at the very start of a
475     pattern, and that they must be in upper case. If more than one of them is
476     present, the last one is used. They can be combined with a change of newline
477     convention, for example, a pattern can start with:
478 ph10 246 .sp
479     (*ANY)(*BSR_ANYCRLF)
480     .sp
481 nigel 93 Inside a character class, \eR matches the letter "R".
482     .
483     .
484 nigel 75 .\" HTML <a name="uniextseq"></a>
485     .SS Unicode character properties
486     .rs
487     .sp
488     When PCRE is built with Unicode character property support, three additional
489 ph10 184 escape sequences that match characters with specific properties are available.
490     When not in UTF-8 mode, these sequences are of course limited to testing
491     characters whose codepoints are less than 256, but they do work in this mode.
492     The extra escape sequences are:
493 nigel 75 .sp
494 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
495     \eP{\fIxx\fP} a character without the \fIxx\fP property
496     \eX an extended Unicode sequence
497 nigel 75 .sp
498 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
499     script names, the general category properties, and "Any", which matches any
500     character (including newline). Other properties such as "InMusicalSymbols" are
501     not currently supported by PCRE. Note that \eP{Any} does not match any
502     characters, so always causes a match failure.
503 nigel 75 .P
504 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
505     character from one of these sets can be matched using a script name. For
506     example:
507 nigel 75 .sp
508 nigel 87 \ep{Greek}
509     \eP{Han}
510     .sp
511     Those that are not part of an identified script are lumped together as
512     "Common". The current list of scripts is:
513     .P
514     Arabic,
515     Armenian,
516 ph10 491 Avestan,
517 nigel 93 Balinese,
518 ph10 491 Bamum,
519 nigel 87 Bengali,
520     Bopomofo,
521     Braille,
522     Buginese,
523     Buhid,
524     Canadian_Aboriginal,
525 ph10 491 Carian,
526     Cham,
527 nigel 87 Cherokee,
528     Common,
529     Coptic,
530 nigel 93 Cuneiform,
531 nigel 87 Cypriot,
532     Cyrillic,
533     Deseret,
534     Devanagari,
535 ph10 491 Egyptian_Hieroglyphs,
536 nigel 87 Ethiopic,
537     Georgian,
538     Glagolitic,
539     Gothic,
540     Greek,
541     Gujarati,
542     Gurmukhi,
543     Han,
544     Hangul,
545     Hanunoo,
546     Hebrew,
547     Hiragana,
548 ph10 491 Imperial_Aramaic,
549 nigel 87 Inherited,
550 ph10 491 Inscriptional_Pahlavi,
551     Inscriptional_Parthian,
552     Javanese,
553     Kaithi,
554 nigel 87 Kannada,
555     Katakana,
556 ph10 491 Kayah_Li,
557 nigel 87 Kharoshthi,
558     Khmer,
559     Lao,
560     Latin,
561 ph10 491 Lepcha,
562 nigel 87 Limbu,
563     Linear_B,
564 ph10 491 Lisu,
565     Lycian,
566     Lydian,
567 nigel 87 Malayalam,
568 ph10 491 Meetei_Mayek,
569 nigel 87 Mongolian,
570     Myanmar,
571     New_Tai_Lue,
572 nigel 93 Nko,
573 nigel 87 Ogham,
574     Old_Italic,
575     Old_Persian,
576 ph10 491 Old_South_Arabian,
577     Old_Turkic,
578     Ol_Chiki,
579 nigel 87 Oriya,
580     Osmanya,
581 nigel 93 Phags_Pa,
582     Phoenician,
583 ph10 491 Rejang,
584 nigel 87 Runic,
585 ph10 491 Samaritan,
586     Saurashtra,
587 nigel 87 Shavian,
588     Sinhala,
589 ph10 491 Sundanese,
590 nigel 87 Syloti_Nagri,
591     Syriac,
592     Tagalog,
593     Tagbanwa,
594     Tai_Le,
595 ph10 491 Tai_Tham,
596     Tai_Viet,
597 nigel 87 Tamil,
598     Telugu,
599     Thaana,
600     Thai,
601     Tibetan,
602     Tifinagh,
603     Ugaritic,
604 ph10 491 Vai,
605 nigel 87 Yi.
606     .P
607     Each character has exactly one general category property, specified by a
608     two-letter abbreviation. For compatibility with Perl, negation can be specified
609     by including a circumflex between the opening brace and the property name. For
610     example, \ep{^Lu} is the same as \eP{Lu}.
611     .P
612     If only one letter is specified with \ep or \eP, it includes all the general
613     category properties that start with that letter. In this case, in the absence
614     of negation, the curly brackets in the escape sequence are optional; these two
615     examples have the same effect:
616     .sp
617 nigel 75 \ep{L}
618     \epL
619     .sp
620 nigel 87 The following general category property codes are supported:
621 nigel 75 .sp
622     C Other
623     Cc Control
624     Cf Format
625     Cn Unassigned
626     Co Private use
627     Cs Surrogate
628     .sp
629     L Letter
630     Ll Lower case letter
631     Lm Modifier letter
632     Lo Other letter
633     Lt Title case letter
634     Lu Upper case letter
635     .sp
636     M Mark
637     Mc Spacing mark
638     Me Enclosing mark
639     Mn Non-spacing mark
640     .sp
641     N Number
642     Nd Decimal number
643     Nl Letter number
644     No Other number
645     .sp
646     P Punctuation
647     Pc Connector punctuation
648     Pd Dash punctuation
649     Pe Close punctuation
650     Pf Final punctuation
651     Pi Initial punctuation
652     Po Other punctuation
653     Ps Open punctuation
654     .sp
655     S Symbol
656     Sc Currency symbol
657     Sk Modifier symbol
658     Sm Mathematical symbol
659     So Other symbol
660     .sp
661     Z Separator
662     Zl Line separator
663     Zp Paragraph separator
664     Zs Space separator
665     .sp
666 nigel 87 The special property L& is also supported: it matches a character that has
667     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
668     a modifier or "other".
669 nigel 75 .P
670 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
671     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
672     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
673     (see the discussion of PCRE_NO_UTF8_CHECK in the
674     .\" HREF
675     \fBpcreapi\fP
676     .\"
677 ph10 451 page). Perl does not support the Cs property.
678 ph10 211 .P
679 ph10 451 The long synonyms for property names that Perl supports (such as \ep{Letter})
680 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
681 nigel 87 properties with "Is".
682     .P
683     No character that is in the Unicode table has the Cn (unassigned) property.
684     Instead, this property is assumed for any code point that is not in the
685     Unicode table.
686     .P
687 nigel 75 Specifying caseless matching does not affect these escape sequences. For
688     example, \ep{Lu} always matches only upper case letters.
689     .P
690     The \eX escape matches any number of Unicode characters that form an extended
691     Unicode sequence. \eX is equivalent to
692     .sp
693     (?>\ePM\epM*)
694     .sp
695     That is, it matches a character without the "mark" property, followed by zero
696     or more characters with the "mark" property, and treats the sequence as an
697     atomic group
698     .\" HTML <a href="#atomicgroup">
699     .\" </a>
700     (see below).
701     .\"
702     Characters with the "mark" property are typically accents that affect the
703 ph10 185 preceding character. None of them have codepoints less than 256, so in
704 ph10 184 non-UTF-8 mode \eX matches any one character.
705 nigel 75 .P
706     Matching characters by Unicode property is not fast, because PCRE has to search
707     a structure that contains data for over fifteen thousand characters. That is
708     why the traditional escape sequences such as \ed and \ew do not use Unicode
709     properties in PCRE.
710     .
711     .
712 ph10 168 .\" HTML <a name="resetmatchstart"></a>
713     .SS "Resetting the match start"
714     .rs
715     .sp
716 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
717     matched characters not to be included in the final matched sequence. For
718 ph10 168 example, the pattern:
719     .sp
720     foo\eKbar
721     .sp
722 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
723 ph10 168 similar to a lookbehind assertion
724     .\" HTML <a href="#lookbehind">
725     .\" </a>
726     (described below).
727     .\"
728 ph10 172 However, in this case, the part of the subject before the real match does not
729     have to be of fixed length, as lookbehind assertions do. The use of \eK does
730 ph10 168 not interfere with the setting of
731     .\" HTML <a href="#subpattern">
732     .\" </a>
733     captured substrings.
734 ph10 172 .\"
735 ph10 168 For example, when the pattern
736     .sp
737     (foo)\eKbar
738     .sp
739 ph10 172 matches "foobar", the first substring is still set to "foo".
740 ph10 500 .P
741 ph10 507 Perl documents that the use of \eK within assertions is "not well defined". In
742     PCRE, \eK is acted upon when it occurs inside positive assertions, but is
743 ph10 500 ignored in negative assertions.
744 ph10 168 .
745     .
746 nigel 75 .\" HTML <a name="smallassertions"></a>
747     .SS "Simple assertions"
748     .rs
749     .sp
750 nigel 93 The final use of backslash is for certain simple assertions. An assertion
751 nigel 63 specifies a condition that has to be met at a particular point in a match,
752     without consuming any characters from the subject string. The use of
753 nigel 75 subpatterns for more complicated assertions is described
754     .\" HTML <a href="#bigassertions">
755     .\" </a>
756     below.
757     .\"
758 nigel 91 The backslashed assertions are:
759 nigel 75 .sp
760     \eb matches at a word boundary
761     \eB matches when not at a word boundary
762 nigel 93 \eA matches at the start of the subject
763     \eZ matches at the end of the subject
764     also matches before a newline at the end of the subject
765     \ez matches only at the end of the subject
766     \eG matches at the first matching position in the subject
767 nigel 75 .sp
768     These assertions may not appear in character classes (but note that \eb has a
769 nigel 63 different meaning, namely the backspace character, inside a character class).
770 nigel 75 .P
771 nigel 63 A word boundary is a position in the subject string where the current character
772 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
773     \ew and the other matches \eW), or the start or end of the string if the
774 ph10 461 first or last character matches \ew, respectively. Neither PCRE nor Perl has a
775     separte "start of word" or "end of word" metasequence. However, whatever
776     follows \eb normally determines which it is. For example, the fragment
777 ph10 456 \eba matches "a" at the start of a word.
778 nigel 75 .P
779     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
780     dollar (described in the next section) in that they only ever match at the very
781     start and end of the subject string, whatever options are set. Thus, they are
782     independent of multiline mode. These three assertions are not affected by the
783     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
784     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
785     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
786     at a point other than the beginning of the subject, \eA can never match. The
787 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
788     of the string as well as at the very end, whereas \ez matches only at the end.
789 nigel 75 .P
790     The \eG assertion is true only when the current matching position is at the
791     start point of the match, as specified by the \fIstartoffset\fP argument of
792     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
793     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
794 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
795 nigel 75 implementation where \eG can be useful.
796     .P
797     Note, however, that PCRE's interpretation of \eG, as the start of the current
798 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
799     previous match. In Perl, these can be different when the previously matched
800     string was empty. Because PCRE does just one match at a time, it cannot
801     reproduce this behaviour.
802 nigel 75 .P
803     If all the alternatives of a pattern begin with \eG, the expression is anchored
804 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
805     regular expression.
806 nigel 75 .
807     .
808     .SH "CIRCUMFLEX AND DOLLAR"
809 nigel 63 .rs
810     .sp
811     Outside a character class, in the default matching mode, the circumflex
812 nigel 75 character is an assertion that is true only if the current matching point is
813     at the start of the subject string. If the \fIstartoffset\fP argument of
814     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
815 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
816 nigel 75 meaning
817     .\" HTML <a href="#characterclass">
818     .\" </a>
819     (see below).
820     .\"
821     .P
822 nigel 63 Circumflex need not be the first character of the pattern if a number of
823     alternatives are involved, but it should be the first thing in each alternative
824     in which it appears if the pattern is ever to match that branch. If all
825     possible alternatives start with a circumflex, that is, if the pattern is
826     constrained to match only at the start of the subject, it is said to be an
827     "anchored" pattern. (There are also other constructs that can cause a pattern
828     to be anchored.)
829 nigel 75 .P
830     A dollar character is an assertion that is true only if the current matching
831 nigel 63 point is at the end of the subject string, or immediately before a newline
832 nigel 91 at the end of the string (by default). Dollar need not be the last character of
833     the pattern if a number of alternatives are involved, but it should be the last
834     item in any branch in which it appears. Dollar has no special meaning in a
835     character class.
836 nigel 75 .P
837 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
838     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
839 nigel 75 does not affect the \eZ assertion.
840     .P
841 nigel 63 The meanings of the circumflex and dollar characters are changed if the
842 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
843     immediately after internal newlines as well as at the start of the subject
844     string. It does not match after a newline that ends the string. A dollar
845     matches before any newlines in the string, as well as at the very end, when
846     PCRE_MULTILINE is set. When newline is specified as the two-character
847     sequence CRLF, isolated CR and LF characters do not indicate newlines.
848 nigel 75 .P
849 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
850     \en represents a newline) in multiline mode, but not otherwise. Consequently,
851     patterns that are anchored in single line mode because all branches start with
852     ^ are not anchored in multiline mode, and a match for circumflex is possible
853     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
854     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
855     .P
856 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
857 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
858 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
859 nigel 75 .
860     .
861     .SH "FULL STOP (PERIOD, DOT)"
862 nigel 63 .rs
863     .sp
864     Outside a character class, a dot in the pattern matches any one character in
865 nigel 91 the subject string except (by default) a character that signifies the end of a
866 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
867 nigel 91 .P
868 nigel 93 When a line ending is defined as a single character, dot never matches that
869     character; when the two-character sequence CRLF is used, dot does not match CR
870     if it is immediately followed by LF, but otherwise it matches all characters
871     (including isolated CRs and LFs). When any Unicode line endings are being
872     recognized, dot does not match CR or LF or any of the other line ending
873     characters.
874     .P
875 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
876 nigel 93 option is set, a dot matches any one character, without exception. If the
877     two-character sequence CRLF is present in the subject string, it takes two dots
878     to match it.
879 nigel 91 .P
880     The handling of dot is entirely independent of the handling of circumflex and
881     dollar, the only relationship being that they both involve newlines. Dot has no
882     special meaning in a character class.
883 nigel 75 .
884     .
885     .SH "MATCHING A SINGLE BYTE"
886 nigel 63 .rs
887     .sp
888 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
889 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
890     characters. The feature is provided in Perl in order to match individual bytes
891     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
892     what remains in the string may be a malformed UTF-8 string. For this reason,
893     the \eC escape sequence is best avoided.
894 nigel 75 .P
895     PCRE does not allow \eC to appear in lookbehind assertions
896     .\" HTML <a href="#lookbehind">
897     .\" </a>
898     (described below),
899     .\"
900     because in UTF-8 mode this would make it impossible to calculate the length of
901     the lookbehind.
902     .
903     .
904     .\" HTML <a name="characterclass"></a>
905     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
906 nigel 63 .rs
907     .sp
908     An opening square bracket introduces a character class, terminated by a closing
909 ph10 461 square bracket. A closing square bracket on its own is not special by default.
910     However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
911 ph10 456 bracket causes a compile-time error. If a closing square bracket is required as
912     a member of the class, it should be the first data character in the class
913     (after an initial circumflex, if present) or escaped with a backslash.
914 nigel 75 .P
915 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
916 ph10 456 character may be more than one byte long. A matched character must be in the
917     set of characters defined by the class, unless the first character in the class
918 nigel 63 definition is a circumflex, in which case the subject character must not be in
919     the set defined by the class. If a circumflex is actually required as a member
920     of the class, ensure it is not the first character, or escape it with a
921     backslash.
922 nigel 75 .P
923 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
924     [^aeiou] matches any character that is not a lower case vowel. Note that a
925 nigel 75 circumflex is just a convenient notation for specifying the characters that
926     are in the class by enumerating those that are not. A class that starts with a
927 ph10 456 circumflex is not an assertion; it still consumes a character from the subject
928 nigel 75 string, and therefore it fails if the current pointer is at the end of the
929     string.
930     .P
931 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
932 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
933     .P
934 nigel 63 When caseless matching is set, any letters in a class represent both their
935     upper case and lower case versions, so for example, a caseless [aeiou] matches
936     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
937 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
938     case for characters whose values are less than 128, so caseless matching is
939     always possible. For characters with higher values, the concept of case is
940     supported if PCRE is compiled with Unicode property support, but not otherwise.
941 ph10 456 If you want to use caseless matching in UTF8-mode for characters 128 and above,
942     you must ensure that PCRE is compiled with Unicode property support as well as
943     with UTF-8 support.
944 nigel 75 .P
945 nigel 93 Characters that might indicate line breaks are never treated in any special way
946     when matching character classes, whatever line-ending sequence is in use, and
947     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
948     such as [^a] always matches one of these characters.
949 nigel 75 .P
950 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
951     character class. For example, [d-m] matches any letter between d and m,
952     inclusive. If a minus character is required in a class, it must be escaped with
953     a backslash or appear in a position where it cannot be interpreted as
954     indicating a range, typically as the first or last character in the class.
955 nigel 75 .P
956 nigel 63 It is not possible to have the literal character "]" as the end character of a
957     range. A pattern such as [W-]46] is interpreted as a class of two characters
958     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
959     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
960 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
961     followed by two other characters. The octal or hexadecimal representation of
962     "]" can also be used to end a range.
963     .P
964 nigel 63 Ranges operate in the collating sequence of character values. They can also be
965 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
966 nigel 63 mode, ranges can include characters whose values are greater than 255, for
967 nigel 75 example [\ex{100}-\ex{2ff}].
968     .P
969 nigel 63 If a range that includes letters is used when caseless matching is set, it
970     matches the letters in either case. For example, [W-c] is equivalent to
971 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
972 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
973 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
974     characters with values greater than 128 only when it is compiled with Unicode
975     property support.
976     .P
977     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
978     in a character class, and add the characters that they match to the class. For
979     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
980 nigel 63 conveniently be used with the upper case character types to specify a more
981     restricted set of characters than the matching lower case type. For example,
982 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
983     .P
984     The only metacharacters that are recognized in character classes are backslash,
985     hyphen (only where it can be interpreted as specifying a range), circumflex
986     (only at the start), opening square bracket (only when it can be interpreted as
987     introducing a POSIX class name - see the next section), and the terminating
988     closing square bracket. However, escaping other non-alphanumeric characters
989     does no harm.
990     .
991     .
992     .SH "POSIX CHARACTER CLASSES"
993 nigel 63 .rs
994     .sp
995 nigel 75 Perl supports the POSIX notation for character classes. This uses names
996 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
997     this notation. For example,
998 nigel 75 .sp
999 nigel 63 [01[:alpha:]%]
1000 nigel 75 .sp
1001 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
1002     are
1003 nigel 75 .sp
1004 nigel 63 alnum letters and digits
1005     alpha letters
1006     ascii character codes 0 - 127
1007     blank space or tab only
1008     cntrl control characters
1009 nigel 75 digit decimal digits (same as \ed)
1010 nigel 63 graph printing characters, excluding space
1011     lower lower case letters
1012     print printing characters, including space
1013     punct printing characters, excluding letters and digits
1014 nigel 75 space white space (not quite the same as \es)
1015 nigel 63 upper upper case letters
1016 nigel 75 word "word" characters (same as \ew)
1017 nigel 63 xdigit hexadecimal digits
1018 nigel 75 .sp
1019 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1020     space (32). Notice that this list includes the VT character (code 11). This
1021 nigel 75 makes "space" different to \es, which does not include VT (for Perl
1022 nigel 63 compatibility).
1023 nigel 75 .P
1024 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1025     5.8. Another Perl extension is negation, which is indicated by a ^ character
1026     after the colon. For example,
1027 nigel 75 .sp
1028 nigel 63 [12[:^digit:]]
1029 nigel 75 .sp
1030 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1031     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1032     supported, and an error is given if they are encountered.
1033 nigel 75 .P
1034     In UTF-8 mode, characters with values greater than 128 do not match any of
1035 nigel 63 the POSIX character classes.
1036 nigel 75 .
1037     .
1038     .SH "VERTICAL BAR"
1039 nigel 63 .rs
1040     .sp
1041     Vertical bar characters are used to separate alternative patterns. For example,
1042     the pattern
1043 nigel 75 .sp
1044 nigel 63 gilbert|sullivan
1045 nigel 75 .sp
1046 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1047 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1048     process tries each alternative in turn, from left to right, and the first one
1049     that succeeds is used. If the alternatives are within a subpattern
1050 nigel 75 .\" HTML <a href="#subpattern">
1051     .\" </a>
1052     (defined below),
1053     .\"
1054     "succeeds" means matching the rest of the main pattern as well as the
1055     alternative in the subpattern.
1056     .
1057     .
1058     .SH "INTERNAL OPTION SETTING"
1059 nigel 63 .rs
1060     .sp
1061     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1062 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1063     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1064     The option letters are
1065 nigel 75 .sp
1066 nigel 63 i for PCRE_CASELESS
1067     m for PCRE_MULTILINE
1068     s for PCRE_DOTALL
1069     x for PCRE_EXTENDED
1070 nigel 75 .sp
1071 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1072     unset these options by preceding the letter with a hyphen, and a combined
1073     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1074     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1075     permitted. If a letter appears both before and after the hyphen, the option is
1076     unset.
1077 nigel 75 .P
1078 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1079     changed in the same way as the Perl-compatible options by using the characters
1080     J, U and X respectively.
1081     .P
1082 ph10 412 When one of these option changes occurs at top level (that is, not inside
1083     subpattern parentheses), the change applies to the remainder of the pattern
1084     that follows. If the change is placed right at the start of a pattern, PCRE
1085     extracts it into the global options (and it will therefore show up in data
1086     extracted by the \fBpcre_fullinfo()\fP function).
1087 nigel 75 .P
1088 nigel 93 An option change within a subpattern (see below for a description of
1089     subpatterns) affects only that part of the current pattern that follows it, so
1090 nigel 75 .sp
1091 nigel 63 (a(?i)b)c
1092 nigel 75 .sp
1093 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1094     By this means, options can be made to have different settings in different
1095     parts of the pattern. Any changes made in one alternative do carry on
1096     into subsequent branches within the same subpattern. For example,
1097 nigel 75 .sp
1098 nigel 63 (a(?i)b|c)
1099 nigel 75 .sp
1100 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1101     branch is abandoned before the option setting. This is because the effects of
1102     option settings happen at compile time. There would be some very weird
1103     behaviour otherwise.
1104 ph10 251 .P
1105     \fBNote:\fP There are other PCRE-specific options that can be set by the
1106     application when the compile or match functions are called. In some cases the
1107 ph10 412 pattern can contain special leading sequences such as (*CRLF) to override what
1108     the application has set or what has been defaulted. Details are given in the
1109     section entitled
1110 ph10 251 .\" HTML <a href="#newlineseq">
1111     .\" </a>
1112     "Newline sequences"
1113     .\"
1114 ph10 416 above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1115 ph10 412 mode; this is equivalent to setting the PCRE_UTF8 option.
1116 nigel 75 .
1117     .
1118     .\" HTML <a name="subpattern"></a>
1119 nigel 63 .SH SUBPATTERNS
1120     .rs
1121     .sp
1122     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1123 nigel 75 Turning part of a pattern into a subpattern does two things:
1124     .sp
1125 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1126 nigel 75 .sp
1127 nigel 63 cat(aract|erpillar|)
1128 nigel 75 .sp
1129 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1130 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1131 nigel 75 .sp
1132     2. It sets up the subpattern as a capturing subpattern. This means that, when
1133     the whole pattern matches, that portion of the subject string that matched the
1134     subpattern is passed back to the caller via the \fIovector\fP argument of
1135     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1136     from 1) to obtain numbers for the capturing subpatterns.
1137     .P
1138 nigel 63 For example, if the string "the red king" is matched against the pattern
1139 nigel 75 .sp
1140 nigel 63 the ((red|white) (king|queen))
1141 nigel 75 .sp
1142 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1143     2, and 3, respectively.
1144 nigel 75 .P
1145 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1146     There are often times when a grouping subpattern is required without a
1147     capturing requirement. If an opening parenthesis is followed by a question mark
1148     and a colon, the subpattern does not do any capturing, and is not counted when
1149     computing the number of any subsequent capturing subpatterns. For example, if
1150     the string "the white queen" is matched against the pattern
1151 nigel 75 .sp
1152 nigel 63 the ((?:red|white) (king|queen))
1153 nigel 75 .sp
1154 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1155 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1156 nigel 75 .P
1157 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1158     a non-capturing subpattern, the option letters may appear between the "?" and
1159     the ":". Thus the two patterns
1160 nigel 75 .sp
1161 nigel 63 (?i:saturday|sunday)
1162     (?:(?i)saturday|sunday)
1163 nigel 75 .sp
1164 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1165     from left to right, and options are not reset until the end of the subpattern
1166     is reached, an option setting in one branch does affect subsequent branches, so
1167     the above patterns match "SUNDAY" as well as "Saturday".
1168 nigel 75 .
1169     .
1170 ph10 456 .\" HTML <a name="dupsubpatternnumber"></a>
1171 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1172     .rs
1173     .sp
1174 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1175     the same numbers for its capturing parentheses. Such a subpattern starts with
1176     (?| and is itself a non-capturing subpattern. For example, consider this
1177 ph10 175 pattern:
1178     .sp
1179     (?|(Sat)ur|(Sun))day
1180 ph10 182 .sp
1181     Because the two alternatives are inside a (?| group, both sets of capturing
1182     parentheses are numbered one. Thus, when the pattern matches, you can look
1183     at captured substring number one, whichever alternative matched. This construct
1184     is useful when you want to capture part, but not all, of one of a number of
1185     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1186 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1187 ph10 182 buffers that follow the subpattern start after the highest number used in any
1188     branch. The following example is taken from the Perl documentation.
1189 ph10 175 The numbers underneath show in which buffer the captured content will be
1190     stored.
1191     .sp
1192     # before ---------------branch-reset----------- after
1193     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1194     # 1 2 2 3 2 3 4
1195 ph10 182 .sp
1196 ph10 488 A back reference to a numbered subpattern uses the most recent value that is
1197     set for that number by any subpattern. The following pattern matches "abcabc"
1198     or "defdef":
1199 ph10 456 .sp
1200 ph10 461 /(?|(abc)|(def))\e1/
1201 ph10 456 .sp
1202     In contrast, a recursive or "subroutine" call to a numbered subpattern always
1203 ph10 461 refers to the first one in the pattern with the given number. The following
1204 ph10 456 pattern matches "abcabc" or "defabc":
1205     .sp
1206     /(?|(abc)|(def))(?1)/
1207     .sp
1208 ph10 459 If a
1209     .\" HTML <a href="#conditions">
1210     .\" </a>
1211     condition test
1212     .\"
1213     for a subpattern's having matched refers to a non-unique number, the test is
1214     true if any of the subpatterns of that number have matched.
1215     .P
1216     An alternative approach to using this "branch reset" feature is to use
1217 ph10 175 duplicate named subpatterns, as described in the next section.
1218     .
1219     .
1220 nigel 75 .SH "NAMED SUBPATTERNS"
1221 nigel 63 .rs
1222     .sp
1223     Identifying capturing parentheses by number is simple, but it can be very hard
1224     to keep track of the numbers in complicated regular expressions. Furthermore,
1225 nigel 75 if an expression is modified, the numbers may change. To help with this
1226 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1227     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1228     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1229 ph10 459 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1230     have different names, but PCRE does not.
1231 nigel 93 .P
1232     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1233     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1234 nigel 91 parentheses from other parts of the pattern, such as
1235     .\" HTML <a href="#backreferences">
1236     .\" </a>
1237 ph10 488 back references,
1238 nigel 91 .\"
1239     .\" HTML <a href="#recursion">
1240     .\" </a>
1241     recursion,
1242     .\"
1243     and
1244     .\" HTML <a href="#conditions">
1245     .\" </a>
1246     conditions,
1247     .\"
1248     can be made by name as well as by number.
1249 nigel 75 .P
1250 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1251 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1252     if the names were not present. The PCRE API provides function calls for
1253     extracting the name-to-number translation table from a compiled pattern. There
1254     is also a convenience function for extracting a captured substring by name.
1255 nigel 91 .P
1256     By default, a name must be unique within a pattern, but it is possible to relax
1257 ph10 457 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1258 ph10 461 names are also always permitted for subpatterns with the same number, set up as
1259 ph10 457 described in the previous section.) Duplicate names can be useful for patterns
1260     where only one instance of the named parentheses can match. Suppose you want to
1261     match the name of a weekday, either as a 3-letter abbreviation or as the full
1262     name, and in both cases you want to extract the abbreviation. This pattern
1263     (ignoring the line breaks) does the job:
1264 nigel 91 .sp
1265 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1266     (?<DN>Tue)(?:sday)?|
1267     (?<DN>Wed)(?:nesday)?|
1268     (?<DN>Thu)(?:rsday)?|
1269     (?<DN>Sat)(?:urday)?
1270 nigel 91 .sp
1271     There are five capturing substrings, but only one is ever set after a match.
1272 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1273 ph10 175 subpattern, as described in the previous section.)
1274     .P
1275 nigel 91 The convenience function for extracting the data by name returns the substring
1276 nigel 93 for the first (and in this example, the only) subpattern of that name that
1277 ph10 461 matched. This saves searching to find which numbered subpattern it was.
1278 ph10 459 .P
1279 ph10 488 If you make a back reference to a non-unique named subpattern from elsewhere in
1280 ph10 459 the pattern, the one that corresponds to the first occurrence of the name is
1281     used. In the absence of duplicate numbers (see the previous section) this is
1282     the one with the lowest number. If you use a named reference in a condition
1283     test (see the
1284     .\"
1285     .\" HTML <a href="#conditions">
1286     .\" </a>
1287     section about conditions
1288     .\"
1289 ph10 461 below), either to check whether a subpattern has matched, or to check for
1290 ph10 459 recursion, all subpatterns with the same name are tested. If the condition is
1291     true for any one of them, the overall condition is true. This is the same
1292     behaviour as testing by number. For further details of the interfaces for
1293     handling named subpatterns, see the
1294 nigel 63 .\" HREF
1295 nigel 75 \fBpcreapi\fP
1296 nigel 63 .\"
1297     documentation.
1298 ph10 385 .P
1299     \fBWarning:\fP You cannot use different names to distinguish between two
1300 ph10 457 subpatterns with the same number because PCRE uses only the numbers when
1301     matching. For this reason, an error is given at compile time if different names
1302     are given to subpatterns with the same number. However, you can give the same
1303     name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1304 nigel 75 .
1305     .
1306 nigel 63 .SH REPETITION
1307     .rs
1308     .sp
1309     Repetition is specified by quantifiers, which can follow any of the following
1310     items:
1311 nigel 75 .sp
1312 nigel 63 a literal data character
1313 nigel 93 the dot metacharacter
1314 nigel 75 the \eC escape sequence
1315     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1316 nigel 93 the \eR escape sequence
1317 nigel 75 an escape such as \ed that matches a single character
1318 nigel 63 a character class
1319     a back reference (see next section)
1320     a parenthesized subpattern (unless it is an assertion)
1321 ph10 461 a recursive or "subroutine" call to a subpattern
1322 nigel 75 .sp
1323 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1324     permitted matches, by giving the two numbers in curly brackets (braces),
1325     separated by a comma. The numbers must be less than 65536, and the first must
1326     be less than or equal to the second. For example:
1327 nigel 75 .sp
1328 nigel 63 z{2,4}
1329 nigel 75 .sp
1330 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1331     character. If the second number is omitted, but the comma is present, there is
1332     no upper limit; if the second number and the comma are both omitted, the
1333     quantifier specifies an exact number of required matches. Thus
1334 nigel 75 .sp
1335 nigel 63 [aeiou]{3,}
1336 nigel 75 .sp
1337 nigel 63 matches at least 3 successive vowels, but may match many more, while
1338 nigel 75 .sp
1339     \ed{8}
1340     .sp
1341 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1342     where a quantifier is not allowed, or one that does not match the syntax of a
1343     quantifier, is taken as a literal character. For example, {,6} is not a
1344     quantifier, but a literal string of four characters.
1345 nigel 75 .P
1346 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1347 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1348     which is represented by a two-byte sequence. Similarly, when Unicode property
1349     support is available, \eX{3} matches three Unicode extended sequences, each of
1350     which may be several bytes long (and they may be of different lengths).
1351     .P
1352 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1353 ph10 345 previous item and the quantifier were not present. This may be useful for
1354     subpatterns that are referenced as
1355 ph10 335 .\" HTML <a href="#subpatternsassubroutines">
1356     .\" </a>
1357     subroutines
1358     .\"
1359 ph10 345 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1360 ph10 335 quantifier are omitted from the compiled pattern.
1361 nigel 75 .P
1362 nigel 93 For convenience, the three most common quantifiers have single-character
1363     abbreviations:
1364 nigel 75 .sp
1365 nigel 63 * is equivalent to {0,}
1366     + is equivalent to {1,}
1367     ? is equivalent to {0,1}
1368 nigel 75 .sp
1369 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1370     match no characters with a quantifier that has no upper limit, for example:
1371 nigel 75 .sp
1372 nigel 63 (a?)*
1373 nigel 75 .sp
1374 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1375     such patterns. However, because there are cases where this can be useful, such
1376     patterns are now accepted, but if any repetition of the subpattern does in fact
1377     match no characters, the loop is forcibly broken.
1378 nigel 75 .P
1379 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1380     possible (up to the maximum number of permitted times), without causing the
1381     rest of the pattern to fail. The classic example of where this gives problems
1382 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1383     and within the comment, individual * and / characters may appear. An attempt to
1384     match C comments by applying the pattern
1385     .sp
1386     /\e*.*\e*/
1387     .sp
1388 nigel 63 to the string
1389 nigel 75 .sp
1390     /* first comment */ not comment /* second comment */
1391     .sp
1392 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1393     item.
1394 nigel 75 .P
1395 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1396     greedy, and instead matches the minimum number of times possible, so the
1397     pattern
1398 nigel 75 .sp
1399     /\e*.*?\e*/
1400     .sp
1401 nigel 63 does the right thing with the C comments. The meaning of the various
1402     quantifiers is not otherwise changed, just the preferred number of matches.
1403     Do not confuse this use of question mark with its use as a quantifier in its
1404     own right. Because it has two uses, it can sometimes appear doubled, as in
1405 nigel 75 .sp
1406     \ed??\ed
1407     .sp
1408 nigel 63 which matches one digit by preference, but can match two if that is the only
1409     way the rest of the pattern matches.
1410 nigel 75 .P
1411 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1412 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1413     greedy by following them with a question mark. In other words, it inverts the
1414     default behaviour.
1415 nigel 75 .P
1416 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1417 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1418 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1419 nigel 75 .P
1420 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1421 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1422 nigel 63 implicitly anchored, because whatever follows will be tried against every
1423     character position in the subject string, so there is no point in retrying the
1424     overall match at any position after the first. PCRE normally treats such a
1425 nigel 75 pattern as though it were preceded by \eA.
1426     .P
1427 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1428     worth setting PCRE_DOTALL in order to obtain this optimization, or
1429     alternatively using ^ to indicate anchoring explicitly.
1430 nigel 75 .P
1431 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1432 ph10 488 is inside capturing parentheses that are the subject of a back reference
1433 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1434     succeeds. Consider, for example:
1435 nigel 75 .sp
1436     (.*)abc\e1
1437     .sp
1438 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1439     this reason, such a pattern is not implicitly anchored.
1440 nigel 75 .P
1441 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1442     that matched the final iteration. For example, after
1443 nigel 75 .sp
1444     (tweedle[dume]{3}\es*)+
1445     .sp
1446 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1447     "tweedledee". However, if there are nested capturing subpatterns, the
1448     corresponding captured values may have been set in previous iterations. For
1449     example, after
1450 nigel 75 .sp
1451 nigel 63 /(a|(b))+/
1452 nigel 75 .sp
1453 nigel 63 matches "aba" the value of the second captured substring is "b".
1454 nigel 75 .
1455     .
1456     .\" HTML <a name="atomicgroup"></a>
1457     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1458 nigel 63 .rs
1459     .sp
1460 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1461     repetition, failure of what follows normally causes the repeated item to be
1462     re-evaluated to see if a different number of repeats allows the rest of the
1463     pattern to match. Sometimes it is useful to prevent this, either to change the
1464     nature of the match, or to cause it fail earlier than it otherwise might, when
1465     the author of the pattern knows there is no point in carrying on.
1466 nigel 75 .P
1467     Consider, for example, the pattern \ed+foo when applied to the subject line
1468     .sp
1469 nigel 63 123456bar
1470 nigel 75 .sp
1471 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1472 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1473 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1474     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1475     that once a subpattern has matched, it is not to be re-evaluated in this way.
1476 nigel 75 .P
1477 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1478 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1479     special parenthesis, starting with (?> as in this example:
1480 nigel 75 .sp
1481     (?>\ed+)foo
1482     .sp
1483 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1484     it has matched, and a failure further into the pattern is prevented from
1485     backtracking into it. Backtracking past it to previous items, however, works as
1486     normal.
1487 nigel 75 .P
1488 nigel 63 An alternative description is that a subpattern of this type matches the string
1489     of characters that an identical standalone pattern would match, if anchored at
1490     the current point in the subject string.
1491 nigel 75 .P
1492 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1493     the above example can be thought of as a maximizing repeat that must swallow
1494 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1495 nigel 63 number of digits they match in order to make the rest of the pattern match,
1496 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1497     .P
1498 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1499     subpatterns, and can be nested. However, when the subpattern for an atomic
1500     group is just a single repeated item, as in the example above, a simpler
1501     notation, called a "possessive quantifier" can be used. This consists of an
1502     additional + character following a quantifier. Using this notation, the
1503     previous example can be rewritten as
1504 nigel 75 .sp
1505     \ed++foo
1506     .sp
1507 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1508     example:
1509     .sp
1510     (abc|xyz){2,3}+
1511     .sp
1512 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1513     option is ignored. They are a convenient notation for the simpler forms of
1514 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1515     quantifier and the equivalent atomic group, though there may be a performance
1516     difference; possessive quantifiers should be slightly faster.
1517 nigel 75 .P
1518 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1519     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1520     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1521     package, and PCRE copied it from there. It ultimately found its way into Perl
1522     at release 5.10.
1523 nigel 75 .P
1524 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1525     pattern constructs. For example, the sequence A+B is treated as A++B because
1526     there is no point in backtracking into a sequence of A's when B must follow.
1527     .P
1528 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1529     be repeated an unlimited number of times, the use of an atomic group is the
1530     only way to avoid some failing matches taking a very long time indeed. The
1531     pattern
1532 nigel 75 .sp
1533     (\eD+|<\ed+>)*[!?]
1534     .sp
1535 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1536     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1537     quickly. However, if it is applied to
1538 nigel 75 .sp
1539 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1540 nigel 75 .sp
1541 nigel 63 it takes a long time before reporting failure. This is because the string can
1542 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1543     large number of ways, and all have to be tried. (The example uses [!?] rather
1544     than a single character at the end, because both PCRE and Perl have an
1545     optimization that allows for fast failure when a single character is used. They
1546     remember the last single character that is required for a match, and fail early
1547     if it is not present in the string.) If the pattern is changed so that it uses
1548     an atomic group, like this:
1549     .sp
1550     ((?>\eD+)|<\ed+>)*[!?]
1551     .sp
1552 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1553 nigel 75 .
1554     .
1555     .\" HTML <a name="backreferences"></a>
1556     .SH "BACK REFERENCES"
1557 nigel 63 .rs
1558     .sp
1559     Outside a character class, a backslash followed by a digit greater than 0 (and
1560     possibly further digits) is a back reference to a capturing subpattern earlier
1561     (that is, to its left) in the pattern, provided there have been that many
1562     previous capturing left parentheses.
1563 nigel 75 .P
1564 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1565     always taken as a back reference, and causes an error only if there are not
1566     that many capturing left parentheses in the entire pattern. In other words, the
1567     parentheses that are referenced need not be to the left of the reference for
1568 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1569     when a repetition is involved and the subpattern to the right has participated
1570     in an earlier iteration.
1571     .P
1572 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1573     whose number is 10 or more using this syntax because a sequence such as \e50 is
1574     interpreted as a character defined in octal. See the subsection entitled
1575 nigel 91 "Non-printing characters"
1576 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1577     .\" </a>
1578     above
1579     .\"
1580 nigel 93 for further details of the handling of digits following a backslash. There is
1581     no such problem when named parentheses are used. A back reference to any
1582     subpattern is possible using named parentheses (see below).
1583 nigel 75 .P
1584 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1585     backslash is to use the \eg escape sequence, which is a feature introduced in
1586 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1587     number, optionally enclosed in braces. These examples are all identical:
1588 nigel 93 .sp
1589     (ring), \e1
1590     (ring), \eg1
1591     (ring), \eg{1}
1592     .sp
1593 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1594     is present in the older syntax. It is also useful when literal digits follow
1595     the reference. A negative number is a relative reference. Consider this
1596     example:
1597 nigel 93 .sp
1598     (abc(def)ghi)\eg{-1}
1599     .sp
1600     The sequence \eg{-1} is a reference to the most recently started capturing
1601     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1602     would be equivalent to \e1. The use of relative references can be helpful in
1603     long patterns, and also in patterns that are created by joining together
1604     fragments that contain references within themselves.
1605     .P
1606 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1607     the current subject string, rather than anything matching the subpattern
1608     itself (see
1609     .\" HTML <a href="#subpatternsassubroutines">
1610     .\" </a>
1611     "Subpatterns as subroutines"
1612     .\"
1613     below for a way of doing that). So the pattern
1614 nigel 75 .sp
1615     (sens|respons)e and \e1ibility
1616     .sp
1617 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1618     "sense and responsibility". If caseful matching is in force at the time of the
1619     back reference, the case of letters is relevant. For example,
1620 nigel 75 .sp
1621     ((?i)rah)\es+\e1
1622     .sp
1623 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1624     capturing subpattern is matched caselessly.
1625 nigel 75 .P
1626 ph10 171 There are several different ways of writing back references to named
1627     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1628     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1629     back reference syntax, in which \eg can be used for both numeric and named
1630     references, is also supported. We could rewrite the above example in any of
1631 nigel 93 the following ways:
1632 nigel 75 .sp
1633 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1634 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1635 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1636 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1637 nigel 75 .sp
1638 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1639     after the reference.
1640     .P
1641 nigel 63 There may be more than one back reference to the same subpattern. If a
1642     subpattern has not actually been used in a particular match, any back
1643 ph10 456 references to it always fail by default. For example, the pattern
1644 nigel 75 .sp
1645     (a|(bc))\e2
1646     .sp
1647 ph10 461 always fails if it starts to match "a" rather than "bc". However, if the
1648     PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1649 ph10 456 unset value matches an empty string.
1650     .P
1651     Because there may be many capturing parentheses in a pattern, all digits
1652     following a backslash are taken as part of a potential back reference number.
1653     If the pattern continues with a digit character, some delimiter must be used to
1654     terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1655     whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1656 nigel 75 .\" HTML <a href="#comments">
1657     .\" </a>
1658     "Comments"
1659     .\"
1660     below) can be used.
1661 ph10 488 .
1662     .SS "Recursive back references"
1663     .rs
1664     .sp
1665 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1666 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1667 nigel 63 However, such references can be useful inside repeated subpatterns. For
1668     example, the pattern
1669 nigel 75 .sp
1670     (a|b\e1)+
1671     .sp
1672 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1673     the subpattern, the back reference matches the character string corresponding
1674     to the previous iteration. In order for this to work, the pattern must be such
1675     that the first iteration does not need to match the back reference. This can be
1676     done using alternation, as in the example above, or by a quantifier with a
1677     minimum of zero.
1678 ph10 488 .P
1679     Back references of this type cause the group that they reference to be treated
1680     as an
1681     .\" HTML <a href="#atomicgroup">
1682     .\" </a>
1683     atomic group.
1684     .\"
1685     Once the whole group has been matched, a subsequent matching failure cannot
1686     cause backtracking into the middle of the group.
1687 nigel 75 .
1688     .
1689     .\" HTML <a name="bigassertions"></a>
1690 nigel 63 .SH ASSERTIONS
1691     .rs
1692     .sp
1693     An assertion is a test on the characters following or preceding the current
1694     matching point that does not actually consume any characters. The simple
1695 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1696     .\" HTML <a href="#smallassertions">
1697     .\" </a>
1698     above.
1699     .\"
1700     .P
1701 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1702     those that look ahead of the current position in the subject string, and those
1703 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1704     except that it does not cause the current matching position to be changed.
1705     .P
1706     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1707     because it makes no sense to assert the same thing several times. If any kind
1708     of assertion contains capturing subpatterns within it, these are counted for
1709     the purposes of numbering the capturing subpatterns in the whole pattern.
1710     However, substring capturing is carried out only for positive assertions,
1711     because it does not make sense for negative assertions.
1712     .
1713     .
1714     .SS "Lookahead assertions"
1715     .rs
1716     .sp
1717 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1718     negative assertions. For example,
1719 nigel 75 .sp
1720     \ew+(?=;)
1721     .sp
1722 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1723     the match, and
1724 nigel 75 .sp
1725 nigel 63 foo(?!bar)
1726 nigel 75 .sp
1727 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1728     apparently similar pattern
1729 nigel 75 .sp
1730 nigel 63 (?!foo)bar
1731 nigel 75 .sp
1732 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1733     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1734     (?!foo) is always true when the next three characters are "bar". A
1735 nigel 75 lookbehind assertion is needed to achieve the other effect.
1736     .P
1737 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1738     convenient way to do it is with (?!) because an empty string always matches, so
1739     an assertion that requires there not to be an empty string must always fail.
1740 ph10 456 The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1741     synonym for (?!).
1742 nigel 75 .
1743     .
1744     .\" HTML <a name="lookbehind"></a>
1745     .SS "Lookbehind assertions"
1746     .rs
1747     .sp
1748 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1749     negative assertions. For example,
1750 nigel 75 .sp
1751 nigel 63 (?<!foo)bar
1752 nigel 75 .sp
1753 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1754     a lookbehind assertion are restricted such that all the strings it matches must
1755 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1756     do not all have to have the same fixed length. Thus
1757 nigel 75 .sp
1758 nigel 63 (?<=bullock|donkey)
1759 nigel 75 .sp
1760 nigel 63 is permitted, but
1761 nigel 75 .sp
1762 nigel 63 (?<!dogs?|cats?)
1763 nigel 75 .sp
1764 nigel 63 causes an error at compile time. Branches that match different length strings
1765     are permitted only at the top level of a lookbehind assertion. This is an
1766 ph10 454 extension compared with Perl (5.8 and 5.10), which requires all branches to
1767 nigel 63 match the same length of string. An assertion such as
1768 nigel 75 .sp
1769 nigel 63 (?<=ab(c|de))
1770 nigel 75 .sp
1771 nigel 63 is not permitted, because its single top-level branch can match two different
1772 ph10 454 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1773     branches:
1774 nigel 75 .sp
1775 nigel 63 (?<=abc|abde)
1776 nigel 75 .sp
1777 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1778 ph10 168 .\" HTML <a href="#resetmatchstart">
1779     .\" </a>
1780     (see above)
1781     .\"
1782 ph10 461 can be used instead of a lookbehind assertion to get round the fixed-length
1783 ph10 454 restriction.
1784 ph10 168 .P
1785 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1786 nigel 93 temporarily move the current position back by the fixed length and then try to
1787 nigel 63 match. If there are insufficient characters before the current position, the
1788 nigel 93 assertion fails.
1789 nigel 75 .P
1790     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1791 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1792 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1793     different numbers of bytes, are also not permitted.
1794 nigel 75 .P
1795 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
1796     .\" </a>
1797     "Subroutine"
1798     .\"
1799     calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1800 ph10 461 as the subpattern matches a fixed-length string.
1801 ph10 454 .\" HTML <a href="#recursion">
1802     .\" </a>
1803     Recursion,
1804     .\"
1805     however, is not supported.
1806     .P
1807 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1808 ph10 456 specify efficient matching of fixed-length strings at the end of subject
1809     strings. Consider a simple pattern such as
1810 nigel 75 .sp
1811 nigel 63 abcd$
1812 nigel 75 .sp
1813 nigel 63 when applied to a long string that does not match. Because matching proceeds
1814     from left to right, PCRE will look for each "a" in the subject and then see if
1815     what follows matches the rest of the pattern. If the pattern is specified as
1816 nigel 75 .sp
1817 nigel 63 ^.*abcd$
1818 nigel 75 .sp
1819 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1820     there is no following "a"), it backtracks to match all but the last character,
1821     then all but the last two characters, and so on. Once again the search for "a"
1822     covers the entire string, from right to left, so we are no better off. However,
1823     if the pattern is written as
1824 nigel 75 .sp
1825 nigel 63 ^.*+(?<=abcd)
1826 nigel 75 .sp
1827 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1828 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1829     characters. If it fails, the match fails immediately. For long strings, this
1830     approach makes a significant difference to the processing time.
1831 nigel 75 .
1832     .
1833     .SS "Using multiple assertions"
1834     .rs
1835     .sp
1836 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1837 nigel 75 .sp
1838     (?<=\ed{3})(?<!999)foo
1839     .sp
1840 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1841     the assertions is applied independently at the same point in the subject
1842     string. First there is a check that the previous three characters are all
1843     digits, and then there is a check that the same three characters are not "999".
1844 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1845 nigel 63 of which are digits and the last three of which are not "999". For example, it
1846     doesn't match "123abcfoo". A pattern to do that is
1847 nigel 75 .sp
1848     (?<=\ed{3}...)(?<!999)foo
1849     .sp
1850 nigel 63 This time the first assertion looks at the preceding six characters, checking
1851     that the first three are digits, and then the second assertion checks that the
1852     preceding three characters are not "999".
1853 nigel 75 .P
1854 nigel 63 Assertions can be nested in any combination. For example,
1855 nigel 75 .sp
1856 nigel 63 (?<=(?<!foo)bar)baz
1857 nigel 75 .sp
1858 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1859     preceded by "foo", while
1860 nigel 75 .sp
1861     (?<=\ed{3}(?!999)...)foo
1862     .sp
1863     is another pattern that matches "foo" preceded by three digits and any three
1864 nigel 63 characters that are not "999".
1865 nigel 75 .
1866     .
1867 nigel 91 .\" HTML <a name="conditions"></a>
1868 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1869 nigel 63 .rs
1870     .sp
1871     It is possible to cause the matching process to obey a subpattern
1872     conditionally or to choose between two alternative subpatterns, depending on
1873 ph10 461 the result of an assertion, or whether a specific capturing subpattern has
1874 ph10 456 already been matched. The two possible forms of conditional subpattern are:
1875 nigel 75 .sp
1876 nigel 63 (?(condition)yes-pattern)
1877     (?(condition)yes-pattern|no-pattern)
1878 nigel 75 .sp
1879 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1880     no-pattern (if present) is used. If there are more than two alternatives in the
1881     subpattern, a compile-time error occurs.
1882 nigel 75 .P
1883 nigel 93 There are four kinds of condition: references to subpatterns, references to
1884     recursion, a pseudo-condition called DEFINE, and assertions.
1885     .
1886     .SS "Checking for a used subpattern by number"
1887     .rs
1888     .sp
1889     If the text between the parentheses consists of a sequence of digits, the
1890 ph10 456 condition is true if a capturing subpattern of that number has previously
1891 ph10 461 matched. If there is more than one capturing subpattern with the same number
1892     (see the earlier
1893 ph10 456 .\"
1894     .\" HTML <a href="#recursion">
1895     .\" </a>
1896     section about duplicate subpattern numbers),
1897     .\"
1898     the condition is true if any of them have been set. An alternative notation is
1899     to precede the digits with a plus or minus sign. In this case, the subpattern
1900     number is relative rather than absolute. The most recently opened parentheses
1901     can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1902     looping constructs it can also make sense to refer to subsequent groups with
1903     constructs such as (?(+2).
1904 nigel 91 .P
1905     Consider the following pattern, which contains non-significant white space to
1906     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1907     three parts for ease of discussion:
1908 nigel 75 .sp
1909     ( \e( )? [^()]+ (?(1) \e) )
1910     .sp
1911 nigel 63 The first part matches an optional opening parenthesis, and if that
1912     character is present, sets it as the first captured substring. The second part
1913     matches one or more characters that are not parentheses. The third part is a
1914     conditional subpattern that tests whether the first set of parentheses matched
1915     or not. If they did, that is, if subject started with an opening parenthesis,
1916     the condition is true, and so the yes-pattern is executed and a closing
1917     parenthesis is required. Otherwise, since no-pattern is not present, the
1918     subpattern matches nothing. In other words, this pattern matches a sequence of
1919 nigel 93 non-parentheses, optionally enclosed in parentheses.
1920 ph10 167 .P
1921 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1922 ph10 167 reference:
1923     .sp
1924     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1925     .sp
1926     This makes the fragment independent of the parentheses in the larger pattern.
1927 nigel 93 .
1928     .SS "Checking for a used subpattern by name"
1929     .rs
1930 nigel 91 .sp
1931 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1932     subpattern by name. For compatibility with earlier versions of PCRE, which had
1933     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1934     there is a possible ambiguity with this syntax, because subpattern names may
1935     consist entirely of digits. PCRE looks first for a named subpattern; if it
1936     cannot find one and the name consists entirely of digits, PCRE looks for a
1937     subpattern of that number, which must be greater than zero. Using subpattern
1938     names that consist entirely of digits is not recommended.
1939     .P
1940     Rewriting the above example to use a named subpattern gives this:
1941 nigel 91 .sp
1942 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1943     .sp
1944 ph10 461 If the name used in a condition of this kind is a duplicate, the test is
1945     applied to all subpatterns of the same name, and is true if any one of them has
1946 ph10 459 matched.
1947 nigel 93 .
1948     .SS "Checking for pattern recursion"
1949     .rs
1950     .sp
1951 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1952 nigel 93 the condition is true if a recursive call to the whole pattern or any
1953     subpattern has been made. If digits or a name preceded by ampersand follow the
1954     letter R, for example:
1955     .sp
1956     (?(R3)...) or (?(R&name)...)
1957     .sp
1958 ph10 456 the condition is true if the most recent recursion is into a subpattern whose
1959 nigel 93 number or name is given. This condition does not check the entire recursion
1960 ph10 461 stack. If the name used in a condition of this kind is a duplicate, the test is
1961     applied to all subpatterns of the same name, and is true if any one of them is
1962     the most recent recursion.
1963 nigel 75 .P
1964 ph10 461 At "top level", all these recursion test conditions are false.
1965 ph10 454 .\" HTML <a href="#recursion">
1966     .\" </a>
1967 ph10 459 The syntax for recursive patterns
1968 ph10 454 .\"
1969 ph10 459 is described below.
1970 nigel 93 .
1971     .SS "Defining subpatterns for use by reference only"
1972     .rs
1973     .sp
1974     If the condition is the string (DEFINE), and there is no subpattern with the
1975     name DEFINE, the condition is always false. In this case, there may be only one
1976     alternative in the subpattern. It is always skipped if control reaches this
1977     point in the pattern; the idea of DEFINE is that it can be used to define
1978 ph10 461 "subroutines" that can be referenced from elsewhere. (The use of
1979 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
1980     .\" </a>
1981     "subroutines"
1982     .\"
1983 nigel 93 is described below.) For example, a pattern to match an IPv4 address could be
1984     written like this (ignore whitespace and line breaks):
1985     .sp
1986     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1987     \eb (?&byte) (\e.(?&byte)){3} \eb
1988     .sp
1989     The first part of the pattern is a DEFINE group inside which a another group
1990     named "byte" is defined. This matches an individual component of an IPv4
1991     address (a number less than 256). When matching takes place, this part of the
1992 ph10 456 pattern is skipped because DEFINE acts like a false condition. The rest of the
1993     pattern uses references to the named group to match the four dot-separated
1994     components of an IPv4 address, insisting on a word boundary at each end.
1995 nigel 93 .
1996     .SS "Assertion conditions"
1997     .rs
1998     .sp
1999     If the condition is not in any of the above formats, it must be an assertion.
2000 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
2001     this pattern, again containing non-significant white space, and with the two
2002     alternatives on the second line:
2003 nigel 75 .sp
2004 nigel 63 (?(?=[^a-z]*[a-z])
2005 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2006     .sp
2007 nigel 63 The condition is a positive lookahead assertion that matches an optional
2008     sequence of non-letters followed by a letter. In other words, it tests for the
2009     presence of at least one letter in the subject. If a letter is found, the
2010     subject is matched against the first alternative; otherwise it is matched
2011     against the second. This pattern matches strings in one of the two forms
2012     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2013 nigel 75 .
2014     .
2015     .\" HTML <a name="comments"></a>
2016 nigel 63 .SH COMMENTS
2017     .rs
2018     .sp
2019 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
2020 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
2021     that make up a comment play no part in the pattern matching at all.
2022 nigel 75 .P
2023 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
2024 nigel 91 character class introduces a comment that continues to immediately after the
2025     next newline in the pattern.
2026 nigel 75 .
2027     .
2028 nigel 91 .\" HTML <a name="recursion"></a>
2029 nigel 75 .SH "RECURSIVE PATTERNS"
2030 nigel 63 .rs
2031     .sp
2032     Consider the problem of matching a string in parentheses, allowing for
2033     unlimited nested parentheses. Without the use of recursion, the best that can
2034     be done is to use a pattern that matches up to some fixed depth of nesting. It
2035 nigel 93 is not possible to handle an arbitrary nesting depth.
2036     .P
2037     For some time, Perl has provided a facility that allows regular expressions to
2038     recurse (amongst other things). It does this by interpolating Perl code in the
2039     expression at run time, and the code can refer to the expression itself. A Perl
2040     pattern using code interpolation to solve the parentheses problem can be
2041     created like this:
2042 nigel 75 .sp
2043     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2044     .sp
2045 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2046 nigel 93 recursively to the pattern in which it appears.
2047 nigel 75 .P
2048 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2049     supports special syntax for recursion of the entire pattern, and also for
2050     individual subpattern recursion. After its introduction in PCRE and Python,
2051 ph10 453 this kind of recursion was subsequently introduced into Perl at release 5.10.
2052 nigel 75 .P
2053 nigel 93 A special item that consists of (? followed by a number greater than zero and a
2054     closing parenthesis is a recursive call of the subpattern of the given number,
2055 ph10 461 provided that it occurs inside that subpattern. (If not, it is a
2056 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2057     .\" </a>
2058     "subroutine"
2059     .\"
2060 nigel 93 call, which is described in the next section.) The special item (?R) or (?0) is
2061     a recursive call of the entire regular expression.
2062 nigel 87 .P
2063     This PCRE pattern solves the nested parentheses problem (assume the
2064     PCRE_EXTENDED option is set so that white space is ignored):
2065 nigel 75 .sp
2066 ph10 456 \e( ( [^()]++ | (?R) )* \e)
2067 nigel 75 .sp
2068 nigel 63 First it matches an opening parenthesis. Then it matches any number of
2069     substrings which can either be a sequence of non-parentheses, or a recursive
2070 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
2071 ph10 461 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2072 ph10 456 to avoid backtracking into sequences of non-parentheses.
2073 nigel 75 .P
2074 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
2075     pattern, so instead you could use this:
2076 nigel 75 .sp
2077 ph10 456 ( \e( ( [^()]++ | (?1) )* \e) )
2078 nigel 75 .sp
2079 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
2080 ph10 172 them instead of the whole pattern.
2081 ph10 166 .P
2082     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2083 ph10 456 is made easier by the use of relative references (a Perl 5.10 feature).
2084 ph10 166 Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2085     most recently opened parentheses preceding the recursion. In other words, a
2086     negative number counts capturing parentheses leftwards from the point at which
2087     it is encountered.
2088     .P
2089     It is also possible to refer to subsequently opened parentheses, by writing
2090     references such as (?+2). However, these cannot be recursive because the
2091     reference is not inside the parentheses that are referenced. They are always
2092 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2093     .\" </a>
2094     "subroutine"
2095     .\"
2096     calls, as described in the next section.
2097 ph10 166 .P
2098     An alternative approach is to use named parentheses instead. The Perl syntax
2099     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2100     could rewrite the above example as follows:
2101 nigel 75 .sp
2102 ph10 456 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2103 nigel 75 .sp
2104 nigel 93 If there is more than one subpattern with the same name, the earliest one is
2105 ph10 172 used.
2106 ph10 166 .P
2107     This particular example pattern that we have been looking at contains nested
2108 ph10 456 unlimited repeats, and so the use of a possessive quantifier for matching
2109     strings of non-parentheses is important when applying the pattern to strings
2110     that do not match. For example, when this pattern is applied to
2111 nigel 75 .sp
2112 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2113 nigel 75 .sp
2114 ph10 456 it yields "no match" quickly. However, if a possessive quantifier is not used,
2115 nigel 63 the match runs for a very long time indeed because there are so many different
2116     ways the + and * repeats can carve up the subject, and all have to be tested
2117     before failure can be reported.
2118 nigel 75 .P
2119 ph10 464 At the end of a match, the values of capturing parentheses are those from
2120     the outermost level. If you want to obtain intermediate values, a callout
2121     function can be used (see below and the
2122 nigel 63 .\" HREF
2123 nigel 75 \fBpcrecallout\fP
2124 nigel 63 .\"
2125     documentation). If the pattern above is matched against
2126 nigel 75 .sp
2127 nigel 63 (ab(cd)ef)
2128 nigel 75 .sp
2129 ph10 464 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2130     the last value taken on at the top level. If a capturing subpattern is not
2131     matched at the top level, its final value is unset, even if it is (temporarily)
2132     set at a deeper level.
2133 nigel 75 .P
2134 ph10 464 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2135     obtain extra memory to store data during a recursion, which it does by using
2136     \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2137     be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2138     .P
2139 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2140     Consider this pattern, which matches text in angle brackets, allowing for
2141     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2142     recursing), whereas any characters are permitted at the outer level.
2143 nigel 75 .sp
2144     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2145     .sp
2146 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2147     different alternatives for the recursive and non-recursive cases. The (?R) item
2148     is the actual recursive call.
2149 nigel 75 .
2150     .
2151 ph10 453 .\" HTML <a name="recursiondifference"></a>
2152     .SS "Recursion difference from Perl"
2153     .rs
2154     .sp
2155     In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2156     treated as an atomic group. That is, once it has matched some of the subject
2157     string, it is never re-entered, even if it contains untried alternatives and
2158 ph10 461 there is a subsequent matching failure. This can be illustrated by the
2159     following pattern, which purports to match a palindromic string that contains
2160 ph10 453 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2161     .sp
2162     ^(.|(.)(?1)\e2)$
2163     .sp
2164 ph10 461 The idea is that it either matches a single character, or two identical
2165     characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2166 ph10 453 it does not if the pattern is longer than three characters. Consider the
2167     subject string "abcba":
2168     .P
2169 ph10 461 At the top level, the first character is matched, but as it is not at the end
2170 ph10 453 of the string, the first alternative fails; the second alternative is taken
2171     and the recursion kicks in. The recursive call to subpattern 1 successfully
2172     matches the next character ("b"). (Note that the beginning and end of line
2173     tests are not part of the recursion).
2174     .P
2175     Back at the top level, the next character ("c") is compared with what
2176 ph10 461 subpattern 2 matched, which was "a". This fails. Because the recursion is
2177 ph10 453 treated as an atomic group, there are now no backtracking points, and so the
2178     entire match fails. (Perl is able, at this point, to re-enter the recursion and
2179     try the second alternative.) However, if the pattern is written with the
2180     alternatives in the other order, things are different:
2181     .sp
2182     ^((.)(?1)\e2|.)$
2183     .sp
2184 ph10 461 This time, the recursing alternative is tried first, and continues to recurse
2185     until it runs out of characters, at which point the recursion fails. But this
2186     time we do have another alternative to try at the higher level. That is the big
2187 ph10 453 difference: in the previous case the remaining alternative is at a deeper
2188     recursion level, which PCRE cannot use.
2189     .P
2190 ph10 461 To change the pattern so that matches all palindromic strings, not just those
2191 ph10 453 with an odd number of characters, it is tempting to change the pattern to this:
2192     .sp
2193     ^((.)(?1)\e2|.?)$
2194     .sp
2195 ph10 461 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2196     deeper recursion has matched a single character, it cannot be entered again in
2197     order to match an empty string. The solution is to separate the two cases, and
2198 ph10 453 write out the odd and even cases as alternatives at the higher level:
2199     .sp
2200     ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2201 ph10 461 .sp
2202     If you want to match typical palindromic phrases, the pattern has to ignore all
2203 ph10 453 non-word characters, which can be done like this:
2204     .sp
2205 ph10 461 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2206 ph10 453 .sp
2207 ph10 461 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2208     man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2209     the use of the possessive quantifier *+ to avoid backtracking into sequences of
2210 ph10 453 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2211     more) to match typical phrases, and Perl takes so long that you think it has
2212     gone into a loop.
2213 ph10 456 .P
2214     \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2215     string does not start with a palindrome that is shorter than the entire string.
2216     For example, although "abcba" is correctly matched, if the subject is "ababa",
2217     PCRE finds the palindrome "aba" at the start, then fails at top level because
2218     the end of the string does not follow. Once again, it cannot jump back into the
2219     recursion to try other alternatives, so the entire match fails.
2220 ph10 453 .
2221     .
2222 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
2223 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
2224 nigel 63 .rs
2225     .sp
2226     If the syntax for a recursive subpattern reference (either by number or by
2227     name) is used outside the parentheses to which it refers, it operates like a
2228 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
2229 ph10 166 before or after the reference. A numbered reference can be absolute or
2230     relative, as in these examples:
2231 nigel 75 .sp
2232 ph10 166 (...(absolute)...)...(?2)...
2233     (...(relative)...)...(?-1)...
2234 ph10 172 (...(?+1)...(relative)...
2235 ph10 166 .sp
2236     An earlier example pointed out that the pattern
2237     .sp
2238 nigel 75 (sens|respons)e and \e1ibility
2239     .sp
2240 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2241     "sense and responsibility". If instead the pattern
2242 nigel 75 .sp
2243 nigel 63 (sens|respons)e and (?1)ibility
2244 nigel 75 .sp
2245 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2246 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2247 nigel 87 .P
2248 ph10 464 Like recursive subpatterns, a subroutine call is always treated as an atomic
2249 nigel 87 group. That is, once it has matched some of the subject string, it is never
2250     re-entered, even if it contains untried alternatives and there is a subsequent
2251 ph10 469 matching failure. Any capturing parentheses that are set during the subroutine
2252 ph10 464 call revert to their previous values afterwards.
2253 nigel 93 .P
2254     When a subpattern is used as a subroutine, processing options such as
2255     case-independence are fixed when the subpattern is defined. They cannot be
2256     changed for different calls. For example, consider this pattern:
2257     .sp
2258 ph10 166 (abc)(?i:(?-1))
2259 nigel 93 .sp
2260     It matches "abcabc". It does not match "abcABC" because the change of
2261     processing option does not affect the called subpattern.
2262 nigel 75 .
2263     .
2264 ph10 333 .\" HTML <a name="onigurumasubroutines"></a>
2265     .SH "ONIGURUMA SUBROUTINE SYNTAX"
2266     .rs
2267     .sp
2268 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2269     a number enclosed either in angle brackets or single quotes, is an alternative
2270     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2271 ph10 333 are two of the examples used above, rewritten using this syntax:
2272     .sp
2273     (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2274     (sens|respons)e and \eg'1'ibility
2275     .sp
2276 ph10 345 PCRE supports an extension to Oniguruma: if a number is preceded by a
2277 ph10 333 plus or a minus sign it is taken as a relative reference. For example:
2278     .sp
2279     (abc)(?i:\eg<-1>)
2280     .sp
2281 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2282 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
2283     .
2284     .
2285 nigel 63 .SH CALLOUTS
2286     .rs
2287     .sp
2288     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2289     code to be obeyed in the middle of matching a regular expression. This makes it
2290     possible, amongst other things, to extract different substrings that match the
2291     same pair of parentheses when there is a repetition.
2292 nigel 75 .P
2293 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2294     code. The feature is called "callout". The caller of PCRE provides an external
2295 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
2296 nigel 63 By default, this variable contains NULL, which disables all calling out.
2297 nigel 75 .P
2298 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2299     function is to be called. If you want to identify different callout points, you
2300     can put a number less than 256 after the letter C. The default value is zero.
2301     For example, this pattern has two callout points:
2302 nigel 75 .sp
2303 ph10 155 (?C1)abc(?C2)def
2304 nigel 75 .sp
2305     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2306     automatically installed before each item in the pattern. They are all numbered
2307     255.
2308     .P
2309     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2310 nigel 63 set), the external function is called. It is provided with the number of the
2311 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2312     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2313     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2314     description of the interface to the callout function is given in the
2315 nigel 63 .\" HREF
2316 nigel 75 \fBpcrecallout\fP
2317 nigel 63 .\"
2318     documentation.
2319 nigel 93 .
2320     .
2321 ph10 235 .SH "BACKTRACKING CONTROL"
2322 ph10 210 .rs
2323     .sp
2324 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2325 ph10 210 are described in the Perl documentation as "experimental and subject to change
2326 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
2327     production code should be noted to avoid problems during upgrades." The same
2328 ph10 210 remarks apply to the PCRE features described in this section.
2329     .P
2330 ph10 341 Since these verbs are specifically related to backtracking, most of them can be
2331     used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2332 ph10 345 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2333 ph10 341 failing negative assertion, they cause an error if encountered by
2334 ph10 210 \fBpcre_dfa_exec()\fP.
2335     .P
2336 ph10 469 If any of these verbs are used in an assertion or subroutine subpattern
2337 ph10 464 (including recursive subpatterns), their effect is confined to that subpattern;
2338     it does not extend to the surrounding pattern. Note that such subpatterns are
2339     processed as anchored at the point where they are tested.
2340 ph10 445 .P
2341 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2342 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form
2343     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2344     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2345     are two kinds:
2346     .
2347     .SS "Verbs that act immediately"
2348     .rs
2349     .sp
2350     The following verbs act as soon as they are encountered:
2351     .sp
2352     (*ACCEPT)
2353     .sp
2354     This verb causes the match to end successfully, skipping the remainder of the
2355 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2356 ph10 456 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2357     captured. (This feature was added to PCRE at release 8.00.) For example:
2358 ph10 210 .sp
2359 ph10 447 A((?:A|B(*ACCEPT)|C)D)
2360 ph10 210 .sp
2361 ph10 461 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2362 ph10 447 the outer parentheses.
2363 ph10 210 .sp
2364     (*FAIL) or (*F)
2365     .sp
2366 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2367 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2368     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2369     Perl features that are not present in PCRE. The nearest equivalent is the
2370     callout feature, as for example in this pattern:
2371     .sp
2372     a+(?C)(*FAIL)
2373     .sp
2374 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2375     each backtrack happens (in this example, 10 times).
2376 ph10 210 .
2377     .SS "Verbs that act after backtracking"
2378     .rs
2379     .sp
2380 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2381     with what follows, but if there is no subsequent match, a failure is forced.
2382 ph10 210 The verbs differ in exactly what kind of failure occurs.
2383     .sp
2384     (*COMMIT)
2385     .sp
2386 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern
2387 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find
2388 ph10 456 a match by advancing the starting point take place. Once (*COMMIT) has been
2389 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2390 ph10 210 starting point, or not at all. For example:
2391     .sp
2392     a+(*COMMIT)b
2393     .sp
2394 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2395 ph10 210 dynamic anchor, or "I've started, so I must finish."
2396     .sp
2397     (*PRUNE)
2398     .sp
2399 ph10 211 This verb causes the match to fail at the current position if the rest of the
2400     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2401 ph10 210 advance to the next starting character then happens. Backtracking can occur as
2402     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2403     if there is no match to the right, backtracking cannot cross (*PRUNE).
2404 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2405 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2406     be expressed in any other way.
2407     .sp
2408     (*SKIP)
2409     .sp
2410 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2411 ph10 210 "bumpalong" advance is not to the next character, but to the position in the
2412     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2413     was matched leading up to it cannot be part of a successful match. Consider:
2414     .sp
2415     a+(*SKIP)b
2416     .sp
2417 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2418 ph10 210 the first character in the string), the starting point skips on to start the
2419 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2420 ph10 456 effect as this example; although it would suppress backtracking during the
2421 ph10 210 first match attempt, the second attempt would start at the second character
2422     instead of skipping on to "c".
2423     .sp
2424     (*THEN)
2425 ph10 211 .sp
2426 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2427     not match. That is, it cancels pending backtracking, but only within the
2428     current alternation. Its name comes from the observation that it can be used
2429     for a pattern-based if-then-else block:
2430     .sp
2431     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2432     .sp
2433 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2434 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2435 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2436 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE).
2437     .
2438     .
2439 nigel 93 .SH "SEE ALSO"
2440     .rs
2441     .sp
2442 ph10 461 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2443 ph10 456 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2444 ph10 99 .
2445     .
2446     .SH AUTHOR
2447     .rs
2448     .sp
2449     .nf
2450     Philip Hazel
2451     University Computing Service
2452     Cambridge CB2 3QH, England.
2453     .fi
2454     .
2455     .
2456     .SH REVISION
2457     .rs
2458     .sp
2459     .nf
2460 ph10 500 Last updated: 06 March 2010
2461 ph10 488 Copyright (c) 1997-2010 University of Cambridge.
2462 ph10 99 .fi

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