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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     there is now also support for UTF-8 character strings. To use this, you must
25     build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
26 ph10 416 the PCRE_UTF8 option. There is also a special sequence that can be given at the
27 ph10 412 start of a pattern:
28     .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     These override the default and the options given to \fBpcre_compile()\fP. For
87     example, on a Unix system where LF is the default newline sequence, the pattern
88     .sp
89     (*CR)a.b
90     .sp
91     changes the convention to CR. That pattern matches "a\enb" because LF is no
92     longer a newline. Note that these special settings, which are not
93     Perl-compatible, are recognized only at the very start of a pattern, and that
94 ph10 231 they must be in upper case. If more than one of them is present, the last one
95     is used.
96     .P
97     The newline convention does not affect what the \eR escape sequence matches. By
98     default, this is any Unicode newline sequence, for Perl compatibility. However,
99     this can be changed; see the description of \eR in the section entitled
100     .\" HTML <a href="#newlineseq">
101     .\" </a>
102     "Newline sequences"
103     .\"
104 ph10 247 below. A change of \eR setting can be combined with a change of newline
105 ph10 246 convention.
106 ph10 227 .
107     .
108 nigel 93 .SH "CHARACTERS AND METACHARACTERS"
109     .rs
110     .sp
111 nigel 63 A regular expression is a pattern that is matched against a subject string from
112     left to right. Most characters stand for themselves in a pattern, and match the
113     corresponding characters in the subject. As a trivial example, the pattern
114 nigel 75 .sp
115 nigel 63 The quick brown fox
116 nigel 75 .sp
117 nigel 77 matches a portion of a subject string that is identical to itself. When
118     caseless matching is specified (the PCRE_CASELESS option), letters are matched
119     independently of case. In UTF-8 mode, PCRE always understands the concept of
120     case for characters whose values are less than 128, so caseless matching is
121     always possible. For characters with higher values, the concept of case is
122     supported if PCRE is compiled with Unicode property support, but not otherwise.
123     If you want to use caseless matching for characters 128 and above, you must
124     ensure that PCRE is compiled with Unicode property support as well as with
125     UTF-8 support.
126     .P
127     The power of regular expressions comes from the ability to include alternatives
128     and repetitions in the pattern. These are encoded in the pattern by the use of
129 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
130 nigel 63 interpreted in some special way.
131 nigel 75 .P
132     There are two different sets of metacharacters: those that are recognized
133 nigel 63 anywhere in the pattern except within square brackets, and those that are
134 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
135     are as follows:
136 nigel 75 .sp
137     \e general escape character with several uses
138 nigel 63 ^ assert start of string (or line, in multiline mode)
139     $ assert end of string (or line, in multiline mode)
140     . match any character except newline (by default)
141     [ start character class definition
142     | start of alternative branch
143     ( start subpattern
144     ) end subpattern
145     ? extends the meaning of (
146     also 0 or 1 quantifier
147     also quantifier minimizer
148     * 0 or more quantifier
149     + 1 or more quantifier
150     also "possessive quantifier"
151     { start min/max quantifier
152 nigel 75 .sp
153 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
154 nigel 75 a character class the only metacharacters are:
155     .sp
156     \e general escape character
157 nigel 63 ^ negate the class, but only if the first character
158     - indicates character range
159 nigel 75 .\" JOIN
160 nigel 63 [ POSIX character class (only if followed by POSIX
161     syntax)
162     ] terminates the character class
163 nigel 75 .sp
164     The following sections describe the use of each of the metacharacters.
165     .
166 nigel 93 .
167 nigel 63 .SH BACKSLASH
168     .rs
169     .sp
170     The backslash character has several uses. Firstly, if it is followed by a
171 nigel 91 non-alphanumeric character, it takes away any special meaning that character
172     may have. This use of backslash as an escape character applies both inside and
173 nigel 63 outside character classes.
174 nigel 75 .P
175     For example, if you want to match a * character, you write \e* in the pattern.
176 nigel 63 This escaping action applies whether or not the following character would
177 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
178     non-alphanumeric with backslash to specify that it stands for itself. In
179     particular, if you want to match a backslash, you write \e\e.
180     .P
181 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
182     pattern (other than in a character class) and characters between a # outside
183 nigel 91 a character class and the next newline are ignored. An escaping backslash can
184     be used to include a whitespace or # character as part of the pattern.
185 nigel 75 .P
186 nigel 63 If you want to remove the special meaning from a sequence of characters, you
187 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
188     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
189 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
190 nigel 75 .sp
191 nigel 63 Pattern PCRE matches Perl matches
192 nigel 75 .sp
193     .\" JOIN
194     \eQabc$xyz\eE abc$xyz abc followed by the
195 nigel 63 contents of $xyz
196 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
197     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
198     .sp
199     The \eQ...\eE sequence is recognized both inside and outside character classes.
200     .
201     .
202     .\" HTML <a name="digitsafterbackslash"></a>
203     .SS "Non-printing characters"
204     .rs
205     .sp
206 nigel 63 A second use of backslash provides a way of encoding non-printing characters
207     in patterns in a visible manner. There is no restriction on the appearance of
208     non-printing characters, apart from the binary zero that terminates a pattern,
209     but when a pattern is being prepared by text editing, it is usually easier to
210     use one of the following escape sequences than the binary character it
211     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     \eddd character with octal code ddd, or backreference
221     \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 454 synonymous. The former is a back reference; the latter is a
337     .\" 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     These override the default and the options given to \fBpcre_compile()\fP, but
472     they can be overridden by options given to \fBpcre_exec()\fP. Note that these
473     special settings, which are not Perl-compatible, are recognized only at the
474     very start of a pattern, and that they must be in upper case. If more than one
475 ph10 247 of them is present, the last one is used. They can be combined with a change of
476 ph10 246 newline convention, for example, a pattern can start with:
477     .sp
478     (*ANY)(*BSR_ANYCRLF)
479     .sp
480 nigel 93 Inside a character class, \eR matches the letter "R".
481     .
482     .
483 nigel 75 .\" HTML <a name="uniextseq"></a>
484     .SS Unicode character properties
485     .rs
486     .sp
487     When PCRE is built with Unicode character property support, three additional
488 ph10 184 escape sequences that match characters with specific properties are available.
489     When not in UTF-8 mode, these sequences are of course limited to testing
490     characters whose codepoints are less than 256, but they do work in this mode.
491     The extra escape sequences are:
492 nigel 75 .sp
493 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
494     \eP{\fIxx\fP} a character without the \fIxx\fP property
495     \eX an extended Unicode sequence
496 nigel 75 .sp
497 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
498     script names, the general category properties, and "Any", which matches any
499     character (including newline). Other properties such as "InMusicalSymbols" are
500     not currently supported by PCRE. Note that \eP{Any} does not match any
501     characters, so always causes a match failure.
502 nigel 75 .P
503 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
504     character from one of these sets can be matched using a script name. For
505     example:
506 nigel 75 .sp
507 nigel 87 \ep{Greek}
508     \eP{Han}
509     .sp
510     Those that are not part of an identified script are lumped together as
511     "Common". The current list of scripts is:
512     .P
513     Arabic,
514     Armenian,
515 nigel 93 Balinese,
516 nigel 87 Bengali,
517     Bopomofo,
518     Braille,
519     Buginese,
520     Buhid,
521     Canadian_Aboriginal,
522     Cherokee,
523     Common,
524     Coptic,
525 nigel 93 Cuneiform,
526 nigel 87 Cypriot,
527     Cyrillic,
528     Deseret,
529     Devanagari,
530     Ethiopic,
531     Georgian,
532     Glagolitic,
533     Gothic,
534     Greek,
535     Gujarati,
536     Gurmukhi,
537     Han,
538     Hangul,
539     Hanunoo,
540     Hebrew,
541     Hiragana,
542     Inherited,
543     Kannada,
544     Katakana,
545     Kharoshthi,
546     Khmer,
547     Lao,
548     Latin,
549     Limbu,
550     Linear_B,
551     Malayalam,
552     Mongolian,
553     Myanmar,
554     New_Tai_Lue,
555 nigel 93 Nko,
556 nigel 87 Ogham,
557     Old_Italic,
558     Old_Persian,
559     Oriya,
560     Osmanya,
561 nigel 93 Phags_Pa,
562     Phoenician,
563 nigel 87 Runic,
564     Shavian,
565     Sinhala,
566     Syloti_Nagri,
567     Syriac,
568     Tagalog,
569     Tagbanwa,
570     Tai_Le,
571     Tamil,
572     Telugu,
573     Thaana,
574     Thai,
575     Tibetan,
576     Tifinagh,
577     Ugaritic,
578     Yi.
579     .P
580     Each character has exactly one general category property, specified by a
581     two-letter abbreviation. For compatibility with Perl, negation can be specified
582     by including a circumflex between the opening brace and the property name. For
583     example, \ep{^Lu} is the same as \eP{Lu}.
584     .P
585     If only one letter is specified with \ep or \eP, it includes all the general
586     category properties that start with that letter. In this case, in the absence
587     of negation, the curly brackets in the escape sequence are optional; these two
588     examples have the same effect:
589     .sp
590 nigel 75 \ep{L}
591     \epL
592     .sp
593 nigel 87 The following general category property codes are supported:
594 nigel 75 .sp
595     C Other
596     Cc Control
597     Cf Format
598     Cn Unassigned
599     Co Private use
600     Cs Surrogate
601     .sp
602     L Letter
603     Ll Lower case letter
604     Lm Modifier letter
605     Lo Other letter
606     Lt Title case letter
607     Lu Upper case letter
608     .sp
609     M Mark
610     Mc Spacing mark
611     Me Enclosing mark
612     Mn Non-spacing mark
613     .sp
614     N Number
615     Nd Decimal number
616     Nl Letter number
617     No Other number
618     .sp
619     P Punctuation
620     Pc Connector punctuation
621     Pd Dash punctuation
622     Pe Close punctuation
623     Pf Final punctuation
624     Pi Initial punctuation
625     Po Other punctuation
626     Ps Open punctuation
627     .sp
628     S Symbol
629     Sc Currency symbol
630     Sk Modifier symbol
631     Sm Mathematical symbol
632     So Other symbol
633     .sp
634     Z Separator
635     Zl Line separator
636     Zp Paragraph separator
637     Zs Space separator
638     .sp
639 nigel 87 The special property L& is also supported: it matches a character that has
640     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
641     a modifier or "other".
642 nigel 75 .P
643 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
644     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
645     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
646     (see the discussion of PCRE_NO_UTF8_CHECK in the
647     .\" HREF
648     \fBpcreapi\fP
649     .\"
650 ph10 451 page). Perl does not support the Cs property.
651 ph10 211 .P
652 ph10 451 The long synonyms for property names that Perl supports (such as \ep{Letter})
653 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
654 nigel 87 properties with "Is".
655     .P
656     No character that is in the Unicode table has the Cn (unassigned) property.
657     Instead, this property is assumed for any code point that is not in the
658     Unicode table.
659     .P
660 nigel 75 Specifying caseless matching does not affect these escape sequences. For
661     example, \ep{Lu} always matches only upper case letters.
662     .P
663     The \eX escape matches any number of Unicode characters that form an extended
664     Unicode sequence. \eX is equivalent to
665     .sp
666     (?>\ePM\epM*)
667     .sp
668     That is, it matches a character without the "mark" property, followed by zero
669     or more characters with the "mark" property, and treats the sequence as an
670     atomic group
671     .\" HTML <a href="#atomicgroup">
672     .\" </a>
673     (see below).
674     .\"
675     Characters with the "mark" property are typically accents that affect the
676 ph10 185 preceding character. None of them have codepoints less than 256, so in
677 ph10 184 non-UTF-8 mode \eX matches any one character.
678 nigel 75 .P
679     Matching characters by Unicode property is not fast, because PCRE has to search
680     a structure that contains data for over fifteen thousand characters. That is
681     why the traditional escape sequences such as \ed and \ew do not use Unicode
682     properties in PCRE.
683     .
684     .
685 ph10 168 .\" HTML <a name="resetmatchstart"></a>
686     .SS "Resetting the match start"
687     .rs
688     .sp
689 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
690     matched characters not to be included in the final matched sequence. For
691 ph10 168 example, the pattern:
692     .sp
693     foo\eKbar
694     .sp
695 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
696 ph10 168 similar to a lookbehind assertion
697     .\" HTML <a href="#lookbehind">
698     .\" </a>
699     (described below).
700     .\"
701 ph10 172 However, in this case, the part of the subject before the real match does not
702     have to be of fixed length, as lookbehind assertions do. The use of \eK does
703 ph10 168 not interfere with the setting of
704     .\" HTML <a href="#subpattern">
705     .\" </a>
706     captured substrings.
707 ph10 172 .\"
708 ph10 168 For example, when the pattern
709     .sp
710     (foo)\eKbar
711     .sp
712 ph10 172 matches "foobar", the first substring is still set to "foo".
713 ph10 168 .
714     .
715 nigel 75 .\" HTML <a name="smallassertions"></a>
716     .SS "Simple assertions"
717     .rs
718     .sp
719 nigel 93 The final use of backslash is for certain simple assertions. An assertion
720 nigel 63 specifies a condition that has to be met at a particular point in a match,
721     without consuming any characters from the subject string. The use of
722 nigel 75 subpatterns for more complicated assertions is described
723     .\" HTML <a href="#bigassertions">
724     .\" </a>
725     below.
726     .\"
727 nigel 91 The backslashed assertions are:
728 nigel 75 .sp
729     \eb matches at a word boundary
730     \eB matches when not at a word boundary
731 nigel 93 \eA matches at the start of the subject
732     \eZ matches at the end of the subject
733     also matches before a newline at the end of the subject
734     \ez matches only at the end of the subject
735     \eG matches at the first matching position in the subject
736 nigel 75 .sp
737     These assertions may not appear in character classes (but note that \eb has a
738 nigel 63 different meaning, namely the backspace character, inside a character class).
739 nigel 75 .P
740 nigel 63 A word boundary is a position in the subject string where the current character
741 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
742     \ew and the other matches \eW), or the start or end of the string if the
743     first or last character matches \ew, respectively.
744     .P
745     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
746     dollar (described in the next section) in that they only ever match at the very
747     start and end of the subject string, whatever options are set. Thus, they are
748     independent of multiline mode. These three assertions are not affected by the
749     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
750     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
751     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
752     at a point other than the beginning of the subject, \eA can never match. The
753 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
754     of the string as well as at the very end, whereas \ez matches only at the end.
755 nigel 75 .P
756     The \eG assertion is true only when the current matching position is at the
757     start point of the match, as specified by the \fIstartoffset\fP argument of
758     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
759     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
760 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
761 nigel 75 implementation where \eG can be useful.
762     .P
763     Note, however, that PCRE's interpretation of \eG, as the start of the current
764 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
765     previous match. In Perl, these can be different when the previously matched
766     string was empty. Because PCRE does just one match at a time, it cannot
767     reproduce this behaviour.
768 nigel 75 .P
769     If all the alternatives of a pattern begin with \eG, the expression is anchored
770 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
771     regular expression.
772 nigel 75 .
773     .
774     .SH "CIRCUMFLEX AND DOLLAR"
775 nigel 63 .rs
776     .sp
777     Outside a character class, in the default matching mode, the circumflex
778 nigel 75 character is an assertion that is true only if the current matching point is
779     at the start of the subject string. If the \fIstartoffset\fP argument of
780     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
781 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
782 nigel 75 meaning
783     .\" HTML <a href="#characterclass">
784     .\" </a>
785     (see below).
786     .\"
787     .P
788 nigel 63 Circumflex need not be the first character of the pattern if a number of
789     alternatives are involved, but it should be the first thing in each alternative
790     in which it appears if the pattern is ever to match that branch. If all
791     possible alternatives start with a circumflex, that is, if the pattern is
792     constrained to match only at the start of the subject, it is said to be an
793     "anchored" pattern. (There are also other constructs that can cause a pattern
794     to be anchored.)
795 nigel 75 .P
796     A dollar character is an assertion that is true only if the current matching
797 nigel 63 point is at the end of the subject string, or immediately before a newline
798 nigel 91 at the end of the string (by default). Dollar need not be the last character of
799     the pattern if a number of alternatives are involved, but it should be the last
800     item in any branch in which it appears. Dollar has no special meaning in a
801     character class.
802 nigel 75 .P
803 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
804     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
805 nigel 75 does not affect the \eZ assertion.
806     .P
807 nigel 63 The meanings of the circumflex and dollar characters are changed if the
808 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
809     immediately after internal newlines as well as at the start of the subject
810     string. It does not match after a newline that ends the string. A dollar
811     matches before any newlines in the string, as well as at the very end, when
812     PCRE_MULTILINE is set. When newline is specified as the two-character
813     sequence CRLF, isolated CR and LF characters do not indicate newlines.
814 nigel 75 .P
815 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
816     \en represents a newline) in multiline mode, but not otherwise. Consequently,
817     patterns that are anchored in single line mode because all branches start with
818     ^ are not anchored in multiline mode, and a match for circumflex is possible
819     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
820     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
821     .P
822 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
823 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
824 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
825 nigel 75 .
826     .
827     .SH "FULL STOP (PERIOD, DOT)"
828 nigel 63 .rs
829     .sp
830     Outside a character class, a dot in the pattern matches any one character in
831 nigel 91 the subject string except (by default) a character that signifies the end of a
832 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
833 nigel 91 .P
834 nigel 93 When a line ending is defined as a single character, dot never matches that
835     character; when the two-character sequence CRLF is used, dot does not match CR
836     if it is immediately followed by LF, but otherwise it matches all characters
837     (including isolated CRs and LFs). When any Unicode line endings are being
838     recognized, dot does not match CR or LF or any of the other line ending
839     characters.
840     .P
841 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
842 nigel 93 option is set, a dot matches any one character, without exception. If the
843     two-character sequence CRLF is present in the subject string, it takes two dots
844     to match it.
845 nigel 91 .P
846     The handling of dot is entirely independent of the handling of circumflex and
847     dollar, the only relationship being that they both involve newlines. Dot has no
848     special meaning in a character class.
849 nigel 75 .
850     .
851     .SH "MATCHING A SINGLE BYTE"
852 nigel 63 .rs
853     .sp
854 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
855 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
856     characters. The feature is provided in Perl in order to match individual bytes
857     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
858     what remains in the string may be a malformed UTF-8 string. For this reason,
859     the \eC escape sequence is best avoided.
860 nigel 75 .P
861     PCRE does not allow \eC to appear in lookbehind assertions
862     .\" HTML <a href="#lookbehind">
863     .\" </a>
864     (described below),
865     .\"
866     because in UTF-8 mode this would make it impossible to calculate the length of
867     the lookbehind.
868     .
869     .
870     .\" HTML <a name="characterclass"></a>
871     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
872 nigel 63 .rs
873     .sp
874     An opening square bracket introduces a character class, terminated by a closing
875     square bracket. A closing square bracket on its own is not special. If a
876     closing square bracket is required as a member of the class, it should be the
877     first data character in the class (after an initial circumflex, if present) or
878     escaped with a backslash.
879 nigel 75 .P
880 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
881     character may occupy more than one byte. A matched character must be in the set
882     of characters defined by the class, unless the first character in the class
883     definition is a circumflex, in which case the subject character must not be in
884     the set defined by the class. If a circumflex is actually required as a member
885     of the class, ensure it is not the first character, or escape it with a
886     backslash.
887 nigel 75 .P
888 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
889     [^aeiou] matches any character that is not a lower case vowel. Note that a
890 nigel 75 circumflex is just a convenient notation for specifying the characters that
891     are in the class by enumerating those that are not. A class that starts with a
892     circumflex is not an assertion: it still consumes a character from the subject
893     string, and therefore it fails if the current pointer is at the end of the
894     string.
895     .P
896 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
897 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
898     .P
899 nigel 63 When caseless matching is set, any letters in a class represent both their
900     upper case and lower case versions, so for example, a caseless [aeiou] matches
901     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
902 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
903     case for characters whose values are less than 128, so caseless matching is
904     always possible. For characters with higher values, the concept of case is
905     supported if PCRE is compiled with Unicode property support, but not otherwise.
906     If you want to use caseless matching for characters 128 and above, you must
907     ensure that PCRE is compiled with Unicode property support as well as with
908     UTF-8 support.
909 nigel 75 .P
910 nigel 93 Characters that might indicate line breaks are never treated in any special way
911     when matching character classes, whatever line-ending sequence is in use, and
912     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
913     such as [^a] always matches one of these characters.
914 nigel 75 .P
915 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
916     character class. For example, [d-m] matches any letter between d and m,
917     inclusive. If a minus character is required in a class, it must be escaped with
918     a backslash or appear in a position where it cannot be interpreted as
919     indicating a range, typically as the first or last character in the class.
920 nigel 75 .P
921 nigel 63 It is not possible to have the literal character "]" as the end character of a
922     range. A pattern such as [W-]46] is interpreted as a class of two characters
923     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
924     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
925 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
926     followed by two other characters. The octal or hexadecimal representation of
927     "]" can also be used to end a range.
928     .P
929 nigel 63 Ranges operate in the collating sequence of character values. They can also be
930 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
931 nigel 63 mode, ranges can include characters whose values are greater than 255, for
932 nigel 75 example [\ex{100}-\ex{2ff}].
933     .P
934 nigel 63 If a range that includes letters is used when caseless matching is set, it
935     matches the letters in either case. For example, [W-c] is equivalent to
936 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
937 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
938 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
939     characters with values greater than 128 only when it is compiled with Unicode
940     property support.
941     .P
942     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
943     in a character class, and add the characters that they match to the class. For
944     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
945 nigel 63 conveniently be used with the upper case character types to specify a more
946     restricted set of characters than the matching lower case type. For example,
947 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
948     .P
949     The only metacharacters that are recognized in character classes are backslash,
950     hyphen (only where it can be interpreted as specifying a range), circumflex
951     (only at the start), opening square bracket (only when it can be interpreted as
952     introducing a POSIX class name - see the next section), and the terminating
953     closing square bracket. However, escaping other non-alphanumeric characters
954     does no harm.
955     .
956     .
957     .SH "POSIX CHARACTER CLASSES"
958 nigel 63 .rs
959     .sp
960 nigel 75 Perl supports the POSIX notation for character classes. This uses names
961 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
962     this notation. For example,
963 nigel 75 .sp
964 nigel 63 [01[:alpha:]%]
965 nigel 75 .sp
966 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
967     are
968 nigel 75 .sp
969 nigel 63 alnum letters and digits
970     alpha letters
971     ascii character codes 0 - 127
972     blank space or tab only
973     cntrl control characters
974 nigel 75 digit decimal digits (same as \ed)
975 nigel 63 graph printing characters, excluding space
976     lower lower case letters
977     print printing characters, including space
978     punct printing characters, excluding letters and digits
979 nigel 75 space white space (not quite the same as \es)
980 nigel 63 upper upper case letters
981 nigel 75 word "word" characters (same as \ew)
982 nigel 63 xdigit hexadecimal digits
983 nigel 75 .sp
984 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
985     space (32). Notice that this list includes the VT character (code 11). This
986 nigel 75 makes "space" different to \es, which does not include VT (for Perl
987 nigel 63 compatibility).
988 nigel 75 .P
989 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
990     5.8. Another Perl extension is negation, which is indicated by a ^ character
991     after the colon. For example,
992 nigel 75 .sp
993 nigel 63 [12[:^digit:]]
994 nigel 75 .sp
995 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
996     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
997     supported, and an error is given if they are encountered.
998 nigel 75 .P
999     In UTF-8 mode, characters with values greater than 128 do not match any of
1000 nigel 63 the POSIX character classes.
1001 nigel 75 .
1002     .
1003     .SH "VERTICAL BAR"
1004 nigel 63 .rs
1005     .sp
1006     Vertical bar characters are used to separate alternative patterns. For example,
1007     the pattern
1008 nigel 75 .sp
1009 nigel 63 gilbert|sullivan
1010 nigel 75 .sp
1011 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1012 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1013     process tries each alternative in turn, from left to right, and the first one
1014     that succeeds is used. If the alternatives are within a subpattern
1015 nigel 75 .\" HTML <a href="#subpattern">
1016     .\" </a>
1017     (defined below),
1018     .\"
1019     "succeeds" means matching the rest of the main pattern as well as the
1020     alternative in the subpattern.
1021     .
1022     .
1023     .SH "INTERNAL OPTION SETTING"
1024 nigel 63 .rs
1025     .sp
1026     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1027 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1028     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1029     The option letters are
1030 nigel 75 .sp
1031 nigel 63 i for PCRE_CASELESS
1032     m for PCRE_MULTILINE
1033     s for PCRE_DOTALL
1034     x for PCRE_EXTENDED
1035 nigel 75 .sp
1036 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1037     unset these options by preceding the letter with a hyphen, and a combined
1038     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1039     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1040     permitted. If a letter appears both before and after the hyphen, the option is
1041     unset.
1042 nigel 75 .P
1043 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1044     changed in the same way as the Perl-compatible options by using the characters
1045     J, U and X respectively.
1046     .P
1047 ph10 412 When one of these option changes occurs at top level (that is, not inside
1048     subpattern parentheses), the change applies to the remainder of the pattern
1049     that follows. If the change is placed right at the start of a pattern, PCRE
1050     extracts it into the global options (and it will therefore show up in data
1051     extracted by the \fBpcre_fullinfo()\fP function).
1052 nigel 75 .P
1053 nigel 93 An option change within a subpattern (see below for a description of
1054     subpatterns) affects only that part of the current pattern that follows it, so
1055 nigel 75 .sp
1056 nigel 63 (a(?i)b)c
1057 nigel 75 .sp
1058 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1059     By this means, options can be made to have different settings in different
1060     parts of the pattern. Any changes made in one alternative do carry on
1061     into subsequent branches within the same subpattern. For example,
1062 nigel 75 .sp
1063 nigel 63 (a(?i)b|c)
1064 nigel 75 .sp
1065 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1066     branch is abandoned before the option setting. This is because the effects of
1067     option settings happen at compile time. There would be some very weird
1068     behaviour otherwise.
1069 ph10 251 .P
1070     \fBNote:\fP There are other PCRE-specific options that can be set by the
1071     application when the compile or match functions are called. In some cases the
1072 ph10 412 pattern can contain special leading sequences such as (*CRLF) to override what
1073     the application has set or what has been defaulted. Details are given in the
1074     section entitled
1075 ph10 251 .\" HTML <a href="#newlineseq">
1076     .\" </a>
1077     "Newline sequences"
1078     .\"
1079 ph10 416 above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1080 ph10 412 mode; this is equivalent to setting the PCRE_UTF8 option.
1081 nigel 75 .
1082     .
1083     .\" HTML <a name="subpattern"></a>
1084 nigel 63 .SH SUBPATTERNS
1085     .rs
1086     .sp
1087     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1088 nigel 75 Turning part of a pattern into a subpattern does two things:
1089     .sp
1090 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1091 nigel 75 .sp
1092 nigel 63 cat(aract|erpillar|)
1093 nigel 75 .sp
1094 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1095 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1096 nigel 75 .sp
1097     2. It sets up the subpattern as a capturing subpattern. This means that, when
1098     the whole pattern matches, that portion of the subject string that matched the
1099     subpattern is passed back to the caller via the \fIovector\fP argument of
1100     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1101     from 1) to obtain numbers for the capturing subpatterns.
1102     .P
1103 nigel 63 For example, if the string "the red king" is matched against the pattern
1104 nigel 75 .sp
1105 nigel 63 the ((red|white) (king|queen))
1106 nigel 75 .sp
1107 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1108     2, and 3, respectively.
1109 nigel 75 .P
1110 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1111     There are often times when a grouping subpattern is required without a
1112     capturing requirement. If an opening parenthesis is followed by a question mark
1113     and a colon, the subpattern does not do any capturing, and is not counted when
1114     computing the number of any subsequent capturing subpatterns. For example, if
1115     the string "the white queen" is matched against the pattern
1116 nigel 75 .sp
1117 nigel 63 the ((?:red|white) (king|queen))
1118 nigel 75 .sp
1119 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1120 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1121 nigel 75 .P
1122 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1123     a non-capturing subpattern, the option letters may appear between the "?" and
1124     the ":". Thus the two patterns
1125 nigel 75 .sp
1126 nigel 63 (?i:saturday|sunday)
1127     (?:(?i)saturday|sunday)
1128 nigel 75 .sp
1129 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1130     from left to right, and options are not reset until the end of the subpattern
1131     is reached, an option setting in one branch does affect subsequent branches, so
1132     the above patterns match "SUNDAY" as well as "Saturday".
1133 nigel 75 .
1134     .
1135 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1136     .rs
1137     .sp
1138 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1139     the same numbers for its capturing parentheses. Such a subpattern starts with
1140     (?| and is itself a non-capturing subpattern. For example, consider this
1141 ph10 175 pattern:
1142     .sp
1143     (?|(Sat)ur|(Sun))day
1144 ph10 182 .sp
1145     Because the two alternatives are inside a (?| group, both sets of capturing
1146     parentheses are numbered one. Thus, when the pattern matches, you can look
1147     at captured substring number one, whichever alternative matched. This construct
1148     is useful when you want to capture part, but not all, of one of a number of
1149     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1150 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1151 ph10 182 buffers that follow the subpattern start after the highest number used in any
1152     branch. The following example is taken from the Perl documentation.
1153 ph10 175 The numbers underneath show in which buffer the captured content will be
1154     stored.
1155     .sp
1156     # before ---------------branch-reset----------- after
1157     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1158     # 1 2 2 3 2 3 4
1159 ph10 182 .sp
1160 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1161     the first one in the pattern with the given number.
1162     .P
1163     An alternative approach to using this "branch reset" feature is to use
1164     duplicate named subpatterns, as described in the next section.
1165     .
1166     .
1167 nigel 75 .SH "NAMED SUBPATTERNS"
1168 nigel 63 .rs
1169     .sp
1170     Identifying capturing parentheses by number is simple, but it can be very hard
1171     to keep track of the numbers in complicated regular expressions. Furthermore,
1172 nigel 75 if an expression is modified, the numbers may change. To help with this
1173 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1174     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1175     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1176     the Perl and the Python syntax.
1177     .P
1178     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1179     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1180 nigel 91 parentheses from other parts of the pattern, such as
1181     .\" HTML <a href="#backreferences">
1182     .\" </a>
1183     backreferences,
1184     .\"
1185     .\" HTML <a href="#recursion">
1186     .\" </a>
1187     recursion,
1188     .\"
1189     and
1190     .\" HTML <a href="#conditions">
1191     .\" </a>
1192     conditions,
1193     .\"
1194     can be made by name as well as by number.
1195 nigel 75 .P
1196 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1197 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1198     if the names were not present. The PCRE API provides function calls for
1199     extracting the name-to-number translation table from a compiled pattern. There
1200     is also a convenience function for extracting a captured substring by name.
1201 nigel 91 .P
1202     By default, a name must be unique within a pattern, but it is possible to relax
1203     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1204     be useful for patterns where only one instance of the named parentheses can
1205     match. Suppose you want to match the name of a weekday, either as a 3-letter
1206     abbreviation or as the full name, and in both cases you want to extract the
1207     abbreviation. This pattern (ignoring the line breaks) does the job:
1208     .sp
1209 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1210     (?<DN>Tue)(?:sday)?|
1211     (?<DN>Wed)(?:nesday)?|
1212     (?<DN>Thu)(?:rsday)?|
1213     (?<DN>Sat)(?:urday)?
1214 nigel 91 .sp
1215     There are five capturing substrings, but only one is ever set after a match.
1216 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1217 ph10 175 subpattern, as described in the previous section.)
1218     .P
1219 nigel 91 The convenience function for extracting the data by name returns the substring
1220 nigel 93 for the first (and in this example, the only) subpattern of that name that
1221 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1222     make a reference to a non-unique named subpattern from elsewhere in the
1223     pattern, the one that corresponds to the lowest number is used. For further
1224     details of the interfaces for handling named subpatterns, see the
1225 nigel 63 .\" HREF
1226 nigel 75 \fBpcreapi\fP
1227 nigel 63 .\"
1228     documentation.
1229 ph10 385 .P
1230     \fBWarning:\fP You cannot use different names to distinguish between two
1231     subpatterns with the same number (see the previous section) because PCRE uses
1232     only the numbers when matching.
1233 nigel 75 .
1234     .
1235 nigel 63 .SH REPETITION
1236     .rs
1237     .sp
1238     Repetition is specified by quantifiers, which can follow any of the following
1239     items:
1240 nigel 75 .sp
1241 nigel 63 a literal data character
1242 nigel 93 the dot metacharacter
1243 nigel 75 the \eC escape sequence
1244     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1245 nigel 93 the \eR escape sequence
1246 nigel 75 an escape such as \ed that matches a single character
1247 nigel 63 a character class
1248     a back reference (see next section)
1249     a parenthesized subpattern (unless it is an assertion)
1250 nigel 75 .sp
1251 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1252     permitted matches, by giving the two numbers in curly brackets (braces),
1253     separated by a comma. The numbers must be less than 65536, and the first must
1254     be less than or equal to the second. For example:
1255 nigel 75 .sp
1256 nigel 63 z{2,4}
1257 nigel 75 .sp
1258 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1259     character. If the second number is omitted, but the comma is present, there is
1260     no upper limit; if the second number and the comma are both omitted, the
1261     quantifier specifies an exact number of required matches. Thus
1262 nigel 75 .sp
1263 nigel 63 [aeiou]{3,}
1264 nigel 75 .sp
1265 nigel 63 matches at least 3 successive vowels, but may match many more, while
1266 nigel 75 .sp
1267     \ed{8}
1268     .sp
1269 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1270     where a quantifier is not allowed, or one that does not match the syntax of a
1271     quantifier, is taken as a literal character. For example, {,6} is not a
1272     quantifier, but a literal string of four characters.
1273 nigel 75 .P
1274 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1275 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1276     which is represented by a two-byte sequence. Similarly, when Unicode property
1277     support is available, \eX{3} matches three Unicode extended sequences, each of
1278     which may be several bytes long (and they may be of different lengths).
1279     .P
1280 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1281 ph10 345 previous item and the quantifier were not present. This may be useful for
1282     subpatterns that are referenced as
1283 ph10 335 .\" HTML <a href="#subpatternsassubroutines">
1284     .\" </a>
1285     subroutines
1286     .\"
1287 ph10 345 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1288 ph10 335 quantifier are omitted from the compiled pattern.
1289 nigel 75 .P
1290 nigel 93 For convenience, the three most common quantifiers have single-character
1291     abbreviations:
1292 nigel 75 .sp
1293 nigel 63 * is equivalent to {0,}
1294     + is equivalent to {1,}
1295     ? is equivalent to {0,1}
1296 nigel 75 .sp
1297 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1298     match no characters with a quantifier that has no upper limit, for example:
1299 nigel 75 .sp
1300 nigel 63 (a?)*
1301 nigel 75 .sp
1302 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1303     such patterns. However, because there are cases where this can be useful, such
1304     patterns are now accepted, but if any repetition of the subpattern does in fact
1305     match no characters, the loop is forcibly broken.
1306 nigel 75 .P
1307 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1308     possible (up to the maximum number of permitted times), without causing the
1309     rest of the pattern to fail. The classic example of where this gives problems
1310 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1311     and within the comment, individual * and / characters may appear. An attempt to
1312     match C comments by applying the pattern
1313     .sp
1314     /\e*.*\e*/
1315     .sp
1316 nigel 63 to the string
1317 nigel 75 .sp
1318     /* first comment */ not comment /* second comment */
1319     .sp
1320 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1321     item.
1322 nigel 75 .P
1323 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1324     greedy, and instead matches the minimum number of times possible, so the
1325     pattern
1326 nigel 75 .sp
1327     /\e*.*?\e*/
1328     .sp
1329 nigel 63 does the right thing with the C comments. The meaning of the various
1330     quantifiers is not otherwise changed, just the preferred number of matches.
1331     Do not confuse this use of question mark with its use as a quantifier in its
1332     own right. Because it has two uses, it can sometimes appear doubled, as in
1333 nigel 75 .sp
1334     \ed??\ed
1335     .sp
1336 nigel 63 which matches one digit by preference, but can match two if that is the only
1337     way the rest of the pattern matches.
1338 nigel 75 .P
1339 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1340 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1341     greedy by following them with a question mark. In other words, it inverts the
1342     default behaviour.
1343 nigel 75 .P
1344 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1345 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1346 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1347 nigel 75 .P
1348 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1349 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1350 nigel 63 implicitly anchored, because whatever follows will be tried against every
1351     character position in the subject string, so there is no point in retrying the
1352     overall match at any position after the first. PCRE normally treats such a
1353 nigel 75 pattern as though it were preceded by \eA.
1354     .P
1355 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1356     worth setting PCRE_DOTALL in order to obtain this optimization, or
1357     alternatively using ^ to indicate anchoring explicitly.
1358 nigel 75 .P
1359 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1360     is inside capturing parentheses that are the subject of a backreference
1361 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1362     succeeds. Consider, for example:
1363 nigel 75 .sp
1364     (.*)abc\e1
1365     .sp
1366 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1367     this reason, such a pattern is not implicitly anchored.
1368 nigel 75 .P
1369 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1370     that matched the final iteration. For example, after
1371 nigel 75 .sp
1372     (tweedle[dume]{3}\es*)+
1373     .sp
1374 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1375     "tweedledee". However, if there are nested capturing subpatterns, the
1376     corresponding captured values may have been set in previous iterations. For
1377     example, after
1378 nigel 75 .sp
1379 nigel 63 /(a|(b))+/
1380 nigel 75 .sp
1381 nigel 63 matches "aba" the value of the second captured substring is "b".
1382 nigel 75 .
1383     .
1384     .\" HTML <a name="atomicgroup"></a>
1385     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1386 nigel 63 .rs
1387     .sp
1388 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1389     repetition, failure of what follows normally causes the repeated item to be
1390     re-evaluated to see if a different number of repeats allows the rest of the
1391     pattern to match. Sometimes it is useful to prevent this, either to change the
1392     nature of the match, or to cause it fail earlier than it otherwise might, when
1393     the author of the pattern knows there is no point in carrying on.
1394 nigel 75 .P
1395     Consider, for example, the pattern \ed+foo when applied to the subject line
1396     .sp
1397 nigel 63 123456bar
1398 nigel 75 .sp
1399 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1400 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1401 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1402     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1403     that once a subpattern has matched, it is not to be re-evaluated in this way.
1404 nigel 75 .P
1405 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1406 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1407     special parenthesis, starting with (?> as in this example:
1408 nigel 75 .sp
1409     (?>\ed+)foo
1410     .sp
1411 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1412     it has matched, and a failure further into the pattern is prevented from
1413     backtracking into it. Backtracking past it to previous items, however, works as
1414     normal.
1415 nigel 75 .P
1416 nigel 63 An alternative description is that a subpattern of this type matches the string
1417     of characters that an identical standalone pattern would match, if anchored at
1418     the current point in the subject string.
1419 nigel 75 .P
1420 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1421     the above example can be thought of as a maximizing repeat that must swallow
1422 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1423 nigel 63 number of digits they match in order to make the rest of the pattern match,
1424 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1425     .P
1426 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1427     subpatterns, and can be nested. However, when the subpattern for an atomic
1428     group is just a single repeated item, as in the example above, a simpler
1429     notation, called a "possessive quantifier" can be used. This consists of an
1430     additional + character following a quantifier. Using this notation, the
1431     previous example can be rewritten as
1432 nigel 75 .sp
1433     \ed++foo
1434     .sp
1435 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1436     example:
1437     .sp
1438     (abc|xyz){2,3}+
1439     .sp
1440 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1441     option is ignored. They are a convenient notation for the simpler forms of
1442 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1443     quantifier and the equivalent atomic group, though there may be a performance
1444     difference; possessive quantifiers should be slightly faster.
1445 nigel 75 .P
1446 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1447     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1448     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1449     package, and PCRE copied it from there. It ultimately found its way into Perl
1450     at release 5.10.
1451 nigel 75 .P
1452 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1453     pattern constructs. For example, the sequence A+B is treated as A++B because
1454     there is no point in backtracking into a sequence of A's when B must follow.
1455     .P
1456 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1457     be repeated an unlimited number of times, the use of an atomic group is the
1458     only way to avoid some failing matches taking a very long time indeed. The
1459     pattern
1460 nigel 75 .sp
1461     (\eD+|<\ed+>)*[!?]
1462     .sp
1463 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1464     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1465     quickly. However, if it is applied to
1466 nigel 75 .sp
1467 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1468 nigel 75 .sp
1469 nigel 63 it takes a long time before reporting failure. This is because the string can
1470 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1471     large number of ways, and all have to be tried. (The example uses [!?] rather
1472     than a single character at the end, because both PCRE and Perl have an
1473     optimization that allows for fast failure when a single character is used. They
1474     remember the last single character that is required for a match, and fail early
1475     if it is not present in the string.) If the pattern is changed so that it uses
1476     an atomic group, like this:
1477     .sp
1478     ((?>\eD+)|<\ed+>)*[!?]
1479     .sp
1480 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1481 nigel 75 .
1482     .
1483     .\" HTML <a name="backreferences"></a>
1484     .SH "BACK REFERENCES"
1485 nigel 63 .rs
1486     .sp
1487     Outside a character class, a backslash followed by a digit greater than 0 (and
1488     possibly further digits) is a back reference to a capturing subpattern earlier
1489     (that is, to its left) in the pattern, provided there have been that many
1490     previous capturing left parentheses.
1491 nigel 75 .P
1492 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1493     always taken as a back reference, and causes an error only if there are not
1494     that many capturing left parentheses in the entire pattern. In other words, the
1495     parentheses that are referenced need not be to the left of the reference for
1496 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1497     when a repetition is involved and the subpattern to the right has participated
1498     in an earlier iteration.
1499     .P
1500 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1501     whose number is 10 or more using this syntax because a sequence such as \e50 is
1502     interpreted as a character defined in octal. See the subsection entitled
1503 nigel 91 "Non-printing characters"
1504 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1505     .\" </a>
1506     above
1507     .\"
1508 nigel 93 for further details of the handling of digits following a backslash. There is
1509     no such problem when named parentheses are used. A back reference to any
1510     subpattern is possible using named parentheses (see below).
1511 nigel 75 .P
1512 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1513     backslash is to use the \eg escape sequence, which is a feature introduced in
1514 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1515     number, optionally enclosed in braces. These examples are all identical:
1516 nigel 93 .sp
1517     (ring), \e1
1518     (ring), \eg1
1519     (ring), \eg{1}
1520     .sp
1521 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1522     is present in the older syntax. It is also useful when literal digits follow
1523     the reference. A negative number is a relative reference. Consider this
1524     example:
1525 nigel 93 .sp
1526     (abc(def)ghi)\eg{-1}
1527     .sp
1528     The sequence \eg{-1} is a reference to the most recently started capturing
1529     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1530     would be equivalent to \e1. The use of relative references can be helpful in
1531     long patterns, and also in patterns that are created by joining together
1532     fragments that contain references within themselves.
1533     .P
1534 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1535     the current subject string, rather than anything matching the subpattern
1536     itself (see
1537     .\" HTML <a href="#subpatternsassubroutines">
1538     .\" </a>
1539     "Subpatterns as subroutines"
1540     .\"
1541     below for a way of doing that). So the pattern
1542 nigel 75 .sp
1543     (sens|respons)e and \e1ibility
1544     .sp
1545 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1546     "sense and responsibility". If caseful matching is in force at the time of the
1547     back reference, the case of letters is relevant. For example,
1548 nigel 75 .sp
1549     ((?i)rah)\es+\e1
1550     .sp
1551 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1552     capturing subpattern is matched caselessly.
1553 nigel 75 .P
1554 ph10 171 There are several different ways of writing back references to named
1555     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1556     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1557     back reference syntax, in which \eg can be used for both numeric and named
1558     references, is also supported. We could rewrite the above example in any of
1559 nigel 93 the following ways:
1560 nigel 75 .sp
1561 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1562 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1563 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1564 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1565 nigel 75 .sp
1566 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1567     after the reference.
1568     .P
1569 nigel 63 There may be more than one back reference to the same subpattern. If a
1570     subpattern has not actually been used in a particular match, any back
1571     references to it always fail. For example, the pattern
1572 nigel 75 .sp
1573     (a|(bc))\e2
1574     .sp
1575 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1576     many capturing parentheses in a pattern, all digits following the backslash are
1577     taken as part of a potential back reference number. If the pattern continues
1578     with a digit character, some delimiter must be used to terminate the back
1579     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1580 nigel 75 Otherwise an empty comment (see
1581     .\" HTML <a href="#comments">
1582     .\" </a>
1583     "Comments"
1584     .\"
1585     below) can be used.
1586     .P
1587 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1588 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1589 nigel 63 However, such references can be useful inside repeated subpatterns. For
1590     example, the pattern
1591 nigel 75 .sp
1592     (a|b\e1)+
1593     .sp
1594 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1595     the subpattern, the back reference matches the character string corresponding
1596     to the previous iteration. In order for this to work, the pattern must be such
1597     that the first iteration does not need to match the back reference. This can be
1598     done using alternation, as in the example above, or by a quantifier with a
1599     minimum of zero.
1600 nigel 75 .
1601     .
1602     .\" HTML <a name="bigassertions"></a>
1603 nigel 63 .SH ASSERTIONS
1604     .rs
1605     .sp
1606     An assertion is a test on the characters following or preceding the current
1607     matching point that does not actually consume any characters. The simple
1608 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1609     .\" HTML <a href="#smallassertions">
1610     .\" </a>
1611     above.
1612     .\"
1613     .P
1614 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1615     those that look ahead of the current position in the subject string, and those
1616 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1617     except that it does not cause the current matching position to be changed.
1618     .P
1619     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1620     because it makes no sense to assert the same thing several times. If any kind
1621     of assertion contains capturing subpatterns within it, these are counted for
1622     the purposes of numbering the capturing subpatterns in the whole pattern.
1623     However, substring capturing is carried out only for positive assertions,
1624     because it does not make sense for negative assertions.
1625     .
1626     .
1627     .SS "Lookahead assertions"
1628     .rs
1629     .sp
1630 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1631     negative assertions. For example,
1632 nigel 75 .sp
1633     \ew+(?=;)
1634     .sp
1635 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1636     the match, and
1637 nigel 75 .sp
1638 nigel 63 foo(?!bar)
1639 nigel 75 .sp
1640 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1641     apparently similar pattern
1642 nigel 75 .sp
1643 nigel 63 (?!foo)bar
1644 nigel 75 .sp
1645 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1646     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1647     (?!foo) is always true when the next three characters are "bar". A
1648 nigel 75 lookbehind assertion is needed to achieve the other effect.
1649     .P
1650 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1651     convenient way to do it is with (?!) because an empty string always matches, so
1652     an assertion that requires there not to be an empty string must always fail.
1653 nigel 75 .
1654     .
1655     .\" HTML <a name="lookbehind"></a>
1656     .SS "Lookbehind assertions"
1657     .rs
1658     .sp
1659 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1660     negative assertions. For example,
1661 nigel 75 .sp
1662 nigel 63 (?<!foo)bar
1663 nigel 75 .sp
1664 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1665     a lookbehind assertion are restricted such that all the strings it matches must
1666 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1667     do not all have to have the same fixed length. Thus
1668 nigel 75 .sp
1669 nigel 63 (?<=bullock|donkey)
1670 nigel 75 .sp
1671 nigel 63 is permitted, but
1672 nigel 75 .sp
1673 nigel 63 (?<!dogs?|cats?)
1674 nigel 75 .sp
1675 nigel 63 causes an error at compile time. Branches that match different length strings
1676     are permitted only at the top level of a lookbehind assertion. This is an
1677 ph10 454 extension compared with Perl (5.8 and 5.10), which requires all branches to
1678 nigel 63 match the same length of string. An assertion such as
1679 nigel 75 .sp
1680 nigel 63 (?<=ab(c|de))
1681 nigel 75 .sp
1682 nigel 63 is not permitted, because its single top-level branch can match two different
1683 ph10 454 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1684     branches:
1685 nigel 75 .sp
1686 nigel 63 (?<=abc|abde)
1687 nigel 75 .sp
1688 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1689 ph10 168 .\" HTML <a href="#resetmatchstart">
1690     .\" </a>
1691     (see above)
1692     .\"
1693 ph10 454 can be used instead of a lookbehind assertion to get round the fixed-length
1694     restriction.
1695 ph10 168 .P
1696 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1697 nigel 93 temporarily move the current position back by the fixed length and then try to
1698 nigel 63 match. If there are insufficient characters before the current position, the
1699 nigel 93 assertion fails.
1700 nigel 75 .P
1701     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1702 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1703 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1704     different numbers of bytes, are also not permitted.
1705 nigel 75 .P
1706 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
1707     .\" </a>
1708     "Subroutine"
1709     .\"
1710     calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1711     as the subpattern matches a fixed-length string.
1712     .\" HTML <a href="#recursion">
1713     .\" </a>
1714     Recursion,
1715     .\"
1716     however, is not supported.
1717     .P
1718 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1719     specify efficient matching at the end of the subject string. Consider a simple
1720     pattern such as
1721 nigel 75 .sp
1722 nigel 63 abcd$
1723 nigel 75 .sp
1724 nigel 63 when applied to a long string that does not match. Because matching proceeds
1725     from left to right, PCRE will look for each "a" in the subject and then see if
1726     what follows matches the rest of the pattern. If the pattern is specified as
1727 nigel 75 .sp
1728 nigel 63 ^.*abcd$
1729 nigel 75 .sp
1730 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1731     there is no following "a"), it backtracks to match all but the last character,
1732     then all but the last two characters, and so on. Once again the search for "a"
1733     covers the entire string, from right to left, so we are no better off. However,
1734     if the pattern is written as
1735 nigel 75 .sp
1736 nigel 63 ^.*+(?<=abcd)
1737 nigel 75 .sp
1738 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1739 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1740     characters. If it fails, the match fails immediately. For long strings, this
1741     approach makes a significant difference to the processing time.
1742 nigel 75 .
1743     .
1744     .SS "Using multiple assertions"
1745     .rs
1746     .sp
1747 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1748 nigel 75 .sp
1749     (?<=\ed{3})(?<!999)foo
1750     .sp
1751 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1752     the assertions is applied independently at the same point in the subject
1753     string. First there is a check that the previous three characters are all
1754     digits, and then there is a check that the same three characters are not "999".
1755 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1756 nigel 63 of which are digits and the last three of which are not "999". For example, it
1757     doesn't match "123abcfoo". A pattern to do that is
1758 nigel 75 .sp
1759     (?<=\ed{3}...)(?<!999)foo
1760     .sp
1761 nigel 63 This time the first assertion looks at the preceding six characters, checking
1762     that the first three are digits, and then the second assertion checks that the
1763     preceding three characters are not "999".
1764 nigel 75 .P
1765 nigel 63 Assertions can be nested in any combination. For example,
1766 nigel 75 .sp
1767 nigel 63 (?<=(?<!foo)bar)baz
1768 nigel 75 .sp
1769 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1770     preceded by "foo", while
1771 nigel 75 .sp
1772     (?<=\ed{3}(?!999)...)foo
1773     .sp
1774     is another pattern that matches "foo" preceded by three digits and any three
1775 nigel 63 characters that are not "999".
1776 nigel 75 .
1777     .
1778 nigel 91 .\" HTML <a name="conditions"></a>
1779 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1780 nigel 63 .rs
1781     .sp
1782     It is possible to cause the matching process to obey a subpattern
1783     conditionally or to choose between two alternative subpatterns, depending on
1784     the result of an assertion, or whether a previous capturing subpattern matched
1785     or not. The two possible forms of conditional subpattern are
1786 nigel 75 .sp
1787 nigel 63 (?(condition)yes-pattern)
1788     (?(condition)yes-pattern|no-pattern)
1789 nigel 75 .sp
1790 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1791     no-pattern (if present) is used. If there are more than two alternatives in the
1792     subpattern, a compile-time error occurs.
1793 nigel 75 .P
1794 nigel 93 There are four kinds of condition: references to subpatterns, references to
1795     recursion, a pseudo-condition called DEFINE, and assertions.
1796     .
1797     .SS "Checking for a used subpattern by number"
1798     .rs
1799     .sp
1800     If the text between the parentheses consists of a sequence of digits, the
1801     condition is true if the capturing subpattern of that number has previously
1802 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1803 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1804 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1805 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1806     refer to subsequent groups with constructs such as (?(+2).
1807 nigel 91 .P
1808     Consider the following pattern, which contains non-significant white space to
1809     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1810     three parts for ease of discussion:
1811 nigel 75 .sp
1812     ( \e( )? [^()]+ (?(1) \e) )
1813     .sp
1814 nigel 63 The first part matches an optional opening parenthesis, and if that
1815     character is present, sets it as the first captured substring. The second part
1816     matches one or more characters that are not parentheses. The third part is a
1817     conditional subpattern that tests whether the first set of parentheses matched
1818     or not. If they did, that is, if subject started with an opening parenthesis,
1819     the condition is true, and so the yes-pattern is executed and a closing
1820     parenthesis is required. Otherwise, since no-pattern is not present, the
1821     subpattern matches nothing. In other words, this pattern matches a sequence of
1822 nigel 93 non-parentheses, optionally enclosed in parentheses.
1823 ph10 167 .P
1824 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1825 ph10 167 reference:
1826     .sp
1827     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1828     .sp
1829     This makes the fragment independent of the parentheses in the larger pattern.
1830 nigel 93 .
1831     .SS "Checking for a used subpattern by name"
1832     .rs
1833 nigel 91 .sp
1834 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1835     subpattern by name. For compatibility with earlier versions of PCRE, which had
1836     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1837     there is a possible ambiguity with this syntax, because subpattern names may
1838     consist entirely of digits. PCRE looks first for a named subpattern; if it
1839     cannot find one and the name consists entirely of digits, PCRE looks for a
1840     subpattern of that number, which must be greater than zero. Using subpattern
1841     names that consist entirely of digits is not recommended.
1842     .P
1843     Rewriting the above example to use a named subpattern gives this:
1844 nigel 91 .sp
1845 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1846     .sp
1847     .
1848     .SS "Checking for pattern recursion"
1849     .rs
1850     .sp
1851 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1852 nigel 93 the condition is true if a recursive call to the whole pattern or any
1853     subpattern has been made. If digits or a name preceded by ampersand follow the
1854     letter R, for example:
1855     .sp
1856     (?(R3)...) or (?(R&name)...)
1857     .sp
1858     the condition is true if the most recent recursion is into the subpattern whose
1859     number or name is given. This condition does not check the entire recursion
1860     stack.
1861 nigel 75 .P
1862 ph10 454 At "top level", all these recursion test conditions are false.
1863     .\" HTML <a href="#recursion">
1864     .\" </a>
1865     Recursive patterns
1866     .\"
1867     are described below.
1868 nigel 93 .
1869     .SS "Defining subpatterns for use by reference only"
1870     .rs
1871     .sp
1872     If the condition is the string (DEFINE), and there is no subpattern with the
1873     name DEFINE, the condition is always false. In this case, there may be only one
1874     alternative in the subpattern. It is always skipped if control reaches this
1875     point in the pattern; the idea of DEFINE is that it can be used to define
1876 ph10 454 "subroutines" that can be referenced from elsewhere. (The use of
1877     .\" HTML <a href="#subpatternsassubroutines">
1878     .\" </a>
1879     "subroutines"
1880     .\"
1881 nigel 93 is described below.) For example, a pattern to match an IPv4 address could be
1882     written like this (ignore whitespace and line breaks):
1883     .sp
1884     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1885     \eb (?&byte) (\e.(?&byte)){3} \eb
1886     .sp
1887     The first part of the pattern is a DEFINE group inside which a another group
1888     named "byte" is defined. This matches an individual component of an IPv4
1889     address (a number less than 256). When matching takes place, this part of the
1890     pattern is skipped because DEFINE acts like a false condition.
1891     .P
1892     The rest of the pattern uses references to the named group to match the four
1893     dot-separated components of an IPv4 address, insisting on a word boundary at
1894     each end.
1895     .
1896     .SS "Assertion conditions"
1897     .rs
1898     .sp
1899     If the condition is not in any of the above formats, it must be an assertion.
1900 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1901     this pattern, again containing non-significant white space, and with the two
1902     alternatives on the second line:
1903 nigel 75 .sp
1904 nigel 63 (?(?=[^a-z]*[a-z])
1905 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1906     .sp
1907 nigel 63 The condition is a positive lookahead assertion that matches an optional
1908     sequence of non-letters followed by a letter. In other words, it tests for the
1909     presence of at least one letter in the subject. If a letter is found, the
1910     subject is matched against the first alternative; otherwise it is matched
1911     against the second. This pattern matches strings in one of the two forms
1912     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1913 nigel 75 .
1914     .
1915     .\" HTML <a name="comments"></a>
1916 nigel 63 .SH COMMENTS
1917     .rs
1918     .sp
1919 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1920 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1921     that make up a comment play no part in the pattern matching at all.
1922 nigel 75 .P
1923 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1924 nigel 91 character class introduces a comment that continues to immediately after the
1925     next newline in the pattern.
1926 nigel 75 .
1927     .
1928 nigel 91 .\" HTML <a name="recursion"></a>
1929 nigel 75 .SH "RECURSIVE PATTERNS"
1930 nigel 63 .rs
1931     .sp
1932     Consider the problem of matching a string in parentheses, allowing for
1933     unlimited nested parentheses. Without the use of recursion, the best that can
1934     be done is to use a pattern that matches up to some fixed depth of nesting. It
1935 nigel 93 is not possible to handle an arbitrary nesting depth.
1936     .P
1937     For some time, Perl has provided a facility that allows regular expressions to
1938     recurse (amongst other things). It does this by interpolating Perl code in the
1939     expression at run time, and the code can refer to the expression itself. A Perl
1940     pattern using code interpolation to solve the parentheses problem can be
1941     created like this:
1942 nigel 75 .sp
1943     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1944     .sp
1945 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1946 nigel 93 recursively to the pattern in which it appears.
1947 nigel 75 .P
1948 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1949     supports special syntax for recursion of the entire pattern, and also for
1950     individual subpattern recursion. After its introduction in PCRE and Python,
1951 ph10 453 this kind of recursion was subsequently introduced into Perl at release 5.10.
1952 nigel 75 .P
1953 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1954     closing parenthesis is a recursive call of the subpattern of the given number,
1955 ph10 454 provided that it occurs inside that subpattern. (If not, it is a
1956     .\" HTML <a href="#subpatternsassubroutines">
1957     .\" </a>
1958     "subroutine"
1959     .\"
1960 nigel 93 call, which is described in the next section.) The special item (?R) or (?0) is
1961     a recursive call of the entire regular expression.
1962 nigel 87 .P
1963     This PCRE pattern solves the nested parentheses problem (assume the
1964     PCRE_EXTENDED option is set so that white space is ignored):
1965 nigel 75 .sp
1966     \e( ( (?>[^()]+) | (?R) )* \e)
1967     .sp
1968 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1969     substrings which can either be a sequence of non-parentheses, or a recursive
1970 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1971 nigel 63 Finally there is a closing parenthesis.
1972 nigel 75 .P
1973 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1974     pattern, so instead you could use this:
1975 nigel 75 .sp
1976     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1977     .sp
1978 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1979 ph10 172 them instead of the whole pattern.
1980 ph10 166 .P
1981     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1982     is made easier by the use of relative references. (A Perl 5.10 feature.)
1983     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1984     most recently opened parentheses preceding the recursion. In other words, a
1985     negative number counts capturing parentheses leftwards from the point at which
1986     it is encountered.
1987     .P
1988     It is also possible to refer to subsequently opened parentheses, by writing
1989     references such as (?+2). However, these cannot be recursive because the
1990     reference is not inside the parentheses that are referenced. They are always
1991 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
1992     .\" </a>
1993     "subroutine"
1994     .\"
1995     calls, as described in the next section.
1996 ph10 166 .P
1997     An alternative approach is to use named parentheses instead. The Perl syntax
1998     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1999     could rewrite the above example as follows:
2000 nigel 75 .sp
2001 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
2002 nigel 75 .sp
2003 nigel 93 If there is more than one subpattern with the same name, the earliest one is
2004 ph10 172 used.
2005 ph10 166 .P
2006     This particular example pattern that we have been looking at contains nested
2007     unlimited repeats, and so the use of atomic grouping for matching strings of
2008     non-parentheses is important when applying the pattern to strings that do not
2009     match. For example, when this pattern is applied to
2010 nigel 75 .sp
2011 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2012 nigel 75 .sp
2013 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
2014     the match runs for a very long time indeed because there are so many different
2015     ways the + and * repeats can carve up the subject, and all have to be tested
2016     before failure can be reported.
2017 nigel 75 .P
2018 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
2019     from the outermost level of the recursion at which the subpattern value is set.
2020     If you want to obtain intermediate values, a callout function can be used (see
2021 nigel 93 below and the
2022 nigel 63 .\" HREF
2023 nigel 75 \fBpcrecallout\fP
2024 nigel 63 .\"
2025     documentation). If the pattern above is matched against
2026 nigel 75 .sp
2027 nigel 63 (ab(cd)ef)
2028 nigel 75 .sp
2029 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
2030     on at the top level. If additional parentheses are added, giving
2031 nigel 75 .sp
2032     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
2033 nigel 63 ^ ^
2034     ^ ^
2035 nigel 75 .sp
2036 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
2037     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
2038     has to obtain extra memory to store data during a recursion, which it does by
2039 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
2040 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2041 nigel 75 .P
2042 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2043     Consider this pattern, which matches text in angle brackets, allowing for
2044     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2045     recursing), whereas any characters are permitted at the outer level.
2046 nigel 75 .sp
2047     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2048     .sp
2049 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2050     different alternatives for the recursive and non-recursive cases. The (?R) item
2051     is the actual recursive call.
2052 nigel 75 .
2053     .
2054 ph10 453 .\" HTML <a name="recursiondifference"></a>
2055     .SS "Recursion difference from Perl"
2056     .rs
2057     .sp
2058     In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2059     treated as an atomic group. That is, once it has matched some of the subject
2060     string, it is never re-entered, even if it contains untried alternatives and
2061     there is a subsequent matching failure. This can be illustrated by the
2062     following pattern, which purports to match a palindromic string that contains
2063     an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2064     .sp
2065     ^(.|(.)(?1)\e2)$
2066     .sp
2067     The idea is that it either matches a single character, or two identical
2068     characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2069     it does not if the pattern is longer than three characters. Consider the
2070     subject string "abcba":
2071     .P
2072     At the top level, the first character is matched, but as it is not at the end
2073     of the string, the first alternative fails; the second alternative is taken
2074     and the recursion kicks in. The recursive call to subpattern 1 successfully
2075     matches the next character ("b"). (Note that the beginning and end of line
2076     tests are not part of the recursion).
2077     .P
2078     Back at the top level, the next character ("c") is compared with what
2079     subpattern 2 matched, which was "a". This fails. Because the recursion is
2080     treated as an atomic group, there are now no backtracking points, and so the
2081     entire match fails. (Perl is able, at this point, to re-enter the recursion and
2082     try the second alternative.) However, if the pattern is written with the
2083     alternatives in the other order, things are different:
2084     .sp
2085     ^((.)(?1)\e2|.)$
2086     .sp
2087     This time, the recursing alternative is tried first, and continues to recurse
2088     until it runs out of characters, at which point the recursion fails. But this
2089     time we do have another alternative to try at the higher level. That is the big
2090     difference: in the previous case the remaining alternative is at a deeper
2091     recursion level, which PCRE cannot use.
2092     .P
2093     To change the pattern so that matches all palindromic strings, not just those
2094     with an odd number of characters, it is tempting to change the pattern to this:
2095     .sp
2096     ^((.)(?1)\e2|.?)$
2097     .sp
2098     Again, this works in Perl, but not in PCRE, and for the same reason. When a
2099     deeper recursion has matched a single character, it cannot be entered again in
2100     order to match an empty string. The solution is to separate the two cases, and
2101     write out the odd and even cases as alternatives at the higher level:
2102     .sp
2103     ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2104     .sp
2105     If you want to match typical palindromic phrases, the pattern has to ignore all
2106     non-word characters, which can be done like this:
2107     .sp
2108     ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$
2109     .sp
2110     If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2111     man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2112     the use of the possessive quantifier *+ to avoid backtracking into sequences of
2113     non-word characters. Without this, PCRE takes a great deal longer (ten times or
2114     more) to match typical phrases, and Perl takes so long that you think it has
2115     gone into a loop.
2116     .
2117     .
2118 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
2119 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
2120 nigel 63 .rs
2121     .sp
2122     If the syntax for a recursive subpattern reference (either by number or by
2123     name) is used outside the parentheses to which it refers, it operates like a
2124 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
2125 ph10 166 before or after the reference. A numbered reference can be absolute or
2126     relative, as in these examples:
2127 nigel 75 .sp
2128 ph10 166 (...(absolute)...)...(?2)...
2129     (...(relative)...)...(?-1)...
2130 ph10 172 (...(?+1)...(relative)...
2131 ph10 166 .sp
2132     An earlier example pointed out that the pattern
2133     .sp
2134 nigel 75 (sens|respons)e and \e1ibility
2135     .sp
2136 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2137     "sense and responsibility". If instead the pattern
2138 nigel 75 .sp
2139 nigel 63 (sens|respons)e and (?1)ibility
2140 nigel 75 .sp
2141 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2142 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2143 nigel 87 .P
2144     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2145     group. That is, once it has matched some of the subject string, it is never
2146     re-entered, even if it contains untried alternatives and there is a subsequent
2147     matching failure.
2148 nigel 93 .P
2149     When a subpattern is used as a subroutine, processing options such as
2150     case-independence are fixed when the subpattern is defined. They cannot be
2151     changed for different calls. For example, consider this pattern:
2152     .sp
2153 ph10 166 (abc)(?i:(?-1))
2154 nigel 93 .sp
2155     It matches "abcabc". It does not match "abcABC" because the change of
2156     processing option does not affect the called subpattern.
2157 nigel 75 .
2158     .
2159 ph10 333 .\" HTML <a name="onigurumasubroutines"></a>
2160     .SH "ONIGURUMA SUBROUTINE SYNTAX"
2161     .rs
2162     .sp
2163 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2164     a number enclosed either in angle brackets or single quotes, is an alternative
2165     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2166 ph10 333 are two of the examples used above, rewritten using this syntax:
2167     .sp
2168     (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2169     (sens|respons)e and \eg'1'ibility
2170     .sp
2171 ph10 345 PCRE supports an extension to Oniguruma: if a number is preceded by a
2172 ph10 333 plus or a minus sign it is taken as a relative reference. For example:
2173     .sp
2174     (abc)(?i:\eg<-1>)
2175     .sp
2176 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2177 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
2178     .
2179     .
2180 nigel 63 .SH CALLOUTS
2181     .rs
2182     .sp
2183     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2184     code to be obeyed in the middle of matching a regular expression. This makes it
2185     possible, amongst other things, to extract different substrings that match the
2186     same pair of parentheses when there is a repetition.
2187 nigel 75 .P
2188 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2189     code. The feature is called "callout". The caller of PCRE provides an external
2190 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
2191 nigel 63 By default, this variable contains NULL, which disables all calling out.
2192 nigel 75 .P
2193 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2194     function is to be called. If you want to identify different callout points, you
2195     can put a number less than 256 after the letter C. The default value is zero.
2196     For example, this pattern has two callout points:
2197 nigel 75 .sp
2198 ph10 155 (?C1)abc(?C2)def
2199 nigel 75 .sp
2200     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2201     automatically installed before each item in the pattern. They are all numbered
2202     255.
2203     .P
2204     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2205 nigel 63 set), the external function is called. It is provided with the number of the
2206 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2207     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2208     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2209     description of the interface to the callout function is given in the
2210 nigel 63 .\" HREF
2211 nigel 75 \fBpcrecallout\fP
2212 nigel 63 .\"
2213     documentation.
2214 nigel 93 .
2215     .
2216 ph10 235 .SH "BACKTRACKING CONTROL"
2217 ph10 210 .rs
2218     .sp
2219 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2220 ph10 210 are described in the Perl documentation as "experimental and subject to change
2221 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
2222     production code should be noted to avoid problems during upgrades." The same
2223 ph10 210 remarks apply to the PCRE features described in this section.
2224     .P
2225 ph10 341 Since these verbs are specifically related to backtracking, most of them can be
2226     used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2227 ph10 345 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2228 ph10 341 failing negative assertion, they cause an error if encountered by
2229 ph10 210 \fBpcre_dfa_exec()\fP.
2230     .P
2231 ph10 445 If any of these verbs are used in an assertion subpattern, their effect is
2232     confined to that subpattern; it does not extend to the surrounding pattern.
2233     Note that assertion subpatterns are processed as anchored at the point where
2234     they are tested.
2235     .P
2236 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2237 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form
2238     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2239     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2240     are two kinds:
2241     .
2242     .SS "Verbs that act immediately"
2243     .rs
2244     .sp
2245     The following verbs act as soon as they are encountered:
2246     .sp
2247     (*ACCEPT)
2248     .sp
2249     This verb causes the match to end successfully, skipping the remainder of the
2250 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2251 ph10 447 immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far
2252     is captured. (This feature was added to PCRE at release 8.00.) For example:
2253 ph10 210 .sp
2254 ph10 447 A((?:A|B(*ACCEPT)|C)D)
2255 ph10 210 .sp
2256 ph10 447 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2257     the outer parentheses.
2258 ph10 210 .sp
2259     (*FAIL) or (*F)
2260     .sp
2261 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2262 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2263     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2264     Perl features that are not present in PCRE. The nearest equivalent is the
2265     callout feature, as for example in this pattern:
2266     .sp
2267     a+(?C)(*FAIL)
2268     .sp
2269 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2270     each backtrack happens (in this example, 10 times).
2271 ph10 210 .
2272     .SS "Verbs that act after backtracking"
2273     .rs
2274     .sp
2275 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2276     with what follows, but if there is no subsequent match, a failure is forced.
2277 ph10 210 The verbs differ in exactly what kind of failure occurs.
2278     .sp
2279     (*COMMIT)
2280     .sp
2281 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern
2282 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find
2283     a match by advancing the start point take place. Once (*COMMIT) has been
2284 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2285 ph10 210 starting point, or not at all. For example:
2286     .sp
2287     a+(*COMMIT)b
2288     .sp
2289 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2290 ph10 210 dynamic anchor, or "I've started, so I must finish."
2291     .sp
2292     (*PRUNE)
2293     .sp
2294 ph10 211 This verb causes the match to fail at the current position if the rest of the
2295     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2296 ph10 210 advance to the next starting character then happens. Backtracking can occur as
2297     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2298     if there is no match to the right, backtracking cannot cross (*PRUNE).
2299 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2300 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2301     be expressed in any other way.
2302     .sp
2303     (*SKIP)
2304     .sp
2305 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2306 ph10 210 "bumpalong" advance is not to the next character, but to the position in the
2307     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2308     was matched leading up to it cannot be part of a successful match. Consider:
2309     .sp
2310     a+(*SKIP)b
2311     .sp
2312 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2313 ph10 210 the first character in the string), the starting point skips on to start the
2314 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2315 ph10 210 effect in this example; although it would suppress backtracking during the
2316     first match attempt, the second attempt would start at the second character
2317     instead of skipping on to "c".
2318     .sp
2319     (*THEN)
2320 ph10 211 .sp
2321 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2322     not match. That is, it cancels pending backtracking, but only within the
2323     current alternation. Its name comes from the observation that it can be used
2324     for a pattern-based if-then-else block:
2325     .sp
2326     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2327     .sp
2328 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2329 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2330 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2331 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE).
2332     .
2333     .
2334 nigel 93 .SH "SEE ALSO"
2335     .rs
2336     .sp
2337     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2338 ph10 99 .
2339     .
2340     .SH AUTHOR
2341     .rs
2342     .sp
2343     .nf
2344     Philip Hazel
2345     University Computing Service
2346     Cambridge CB2 3QH, England.
2347     .fi
2348     .
2349     .
2350     .SH REVISION
2351     .rs
2352     .sp
2353     .nf
2354 ph10 454 Last updated: 22 September 2009
2355 ph10 385 Copyright (c) 1997-2009 University of Cambridge.
2356 ph10 99 .fi

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