/[pcre]/code/trunk/doc/pcrepattern.3
ViewVC logotype

Contents of /code/trunk/doc/pcrepattern.3

Parent Directory Parent Directory | Revision Log Revision Log


Revision 513 - (hide annotations) (download)
Mon May 3 11:13:37 2010 UTC (4 years, 4 months ago) by ph10
File size: 105791 byte(s)
Make \R and \X in a character class behave more like Perl

1 nigel 79 .TH PCREPATTERN 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 nigel 63 .rs
6     .sp
7 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE
8     are described in detail below. There is a quick-reference syntax summary in the
9     .\" HREF
10     \fBpcresyntax\fP
11     .\"
12 ph10 333 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13     also supports some alternative regular expression syntax (which does not
14     conflict with the Perl syntax) in order to provide some compatibility with
15     regular expressions in Python, .NET, and Oniguruma.
16     .P
17     Perl's regular expressions are described in its own documentation, and
18 ph10 208 regular expressions in general are covered in a number of books, some of which
19     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20     published by O'Reilly, covers regular expressions in great detail. This
21     description of PCRE's regular expressions is intended as reference material.
22 nigel 75 .P
23     The original operation of PCRE was on strings of one-byte characters. However,
24 ph10 461 there is now also support for UTF-8 character strings. To use this,
25 ph10 456 PCRE must be built to include UTF-8 support, and you must call
26     \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27     is also a special sequence that can be given at the start of a pattern:
28 ph10 412 .sp
29     (*UTF8)
30 ph10 416 .sp
31 ph10 412 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32     option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33     pattern matching is mentioned in several places below. There is also a summary
34     of UTF-8 features in the
35 nigel 63 .\" HTML <a href="pcre.html#utf8support">
36     .\" </a>
37     section on UTF-8 support
38     .\"
39     in the main
40     .\" HREF
41 nigel 75 \fBpcre\fP
42 nigel 63 .\"
43     page.
44 nigel 75 .P
45 nigel 77 The remainder of this document discusses the patterns that are supported by
46     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47     From release 6.0, PCRE offers a second matching function,
48     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49 ph10 172 Perl-compatible. Some of the features discussed below are not available when
50 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51     alternative function, and how it differs from the normal function, are
52     discussed in the
53 nigel 77 .\" HREF
54     \fBpcrematching\fP
55     .\"
56     page.
57 nigel 93 .
58     .
59 ph10 227 .SH "NEWLINE CONVENTIONS"
60     .rs
61     .sp
62     PCRE supports five different conventions for indicating line breaks in
63     strings: a single CR (carriage return) character, a single LF (linefeed)
64     character, the two-character sequence CRLF, any of the three preceding, or any
65     Unicode newline sequence. The
66     .\" HREF
67     \fBpcreapi\fP
68     .\"
69     page has
70     .\" HTML <a href="pcreapi.html#newlines">
71     .\" </a>
72     further discussion
73     .\"
74     about newlines, and shows how to set the newline convention in the
75     \fIoptions\fP arguments for the compiling and matching functions.
76     .P
77     It is also possible to specify a newline convention by starting a pattern
78     string with one of the following five sequences:
79     .sp
80     (*CR) carriage return
81     (*LF) linefeed
82     (*CRLF) carriage return, followed by linefeed
83     (*ANYCRLF) any of the three above
84     (*ANY) all Unicode newline sequences
85     .sp
86 ph10 461 These override the default and the options given to \fBpcre_compile()\fP or
87 ph10 456 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88     newline sequence, the pattern
89 ph10 227 .sp
90     (*CR)a.b
91     .sp
92     changes the convention to CR. That pattern matches "a\enb" because LF is no
93     longer a newline. Note that these special settings, which are not
94     Perl-compatible, are recognized only at the very start of a pattern, and that
95 ph10 231 they must be in upper case. If more than one of them is present, the last one
96     is used.
97     .P
98     The newline convention does not affect what the \eR escape sequence matches. By
99     default, this is any Unicode newline sequence, for Perl compatibility. However,
100     this can be changed; see the description of \eR in the section entitled
101     .\" HTML <a href="#newlineseq">
102     .\" </a>
103     "Newline sequences"
104     .\"
105 ph10 247 below. A change of \eR setting can be combined with a change of newline
106 ph10 246 convention.
107 ph10 227 .
108     .
109 nigel 93 .SH "CHARACTERS AND METACHARACTERS"
110     .rs
111     .sp
112 nigel 63 A regular expression is a pattern that is matched against a subject string from
113     left to right. Most characters stand for themselves in a pattern, and match the
114     corresponding characters in the subject. As a trivial example, the pattern
115 nigel 75 .sp
116 nigel 63 The quick brown fox
117 nigel 75 .sp
118 nigel 77 matches a portion of a subject string that is identical to itself. When
119     caseless matching is specified (the PCRE_CASELESS option), letters are matched
120     independently of case. In UTF-8 mode, PCRE always understands the concept of
121     case for characters whose values are less than 128, so caseless matching is
122     always possible. For characters with higher values, the concept of case is
123     supported if PCRE is compiled with Unicode property support, but not otherwise.
124     If you want to use caseless matching for characters 128 and above, you must
125     ensure that PCRE is compiled with Unicode property support as well as with
126     UTF-8 support.
127     .P
128     The power of regular expressions comes from the ability to include alternatives
129     and repetitions in the pattern. These are encoded in the pattern by the use of
130 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
131 nigel 63 interpreted in some special way.
132 nigel 75 .P
133     There are two different sets of metacharacters: those that are recognized
134 nigel 63 anywhere in the pattern except within square brackets, and those that are
135 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
136     are as follows:
137 nigel 75 .sp
138     \e general escape character with several uses
139 nigel 63 ^ assert start of string (or line, in multiline mode)
140     $ assert end of string (or line, in multiline mode)
141     . match any character except newline (by default)
142     [ start character class definition
143     | start of alternative branch
144     ( start subpattern
145     ) end subpattern
146     ? extends the meaning of (
147     also 0 or 1 quantifier
148     also quantifier minimizer
149     * 0 or more quantifier
150     + 1 or more quantifier
151     also "possessive quantifier"
152     { start min/max quantifier
153 nigel 75 .sp
154 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
155 nigel 75 a character class the only metacharacters are:
156     .sp
157     \e general escape character
158 nigel 63 ^ negate the class, but only if the first character
159     - indicates character range
160 nigel 75 .\" JOIN
161 nigel 63 [ POSIX character class (only if followed by POSIX
162     syntax)
163     ] terminates the character class
164 nigel 75 .sp
165     The following sections describe the use of each of the metacharacters.
166     .
167 nigel 93 .
168 nigel 63 .SH BACKSLASH
169     .rs
170     .sp
171     The backslash character has several uses. Firstly, if it is followed by a
172 nigel 91 non-alphanumeric character, it takes away any special meaning that character
173     may have. This use of backslash as an escape character applies both inside and
174 nigel 63 outside character classes.
175 nigel 75 .P
176     For example, if you want to match a * character, you write \e* in the pattern.
177 nigel 63 This escaping action applies whether or not the following character would
178 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
179     non-alphanumeric with backslash to specify that it stands for itself. In
180     particular, if you want to match a backslash, you write \e\e.
181     .P
182 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
183     pattern (other than in a character class) and characters between a # outside
184 nigel 91 a character class and the next newline are ignored. An escaping backslash can
185     be used to include a whitespace or # character as part of the pattern.
186 nigel 75 .P
187 nigel 63 If you want to remove the special meaning from a sequence of characters, you
188 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
189     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
190 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
191 nigel 75 .sp
192 nigel 63 Pattern PCRE matches Perl matches
193 nigel 75 .sp
194     .\" JOIN
195     \eQabc$xyz\eE abc$xyz abc followed by the
196 nigel 63 contents of $xyz
197 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
198     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
199     .sp
200     The \eQ...\eE sequence is recognized both inside and outside character classes.
201     .
202     .
203     .\" HTML <a name="digitsafterbackslash"></a>
204     .SS "Non-printing characters"
205     .rs
206     .sp
207 nigel 63 A second use of backslash provides a way of encoding non-printing characters
208     in patterns in a visible manner. There is no restriction on the appearance of
209     non-printing characters, apart from the binary zero that terminates a pattern,
210 ph10 456 but when a pattern is being prepared by text editing, it is often easier to use
211     one of the following escape sequences than the binary character it represents:
212 nigel 75 .sp
213     \ea alarm, that is, the BEL character (hex 07)
214     \ecx "control-x", where x is any character
215     \ee escape (hex 1B)
216     \ef formfeed (hex 0C)
217 ph10 227 \en linefeed (hex 0A)
218 nigel 75 \er carriage return (hex 0D)
219     \et tab (hex 09)
220 ph10 488 \eddd character with octal code ddd, or back reference
221 nigel 75 \exhh character with hex code hh
222 nigel 87 \ex{hhh..} character with hex code hhh..
223 nigel 75 .sp
224     The precise effect of \ecx is as follows: if x is a lower case letter, it
225 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
226 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
227 nigel 63 7B.
228 nigel 75 .P
229     After \ex, from zero to two hexadecimal digits are read (letters can be in
230 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
231     and }, but the value of the character code must be less than 256 in non-UTF-8
232 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234     point, which is 10FFFF.
235 nigel 75 .P
236 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if
237     there is no terminating }, this form of escape is not recognized. Instead, the
238     initial \ex will be interpreted as a basic hexadecimal escape, with no
239     following digits, giving a character whose value is zero.
240     .P
241 nigel 63 Characters whose value is less than 256 can be defined by either of the two
242 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
243     example, \exdc is exactly the same as \ex{dc}.
244 nigel 75 .P
245 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
246     digits, just those that are present are used. Thus the sequence \e0\ex\e07
247     specifies two binary zeros followed by a BEL character (code value 7). Make
248     sure you supply two digits after the initial zero if the pattern character that
249     follows is itself an octal digit.
250 nigel 75 .P
251 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
252     Outside a character class, PCRE reads it and any following digits as a decimal
253     number. If the number is less than 10, or if there have been at least that many
254     previous capturing left parentheses in the expression, the entire sequence is
255 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
256     .\" HTML <a href="#backreferences">
257     .\" </a>
258     later,
259     .\"
260     following the discussion of
261     .\" HTML <a href="#subpattern">
262     .\" </a>
263     parenthesized subpatterns.
264     .\"
265     .P
266 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
267     have not been that many capturing subpatterns, PCRE re-reads up to three octal
268 nigel 93 digits following the backslash, and uses them to generate a data character. Any
269 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
270     character specified in octal must be less than \e400. In UTF-8 mode, values up
271     to \e777 are permitted. For example:
272 nigel 75 .sp
273     \e040 is another way of writing a space
274     .\" JOIN
275     \e40 is the same, provided there are fewer than 40
276 nigel 63 previous capturing subpatterns
277 nigel 75 \e7 is always a back reference
278     .\" JOIN
279     \e11 might be a back reference, or another way of
280 nigel 63 writing a tab
281 nigel 75 \e011 is always a tab
282     \e0113 is a tab followed by the character "3"
283     .\" JOIN
284     \e113 might be a back reference, otherwise the
285 nigel 63 character with octal code 113
286 nigel 75 .\" JOIN
287     \e377 might be a back reference, otherwise
288 nigel 63 the byte consisting entirely of 1 bits
289 nigel 75 .\" JOIN
290     \e81 is either a back reference, or a binary zero
291 nigel 63 followed by the two characters "8" and "1"
292 nigel 75 .sp
293 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
294     zero, because no more than three octal digits are ever read.
295 nigel 75 .P
296 nigel 91 All the sequences that define a single character value can be used both inside
297     and outside character classes. In addition, inside a character class, the
298 ph10 513 sequence \eb is interpreted as the backspace character (hex 08). The sequences
299     \eB, \eR, and \eX are not special inside a character class. Like any other
300     unrecognized escape sequences, they are treated as the literal characters "B",
301     "R", and "X" by default, but cause an error if the PCRE_EXTRA option is set.
302     Outside a character class, these sequences have different meanings
303 nigel 75 .\" HTML <a href="#uniextseq">
304     .\" </a>
305     (see below).
306     .\"
307     .
308     .
309 nigel 93 .SS "Absolute and relative back references"
310     .rs
311     .sp
312 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
313     enclosed in braces, is an absolute or relative back reference. A named back
314     reference can be coded as \eg{name}. Back references are discussed
315 nigel 93 .\" HTML <a href="#backreferences">
316     .\" </a>
317     later,
318     .\"
319     following the discussion of
320     .\" HTML <a href="#subpattern">
321     .\" </a>
322     parenthesized subpatterns.
323     .\"
324     .
325     .
326 ph10 333 .SS "Absolute and relative subroutine calls"
327     .rs
328     .sp
329 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
330     a number enclosed either in angle brackets or single quotes, is an alternative
331     syntax for referencing a subpattern as a "subroutine". Details are discussed
332 ph10 333 .\" HTML <a href="#onigurumasubroutines">
333     .\" </a>
334     later.
335     .\"
336 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
337 ph10 461 synonymous. The former is a back reference; the latter is a
338 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
339     .\" </a>
340     subroutine
341     .\"
342     call.
343 ph10 333 .
344     .
345 nigel 75 .SS "Generic character types"
346     .rs
347     .sp
348 nigel 93 Another use of backslash is for specifying generic character types. The
349 nigel 75 following are always recognized:
350     .sp
351 ph10 182 \ed any decimal digit
352 nigel 75 \eD any character that is not a decimal digit
353 ph10 178 \eh any horizontal whitespace character
354 ph10 182 \eH any character that is not a horizontal whitespace character
355 nigel 75 \es any whitespace character
356     \eS any character that is not a whitespace character
357 ph10 178 \ev any vertical whitespace character
358 ph10 182 \eV any character that is not a vertical whitespace character
359 nigel 75 \ew any "word" character
360     \eW any "non-word" character
361     .sp
362 nigel 63 Each pair of escape sequences partitions the complete set of characters into
363     two disjoint sets. Any given character matches one, and only one, of each pair.
364 nigel 75 .P
365     These character type sequences can appear both inside and outside character
366     classes. They each match one character of the appropriate type. If the current
367     matching point is at the end of the subject string, all of them fail, since
368     there is no character to match.
369     .P
370     For compatibility with Perl, \es does not match the VT character (code 11).
371     This makes it different from the the POSIX "space" class. The \es characters
372 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
373 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
374 ph10 178 does.
375 nigel 75 .P
376 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
377     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
378 ph10 182 character property support is available. These sequences retain their original
379     meanings from before UTF-8 support was available, mainly for efficiency
380 ph10 406 reasons. Note that this also affects \eb, because it is defined in terms of \ew
381 ph10 394 and \eW.
382 ph10 178 .P
383 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
384 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
385     The horizontal space characters are:
386     .sp
387     U+0009 Horizontal tab
388     U+0020 Space
389     U+00A0 Non-break space
390     U+1680 Ogham space mark
391     U+180E Mongolian vowel separator
392     U+2000 En quad
393     U+2001 Em quad
394     U+2002 En space
395     U+2003 Em space
396     U+2004 Three-per-em space
397     U+2005 Four-per-em space
398     U+2006 Six-per-em space
399     U+2007 Figure space
400     U+2008 Punctuation space
401     U+2009 Thin space
402     U+200A Hair space
403     U+202F Narrow no-break space
404     U+205F Medium mathematical space
405     U+3000 Ideographic space
406     .sp
407     The vertical space characters are:
408     .sp
409     U+000A Linefeed
410     U+000B Vertical tab
411     U+000C Formfeed
412     U+000D Carriage return
413     U+0085 Next line
414     U+2028 Line separator
415     U+2029 Paragraph separator
416     .P
417 nigel 75 A "word" character is an underscore or any character less than 256 that is a
418     letter or digit. The definition of letters and digits is controlled by PCRE's
419     low-valued character tables, and may vary if locale-specific matching is taking
420     place (see
421 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
422     .\" </a>
423     "Locale support"
424     .\"
425     in the
426     .\" HREF
427 nigel 75 \fBpcreapi\fP
428 nigel 63 .\"
429 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
430     or "french" in Windows, some character codes greater than 128 are used for
431 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
432     is discouraged.
433 nigel 75 .
434     .
435 ph10 231 .\" HTML <a name="newlineseq"></a>
436 nigel 93 .SS "Newline sequences"
437     .rs
438     .sp
439 ph10 231 Outside a character class, by default, the escape sequence \eR matches any
440     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
441     equivalent to the following:
442 nigel 93 .sp
443     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
444     .sp
445     This is an example of an "atomic group", details of which are given
446     .\" HTML <a href="#atomicgroup">
447     .\" </a>
448     below.
449     .\"
450     This particular group matches either the two-character sequence CR followed by
451     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
452     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
453     line, U+0085). The two-character sequence is treated as a single unit that
454     cannot be split.
455     .P
456     In UTF-8 mode, two additional characters whose codepoints are greater than 255
457     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
458     Unicode character property support is not needed for these characters to be
459     recognized.
460     .P
461 ph10 231 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
462     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
463 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
464 ph10 246 for "backslash R".) This can be made the default when PCRE is built; if this is
465     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
466     It is also possible to specify these settings by starting a pattern string with
467     one of the following sequences:
468 ph10 231 .sp
469     (*BSR_ANYCRLF) CR, LF, or CRLF only
470     (*BSR_UNICODE) any Unicode newline sequence
471     .sp
472 ph10 461 These override the default and the options given to \fBpcre_compile()\fP or
473 ph10 456 \fBpcre_compile2()\fP, but they can be overridden by options given to
474     \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
475     which are not Perl-compatible, are recognized only at the very start of a
476     pattern, and that they must be in upper case. If more than one of them is
477     present, the last one is used. They can be combined with a change of newline
478     convention, for example, a pattern can start with:
479 ph10 246 .sp
480     (*ANY)(*BSR_ANYCRLF)
481     .sp
482 ph10 513 Inside a character class, \eR is treated as an unrecognized escape sequence,
483     and so matches the letter "R" by default, but causes an error if PCRE_EXTRA is
484     set.
485 nigel 93 .
486     .
487 nigel 75 .\" HTML <a name="uniextseq"></a>
488     .SS Unicode character properties
489     .rs
490     .sp
491     When PCRE is built with Unicode character property support, three additional
492 ph10 184 escape sequences that match characters with specific properties are available.
493     When not in UTF-8 mode, these sequences are of course limited to testing
494     characters whose codepoints are less than 256, but they do work in this mode.
495     The extra escape sequences are:
496 nigel 75 .sp
497 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
498     \eP{\fIxx\fP} a character without the \fIxx\fP property
499     \eX an extended Unicode sequence
500 nigel 75 .sp
501 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
502     script names, the general category properties, and "Any", which matches any
503     character (including newline). Other properties such as "InMusicalSymbols" are
504     not currently supported by PCRE. Note that \eP{Any} does not match any
505     characters, so always causes a match failure.
506 nigel 75 .P
507 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
508     character from one of these sets can be matched using a script name. For
509     example:
510 nigel 75 .sp
511 nigel 87 \ep{Greek}
512     \eP{Han}
513     .sp
514     Those that are not part of an identified script are lumped together as
515     "Common". The current list of scripts is:
516     .P
517     Arabic,
518     Armenian,
519 ph10 491 Avestan,
520 nigel 93 Balinese,
521 ph10 491 Bamum,
522 nigel 87 Bengali,
523     Bopomofo,
524     Braille,
525     Buginese,
526     Buhid,
527     Canadian_Aboriginal,
528 ph10 491 Carian,
529     Cham,
530 nigel 87 Cherokee,
531     Common,
532     Coptic,
533 nigel 93 Cuneiform,
534 nigel 87 Cypriot,
535     Cyrillic,
536     Deseret,
537     Devanagari,
538 ph10 491 Egyptian_Hieroglyphs,
539 nigel 87 Ethiopic,
540     Georgian,
541     Glagolitic,
542     Gothic,
543     Greek,
544     Gujarati,
545     Gurmukhi,
546     Han,
547     Hangul,
548     Hanunoo,
549     Hebrew,
550     Hiragana,
551 ph10 491 Imperial_Aramaic,
552 nigel 87 Inherited,
553 ph10 491 Inscriptional_Pahlavi,
554     Inscriptional_Parthian,
555     Javanese,
556     Kaithi,
557 nigel 87 Kannada,
558     Katakana,
559 ph10 491 Kayah_Li,
560 nigel 87 Kharoshthi,
561     Khmer,
562     Lao,
563     Latin,
564 ph10 491 Lepcha,
565 nigel 87 Limbu,
566     Linear_B,
567 ph10 491 Lisu,
568     Lycian,
569     Lydian,
570 nigel 87 Malayalam,
571 ph10 491 Meetei_Mayek,
572 nigel 87 Mongolian,
573     Myanmar,
574     New_Tai_Lue,
575 nigel 93 Nko,
576 nigel 87 Ogham,
577     Old_Italic,
578     Old_Persian,
579 ph10 491 Old_South_Arabian,
580     Old_Turkic,
581     Ol_Chiki,
582 nigel 87 Oriya,
583     Osmanya,
584 nigel 93 Phags_Pa,
585     Phoenician,
586 ph10 491 Rejang,
587 nigel 87 Runic,
588 ph10 491 Samaritan,
589     Saurashtra,
590 nigel 87 Shavian,
591     Sinhala,
592 ph10 491 Sundanese,
593 nigel 87 Syloti_Nagri,
594     Syriac,
595     Tagalog,
596     Tagbanwa,
597     Tai_Le,
598 ph10 491 Tai_Tham,
599     Tai_Viet,
600 nigel 87 Tamil,
601     Telugu,
602     Thaana,
603     Thai,
604     Tibetan,
605     Tifinagh,
606     Ugaritic,
607 ph10 491 Vai,
608 nigel 87 Yi.
609     .P
610     Each character has exactly one general category property, specified by a
611     two-letter abbreviation. For compatibility with Perl, negation can be specified
612     by including a circumflex between the opening brace and the property name. For
613     example, \ep{^Lu} is the same as \eP{Lu}.
614     .P
615     If only one letter is specified with \ep or \eP, it includes all the general
616     category properties that start with that letter. In this case, in the absence
617     of negation, the curly brackets in the escape sequence are optional; these two
618     examples have the same effect:
619     .sp
620 nigel 75 \ep{L}
621     \epL
622     .sp
623 nigel 87 The following general category property codes are supported:
624 nigel 75 .sp
625     C Other
626     Cc Control
627     Cf Format
628     Cn Unassigned
629     Co Private use
630     Cs Surrogate
631     .sp
632     L Letter
633     Ll Lower case letter
634     Lm Modifier letter
635     Lo Other letter
636     Lt Title case letter
637     Lu Upper case letter
638     .sp
639     M Mark
640     Mc Spacing mark
641     Me Enclosing mark
642     Mn Non-spacing mark
643     .sp
644     N Number
645     Nd Decimal number
646     Nl Letter number
647     No Other number
648     .sp
649     P Punctuation
650     Pc Connector punctuation
651     Pd Dash punctuation
652     Pe Close punctuation
653     Pf Final punctuation
654     Pi Initial punctuation
655     Po Other punctuation
656     Ps Open punctuation
657     .sp
658     S Symbol
659     Sc Currency symbol
660     Sk Modifier symbol
661     Sm Mathematical symbol
662     So Other symbol
663     .sp
664     Z Separator
665     Zl Line separator
666     Zp Paragraph separator
667     Zs Space separator
668     .sp
669 nigel 87 The special property L& is also supported: it matches a character that has
670     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
671     a modifier or "other".
672 nigel 75 .P
673 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
674     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
675     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
676     (see the discussion of PCRE_NO_UTF8_CHECK in the
677     .\" HREF
678     \fBpcreapi\fP
679     .\"
680 ph10 451 page). Perl does not support the Cs property.
681 ph10 211 .P
682 ph10 451 The long synonyms for property names that Perl supports (such as \ep{Letter})
683 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
684 nigel 87 properties with "Is".
685     .P
686     No character that is in the Unicode table has the Cn (unassigned) property.
687     Instead, this property is assumed for any code point that is not in the
688     Unicode table.
689     .P
690 nigel 75 Specifying caseless matching does not affect these escape sequences. For
691     example, \ep{Lu} always matches only upper case letters.
692     .P
693     The \eX escape matches any number of Unicode characters that form an extended
694     Unicode sequence. \eX is equivalent to
695     .sp
696     (?>\ePM\epM*)
697     .sp
698     That is, it matches a character without the "mark" property, followed by zero
699     or more characters with the "mark" property, and treats the sequence as an
700     atomic group
701     .\" HTML <a href="#atomicgroup">
702     .\" </a>
703     (see below).
704     .\"
705     Characters with the "mark" property are typically accents that affect the
706 ph10 185 preceding character. None of them have codepoints less than 256, so in
707 ph10 184 non-UTF-8 mode \eX matches any one character.
708 nigel 75 .P
709     Matching characters by Unicode property is not fast, because PCRE has to search
710     a structure that contains data for over fifteen thousand characters. That is
711     why the traditional escape sequences such as \ed and \ew do not use Unicode
712     properties in PCRE.
713     .
714     .
715 ph10 168 .\" HTML <a name="resetmatchstart"></a>
716     .SS "Resetting the match start"
717     .rs
718     .sp
719 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
720     matched characters not to be included in the final matched sequence. For
721 ph10 168 example, the pattern:
722     .sp
723     foo\eKbar
724     .sp
725 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
726 ph10 168 similar to a lookbehind assertion
727     .\" HTML <a href="#lookbehind">
728     .\" </a>
729     (described below).
730     .\"
731 ph10 172 However, in this case, the part of the subject before the real match does not
732     have to be of fixed length, as lookbehind assertions do. The use of \eK does
733 ph10 168 not interfere with the setting of
734     .\" HTML <a href="#subpattern">
735     .\" </a>
736     captured substrings.
737 ph10 172 .\"
738 ph10 168 For example, when the pattern
739     .sp
740     (foo)\eKbar
741     .sp
742 ph10 172 matches "foobar", the first substring is still set to "foo".
743 ph10 500 .P
744 ph10 507 Perl documents that the use of \eK within assertions is "not well defined". In
745     PCRE, \eK is acted upon when it occurs inside positive assertions, but is
746 ph10 500 ignored in negative assertions.
747 ph10 168 .
748     .
749 nigel 75 .\" HTML <a name="smallassertions"></a>
750     .SS "Simple assertions"
751     .rs
752     .sp
753 nigel 93 The final use of backslash is for certain simple assertions. An assertion
754 nigel 63 specifies a condition that has to be met at a particular point in a match,
755     without consuming any characters from the subject string. The use of
756 nigel 75 subpatterns for more complicated assertions is described
757     .\" HTML <a href="#bigassertions">
758     .\" </a>
759     below.
760     .\"
761 nigel 91 The backslashed assertions are:
762 nigel 75 .sp
763     \eb matches at a word boundary
764     \eB matches when not at a word boundary
765 nigel 93 \eA matches at the start of the subject
766     \eZ matches at the end of the subject
767     also matches before a newline at the end of the subject
768     \ez matches only at the end of the subject
769     \eG matches at the first matching position in the subject
770 nigel 75 .sp
771 ph10 513 Inside a character class, \eb has a different meaning; it matches the backspace
772     character. If any other of these assertions appears in a character class, by
773     default it matches the corresponding literal character (for example, \eB
774     matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
775     escape sequence" error is generated instead.
776 nigel 75 .P
777 nigel 63 A word boundary is a position in the subject string where the current character
778 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
779     \ew and the other matches \eW), or the start or end of the string if the
780 ph10 461 first or last character matches \ew, respectively. Neither PCRE nor Perl has a
781     separte "start of word" or "end of word" metasequence. However, whatever
782     follows \eb normally determines which it is. For example, the fragment
783 ph10 456 \eba matches "a" at the start of a word.
784 nigel 75 .P
785     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
786     dollar (described in the next section) in that they only ever match at the very
787     start and end of the subject string, whatever options are set. Thus, they are
788     independent of multiline mode. These three assertions are not affected by the
789     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
790     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
791     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
792     at a point other than the beginning of the subject, \eA can never match. The
793 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
794     of the string as well as at the very end, whereas \ez matches only at the end.
795 nigel 75 .P
796     The \eG assertion is true only when the current matching position is at the
797     start point of the match, as specified by the \fIstartoffset\fP argument of
798     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
799     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
800 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
801 nigel 75 implementation where \eG can be useful.
802     .P
803     Note, however, that PCRE's interpretation of \eG, as the start of the current
804 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
805     previous match. In Perl, these can be different when the previously matched
806     string was empty. Because PCRE does just one match at a time, it cannot
807     reproduce this behaviour.
808 nigel 75 .P
809     If all the alternatives of a pattern begin with \eG, the expression is anchored
810 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
811     regular expression.
812 nigel 75 .
813     .
814     .SH "CIRCUMFLEX AND DOLLAR"
815 nigel 63 .rs
816     .sp
817     Outside a character class, in the default matching mode, the circumflex
818 nigel 75 character is an assertion that is true only if the current matching point is
819     at the start of the subject string. If the \fIstartoffset\fP argument of
820     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
821 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
822 nigel 75 meaning
823     .\" HTML <a href="#characterclass">
824     .\" </a>
825     (see below).
826     .\"
827     .P
828 nigel 63 Circumflex need not be the first character of the pattern if a number of
829     alternatives are involved, but it should be the first thing in each alternative
830     in which it appears if the pattern is ever to match that branch. If all
831     possible alternatives start with a circumflex, that is, if the pattern is
832     constrained to match only at the start of the subject, it is said to be an
833     "anchored" pattern. (There are also other constructs that can cause a pattern
834     to be anchored.)
835 nigel 75 .P
836     A dollar character is an assertion that is true only if the current matching
837 nigel 63 point is at the end of the subject string, or immediately before a newline
838 nigel 91 at the end of the string (by default). Dollar need not be the last character of
839     the pattern if a number of alternatives are involved, but it should be the last
840     item in any branch in which it appears. Dollar has no special meaning in a
841     character class.
842 nigel 75 .P
843 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
844     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
845 nigel 75 does not affect the \eZ assertion.
846     .P
847 nigel 63 The meanings of the circumflex and dollar characters are changed if the
848 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
849     immediately after internal newlines as well as at the start of the subject
850     string. It does not match after a newline that ends the string. A dollar
851     matches before any newlines in the string, as well as at the very end, when
852     PCRE_MULTILINE is set. When newline is specified as the two-character
853     sequence CRLF, isolated CR and LF characters do not indicate newlines.
854 nigel 75 .P
855 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
856     \en represents a newline) in multiline mode, but not otherwise. Consequently,
857     patterns that are anchored in single line mode because all branches start with
858     ^ are not anchored in multiline mode, and a match for circumflex is possible
859     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
860     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
861     .P
862 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
863 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
864 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
865 nigel 75 .
866     .
867     .SH "FULL STOP (PERIOD, DOT)"
868 nigel 63 .rs
869     .sp
870     Outside a character class, a dot in the pattern matches any one character in
871 nigel 91 the subject string except (by default) a character that signifies the end of a
872 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
873 nigel 91 .P
874 nigel 93 When a line ending is defined as a single character, dot never matches that
875     character; when the two-character sequence CRLF is used, dot does not match CR
876     if it is immediately followed by LF, but otherwise it matches all characters
877     (including isolated CRs and LFs). When any Unicode line endings are being
878     recognized, dot does not match CR or LF or any of the other line ending
879     characters.
880     .P
881 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
882 nigel 93 option is set, a dot matches any one character, without exception. If the
883     two-character sequence CRLF is present in the subject string, it takes two dots
884     to match it.
885 nigel 91 .P
886     The handling of dot is entirely independent of the handling of circumflex and
887     dollar, the only relationship being that they both involve newlines. Dot has no
888     special meaning in a character class.
889 nigel 75 .
890     .
891     .SH "MATCHING A SINGLE BYTE"
892 nigel 63 .rs
893     .sp
894 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
895 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
896     characters. The feature is provided in Perl in order to match individual bytes
897     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
898     what remains in the string may be a malformed UTF-8 string. For this reason,
899     the \eC escape sequence is best avoided.
900 nigel 75 .P
901     PCRE does not allow \eC to appear in lookbehind assertions
902     .\" HTML <a href="#lookbehind">
903     .\" </a>
904     (described below),
905     .\"
906     because in UTF-8 mode this would make it impossible to calculate the length of
907     the lookbehind.
908     .
909     .
910     .\" HTML <a name="characterclass"></a>
911     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
912 nigel 63 .rs
913     .sp
914     An opening square bracket introduces a character class, terminated by a closing
915 ph10 461 square bracket. A closing square bracket on its own is not special by default.
916     However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
917 ph10 456 bracket causes a compile-time error. If a closing square bracket is required as
918     a member of the class, it should be the first data character in the class
919     (after an initial circumflex, if present) or escaped with a backslash.
920 nigel 75 .P
921 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
922 ph10 456 character may be more than one byte long. A matched character must be in the
923     set of characters defined by the class, unless the first character in the class
924 nigel 63 definition is a circumflex, in which case the subject character must not be in
925     the set defined by the class. If a circumflex is actually required as a member
926     of the class, ensure it is not the first character, or escape it with a
927     backslash.
928 nigel 75 .P
929 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
930     [^aeiou] matches any character that is not a lower case vowel. Note that a
931 nigel 75 circumflex is just a convenient notation for specifying the characters that
932     are in the class by enumerating those that are not. A class that starts with a
933 ph10 456 circumflex is not an assertion; it still consumes a character from the subject
934 nigel 75 string, and therefore it fails if the current pointer is at the end of the
935     string.
936     .P
937 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
938 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
939     .P
940 nigel 63 When caseless matching is set, any letters in a class represent both their
941     upper case and lower case versions, so for example, a caseless [aeiou] matches
942     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
943 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
944     case for characters whose values are less than 128, so caseless matching is
945     always possible. For characters with higher values, the concept of case is
946     supported if PCRE is compiled with Unicode property support, but not otherwise.
947 ph10 456 If you want to use caseless matching in UTF8-mode for characters 128 and above,
948     you must ensure that PCRE is compiled with Unicode property support as well as
949     with UTF-8 support.
950 nigel 75 .P
951 nigel 93 Characters that might indicate line breaks are never treated in any special way
952     when matching character classes, whatever line-ending sequence is in use, and
953     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
954     such as [^a] always matches one of these characters.
955 nigel 75 .P
956 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
957     character class. For example, [d-m] matches any letter between d and m,
958     inclusive. If a minus character is required in a class, it must be escaped with
959     a backslash or appear in a position where it cannot be interpreted as
960     indicating a range, typically as the first or last character in the class.
961 nigel 75 .P
962 nigel 63 It is not possible to have the literal character "]" as the end character of a
963     range. A pattern such as [W-]46] is interpreted as a class of two characters
964     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
965     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
966 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
967     followed by two other characters. The octal or hexadecimal representation of
968     "]" can also be used to end a range.
969     .P
970 nigel 63 Ranges operate in the collating sequence of character values. They can also be
971 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
972 nigel 63 mode, ranges can include characters whose values are greater than 255, for
973 nigel 75 example [\ex{100}-\ex{2ff}].
974     .P
975 nigel 63 If a range that includes letters is used when caseless matching is set, it
976     matches the letters in either case. For example, [W-c] is equivalent to
977 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
978 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
979 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
980     characters with values greater than 128 only when it is compiled with Unicode
981     property support.
982     .P
983     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
984     in a character class, and add the characters that they match to the class. For
985     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
986 nigel 63 conveniently be used with the upper case character types to specify a more
987     restricted set of characters than the matching lower case type. For example,
988 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
989     .P
990     The only metacharacters that are recognized in character classes are backslash,
991     hyphen (only where it can be interpreted as specifying a range), circumflex
992     (only at the start), opening square bracket (only when it can be interpreted as
993     introducing a POSIX class name - see the next section), and the terminating
994     closing square bracket. However, escaping other non-alphanumeric characters
995     does no harm.
996     .
997     .
998     .SH "POSIX CHARACTER CLASSES"
999 nigel 63 .rs
1000     .sp
1001 nigel 75 Perl supports the POSIX notation for character classes. This uses names
1002 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1003     this notation. For example,
1004 nigel 75 .sp
1005 nigel 63 [01[:alpha:]%]
1006 nigel 75 .sp
1007 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
1008     are
1009 nigel 75 .sp
1010 nigel 63 alnum letters and digits
1011     alpha letters
1012     ascii character codes 0 - 127
1013     blank space or tab only
1014     cntrl control characters
1015 nigel 75 digit decimal digits (same as \ed)
1016 nigel 63 graph printing characters, excluding space
1017     lower lower case letters
1018     print printing characters, including space
1019     punct printing characters, excluding letters and digits
1020 nigel 75 space white space (not quite the same as \es)
1021 nigel 63 upper upper case letters
1022 nigel 75 word "word" characters (same as \ew)
1023 nigel 63 xdigit hexadecimal digits
1024 nigel 75 .sp
1025 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1026     space (32). Notice that this list includes the VT character (code 11). This
1027 nigel 75 makes "space" different to \es, which does not include VT (for Perl
1028 nigel 63 compatibility).
1029 nigel 75 .P
1030 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1031     5.8. Another Perl extension is negation, which is indicated by a ^ character
1032     after the colon. For example,
1033 nigel 75 .sp
1034 nigel 63 [12[:^digit:]]
1035 nigel 75 .sp
1036 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1037     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1038     supported, and an error is given if they are encountered.
1039 nigel 75 .P
1040     In UTF-8 mode, characters with values greater than 128 do not match any of
1041 nigel 63 the POSIX character classes.
1042 nigel 75 .
1043     .
1044     .SH "VERTICAL BAR"
1045 nigel 63 .rs
1046     .sp
1047     Vertical bar characters are used to separate alternative patterns. For example,
1048     the pattern
1049 nigel 75 .sp
1050 nigel 63 gilbert|sullivan
1051 nigel 75 .sp
1052 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1053 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1054     process tries each alternative in turn, from left to right, and the first one
1055     that succeeds is used. If the alternatives are within a subpattern
1056 nigel 75 .\" HTML <a href="#subpattern">
1057     .\" </a>
1058     (defined below),
1059     .\"
1060     "succeeds" means matching the rest of the main pattern as well as the
1061     alternative in the subpattern.
1062     .
1063     .
1064     .SH "INTERNAL OPTION SETTING"
1065 nigel 63 .rs
1066     .sp
1067     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1068 ph10 231 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1069     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1070     The option letters are
1071 nigel 75 .sp
1072 nigel 63 i for PCRE_CASELESS
1073     m for PCRE_MULTILINE
1074     s for PCRE_DOTALL
1075     x for PCRE_EXTENDED
1076 nigel 75 .sp
1077 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1078     unset these options by preceding the letter with a hyphen, and a combined
1079     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1080     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1081     permitted. If a letter appears both before and after the hyphen, the option is
1082     unset.
1083 nigel 75 .P
1084 ph10 231 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1085     changed in the same way as the Perl-compatible options by using the characters
1086     J, U and X respectively.
1087     .P
1088 ph10 412 When one of these option changes occurs at top level (that is, not inside
1089     subpattern parentheses), the change applies to the remainder of the pattern
1090     that follows. If the change is placed right at the start of a pattern, PCRE
1091     extracts it into the global options (and it will therefore show up in data
1092     extracted by the \fBpcre_fullinfo()\fP function).
1093 nigel 75 .P
1094 nigel 93 An option change within a subpattern (see below for a description of
1095     subpatterns) affects only that part of the current pattern that follows it, so
1096 nigel 75 .sp
1097 nigel 63 (a(?i)b)c
1098 nigel 75 .sp
1099 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1100     By this means, options can be made to have different settings in different
1101     parts of the pattern. Any changes made in one alternative do carry on
1102     into subsequent branches within the same subpattern. For example,
1103 nigel 75 .sp
1104 nigel 63 (a(?i)b|c)
1105 nigel 75 .sp
1106 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1107     branch is abandoned before the option setting. This is because the effects of
1108     option settings happen at compile time. There would be some very weird
1109     behaviour otherwise.
1110 ph10 251 .P
1111     \fBNote:\fP There are other PCRE-specific options that can be set by the
1112     application when the compile or match functions are called. In some cases the
1113 ph10 412 pattern can contain special leading sequences such as (*CRLF) to override what
1114     the application has set or what has been defaulted. Details are given in the
1115     section entitled
1116 ph10 251 .\" HTML <a href="#newlineseq">
1117     .\" </a>
1118     "Newline sequences"
1119     .\"
1120 ph10 416 above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1121 ph10 412 mode; this is equivalent to setting the PCRE_UTF8 option.
1122 nigel 75 .
1123     .
1124     .\" HTML <a name="subpattern"></a>
1125 nigel 63 .SH SUBPATTERNS
1126     .rs
1127     .sp
1128     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1129 nigel 75 Turning part of a pattern into a subpattern does two things:
1130     .sp
1131 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1132 nigel 75 .sp
1133 nigel 63 cat(aract|erpillar|)
1134 nigel 75 .sp
1135 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1136 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1137 nigel 75 .sp
1138     2. It sets up the subpattern as a capturing subpattern. This means that, when
1139     the whole pattern matches, that portion of the subject string that matched the
1140     subpattern is passed back to the caller via the \fIovector\fP argument of
1141     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1142     from 1) to obtain numbers for the capturing subpatterns.
1143     .P
1144 nigel 63 For example, if the string "the red king" is matched against the pattern
1145 nigel 75 .sp
1146 nigel 63 the ((red|white) (king|queen))
1147 nigel 75 .sp
1148 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1149     2, and 3, respectively.
1150 nigel 75 .P
1151 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
1152     There are often times when a grouping subpattern is required without a
1153     capturing requirement. If an opening parenthesis is followed by a question mark
1154     and a colon, the subpattern does not do any capturing, and is not counted when
1155     computing the number of any subsequent capturing subpatterns. For example, if
1156     the string "the white queen" is matched against the pattern
1157 nigel 75 .sp
1158 nigel 63 the ((?:red|white) (king|queen))
1159 nigel 75 .sp
1160 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1161 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1162 nigel 75 .P
1163 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1164     a non-capturing subpattern, the option letters may appear between the "?" and
1165     the ":". Thus the two patterns
1166 nigel 75 .sp
1167 nigel 63 (?i:saturday|sunday)
1168     (?:(?i)saturday|sunday)
1169 nigel 75 .sp
1170 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1171     from left to right, and options are not reset until the end of the subpattern
1172     is reached, an option setting in one branch does affect subsequent branches, so
1173     the above patterns match "SUNDAY" as well as "Saturday".
1174 nigel 75 .
1175     .
1176 ph10 456 .\" HTML <a name="dupsubpatternnumber"></a>
1177 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1178     .rs
1179     .sp
1180 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1181     the same numbers for its capturing parentheses. Such a subpattern starts with
1182     (?| and is itself a non-capturing subpattern. For example, consider this
1183 ph10 175 pattern:
1184     .sp
1185     (?|(Sat)ur|(Sun))day
1186 ph10 182 .sp
1187     Because the two alternatives are inside a (?| group, both sets of capturing
1188     parentheses are numbered one. Thus, when the pattern matches, you can look
1189     at captured substring number one, whichever alternative matched. This construct
1190     is useful when you want to capture part, but not all, of one of a number of
1191     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1192 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1193 ph10 182 buffers that follow the subpattern start after the highest number used in any
1194     branch. The following example is taken from the Perl documentation.
1195 ph10 175 The numbers underneath show in which buffer the captured content will be
1196     stored.
1197     .sp
1198     # before ---------------branch-reset----------- after
1199     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1200     # 1 2 2 3 2 3 4
1201 ph10 182 .sp
1202 ph10 488 A back reference to a numbered subpattern uses the most recent value that is
1203     set for that number by any subpattern. The following pattern matches "abcabc"
1204     or "defdef":
1205 ph10 456 .sp
1206 ph10 461 /(?|(abc)|(def))\e1/
1207 ph10 456 .sp
1208     In contrast, a recursive or "subroutine" call to a numbered subpattern always
1209 ph10 461 refers to the first one in the pattern with the given number. The following
1210 ph10 456 pattern matches "abcabc" or "defabc":
1211     .sp
1212     /(?|(abc)|(def))(?1)/
1213     .sp
1214 ph10 459 If a
1215     .\" HTML <a href="#conditions">
1216     .\" </a>
1217     condition test
1218     .\"
1219     for a subpattern's having matched refers to a non-unique number, the test is
1220     true if any of the subpatterns of that number have matched.
1221     .P
1222     An alternative approach to using this "branch reset" feature is to use
1223 ph10 175 duplicate named subpatterns, as described in the next section.
1224     .
1225     .
1226 nigel 75 .SH "NAMED SUBPATTERNS"
1227 nigel 63 .rs
1228     .sp
1229     Identifying capturing parentheses by number is simple, but it can be very hard
1230     to keep track of the numbers in complicated regular expressions. Furthermore,
1231 nigel 75 if an expression is modified, the numbers may change. To help with this
1232 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1233     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1234     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1235 ph10 459 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1236     have different names, but PCRE does not.
1237 nigel 93 .P
1238     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1239     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1240 nigel 91 parentheses from other parts of the pattern, such as
1241     .\" HTML <a href="#backreferences">
1242     .\" </a>
1243 ph10 488 back references,
1244 nigel 91 .\"
1245     .\" HTML <a href="#recursion">
1246     .\" </a>
1247     recursion,
1248     .\"
1249     and
1250     .\" HTML <a href="#conditions">
1251     .\" </a>
1252     conditions,
1253     .\"
1254     can be made by name as well as by number.
1255 nigel 75 .P
1256 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1257 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1258     if the names were not present. The PCRE API provides function calls for
1259     extracting the name-to-number translation table from a compiled pattern. There
1260     is also a convenience function for extracting a captured substring by name.
1261 nigel 91 .P
1262     By default, a name must be unique within a pattern, but it is possible to relax
1263 ph10 457 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1264 ph10 461 names are also always permitted for subpatterns with the same number, set up as
1265 ph10 457 described in the previous section.) Duplicate names can be useful for patterns
1266     where only one instance of the named parentheses can match. Suppose you want to
1267     match the name of a weekday, either as a 3-letter abbreviation or as the full
1268     name, and in both cases you want to extract the abbreviation. This pattern
1269     (ignoring the line breaks) does the job:
1270 nigel 91 .sp
1271 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1272     (?<DN>Tue)(?:sday)?|
1273     (?<DN>Wed)(?:nesday)?|
1274     (?<DN>Thu)(?:rsday)?|
1275     (?<DN>Sat)(?:urday)?
1276 nigel 91 .sp
1277     There are five capturing substrings, but only one is ever set after a match.
1278 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1279 ph10 175 subpattern, as described in the previous section.)
1280     .P
1281 nigel 91 The convenience function for extracting the data by name returns the substring
1282 nigel 93 for the first (and in this example, the only) subpattern of that name that
1283 ph10 461 matched. This saves searching to find which numbered subpattern it was.
1284 ph10 459 .P
1285 ph10 488 If you make a back reference to a non-unique named subpattern from elsewhere in
1286 ph10 459 the pattern, the one that corresponds to the first occurrence of the name is
1287     used. In the absence of duplicate numbers (see the previous section) this is
1288     the one with the lowest number. If you use a named reference in a condition
1289     test (see the
1290     .\"
1291     .\" HTML <a href="#conditions">
1292     .\" </a>
1293     section about conditions
1294     .\"
1295 ph10 461 below), either to check whether a subpattern has matched, or to check for
1296 ph10 459 recursion, all subpatterns with the same name are tested. If the condition is
1297     true for any one of them, the overall condition is true. This is the same
1298     behaviour as testing by number. For further details of the interfaces for
1299     handling named subpatterns, see the
1300 nigel 63 .\" HREF
1301 nigel 75 \fBpcreapi\fP
1302 nigel 63 .\"
1303     documentation.
1304 ph10 385 .P
1305     \fBWarning:\fP You cannot use different names to distinguish between two
1306 ph10 457 subpatterns with the same number because PCRE uses only the numbers when
1307     matching. For this reason, an error is given at compile time if different names
1308     are given to subpatterns with the same number. However, you can give the same
1309     name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1310 nigel 75 .
1311     .
1312 nigel 63 .SH REPETITION
1313     .rs
1314     .sp
1315     Repetition is specified by quantifiers, which can follow any of the following
1316     items:
1317 nigel 75 .sp
1318 nigel 63 a literal data character
1319 nigel 93 the dot metacharacter
1320 nigel 75 the \eC escape sequence
1321     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1322 nigel 93 the \eR escape sequence
1323 nigel 75 an escape such as \ed that matches a single character
1324 nigel 63 a character class
1325     a back reference (see next section)
1326     a parenthesized subpattern (unless it is an assertion)
1327 ph10 461 a recursive or "subroutine" call to a subpattern
1328 nigel 75 .sp
1329 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1330     permitted matches, by giving the two numbers in curly brackets (braces),
1331     separated by a comma. The numbers must be less than 65536, and the first must
1332     be less than or equal to the second. For example:
1333 nigel 75 .sp
1334 nigel 63 z{2,4}
1335 nigel 75 .sp
1336 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1337     character. If the second number is omitted, but the comma is present, there is
1338     no upper limit; if the second number and the comma are both omitted, the
1339     quantifier specifies an exact number of required matches. Thus
1340 nigel 75 .sp
1341 nigel 63 [aeiou]{3,}
1342 nigel 75 .sp
1343 nigel 63 matches at least 3 successive vowels, but may match many more, while
1344 nigel 75 .sp
1345     \ed{8}
1346     .sp
1347 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1348     where a quantifier is not allowed, or one that does not match the syntax of a
1349     quantifier, is taken as a literal character. For example, {,6} is not a
1350     quantifier, but a literal string of four characters.
1351 nigel 75 .P
1352 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1353 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1354     which is represented by a two-byte sequence. Similarly, when Unicode property
1355     support is available, \eX{3} matches three Unicode extended sequences, each of
1356     which may be several bytes long (and they may be of different lengths).
1357     .P
1358 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1359 ph10 345 previous item and the quantifier were not present. This may be useful for
1360     subpatterns that are referenced as
1361 ph10 335 .\" HTML <a href="#subpatternsassubroutines">
1362     .\" </a>
1363     subroutines
1364     .\"
1365 ph10 345 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1366 ph10 335 quantifier are omitted from the compiled pattern.
1367 nigel 75 .P
1368 nigel 93 For convenience, the three most common quantifiers have single-character
1369     abbreviations:
1370 nigel 75 .sp
1371 nigel 63 * is equivalent to {0,}
1372     + is equivalent to {1,}
1373     ? is equivalent to {0,1}
1374 nigel 75 .sp
1375 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1376     match no characters with a quantifier that has no upper limit, for example:
1377 nigel 75 .sp
1378 nigel 63 (a?)*
1379 nigel 75 .sp
1380 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1381     such patterns. However, because there are cases where this can be useful, such
1382     patterns are now accepted, but if any repetition of the subpattern does in fact
1383     match no characters, the loop is forcibly broken.
1384 nigel 75 .P
1385 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1386     possible (up to the maximum number of permitted times), without causing the
1387     rest of the pattern to fail. The classic example of where this gives problems
1388 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1389     and within the comment, individual * and / characters may appear. An attempt to
1390     match C comments by applying the pattern
1391     .sp
1392     /\e*.*\e*/
1393     .sp
1394 nigel 63 to the string
1395 nigel 75 .sp
1396     /* first comment */ not comment /* second comment */
1397     .sp
1398 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1399     item.
1400 nigel 75 .P
1401 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1402     greedy, and instead matches the minimum number of times possible, so the
1403     pattern
1404 nigel 75 .sp
1405     /\e*.*?\e*/
1406     .sp
1407 nigel 63 does the right thing with the C comments. The meaning of the various
1408     quantifiers is not otherwise changed, just the preferred number of matches.
1409     Do not confuse this use of question mark with its use as a quantifier in its
1410     own right. Because it has two uses, it can sometimes appear doubled, as in
1411 nigel 75 .sp
1412     \ed??\ed
1413     .sp
1414 nigel 63 which matches one digit by preference, but can match two if that is the only
1415     way the rest of the pattern matches.
1416 nigel 75 .P
1417 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1418 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1419     greedy by following them with a question mark. In other words, it inverts the
1420     default behaviour.
1421 nigel 75 .P
1422 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1423 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1424 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1425 nigel 75 .P
1426 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1427 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1428 nigel 63 implicitly anchored, because whatever follows will be tried against every
1429     character position in the subject string, so there is no point in retrying the
1430     overall match at any position after the first. PCRE normally treats such a
1431 nigel 75 pattern as though it were preceded by \eA.
1432     .P
1433 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1434     worth setting PCRE_DOTALL in order to obtain this optimization, or
1435     alternatively using ^ to indicate anchoring explicitly.
1436 nigel 75 .P
1437 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1438 ph10 488 is inside capturing parentheses that are the subject of a back reference
1439 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1440     succeeds. Consider, for example:
1441 nigel 75 .sp
1442     (.*)abc\e1
1443     .sp
1444 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1445     this reason, such a pattern is not implicitly anchored.
1446 nigel 75 .P
1447 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1448     that matched the final iteration. For example, after
1449 nigel 75 .sp
1450     (tweedle[dume]{3}\es*)+
1451     .sp
1452 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1453     "tweedledee". However, if there are nested capturing subpatterns, the
1454     corresponding captured values may have been set in previous iterations. For
1455     example, after
1456 nigel 75 .sp
1457 nigel 63 /(a|(b))+/
1458 nigel 75 .sp
1459 nigel 63 matches "aba" the value of the second captured substring is "b".
1460 nigel 75 .
1461     .
1462     .\" HTML <a name="atomicgroup"></a>
1463     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1464 nigel 63 .rs
1465     .sp
1466 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1467     repetition, failure of what follows normally causes the repeated item to be
1468     re-evaluated to see if a different number of repeats allows the rest of the
1469     pattern to match. Sometimes it is useful to prevent this, either to change the
1470     nature of the match, or to cause it fail earlier than it otherwise might, when
1471     the author of the pattern knows there is no point in carrying on.
1472 nigel 75 .P
1473     Consider, for example, the pattern \ed+foo when applied to the subject line
1474     .sp
1475 nigel 63 123456bar
1476 nigel 75 .sp
1477 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1478 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1479 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1480     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1481     that once a subpattern has matched, it is not to be re-evaluated in this way.
1482 nigel 75 .P
1483 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1484 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1485     special parenthesis, starting with (?> as in this example:
1486 nigel 75 .sp
1487     (?>\ed+)foo
1488     .sp
1489 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1490     it has matched, and a failure further into the pattern is prevented from
1491     backtracking into it. Backtracking past it to previous items, however, works as
1492     normal.
1493 nigel 75 .P
1494 nigel 63 An alternative description is that a subpattern of this type matches the string
1495     of characters that an identical standalone pattern would match, if anchored at
1496     the current point in the subject string.
1497 nigel 75 .P
1498 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1499     the above example can be thought of as a maximizing repeat that must swallow
1500 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1501 nigel 63 number of digits they match in order to make the rest of the pattern match,
1502 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1503     .P
1504 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1505     subpatterns, and can be nested. However, when the subpattern for an atomic
1506     group is just a single repeated item, as in the example above, a simpler
1507     notation, called a "possessive quantifier" can be used. This consists of an
1508     additional + character following a quantifier. Using this notation, the
1509     previous example can be rewritten as
1510 nigel 75 .sp
1511     \ed++foo
1512     .sp
1513 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1514     example:
1515     .sp
1516     (abc|xyz){2,3}+
1517     .sp
1518 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1519     option is ignored. They are a convenient notation for the simpler forms of
1520 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1521     quantifier and the equivalent atomic group, though there may be a performance
1522     difference; possessive quantifiers should be slightly faster.
1523 nigel 75 .P
1524 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1525     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1526     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1527     package, and PCRE copied it from there. It ultimately found its way into Perl
1528     at release 5.10.
1529 nigel 75 .P
1530 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1531     pattern constructs. For example, the sequence A+B is treated as A++B because
1532     there is no point in backtracking into a sequence of A's when B must follow.
1533     .P
1534 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1535     be repeated an unlimited number of times, the use of an atomic group is the
1536     only way to avoid some failing matches taking a very long time indeed. The
1537     pattern
1538 nigel 75 .sp
1539     (\eD+|<\ed+>)*[!?]
1540     .sp
1541 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1542     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1543     quickly. However, if it is applied to
1544 nigel 75 .sp
1545 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1546 nigel 75 .sp
1547 nigel 63 it takes a long time before reporting failure. This is because the string can
1548 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1549     large number of ways, and all have to be tried. (The example uses [!?] rather
1550     than a single character at the end, because both PCRE and Perl have an
1551     optimization that allows for fast failure when a single character is used. They
1552     remember the last single character that is required for a match, and fail early
1553     if it is not present in the string.) If the pattern is changed so that it uses
1554     an atomic group, like this:
1555     .sp
1556     ((?>\eD+)|<\ed+>)*[!?]
1557     .sp
1558 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1559 nigel 75 .
1560     .
1561     .\" HTML <a name="backreferences"></a>
1562     .SH "BACK REFERENCES"
1563 nigel 63 .rs
1564     .sp
1565     Outside a character class, a backslash followed by a digit greater than 0 (and
1566     possibly further digits) is a back reference to a capturing subpattern earlier
1567     (that is, to its left) in the pattern, provided there have been that many
1568     previous capturing left parentheses.
1569 nigel 75 .P
1570 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1571     always taken as a back reference, and causes an error only if there are not
1572     that many capturing left parentheses in the entire pattern. In other words, the
1573     parentheses that are referenced need not be to the left of the reference for
1574 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1575     when a repetition is involved and the subpattern to the right has participated
1576     in an earlier iteration.
1577     .P
1578 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1579     whose number is 10 or more using this syntax because a sequence such as \e50 is
1580     interpreted as a character defined in octal. See the subsection entitled
1581 nigel 91 "Non-printing characters"
1582 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1583     .\" </a>
1584     above
1585     .\"
1586 nigel 93 for further details of the handling of digits following a backslash. There is
1587     no such problem when named parentheses are used. A back reference to any
1588     subpattern is possible using named parentheses (see below).
1589 nigel 75 .P
1590 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1591     backslash is to use the \eg escape sequence, which is a feature introduced in
1592 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1593     number, optionally enclosed in braces. These examples are all identical:
1594 nigel 93 .sp
1595     (ring), \e1
1596     (ring), \eg1
1597     (ring), \eg{1}
1598     .sp
1599 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1600     is present in the older syntax. It is also useful when literal digits follow
1601     the reference. A negative number is a relative reference. Consider this
1602     example:
1603 nigel 93 .sp
1604     (abc(def)ghi)\eg{-1}
1605     .sp
1606     The sequence \eg{-1} is a reference to the most recently started capturing
1607     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1608     would be equivalent to \e1. The use of relative references can be helpful in
1609     long patterns, and also in patterns that are created by joining together
1610     fragments that contain references within themselves.
1611     .P
1612 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1613     the current subject string, rather than anything matching the subpattern
1614     itself (see
1615     .\" HTML <a href="#subpatternsassubroutines">
1616     .\" </a>
1617     "Subpatterns as subroutines"
1618     .\"
1619     below for a way of doing that). So the pattern
1620 nigel 75 .sp
1621     (sens|respons)e and \e1ibility
1622     .sp
1623 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1624     "sense and responsibility". If caseful matching is in force at the time of the
1625     back reference, the case of letters is relevant. For example,
1626 nigel 75 .sp
1627     ((?i)rah)\es+\e1
1628     .sp
1629 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1630     capturing subpattern is matched caselessly.
1631 nigel 75 .P
1632 ph10 171 There are several different ways of writing back references to named
1633     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1634     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1635     back reference syntax, in which \eg can be used for both numeric and named
1636     references, is also supported. We could rewrite the above example in any of
1637 nigel 93 the following ways:
1638 nigel 75 .sp
1639 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1640 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1641 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1642 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1643 nigel 75 .sp
1644 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1645     after the reference.
1646     .P
1647 nigel 63 There may be more than one back reference to the same subpattern. If a
1648     subpattern has not actually been used in a particular match, any back
1649 ph10 456 references to it always fail by default. For example, the pattern
1650 nigel 75 .sp
1651     (a|(bc))\e2
1652     .sp
1653 ph10 461 always fails if it starts to match "a" rather than "bc". However, if the
1654     PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1655 ph10 456 unset value matches an empty string.
1656     .P
1657     Because there may be many capturing parentheses in a pattern, all digits
1658     following a backslash are taken as part of a potential back reference number.
1659     If the pattern continues with a digit character, some delimiter must be used to
1660     terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1661     whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1662 nigel 75 .\" HTML <a href="#comments">
1663     .\" </a>
1664     "Comments"
1665     .\"
1666     below) can be used.
1667 ph10 488 .
1668     .SS "Recursive back references"
1669     .rs
1670     .sp
1671 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1672 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1673 nigel 63 However, such references can be useful inside repeated subpatterns. For
1674     example, the pattern
1675 nigel 75 .sp
1676     (a|b\e1)+
1677     .sp
1678 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1679     the subpattern, the back reference matches the character string corresponding
1680     to the previous iteration. In order for this to work, the pattern must be such
1681     that the first iteration does not need to match the back reference. This can be
1682     done using alternation, as in the example above, or by a quantifier with a
1683     minimum of zero.
1684 ph10 488 .P
1685     Back references of this type cause the group that they reference to be treated
1686     as an
1687     .\" HTML <a href="#atomicgroup">
1688     .\" </a>
1689     atomic group.
1690     .\"
1691     Once the whole group has been matched, a subsequent matching failure cannot
1692     cause backtracking into the middle of the group.
1693 nigel 75 .
1694     .
1695     .\" HTML <a name="bigassertions"></a>
1696 nigel 63 .SH ASSERTIONS
1697     .rs
1698     .sp
1699     An assertion is a test on the characters following or preceding the current
1700     matching point that does not actually consume any characters. The simple
1701 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1702     .\" HTML <a href="#smallassertions">
1703     .\" </a>
1704     above.
1705     .\"
1706     .P
1707 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1708     those that look ahead of the current position in the subject string, and those
1709 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1710     except that it does not cause the current matching position to be changed.
1711     .P
1712     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1713     because it makes no sense to assert the same thing several times. If any kind
1714     of assertion contains capturing subpatterns within it, these are counted for
1715     the purposes of numbering the capturing subpatterns in the whole pattern.
1716     However, substring capturing is carried out only for positive assertions,
1717     because it does not make sense for negative assertions.
1718     .
1719     .
1720     .SS "Lookahead assertions"
1721     .rs
1722     .sp
1723 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1724     negative assertions. For example,
1725 nigel 75 .sp
1726     \ew+(?=;)
1727     .sp
1728 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1729     the match, and
1730 nigel 75 .sp
1731 nigel 63 foo(?!bar)
1732 nigel 75 .sp
1733 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1734     apparently similar pattern
1735 nigel 75 .sp
1736 nigel 63 (?!foo)bar
1737 nigel 75 .sp
1738 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1739     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1740     (?!foo) is always true when the next three characters are "bar". A
1741 nigel 75 lookbehind assertion is needed to achieve the other effect.
1742     .P
1743 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1744     convenient way to do it is with (?!) because an empty string always matches, so
1745     an assertion that requires there not to be an empty string must always fail.
1746 ph10 456 The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1747     synonym for (?!).
1748 nigel 75 .
1749     .
1750     .\" HTML <a name="lookbehind"></a>
1751     .SS "Lookbehind assertions"
1752     .rs
1753     .sp
1754 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1755     negative assertions. For example,
1756 nigel 75 .sp
1757 nigel 63 (?<!foo)bar
1758 nigel 75 .sp
1759 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1760     a lookbehind assertion are restricted such that all the strings it matches must
1761 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1762     do not all have to have the same fixed length. Thus
1763 nigel 75 .sp
1764 nigel 63 (?<=bullock|donkey)
1765 nigel 75 .sp
1766 nigel 63 is permitted, but
1767 nigel 75 .sp
1768 nigel 63 (?<!dogs?|cats?)
1769 nigel 75 .sp
1770 nigel 63 causes an error at compile time. Branches that match different length strings
1771     are permitted only at the top level of a lookbehind assertion. This is an
1772 ph10 454 extension compared with Perl (5.8 and 5.10), which requires all branches to
1773 nigel 63 match the same length of string. An assertion such as
1774 nigel 75 .sp
1775 nigel 63 (?<=ab(c|de))
1776 nigel 75 .sp
1777 nigel 63 is not permitted, because its single top-level branch can match two different
1778 ph10 454 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1779     branches:
1780 nigel 75 .sp
1781 nigel 63 (?<=abc|abde)
1782 nigel 75 .sp
1783 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1784 ph10 168 .\" HTML <a href="#resetmatchstart">
1785     .\" </a>
1786     (see above)
1787     .\"
1788 ph10 461 can be used instead of a lookbehind assertion to get round the fixed-length
1789 ph10 454 restriction.
1790 ph10 168 .P
1791 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1792 nigel 93 temporarily move the current position back by the fixed length and then try to
1793 nigel 63 match. If there are insufficient characters before the current position, the
1794 nigel 93 assertion fails.
1795 nigel 75 .P
1796     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1797 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1798 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1799     different numbers of bytes, are also not permitted.
1800 nigel 75 .P
1801 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
1802     .\" </a>
1803     "Subroutine"
1804     .\"
1805     calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1806 ph10 461 as the subpattern matches a fixed-length string.
1807 ph10 454 .\" HTML <a href="#recursion">
1808     .\" </a>
1809     Recursion,
1810     .\"
1811     however, is not supported.
1812     .P
1813 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1814 ph10 456 specify efficient matching of fixed-length strings at the end of subject
1815     strings. Consider a simple pattern such as
1816 nigel 75 .sp
1817 nigel 63 abcd$
1818 nigel 75 .sp
1819 nigel 63 when applied to a long string that does not match. Because matching proceeds
1820     from left to right, PCRE will look for each "a" in the subject and then see if
1821     what follows matches the rest of the pattern. If the pattern is specified as
1822 nigel 75 .sp
1823 nigel 63 ^.*abcd$
1824 nigel 75 .sp
1825 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1826     there is no following "a"), it backtracks to match all but the last character,
1827     then all but the last two characters, and so on. Once again the search for "a"
1828     covers the entire string, from right to left, so we are no better off. However,
1829     if the pattern is written as
1830 nigel 75 .sp
1831 nigel 63 ^.*+(?<=abcd)
1832 nigel 75 .sp
1833 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1834 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1835     characters. If it fails, the match fails immediately. For long strings, this
1836     approach makes a significant difference to the processing time.
1837 nigel 75 .
1838     .
1839     .SS "Using multiple assertions"
1840     .rs
1841     .sp
1842 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1843 nigel 75 .sp
1844     (?<=\ed{3})(?<!999)foo
1845     .sp
1846 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1847     the assertions is applied independently at the same point in the subject
1848     string. First there is a check that the previous three characters are all
1849     digits, and then there is a check that the same three characters are not "999".
1850 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1851 nigel 63 of which are digits and the last three of which are not "999". For example, it
1852     doesn't match "123abcfoo". A pattern to do that is
1853 nigel 75 .sp
1854     (?<=\ed{3}...)(?<!999)foo
1855     .sp
1856 nigel 63 This time the first assertion looks at the preceding six characters, checking
1857     that the first three are digits, and then the second assertion checks that the
1858     preceding three characters are not "999".
1859 nigel 75 .P
1860 nigel 63 Assertions can be nested in any combination. For example,
1861 nigel 75 .sp
1862 nigel 63 (?<=(?<!foo)bar)baz
1863 nigel 75 .sp
1864 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1865     preceded by "foo", while
1866 nigel 75 .sp
1867     (?<=\ed{3}(?!999)...)foo
1868     .sp
1869     is another pattern that matches "foo" preceded by three digits and any three
1870 nigel 63 characters that are not "999".
1871 nigel 75 .
1872     .
1873 nigel 91 .\" HTML <a name="conditions"></a>
1874 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1875 nigel 63 .rs
1876     .sp
1877     It is possible to cause the matching process to obey a subpattern
1878     conditionally or to choose between two alternative subpatterns, depending on
1879 ph10 461 the result of an assertion, or whether a specific capturing subpattern has
1880 ph10 456 already been matched. The two possible forms of conditional subpattern are:
1881 nigel 75 .sp
1882 nigel 63 (?(condition)yes-pattern)
1883     (?(condition)yes-pattern|no-pattern)
1884 nigel 75 .sp
1885 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1886     no-pattern (if present) is used. If there are more than two alternatives in the
1887     subpattern, a compile-time error occurs.
1888 nigel 75 .P
1889 nigel 93 There are four kinds of condition: references to subpatterns, references to
1890     recursion, a pseudo-condition called DEFINE, and assertions.
1891     .
1892     .SS "Checking for a used subpattern by number"
1893     .rs
1894     .sp
1895     If the text between the parentheses consists of a sequence of digits, the
1896 ph10 456 condition is true if a capturing subpattern of that number has previously
1897 ph10 461 matched. If there is more than one capturing subpattern with the same number
1898     (see the earlier
1899 ph10 456 .\"
1900     .\" HTML <a href="#recursion">
1901     .\" </a>
1902     section about duplicate subpattern numbers),
1903     .\"
1904     the condition is true if any of them have been set. An alternative notation is
1905     to precede the digits with a plus or minus sign. In this case, the subpattern
1906     number is relative rather than absolute. The most recently opened parentheses
1907     can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1908     looping constructs it can also make sense to refer to subsequent groups with
1909     constructs such as (?(+2).
1910 nigel 91 .P
1911     Consider the following pattern, which contains non-significant white space to
1912     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1913     three parts for ease of discussion:
1914 nigel 75 .sp
1915     ( \e( )? [^()]+ (?(1) \e) )
1916     .sp
1917 nigel 63 The first part matches an optional opening parenthesis, and if that
1918     character is present, sets it as the first captured substring. The second part
1919     matches one or more characters that are not parentheses. The third part is a
1920     conditional subpattern that tests whether the first set of parentheses matched
1921     or not. If they did, that is, if subject started with an opening parenthesis,
1922     the condition is true, and so the yes-pattern is executed and a closing
1923     parenthesis is required. Otherwise, since no-pattern is not present, the
1924     subpattern matches nothing. In other words, this pattern matches a sequence of
1925 nigel 93 non-parentheses, optionally enclosed in parentheses.
1926 ph10 167 .P
1927 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1928 ph10 167 reference:
1929     .sp
1930     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1931     .sp
1932     This makes the fragment independent of the parentheses in the larger pattern.
1933 nigel 93 .
1934     .SS "Checking for a used subpattern by name"
1935     .rs
1936 nigel 91 .sp
1937 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1938     subpattern by name. For compatibility with earlier versions of PCRE, which had
1939     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1940     there is a possible ambiguity with this syntax, because subpattern names may
1941     consist entirely of digits. PCRE looks first for a named subpattern; if it
1942     cannot find one and the name consists entirely of digits, PCRE looks for a
1943     subpattern of that number, which must be greater than zero. Using subpattern
1944     names that consist entirely of digits is not recommended.
1945     .P
1946     Rewriting the above example to use a named subpattern gives this:
1947 nigel 91 .sp
1948 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1949     .sp
1950 ph10 461 If the name used in a condition of this kind is a duplicate, the test is
1951     applied to all subpatterns of the same name, and is true if any one of them has
1952 ph10 459 matched.
1953 nigel 93 .
1954     .SS "Checking for pattern recursion"
1955     .rs
1956     .sp
1957 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1958 nigel 93 the condition is true if a recursive call to the whole pattern or any
1959     subpattern has been made. If digits or a name preceded by ampersand follow the
1960     letter R, for example:
1961     .sp
1962     (?(R3)...) or (?(R&name)...)
1963     .sp
1964 ph10 456 the condition is true if the most recent recursion is into a subpattern whose
1965 nigel 93 number or name is given. This condition does not check the entire recursion
1966 ph10 461 stack. If the name used in a condition of this kind is a duplicate, the test is
1967     applied to all subpatterns of the same name, and is true if any one of them is
1968     the most recent recursion.
1969 nigel 75 .P
1970 ph10 461 At "top level", all these recursion test conditions are false.
1971 ph10 454 .\" HTML <a href="#recursion">
1972     .\" </a>
1973 ph10 459 The syntax for recursive patterns
1974 ph10 454 .\"
1975 ph10 459 is described below.
1976 nigel 93 .
1977     .SS "Defining subpatterns for use by reference only"
1978     .rs
1979     .sp
1980     If the condition is the string (DEFINE), and there is no subpattern with the
1981     name DEFINE, the condition is always false. In this case, there may be only one
1982     alternative in the subpattern. It is always skipped if control reaches this
1983     point in the pattern; the idea of DEFINE is that it can be used to define
1984 ph10 461 "subroutines" that can be referenced from elsewhere. (The use of
1985 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
1986     .\" </a>
1987     "subroutines"
1988     .\"
1989 nigel 93 is described below.) For example, a pattern to match an IPv4 address could be
1990     written like this (ignore whitespace and line breaks):
1991     .sp
1992     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1993     \eb (?&byte) (\e.(?&byte)){3} \eb
1994     .sp
1995     The first part of the pattern is a DEFINE group inside which a another group
1996     named "byte" is defined. This matches an individual component of an IPv4
1997     address (a number less than 256). When matching takes place, this part of the
1998 ph10 456 pattern is skipped because DEFINE acts like a false condition. The rest of the
1999     pattern uses references to the named group to match the four dot-separated
2000     components of an IPv4 address, insisting on a word boundary at each end.
2001 nigel 93 .
2002     .SS "Assertion conditions"
2003     .rs
2004     .sp
2005     If the condition is not in any of the above formats, it must be an assertion.
2006 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
2007     this pattern, again containing non-significant white space, and with the two
2008     alternatives on the second line:
2009 nigel 75 .sp
2010 nigel 63 (?(?=[^a-z]*[a-z])
2011 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2012     .sp
2013 nigel 63 The condition is a positive lookahead assertion that matches an optional
2014     sequence of non-letters followed by a letter. In other words, it tests for the
2015     presence of at least one letter in the subject. If a letter is found, the
2016     subject is matched against the first alternative; otherwise it is matched
2017     against the second. This pattern matches strings in one of the two forms
2018     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2019 nigel 75 .
2020     .
2021     .\" HTML <a name="comments"></a>
2022 nigel 63 .SH COMMENTS
2023     .rs
2024     .sp
2025 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
2026 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
2027     that make up a comment play no part in the pattern matching at all.
2028 nigel 75 .P
2029 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
2030 nigel 91 character class introduces a comment that continues to immediately after the
2031     next newline in the pattern.
2032 nigel 75 .
2033     .
2034 nigel 91 .\" HTML <a name="recursion"></a>
2035 nigel 75 .SH "RECURSIVE PATTERNS"
2036 nigel 63 .rs
2037     .sp
2038     Consider the problem of matching a string in parentheses, allowing for
2039     unlimited nested parentheses. Without the use of recursion, the best that can
2040     be done is to use a pattern that matches up to some fixed depth of nesting. It
2041 nigel 93 is not possible to handle an arbitrary nesting depth.
2042     .P
2043     For some time, Perl has provided a facility that allows regular expressions to
2044     recurse (amongst other things). It does this by interpolating Perl code in the
2045     expression at run time, and the code can refer to the expression itself. A Perl
2046     pattern using code interpolation to solve the parentheses problem can be
2047     created like this:
2048 nigel 75 .sp
2049     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2050     .sp
2051 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2052 nigel 93 recursively to the pattern in which it appears.
2053 nigel 75 .P
2054 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2055     supports special syntax for recursion of the entire pattern, and also for
2056     individual subpattern recursion. After its introduction in PCRE and Python,
2057 ph10 453 this kind of recursion was subsequently introduced into Perl at release 5.10.
2058 nigel 75 .P
2059 nigel 93 A special item that consists of (? followed by a number greater than zero and a
2060     closing parenthesis is a recursive call of the subpattern of the given number,
2061 ph10 461 provided that it occurs inside that subpattern. (If not, it is a
2062 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2063     .\" </a>
2064     "subroutine"
2065     .\"
2066 nigel 93 call, which is described in the next section.) The special item (?R) or (?0) is
2067     a recursive call of the entire regular expression.
2068 nigel 87 .P
2069     This PCRE pattern solves the nested parentheses problem (assume the
2070     PCRE_EXTENDED option is set so that white space is ignored):
2071 nigel 75 .sp
2072 ph10 456 \e( ( [^()]++ | (?R) )* \e)
2073 nigel 75 .sp
2074 nigel 63 First it matches an opening parenthesis. Then it matches any number of
2075     substrings which can either be a sequence of non-parentheses, or a recursive
2076 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
2077 ph10 461 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2078 ph10 456 to avoid backtracking into sequences of non-parentheses.
2079 nigel 75 .P
2080 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
2081     pattern, so instead you could use this:
2082 nigel 75 .sp
2083 ph10 456 ( \e( ( [^()]++ | (?1) )* \e) )
2084 nigel 75 .sp
2085 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
2086 ph10 172 them instead of the whole pattern.
2087 ph10 166 .P
2088     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2089 ph10 456 is made easier by the use of relative references (a Perl 5.10 feature).
2090 ph10 166 Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2091     most recently opened parentheses preceding the recursion. In other words, a
2092     negative number counts capturing parentheses leftwards from the point at which
2093     it is encountered.
2094     .P
2095     It is also possible to refer to subsequently opened parentheses, by writing
2096     references such as (?+2). However, these cannot be recursive because the
2097     reference is not inside the parentheses that are referenced. They are always
2098 ph10 454 .\" HTML <a href="#subpatternsassubroutines">
2099     .\" </a>
2100     "subroutine"
2101     .\"
2102     calls, as described in the next section.
2103 ph10 166 .P
2104     An alternative approach is to use named parentheses instead. The Perl syntax
2105     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2106     could rewrite the above example as follows:
2107 nigel 75 .sp
2108 ph10 456 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2109 nigel 75 .sp
2110 nigel 93 If there is more than one subpattern with the same name, the earliest one is
2111 ph10 172 used.
2112 ph10 166 .P
2113     This particular example pattern that we have been looking at contains nested
2114 ph10 456 unlimited repeats, and so the use of a possessive quantifier for matching
2115     strings of non-parentheses is important when applying the pattern to strings
2116     that do not match. For example, when this pattern is applied to
2117 nigel 75 .sp
2118 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2119 nigel 75 .sp
2120 ph10 456 it yields "no match" quickly. However, if a possessive quantifier is not used,
2121 nigel 63 the match runs for a very long time indeed because there are so many different
2122     ways the + and * repeats can carve up the subject, and all have to be tested
2123     before failure can be reported.
2124 nigel 75 .P
2125 ph10 464 At the end of a match, the values of capturing parentheses are those from
2126     the outermost level. If you want to obtain intermediate values, a callout
2127     function can be used (see below and the
2128 nigel 63 .\" HREF
2129 nigel 75 \fBpcrecallout\fP
2130 nigel 63 .\"
2131     documentation). If the pattern above is matched against
2132 nigel 75 .sp
2133 nigel 63 (ab(cd)ef)
2134 nigel 75 .sp
2135 ph10 464 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2136     the last value taken on at the top level. If a capturing subpattern is not
2137     matched at the top level, its final value is unset, even if it is (temporarily)
2138     set at a deeper level.
2139 nigel 75 .P
2140 ph10 464 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2141     obtain extra memory to store data during a recursion, which it does by using
2142     \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2143     be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2144     .P
2145 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2146     Consider this pattern, which matches text in angle brackets, allowing for
2147     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2148     recursing), whereas any characters are permitted at the outer level.
2149 nigel 75 .sp
2150     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2151     .sp
2152 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2153     different alternatives for the recursive and non-recursive cases. The (?R) item
2154     is the actual recursive call.
2155 nigel 75 .
2156     .
2157 ph10 453 .\" HTML <a name="recursiondifference"></a>
2158     .SS "Recursion difference from Perl"
2159     .rs
2160     .sp
2161     In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2162     treated as an atomic group. That is, once it has matched some of the subject
2163     string, it is never re-entered, even if it contains untried alternatives and
2164 ph10 461 there is a subsequent matching failure. This can be illustrated by the
2165     following pattern, which purports to match a palindromic string that contains
2166 ph10 453 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2167     .sp
2168     ^(.|(.)(?1)\e2)$
2169     .sp
2170 ph10 461 The idea is that it either matches a single character, or two identical
2171     characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2172 ph10 453 it does not if the pattern is longer than three characters. Consider the
2173     subject string "abcba":
2174     .P
2175 ph10 461 At the top level, the first character is matched, but as it is not at the end
2176 ph10 453 of the string, the first alternative fails; the second alternative is taken
2177     and the recursion kicks in. The recursive call to subpattern 1 successfully
2178     matches the next character ("b"). (Note that the beginning and end of line
2179     tests are not part of the recursion).
2180     .P
2181     Back at the top level, the next character ("c") is compared with what
2182 ph10 461 subpattern 2 matched, which was "a". This fails. Because the recursion is
2183 ph10 453 treated as an atomic group, there are now no backtracking points, and so the
2184     entire match fails. (Perl is able, at this point, to re-enter the recursion and
2185     try the second alternative.) However, if the pattern is written with the
2186     alternatives in the other order, things are different:
2187     .sp
2188     ^((.)(?1)\e2|.)$
2189     .sp
2190 ph10 461 This time, the recursing alternative is tried first, and continues to recurse
2191     until it runs out of characters, at which point the recursion fails. But this
2192     time we do have another alternative to try at the higher level. That is the big
2193 ph10 453 difference: in the previous case the remaining alternative is at a deeper
2194     recursion level, which PCRE cannot use.
2195     .P
2196 ph10 461 To change the pattern so that matches all palindromic strings, not just those
2197 ph10 453 with an odd number of characters, it is tempting to change the pattern to this:
2198     .sp
2199     ^((.)(?1)\e2|.?)$
2200     .sp
2201 ph10 461 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2202     deeper recursion has matched a single character, it cannot be entered again in
2203     order to match an empty string. The solution is to separate the two cases, and
2204 ph10 453 write out the odd and even cases as alternatives at the higher level:
2205     .sp
2206     ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2207 ph10 461 .sp
2208     If you want to match typical palindromic phrases, the pattern has to ignore all
2209 ph10 453 non-word characters, which can be done like this:
2210     .sp
2211 ph10 461 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2212 ph10 453 .sp
2213 ph10 461 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2214     man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2215     the use of the possessive quantifier *+ to avoid backtracking into sequences of
2216 ph10 453 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2217     more) to match typical phrases, and Perl takes so long that you think it has
2218     gone into a loop.
2219 ph10 456 .P
2220     \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2221     string does not start with a palindrome that is shorter than the entire string.
2222     For example, although "abcba" is correctly matched, if the subject is "ababa",
2223     PCRE finds the palindrome "aba" at the start, then fails at top level because
2224     the end of the string does not follow. Once again, it cannot jump back into the
2225     recursion to try other alternatives, so the entire match fails.
2226 ph10 453 .
2227     .
2228 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
2229 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
2230 nigel 63 .rs
2231     .sp
2232     If the syntax for a recursive subpattern reference (either by number or by
2233     name) is used outside the parentheses to which it refers, it operates like a
2234 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
2235 ph10 166 before or after the reference. A numbered reference can be absolute or
2236     relative, as in these examples:
2237 nigel 75 .sp
2238 ph10 166 (...(absolute)...)...(?2)...
2239     (...(relative)...)...(?-1)...
2240 ph10 172 (...(?+1)...(relative)...
2241 ph10 166 .sp
2242     An earlier example pointed out that the pattern
2243     .sp
2244 nigel 75 (sens|respons)e and \e1ibility
2245     .sp
2246 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2247     "sense and responsibility". If instead the pattern
2248 nigel 75 .sp
2249 nigel 63 (sens|respons)e and (?1)ibility
2250 nigel 75 .sp
2251 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2252 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2253 nigel 87 .P
2254 ph10 464 Like recursive subpatterns, a subroutine call is always treated as an atomic
2255 nigel 87 group. That is, once it has matched some of the subject string, it is never
2256     re-entered, even if it contains untried alternatives and there is a subsequent
2257 ph10 469 matching failure. Any capturing parentheses that are set during the subroutine
2258 ph10 464 call revert to their previous values afterwards.
2259 nigel 93 .P
2260     When a subpattern is used as a subroutine, processing options such as
2261     case-independence are fixed when the subpattern is defined. They cannot be
2262     changed for different calls. For example, consider this pattern:
2263     .sp
2264 ph10 166 (abc)(?i:(?-1))
2265 nigel 93 .sp
2266     It matches "abcabc". It does not match "abcABC" because the change of
2267     processing option does not affect the called subpattern.
2268 nigel 75 .
2269     .
2270 ph10 333 .\" HTML <a name="onigurumasubroutines"></a>
2271     .SH "ONIGURUMA SUBROUTINE SYNTAX"
2272     .rs
2273     .sp
2274 ph10 345 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2275     a number enclosed either in angle brackets or single quotes, is an alternative
2276     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2277 ph10 333 are two of the examples used above, rewritten using this syntax:
2278     .sp
2279     (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2280     (sens|respons)e and \eg'1'ibility
2281     .sp
2282 ph10 345 PCRE supports an extension to Oniguruma: if a number is preceded by a
2283 ph10 333 plus or a minus sign it is taken as a relative reference. For example:
2284     .sp
2285     (abc)(?i:\eg<-1>)
2286     .sp
2287 ph10 345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2288 ph10 333 synonymous. The former is a back reference; the latter is a subroutine call.
2289     .
2290     .
2291 nigel 63 .SH CALLOUTS
2292     .rs
2293     .sp
2294     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2295     code to be obeyed in the middle of matching a regular expression. This makes it
2296     possible, amongst other things, to extract different substrings that match the
2297     same pair of parentheses when there is a repetition.
2298 nigel 75 .P
2299 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2300     code. The feature is called "callout". The caller of PCRE provides an external
2301 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
2302 nigel 63 By default, this variable contains NULL, which disables all calling out.
2303 nigel 75 .P
2304 nigel 63 Within a regular expression, (?C) indicates the points at which the external
2305     function is to be called. If you want to identify different callout points, you
2306     can put a number less than 256 after the letter C. The default value is zero.
2307     For example, this pattern has two callout points:
2308 nigel 75 .sp
2309 ph10 155 (?C1)abc(?C2)def
2310 nigel 75 .sp
2311     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2312     automatically installed before each item in the pattern. They are all numbered
2313     255.
2314     .P
2315     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2316 nigel 63 set), the external function is called. It is provided with the number of the
2317 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2318     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2319     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2320     description of the interface to the callout function is given in the
2321 nigel 63 .\" HREF
2322 nigel 75 \fBpcrecallout\fP
2323 nigel 63 .\"
2324     documentation.
2325 nigel 93 .
2326     .
2327 ph10 510 .\" HTML <a name="backtrackcontrol"></a>
2328 ph10 235 .SH "BACKTRACKING CONTROL"
2329 ph10 210 .rs
2330     .sp
2331 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2332 ph10 210 are described in the Perl documentation as "experimental and subject to change
2333 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
2334     production code should be noted to avoid problems during upgrades." The same
2335 ph10 210 remarks apply to the PCRE features described in this section.
2336     .P
2337 ph10 341 Since these verbs are specifically related to backtracking, most of them can be
2338     used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2339 ph10 345 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2340 ph10 341 failing negative assertion, they cause an error if encountered by
2341 ph10 210 \fBpcre_dfa_exec()\fP.
2342     .P
2343 ph10 469 If any of these verbs are used in an assertion or subroutine subpattern
2344 ph10 464 (including recursive subpatterns), their effect is confined to that subpattern;
2345     it does not extend to the surrounding pattern. Note that such subpatterns are
2346     processed as anchored at the point where they are tested.
2347 ph10 445 .P
2348 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2349 ph10 510 parenthesis followed by an asterisk. They are generally of the form
2350 ph10 512 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2351 ph10 510 depending on whether or not an argument is present. An name is a sequence of
2352     letters, digits, and underscores. If the name is empty, that is, if the closing
2353     parenthesis immediately follows the colon, the effect is as if the colon were
2354     not there. Any number of these verbs may occur in a pattern.
2355     .P
2356 ph10 512 PCRE contains some optimizations that are used to speed up matching by running
2357     some checks at the start of each match attempt. For example, it may know the
2358     minimum length of matching subject, or that a particular character must be
2359     present. When one of these optimizations suppresses the running of a match, any
2360     included backtracking verbs will not, of course, be processed. You can suppress
2361     the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2362 ph10 510 when calling \fBpcre_exec()\fP.
2363 ph10 210 .
2364 ph10 510 .
2365 ph10 210 .SS "Verbs that act immediately"
2366     .rs
2367     .sp
2368 ph10 512 The following verbs act as soon as they are encountered. They may not be
2369 ph10 510 followed by a name.
2370 ph10 210 .sp
2371     (*ACCEPT)
2372     .sp
2373     This verb causes the match to end successfully, skipping the remainder of the
2374 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2375 ph10 456 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2376     captured. (This feature was added to PCRE at release 8.00.) For example:
2377 ph10 210 .sp
2378 ph10 447 A((?:A|B(*ACCEPT)|C)D)
2379 ph10 210 .sp
2380 ph10 461 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2381 ph10 447 the outer parentheses.
2382 ph10 210 .sp
2383     (*FAIL) or (*F)
2384     .sp
2385 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2386 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2387     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2388     Perl features that are not present in PCRE. The nearest equivalent is the
2389     callout feature, as for example in this pattern:
2390     .sp
2391     a+(?C)(*FAIL)
2392     .sp
2393 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2394     each backtrack happens (in this example, 10 times).
2395 ph10 210 .
2396 ph10 510 .
2397     .SS "Recording which path was taken"
2398     .rs
2399     .sp
2400 ph10 512 There is one verb whose main purpose is to track how a match was arrived at,
2401     though it also has a secondary use in conjunction with advancing the match
2402 ph10 510 starting point (see (*SKIP) below).
2403     .sp
2404     (*MARK:NAME) or (*:NAME)
2405     .sp
2406     A name is always required with this verb. There may be as many instances of
2407     (*MARK) as you like in a pattern, and their names do not have to be unique.
2408     .P
2409     When a match succeeds, the name of the last-encountered (*MARK) is passed back
2410     to the caller via the \fIpcre_extra\fP data structure, as described in the
2411     .\" HTML <a href="pcreapi.html#extradata">
2412     .\" </a>
2413     section on \fIpcre_extra\fP
2414     .\"
2415 ph10 512 in the
2416 ph10 510 .\" HREF
2417     \fBpcreapi\fP
2418     .\"
2419     documentation. No data is returned for a partial match. Here is an example of
2420     \fBpcretest\fP output, where the /K modifier requests the retrieval and
2421     outputting of (*MARK) data:
2422     .sp
2423     /X(*MARK:A)Y|X(*MARK:B)Z/K
2424     XY
2425     0: XY
2426     MK: A
2427     XZ
2428     0: XZ
2429     MK: B
2430     .sp
2431 ph10 512 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2432     indicates which of the two alternatives matched. This is a more efficient way
2433 ph10 510 of obtaining this information than putting each alternative in its own
2434     capturing parentheses.
2435     .P
2436     A name may also be returned after a failed match if the final path through the
2437     pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2438     (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2439     starting point for matching is advanced, the final check is often with an empty
2440     string, causing a failure before (*MARK) is reached. For example:
2441     .sp
2442     /X(*MARK:A)Y|X(*MARK:B)Z/K
2443     XP
2444     No match
2445     .sp
2446     There are three potential starting points for this match (starting with X,
2447 ph10 512 starting with P, and with an empty string). If the pattern is anchored, the
2448 ph10 510 result is different:
2449     .sp
2450     /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2451     XP
2452     No match, mark = B
2453     .sp
2454 ph10 512 PCRE's start-of-match optimizations can also interfere with this. For example,
2455     if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2456 ph10 510 subject length for a match, a shorter subject will not be scanned at all.
2457     .P
2458 ph10 512 Note that similar anomalies (though different in detail) exist in Perl, no
2459     doubt for the same reasons. The use of (*MARK) data after a failed match of an
2460 ph10 510 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2461     .
2462     .
2463 ph10 210 .SS "Verbs that act after backtracking"
2464     .rs
2465     .sp
2466 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2467 ph10 510 with what follows, but if there is no subsequent match, causing a backtrack to
2468     the verb, a failure is forced. That is, backtracking cannot pass to the left of
2469     the verb. However, when one of these verbs appears inside an atomic group, its
2470     effect is confined to that group, because once the group has been matched,
2471     there is never any backtracking into it. In this situation, backtracking can
2472 ph10 512 "jump back" to the left of the entire atomic group. (Remember also, as stated
2473 ph10 510 above, that this localization also applies in subroutine calls and assertions.)
2474     .P
2475     These verbs differ in exactly what kind of failure occurs when backtracking
2476     reaches them.
2477 ph10 210 .sp
2478     (*COMMIT)
2479     .sp
2480 ph10 510 This verb, which may not be followed by a name, causes the whole match to fail
2481     outright if the rest of the pattern does not match. Even if the pattern is
2482     unanchored, no further attempts to find a match by advancing the starting point
2483     take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2484     finding a match at the current starting point, or not at all. For example:
2485 ph10 210 .sp
2486     a+(*COMMIT)b
2487     .sp
2488 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2489 ph10 512 dynamic anchor, or "I've started, so I must finish." The name of the most
2490     recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2491 ph10 510 match failure.
2492     .P
2493 ph10 512 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2494     unless PCRE's start-of-match optimizations are turned off, as shown in this
2495 ph10 510 \fBpcretest\fP example:
2496 ph10 210 .sp
2497 ph10 510 /(*COMMIT)abc/
2498     xyzabc
2499     0: abc
2500     xyzabc\eY
2501     No match
2502 ph10 210 .sp
2503 ph10 512 PCRE knows that any match must start with "a", so the optimization skips along
2504 ph10 510 the subject to "a" before running the first match attempt, which succeeds. When
2505     the optimization is disabled by the \eY escape in the second subject, the match
2506 ph10 512 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2507 ph10 510 starting points.
2508 ph10 210 .sp
2509 ph10 510 (*PRUNE) or (*PRUNE:NAME)
2510     .sp
2511 ph10 512 This verb causes the match to fail at the current starting position in the
2512 ph10 510 subject if the rest of the pattern does not match. If the pattern is
2513     unanchored, the normal "bumpalong" advance to the next starting character then
2514     happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2515     reached, or when matching to the right of (*PRUNE), but if there is no match to
2516     the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2517     (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2518     but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2519     The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2520     match fails completely; the name is passed back if this is the final attempt.
2521     (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2522     pattern (*PRUNE) has the same effect as (*COMMIT).
2523     .sp
2524 ph10 210 (*SKIP)
2525     .sp
2526 ph10 510 This verb, when given without a name, is like (*PRUNE), except that if the
2527     pattern is unanchored, the "bumpalong" advance is not to the next character,
2528     but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2529     signifies that whatever text was matched leading up to it cannot be part of a
2530     successful match. Consider:
2531 ph10 210 .sp
2532     a+(*SKIP)b
2533     .sp
2534 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2535 ph10 210 the first character in the string), the starting point skips on to start the
2536 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2537 ph10 456 effect as this example; although it would suppress backtracking during the
2538 ph10 210 first match attempt, the second attempt would start at the second character
2539     instead of skipping on to "c".
2540     .sp
2541 ph10 510 (*SKIP:NAME)
2542 ph10 211 .sp
2543 ph10 512 When (*SKIP) has an associated name, its behaviour is modified. If the
2544     following pattern fails to match, the previous path through the pattern is
2545     searched for the most recent (*MARK) that has the same name. If one is found,
2546     the "bumpalong" advance is to the subject position that corresponds to that
2547     (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2548     matching name is found, normal "bumpalong" of one character happens (the
2549 ph10 510 (*SKIP) is ignored).
2550     .sp
2551     (*THEN) or (*THEN:NAME)
2552     .sp
2553 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2554     not match. That is, it cancels pending backtracking, but only within the
2555     current alternation. Its name comes from the observation that it can be used
2556     for a pattern-based if-then-else block:
2557     .sp
2558     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2559     .sp
2560 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2561 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2562 ph10 510 second alternative and tries COND2, without backtracking into COND1. The
2563     behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2564     overall match fails. If (*THEN) is not directly inside an alternation, it acts
2565     like (*PRUNE).
2566 ph10 210 .
2567     .
2568 nigel 93 .SH "SEE ALSO"
2569     .rs
2570     .sp
2571 ph10 461 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2572 ph10 456 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2573 ph10 99 .
2574     .
2575     .SH AUTHOR
2576     .rs
2577     .sp
2578     .nf
2579     Philip Hazel
2580     University Computing Service
2581     Cambridge CB2 3QH, England.
2582     .fi
2583     .
2584     .
2585     .SH REVISION
2586     .rs
2587     .sp
2588     .nf
2589 ph10 513 Last updated: 03 May 2010
2590 ph10 488 Copyright (c) 1997-2010 University of Cambridge.
2591 ph10 99 .fi

Properties

Name Value
svn:eol-style native
svn:keywords "Author Date Id Revision Url"

webmaster@exim.org
ViewVC Help
Powered by ViewVC 1.1.12