ViewVC logotype

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

Parent Directory Parent Directory | Revision Log Revision Log

Revision 259 - (hide annotations) (download) (as text)
Wed Sep 19 09:52:44 2007 UTC (7 years, 6 months ago) by ph10
File MIME type: text/html
File size: 93196 byte(s)
File tidies caused by building a release.

1 nigel 63 <html>
2     <head>
3     <title>pcrepattern specification</title>
4     </head>
5     <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 nigel 75 <h1>pcrepattern man page</h1>
7     <p>
8     Return to the <a href="index.html">PCRE index page</a>.
9     </p>
10 ph10 111 <p>
11 nigel 75 This page is part of the PCRE HTML documentation. It was generated automatically
12     from the original man page. If there is any nonsense in it, please consult the
13     man page, in case the conversion went wrong.
14 ph10 111 <br>
15 nigel 63 <ul>
16     <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 ph10 227 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18     <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19     <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20     <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21     <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
22     <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
23     <li><a name="TOC8" href="#SEC8">SQUARE BRACKETS AND CHARACTER CLASSES</a>
24     <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25     <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26     <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27     <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28     <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29     <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30     <li><a name="TOC15" href="#SEC15">REPETITION</a>
32     <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33     <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34     <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35     <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36     <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37     <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38     <li><a name="TOC23" href="#SEC23">CALLOUTS</a>
39 ph10 243 <li><a name="TOC24" href="#SEC24">BACKTRACKING CONTROL</a>
40 ph10 227 <li><a name="TOC25" href="#SEC25">SEE ALSO</a>
41     <li><a name="TOC26" href="#SEC26">AUTHOR</a>
42     <li><a name="TOC27" href="#SEC27">REVISION</a>
43 nigel 63 </ul>
44     <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
45     <P>
46 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE
47     are described in detail below. There is a quick-reference syntax summary in the
48     <a href="pcresyntax.html"><b>pcresyntax</b></a>
49     page. Perl's regular expressions are described in its own documentation, and
50     regular expressions in general are covered in a number of books, some of which
51     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
52     published by O'Reilly, covers regular expressions in great detail. This
53     description of PCRE's regular expressions is intended as reference material.
54 nigel 63 </P>
55     <P>
56 nigel 75 The original operation of PCRE was on strings of one-byte characters. However,
57     there is now also support for UTF-8 character strings. To use this, you must
58     build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
59     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
60     places below. There is also a summary of UTF-8 features in the
61 nigel 63 <a href="pcre.html#utf8support">section on UTF-8 support</a>
62     in the main
63     <a href="pcre.html"><b>pcre</b></a>
64     page.
65     </P>
66     <P>
67 nigel 77 The remainder of this document discusses the patterns that are supported by
68     PCRE when its main matching function, <b>pcre_exec()</b>, is used.
69     From release 6.0, PCRE offers a second matching function,
70     <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
71 ph10 172 Perl-compatible. Some of the features discussed below are not available when
72     <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
73     alternative function, and how it differs from the normal function, are
74     discussed in the
75 nigel 77 <a href="pcrematching.html"><b>pcrematching</b></a>
76     page.
77     </P>
78 ph10 227 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
79 nigel 77 <P>
80 ph10 227 PCRE supports five different conventions for indicating line breaks in
81     strings: a single CR (carriage return) character, a single LF (linefeed)
82     character, the two-character sequence CRLF, any of the three preceding, or any
83     Unicode newline sequence. The
84     <a href="pcreapi.html"><b>pcreapi</b></a>
85     page has
86     <a href="pcreapi.html#newlines">further discussion</a>
87     about newlines, and shows how to set the newline convention in the
88     <i>options</i> arguments for the compiling and matching functions.
89     </P>
90     <P>
91     It is also possible to specify a newline convention by starting a pattern
92     string with one of the following five sequences:
93     <pre>
94     (*CR) carriage return
95     (*LF) linefeed
96     (*CRLF) carriage return, followed by linefeed
97     (*ANYCRLF) any of the three above
98     (*ANY) all Unicode newline sequences
99     </pre>
100     These override the default and the options given to <b>pcre_compile()</b>. For
101     example, on a Unix system where LF is the default newline sequence, the pattern
102     <pre>
103     (*CR)a.b
104     </pre>
105     changes the convention to CR. That pattern matches "a\nb" because LF is no
106     longer a newline. Note that these special settings, which are not
107     Perl-compatible, are recognized only at the very start of a pattern, and that
108 ph10 231 they must be in upper case. If more than one of them is present, the last one
109     is used.
110 ph10 227 </P>
111 ph10 231 <P>
112     The newline convention does not affect what the \R escape sequence matches. By
113     default, this is any Unicode newline sequence, for Perl compatibility. However,
114     this can be changed; see the description of \R in the section entitled
115     <a href="#newlineseq">"Newline sequences"</a>
116 ph10 247 below. A change of \R setting can be combined with a change of newline
117     convention.
118 ph10 231 </P>
119 ph10 227 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
120     <P>
121 nigel 63 A regular expression is a pattern that is matched against a subject string from
122     left to right. Most characters stand for themselves in a pattern, and match the
123     corresponding characters in the subject. As a trivial example, the pattern
124     <pre>
125     The quick brown fox
126 nigel 75 </pre>
127 nigel 77 matches a portion of a subject string that is identical to itself. When
128     caseless matching is specified (the PCRE_CASELESS option), letters are matched
129     independently of case. In UTF-8 mode, PCRE always understands the concept of
130     case for characters whose values are less than 128, so caseless matching is
131     always possible. For characters with higher values, the concept of case is
132     supported if PCRE is compiled with Unicode property support, but not otherwise.
133     If you want to use caseless matching for characters 128 and above, you must
134     ensure that PCRE is compiled with Unicode property support as well as with
135     UTF-8 support.
136     </P>
137     <P>
138     The power of regular expressions comes from the ability to include alternatives
139     and repetitions in the pattern. These are encoded in the pattern by the use of
140 nigel 75 <i>metacharacters</i>, which do not stand for themselves but instead are
141 nigel 63 interpreted in some special way.
142     </P>
143     <P>
144 nigel 75 There are two different sets of metacharacters: those that are recognized
145 nigel 63 anywhere in the pattern except within square brackets, and those that are
146 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
147     are as follows:
148 nigel 63 <pre>
149     \ general escape character with several uses
150     ^ assert start of string (or line, in multiline mode)
151     $ assert end of string (or line, in multiline mode)
152     . match any character except newline (by default)
153     [ start character class definition
154     | start of alternative branch
155     ( start subpattern
156     ) end subpattern
157     ? extends the meaning of (
158     also 0 or 1 quantifier
159     also quantifier minimizer
160     * 0 or more quantifier
161     + 1 or more quantifier
162     also "possessive quantifier"
163     { start min/max quantifier
164 nigel 75 </pre>
165 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
166 nigel 75 a character class the only metacharacters are:
167 nigel 63 <pre>
168     \ general escape character
169     ^ negate the class, but only if the first character
170     - indicates character range
171 nigel 75 [ POSIX character class (only if followed by POSIX syntax)
172 nigel 63 ] terminates the character class
173 nigel 75 </pre>
174     The following sections describe the use of each of the metacharacters.
175 nigel 63 </P>
176 ph10 227 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
177 nigel 63 <P>
178     The backslash character has several uses. Firstly, if it is followed by a
179 nigel 91 non-alphanumeric character, it takes away any special meaning that character
180     may have. This use of backslash as an escape character applies both inside and
181 nigel 63 outside character classes.
182     </P>
183     <P>
184     For example, if you want to match a * character, you write \* in the pattern.
185     This escaping action applies whether or not the following character would
186 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
187     non-alphanumeric with backslash to specify that it stands for itself. In
188 nigel 63 particular, if you want to match a backslash, you write \\.
189     </P>
190     <P>
191     If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
192     pattern (other than in a character class) and characters between a # outside
193 nigel 91 a character class and the next newline are ignored. An escaping backslash can
194     be used to include a whitespace or # character as part of the pattern.
195 nigel 63 </P>
196     <P>
197     If you want to remove the special meaning from a sequence of characters, you
198     can do so by putting them between \Q and \E. This is different from Perl in
199     that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
200     Perl, $ and @ cause variable interpolation. Note the following examples:
201     <pre>
202     Pattern PCRE matches Perl matches
203 nigel 75
204     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
205 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
206     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
207 nigel 75 </pre>
208 nigel 63 The \Q...\E sequence is recognized both inside and outside character classes.
209 nigel 75 <a name="digitsafterbackslash"></a></P>
210     <br><b>
211     Non-printing characters
212     </b><br>
213 nigel 63 <P>
214     A second use of backslash provides a way of encoding non-printing characters
215     in patterns in a visible manner. There is no restriction on the appearance of
216     non-printing characters, apart from the binary zero that terminates a pattern,
217     but when a pattern is being prepared by text editing, it is usually easier to
218     use one of the following escape sequences than the binary character it
219     represents:
220     <pre>
221     \a alarm, that is, the BEL character (hex 07)
222     \cx "control-x", where x is any character
223     \e escape (hex 1B)
224     \f formfeed (hex 0C)
225 ph10 227 \n linefeed (hex 0A)
226 nigel 63 \r carriage return (hex 0D)
227     \t tab (hex 09)
228     \ddd character with octal code ddd, or backreference
229     \xhh character with hex code hh
230 nigel 87 \x{hhh..} character with hex code hhh..
231 nigel 75 </pre>
232 nigel 63 The precise effect of \cx is as follows: if x is a lower case letter, it
233     is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
234     Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
235     7B.
236     </P>
237     <P>
238     After \x, from zero to two hexadecimal digits are read (letters can be in
239 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \x{
240     and }, but the value of the character code must be less than 256 in non-UTF-8
241 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
242     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
243     point, which is 10FFFF.
244 nigel 63 </P>
245     <P>
246 ph10 211 If characters other than hexadecimal digits appear between \x{ and }, or if
247     there is no terminating }, this form of escape is not recognized. Instead, the
248     initial \x will be interpreted as a basic hexadecimal escape, with no
249     following digits, giving a character whose value is zero.
250     </P>
251     <P>
252 nigel 63 Characters whose value is less than 256 can be defined by either of the two
253 nigel 87 syntaxes for \x. There is no difference in the way they are handled. For
254     example, \xdc is exactly the same as \x{dc}.
255 nigel 63 </P>
256     <P>
257 nigel 91 After \0 up to two further octal digits are read. If there are fewer than two
258     digits, just those that are present are used. Thus the sequence \0\x\07
259     specifies two binary zeros followed by a BEL character (code value 7). Make
260     sure you supply two digits after the initial zero if the pattern character that
261     follows is itself an octal digit.
262 nigel 63 </P>
263     <P>
264     The handling of a backslash followed by a digit other than 0 is complicated.
265     Outside a character class, PCRE reads it and any following digits as a decimal
266     number. If the number is less than 10, or if there have been at least that many
267     previous capturing left parentheses in the expression, the entire sequence is
268     taken as a <i>back reference</i>. A description of how this works is given
269 nigel 75 <a href="#backreferences">later,</a>
270     following the discussion of
271     <a href="#subpattern">parenthesized subpatterns.</a>
272 nigel 63 </P>
273     <P>
274     Inside a character class, or if the decimal number is greater than 9 and there
275     have not been that many capturing subpatterns, PCRE re-reads up to three octal
276 nigel 93 digits following the backslash, and uses them to generate a data character. Any
277 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
278     character specified in octal must be less than \400. In UTF-8 mode, values up
279     to \777 are permitted. For example:
280 nigel 63 <pre>
281     \040 is another way of writing a space
282 nigel 75 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
283 nigel 63 \7 is always a back reference
284 nigel 75 \11 might be a back reference, or another way of writing a tab
285 nigel 63 \011 is always a tab
286     \0113 is a tab followed by the character "3"
287 nigel 75 \113 might be a back reference, otherwise the character with octal code 113
288     \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
289     \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
290     </pre>
291 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
292     zero, because no more than three octal digits are ever read.
293     </P>
294     <P>
295 nigel 91 All the sequences that define a single character value can be used both inside
296     and outside character classes. In addition, inside a character class, the
297     sequence \b is interpreted as the backspace character (hex 08), and the
298 nigel 93 sequences \R and \X are interpreted as the characters "R" and "X",
299     respectively. Outside a character class, these sequences have different
300     meanings
301 nigel 75 <a href="#uniextseq">(see below).</a>
302 nigel 63 </P>
303 nigel 75 <br><b>
304 nigel 93 Absolute and relative back references
305     </b><br>
306     <P>
307 ph10 208 The sequence \g followed by an unsigned or a negative number, optionally
308     enclosed in braces, is an absolute or relative back reference. A named back
309     reference can be coded as \g{name}. Back references are discussed
310 nigel 93 <a href="#backreferences">later,</a>
311     following the discussion of
312     <a href="#subpattern">parenthesized subpatterns.</a>
313     </P>
314     <br><b>
315 nigel 75 Generic character types
316     </b><br>
317 nigel 63 <P>
318 nigel 93 Another use of backslash is for specifying generic character types. The
319 nigel 75 following are always recognized:
320 nigel 63 <pre>
321     \d any decimal digit
322     \D any character that is not a decimal digit
323 ph10 182 \h any horizontal whitespace character
324     \H any character that is not a horizontal whitespace character
325 nigel 63 \s any whitespace character
326     \S any character that is not a whitespace character
327 ph10 182 \v any vertical whitespace character
328     \V any character that is not a vertical whitespace character
329 nigel 63 \w any "word" character
330     \W any "non-word" character
331 nigel 75 </pre>
332 nigel 63 Each pair of escape sequences partitions the complete set of characters into
333     two disjoint sets. Any given character matches one, and only one, of each pair.
334     </P>
335     <P>
336 nigel 75 These character type sequences can appear both inside and outside character
337     classes. They each match one character of the appropriate type. If the current
338     matching point is at the end of the subject string, all of them fail, since
339     there is no character to match.
340 nigel 63 </P>
341     <P>
342     For compatibility with Perl, \s does not match the VT character (code 11).
343     This makes it different from the the POSIX "space" class. The \s characters
344 ph10 182 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
345 nigel 91 included in a Perl script, \s may match the VT character. In PCRE, it never
346 ph10 182 does.
347 nigel 63 </P>
348     <P>
349 ph10 182 In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
350     \w, and always match \D, \S, and \W. This is true even when Unicode
351     character property support is available. These sequences retain their original
352     meanings from before UTF-8 support was available, mainly for efficiency
353     reasons.
354     </P>
355     <P>
356     The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
357     other sequences, these do match certain high-valued codepoints in UTF-8 mode.
358     The horizontal space characters are:
359     <pre>
360     U+0009 Horizontal tab
361     U+0020 Space
362     U+00A0 Non-break space
363     U+1680 Ogham space mark
364     U+180E Mongolian vowel separator
365     U+2000 En quad
366     U+2001 Em quad
367     U+2002 En space
368     U+2003 Em space
369     U+2004 Three-per-em space
370     U+2005 Four-per-em space
371     U+2006 Six-per-em space
372     U+2007 Figure space
373     U+2008 Punctuation space
374     U+2009 Thin space
375     U+200A Hair space
376     U+202F Narrow no-break space
377     U+205F Medium mathematical space
378     U+3000 Ideographic space
379     </pre>
380     The vertical space characters are:
381     <pre>
382     U+000A Linefeed
383     U+000B Vertical tab
384     U+000C Formfeed
385     U+000D Carriage return
386     U+0085 Next line
387     U+2028 Line separator
388     U+2029 Paragraph separator
389     </PRE>
390     </P>
391     <P>
392 nigel 75 A "word" character is an underscore or any character less than 256 that is a
393     letter or digit. The definition of letters and digits is controlled by PCRE's
394     low-valued character tables, and may vary if locale-specific matching is taking
395     place (see
396 nigel 63 <a href="pcreapi.html#localesupport">"Locale support"</a>
397     in the
398     <a href="pcreapi.html"><b>pcreapi</b></a>
399 ph10 142 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
400     or "french" in Windows, some character codes greater than 128 are used for
401 ph10 182 accented letters, and these are matched by \w. The use of locales with Unicode
402     is discouraged.
403 ph10 231 <a name="newlineseq"></a></P>
404 nigel 93 <br><b>
405     Newline sequences
406     </b><br>
407     <P>
408 ph10 231 Outside a character class, by default, the escape sequence \R matches any
409     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
410     equivalent to the following:
411 nigel 93 <pre>
412     (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
413     </pre>
414     This is an example of an "atomic group", details of which are given
415     <a href="#atomicgroup">below.</a>
416     This particular group matches either the two-character sequence CR followed by
417     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
418     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
419     line, U+0085). The two-character sequence is treated as a single unit that
420     cannot be split.
421     </P>
422     <P>
423     In UTF-8 mode, two additional characters whose codepoints are greater than 255
424     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
425     Unicode character property support is not needed for these characters to be
426     recognized.
427     </P>
428     <P>
429 ph10 231 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
430     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
431 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
432     for "backslash R".) This can be made the default when PCRE is built; if this is
433     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
434     It is also possible to specify these settings by starting a pattern string with
435     one of the following sequences:
436 ph10 231 <pre>
437     (*BSR_ANYCRLF) CR, LF, or CRLF only
438     (*BSR_UNICODE) any Unicode newline sequence
439     </pre>
440     These override the default and the options given to <b>pcre_compile()</b>, but
441     they can be overridden by options given to <b>pcre_exec()</b>. Note that these
442     special settings, which are not Perl-compatible, are recognized only at the
443     very start of a pattern, and that they must be in upper case. If more than one
444 ph10 247 of them is present, the last one is used. They can be combined with a change of
445     newline convention, for example, a pattern can start with:
446     <pre>
447     (*ANY)(*BSR_ANYCRLF)
448     </pre>
449 nigel 93 Inside a character class, \R matches the letter "R".
450 nigel 75 <a name="uniextseq"></a></P>
451     <br><b>
452     Unicode character properties
453     </b><br>
454     <P>
455     When PCRE is built with Unicode character property support, three additional
456 ph10 185 escape sequences that match characters with specific properties are available.
457     When not in UTF-8 mode, these sequences are of course limited to testing
458     characters whose codepoints are less than 256, but they do work in this mode.
459     The extra escape sequences are:
460 nigel 75 <pre>
461 nigel 87 \p{<i>xx</i>} a character with the <i>xx</i> property
462     \P{<i>xx</i>} a character without the <i>xx</i> property
463     \X an extended Unicode sequence
464 nigel 75 </pre>
465 nigel 87 The property names represented by <i>xx</i> above are limited to the Unicode
466     script names, the general category properties, and "Any", which matches any
467     character (including newline). Other properties such as "InMusicalSymbols" are
468     not currently supported by PCRE. Note that \P{Any} does not match any
469     characters, so always causes a match failure.
470 nigel 63 </P>
471     <P>
472 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
473     character from one of these sets can be matched using a script name. For
474     example:
475 nigel 75 <pre>
476 nigel 87 \p{Greek}
477     \P{Han}
478     </pre>
479     Those that are not part of an identified script are lumped together as
480     "Common". The current list of scripts is:
481     </P>
482     <P>
483     Arabic,
484     Armenian,
485 nigel 93 Balinese,
486 nigel 87 Bengali,
487     Bopomofo,
488     Braille,
489     Buginese,
490     Buhid,
491     Canadian_Aboriginal,
492     Cherokee,
493     Common,
494     Coptic,
495 nigel 93 Cuneiform,
496 nigel 87 Cypriot,
497     Cyrillic,
498     Deseret,
499     Devanagari,
500     Ethiopic,
501     Georgian,
502     Glagolitic,
503     Gothic,
504     Greek,
505     Gujarati,
506     Gurmukhi,
507     Han,
508     Hangul,
509     Hanunoo,
510     Hebrew,
511     Hiragana,
512     Inherited,
513     Kannada,
514     Katakana,
515     Kharoshthi,
516     Khmer,
517     Lao,
518     Latin,
519     Limbu,
520     Linear_B,
521     Malayalam,
522     Mongolian,
523     Myanmar,
524     New_Tai_Lue,
525 nigel 93 Nko,
526 nigel 87 Ogham,
527     Old_Italic,
528     Old_Persian,
529     Oriya,
530     Osmanya,
531 nigel 93 Phags_Pa,
532     Phoenician,
533 nigel 87 Runic,
534     Shavian,
535     Sinhala,
536     Syloti_Nagri,
537     Syriac,
538     Tagalog,
539     Tagbanwa,
540     Tai_Le,
541     Tamil,
542     Telugu,
543     Thaana,
544     Thai,
545     Tibetan,
546     Tifinagh,
547     Ugaritic,
548     Yi.
549     </P>
550     <P>
551     Each character has exactly one general category property, specified by a
552     two-letter abbreviation. For compatibility with Perl, negation can be specified
553     by including a circumflex between the opening brace and the property name. For
554     example, \p{^Lu} is the same as \P{Lu}.
555     </P>
556     <P>
557     If only one letter is specified with \p or \P, it includes all the general
558     category properties that start with that letter. In this case, in the absence
559     of negation, the curly brackets in the escape sequence are optional; these two
560     examples have the same effect:
561     <pre>
562 nigel 75 \p{L}
563     \pL
564     </pre>
565 nigel 87 The following general category property codes are supported:
566 nigel 75 <pre>
567     C Other
568     Cc Control
569     Cf Format
570     Cn Unassigned
571     Co Private use
572     Cs Surrogate
574     L Letter
575     Ll Lower case letter
576     Lm Modifier letter
577     Lo Other letter
578     Lt Title case letter
579     Lu Upper case letter
581     M Mark
582     Mc Spacing mark
583     Me Enclosing mark
584     Mn Non-spacing mark
586     N Number
587     Nd Decimal number
588     Nl Letter number
589     No Other number
591     P Punctuation
592     Pc Connector punctuation
593     Pd Dash punctuation
594     Pe Close punctuation
595     Pf Final punctuation
596     Pi Initial punctuation
597     Po Other punctuation
598     Ps Open punctuation
600     S Symbol
601     Sc Currency symbol
602     Sk Modifier symbol
603     Sm Mathematical symbol
604     So Other symbol
606     Z Separator
607     Zl Line separator
608     Zp Paragraph separator
609     Zs Space separator
610     </pre>
611 nigel 87 The special property L& is also supported: it matches a character that has
612     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
613     a modifier or "other".
614 nigel 75 </P>
615     <P>
616 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
617     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
618     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
619     (see the discussion of PCRE_NO_UTF8_CHECK in the
620     <a href="pcreapi.html"><b>pcreapi</b></a>
621     page).
622     </P>
623     <P>
624 nigel 87 The long synonyms for these properties that Perl supports (such as \p{Letter})
625 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
626 nigel 87 properties with "Is".
627     </P>
628     <P>
629     No character that is in the Unicode table has the Cn (unassigned) property.
630     Instead, this property is assumed for any code point that is not in the
631     Unicode table.
632     </P>
633     <P>
634 nigel 75 Specifying caseless matching does not affect these escape sequences. For
635     example, \p{Lu} always matches only upper case letters.
636     </P>
637     <P>
638     The \X escape matches any number of Unicode characters that form an extended
639     Unicode sequence. \X is equivalent to
640     <pre>
641     (?&#62;\PM\pM*)
642     </pre>
643     That is, it matches a character without the "mark" property, followed by zero
644     or more characters with the "mark" property, and treats the sequence as an
645     atomic group
646     <a href="#atomicgroup">(see below).</a>
647     Characters with the "mark" property are typically accents that affect the
648 ph10 185 preceding character. None of them have codepoints less than 256, so in
649     non-UTF-8 mode \X matches any one character.
650 nigel 75 </P>
651     <P>
652     Matching characters by Unicode property is not fast, because PCRE has to search
653     a structure that contains data for over fifteen thousand characters. That is
654     why the traditional escape sequences such as \d and \w do not use Unicode
655     properties in PCRE.
656 ph10 172 <a name="resetmatchstart"></a></P>
657     <br><b>
658     Resetting the match start
659     </b><br>
660     <P>
661     The escape sequence \K, which is a Perl 5.10 feature, causes any previously
662     matched characters not to be included in the final matched sequence. For
663     example, the pattern:
664     <pre>
665     foo\Kbar
666     </pre>
667     matches "foobar", but reports that it has matched "bar". This feature is
668     similar to a lookbehind assertion
669     <a href="#lookbehind">(described below).</a>
670     However, in this case, the part of the subject before the real match does not
671     have to be of fixed length, as lookbehind assertions do. The use of \K does
672     not interfere with the setting of
673     <a href="#subpattern">captured substrings.</a>
674     For example, when the pattern
675     <pre>
676     (foo)\Kbar
677     </pre>
678     matches "foobar", the first substring is still set to "foo".
679 nigel 75 <a name="smallassertions"></a></P>
680     <br><b>
681     Simple assertions
682     </b><br>
683     <P>
684 nigel 93 The final use of backslash is for certain simple assertions. An assertion
685 nigel 63 specifies a condition that has to be met at a particular point in a match,
686     without consuming any characters from the subject string. The use of
687 nigel 75 subpatterns for more complicated assertions is described
688     <a href="#bigassertions">below.</a>
689 nigel 91 The backslashed assertions are:
690 nigel 63 <pre>
691     \b matches at a word boundary
692     \B matches when not at a word boundary
693 nigel 93 \A matches at the start of the subject
694     \Z matches at the end of the subject
695     also matches before a newline at the end of the subject
696     \z matches only at the end of the subject
697     \G matches at the first matching position in the subject
698 nigel 75 </pre>
699 nigel 63 These assertions may not appear in character classes (but note that \b has a
700     different meaning, namely the backspace character, inside a character class).
701     </P>
702     <P>
703     A word boundary is a position in the subject string where the current character
704     and the previous character do not both match \w or \W (i.e. one matches
705     \w and the other matches \W), or the start or end of the string if the
706     first or last character matches \w, respectively.
707     </P>
708     <P>
709     The \A, \Z, and \z assertions differ from the traditional circumflex and
710 nigel 75 dollar (described in the next section) in that they only ever match at the very
711     start and end of the subject string, whatever options are set. Thus, they are
712     independent of multiline mode. These three assertions are not affected by the
713     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
714     circumflex and dollar metacharacters. However, if the <i>startoffset</i>
715     argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
716     at a point other than the beginning of the subject, \A can never match. The
717 nigel 91 difference between \Z and \z is that \Z matches before a newline at the end
718     of the string as well as at the very end, whereas \z matches only at the end.
719 nigel 63 </P>
720     <P>
721     The \G assertion is true only when the current matching position is at the
722     start point of the match, as specified by the <i>startoffset</i> argument of
723     <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
724     non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
725     arguments, you can mimic Perl's /g option, and it is in this kind of
726     implementation where \G can be useful.
727     </P>
728     <P>
729     Note, however, that PCRE's interpretation of \G, as the start of the current
730     match, is subtly different from Perl's, which defines it as the end of the
731     previous match. In Perl, these can be different when the previously matched
732     string was empty. Because PCRE does just one match at a time, it cannot
733     reproduce this behaviour.
734     </P>
735     <P>
736     If all the alternatives of a pattern begin with \G, the expression is anchored
737     to the starting match position, and the "anchored" flag is set in the compiled
738     regular expression.
739     </P>
740 ph10 227 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
741 nigel 63 <P>
742     Outside a character class, in the default matching mode, the circumflex
743 nigel 75 character is an assertion that is true only if the current matching point is
744 nigel 63 at the start of the subject string. If the <i>startoffset</i> argument of
745     <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
746     option is unset. Inside a character class, circumflex has an entirely different
747 nigel 75 meaning
748     <a href="#characterclass">(see below).</a>
749 nigel 63 </P>
750     <P>
751     Circumflex need not be the first character of the pattern if a number of
752     alternatives are involved, but it should be the first thing in each alternative
753     in which it appears if the pattern is ever to match that branch. If all
754     possible alternatives start with a circumflex, that is, if the pattern is
755     constrained to match only at the start of the subject, it is said to be an
756     "anchored" pattern. (There are also other constructs that can cause a pattern
757     to be anchored.)
758     </P>
759     <P>
760 nigel 75 A dollar character is an assertion that is true only if the current matching
761 nigel 63 point is at the end of the subject string, or immediately before a newline
762 nigel 91 at the end of the string (by default). Dollar need not be the last character of
763     the pattern if a number of alternatives are involved, but it should be the last
764     item in any branch in which it appears. Dollar has no special meaning in a
765     character class.
766 nigel 63 </P>
767     <P>
768     The meaning of dollar can be changed so that it matches only at the very end of
769     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
770     does not affect the \Z assertion.
771     </P>
772     <P>
773     The meanings of the circumflex and dollar characters are changed if the
774 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
775     immediately after internal newlines as well as at the start of the subject
776     string. It does not match after a newline that ends the string. A dollar
777     matches before any newlines in the string, as well as at the very end, when
778     PCRE_MULTILINE is set. When newline is specified as the two-character
779     sequence CRLF, isolated CR and LF characters do not indicate newlines.
780 nigel 63 </P>
781     <P>
782 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
783     \n represents a newline) in multiline mode, but not otherwise. Consequently,
784     patterns that are anchored in single line mode because all branches start with
785     ^ are not anchored in multiline mode, and a match for circumflex is possible
786     when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
787     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
788     </P>
789     <P>
790 nigel 63 Note that the sequences \A, \Z, and \z can be used to match the start and
791     end of the subject in both modes, and if all branches of a pattern start with
792 nigel 91 \A it is always anchored, whether or not PCRE_MULTILINE is set.
793 nigel 63 </P>
794 ph10 227 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
795 nigel 63 <P>
796     Outside a character class, a dot in the pattern matches any one character in
797 nigel 91 the subject string except (by default) a character that signifies the end of a
798 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
799 nigel 63 </P>
800 nigel 91 <P>
801 nigel 93 When a line ending is defined as a single character, dot never matches that
802     character; when the two-character sequence CRLF is used, dot does not match CR
803     if it is immediately followed by LF, but otherwise it matches all characters
804     (including isolated CRs and LFs). When any Unicode line endings are being
805     recognized, dot does not match CR or LF or any of the other line ending
806     characters.
807     </P>
808     <P>
809 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
810 nigel 93 option is set, a dot matches any one character, without exception. If the
811     two-character sequence CRLF is present in the subject string, it takes two dots
812     to match it.
813 nigel 91 </P>
814     <P>
815     The handling of dot is entirely independent of the handling of circumflex and
816     dollar, the only relationship being that they both involve newlines. Dot has no
817     special meaning in a character class.
818     </P>
819 ph10 227 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
820 nigel 63 <P>
821     Outside a character class, the escape sequence \C matches any one byte, both
822 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
823     characters. The feature is provided in Perl in order to match individual bytes
824     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
825     what remains in the string may be a malformed UTF-8 string. For this reason,
826     the \C escape sequence is best avoided.
827 nigel 63 </P>
828     <P>
829 nigel 75 PCRE does not allow \C to appear in lookbehind assertions
830     <a href="#lookbehind">(described below),</a>
831     because in UTF-8 mode this would make it impossible to calculate the length of
832     the lookbehind.
833     <a name="characterclass"></a></P>
834 ph10 227 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
835 nigel 63 <P>
836     An opening square bracket introduces a character class, terminated by a closing
837     square bracket. A closing square bracket on its own is not special. If a
838     closing square bracket is required as a member of the class, it should be the
839     first data character in the class (after an initial circumflex, if present) or
840     escaped with a backslash.
841     </P>
842     <P>
843     A character class matches a single character in the subject. In UTF-8 mode, the
844     character may occupy more than one byte. A matched character must be in the set
845     of characters defined by the class, unless the first character in the class
846     definition is a circumflex, in which case the subject character must not be in
847     the set defined by the class. If a circumflex is actually required as a member
848     of the class, ensure it is not the first character, or escape it with a
849     backslash.
850     </P>
851     <P>
852     For example, the character class [aeiou] matches any lower case vowel, while
853     [^aeiou] matches any character that is not a lower case vowel. Note that a
854 nigel 75 circumflex is just a convenient notation for specifying the characters that
855     are in the class by enumerating those that are not. A class that starts with a
856     circumflex is not an assertion: it still consumes a character from the subject
857     string, and therefore it fails if the current pointer is at the end of the
858     string.
859 nigel 63 </P>
860     <P>
861     In UTF-8 mode, characters with values greater than 255 can be included in a
862     class as a literal string of bytes, or by using the \x{ escaping mechanism.
863     </P>
864     <P>
865     When caseless matching is set, any letters in a class represent both their
866     upper case and lower case versions, so for example, a caseless [aeiou] matches
867     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
868 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
869     case for characters whose values are less than 128, so caseless matching is
870     always possible. For characters with higher values, the concept of case is
871     supported if PCRE is compiled with Unicode property support, but not otherwise.
872     If you want to use caseless matching for characters 128 and above, you must
873     ensure that PCRE is compiled with Unicode property support as well as with
874     UTF-8 support.
875 nigel 63 </P>
876     <P>
877 nigel 93 Characters that might indicate line breaks are never treated in any special way
878     when matching character classes, whatever line-ending sequence is in use, and
879     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
880     such as [^a] always matches one of these characters.
881 nigel 63 </P>
882     <P>
883     The minus (hyphen) character can be used to specify a range of characters in a
884     character class. For example, [d-m] matches any letter between d and m,
885     inclusive. If a minus character is required in a class, it must be escaped with
886     a backslash or appear in a position where it cannot be interpreted as
887     indicating a range, typically as the first or last character in the class.
888     </P>
889     <P>
890     It is not possible to have the literal character "]" as the end character of a
891     range. A pattern such as [W-]46] is interpreted as a class of two characters
892     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
893     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
894 nigel 75 the end of range, so [W-\]46] is interpreted as a class containing a range
895     followed by two other characters. The octal or hexadecimal representation of
896     "]" can also be used to end a range.
897 nigel 63 </P>
898     <P>
899     Ranges operate in the collating sequence of character values. They can also be
900     used for characters specified numerically, for example [\000-\037]. In UTF-8
901     mode, ranges can include characters whose values are greater than 255, for
902     example [\x{100}-\x{2ff}].
903     </P>
904     <P>
905     If a range that includes letters is used when caseless matching is set, it
906     matches the letters in either case. For example, [W-c] is equivalent to
907 nigel 75 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
908 ph10 142 tables for a French locale are in use, [\xc8-\xcb] matches accented E
909 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
910     characters with values greater than 128 only when it is compiled with Unicode
911     property support.
912 nigel 63 </P>
913     <P>
914 nigel 75 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
915     in a character class, and add the characters that they match to the class. For
916 nigel 63 example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
917     conveniently be used with the upper case character types to specify a more
918     restricted set of characters than the matching lower case type. For example,
919     the class [^\W_] matches any letter or digit, but not underscore.
920     </P>
921     <P>
922 nigel 75 The only metacharacters that are recognized in character classes are backslash,
923     hyphen (only where it can be interpreted as specifying a range), circumflex
924     (only at the start), opening square bracket (only when it can be interpreted as
925     introducing a POSIX class name - see the next section), and the terminating
926     closing square bracket. However, escaping other non-alphanumeric characters
927     does no harm.
928 nigel 63 </P>
929 ph10 227 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
930 nigel 63 <P>
931 nigel 75 Perl supports the POSIX notation for character classes. This uses names
932 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
933     this notation. For example,
934     <pre>
935     [01[:alpha:]%]
936 nigel 75 </pre>
937 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
938     are
939     <pre>
940     alnum letters and digits
941     alpha letters
942     ascii character codes 0 - 127
943     blank space or tab only
944     cntrl control characters
945     digit decimal digits (same as \d)
946     graph printing characters, excluding space
947     lower lower case letters
948     print printing characters, including space
949     punct printing characters, excluding letters and digits
950     space white space (not quite the same as \s)
951     upper upper case letters
952     word "word" characters (same as \w)
953     xdigit hexadecimal digits
954 nigel 75 </pre>
955 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
956     space (32). Notice that this list includes the VT character (code 11). This
957     makes "space" different to \s, which does not include VT (for Perl
958     compatibility).
959     </P>
960     <P>
961     The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
962     5.8. Another Perl extension is negation, which is indicated by a ^ character
963     after the colon. For example,
964     <pre>
965     [12[:^digit:]]
966 nigel 75 </pre>
967 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
968     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
969     supported, and an error is given if they are encountered.
970     </P>
971     <P>
972 nigel 75 In UTF-8 mode, characters with values greater than 128 do not match any of
973 nigel 63 the POSIX character classes.
974     </P>
975 ph10 227 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
976 nigel 63 <P>
977     Vertical bar characters are used to separate alternative patterns. For example,
978     the pattern
979     <pre>
980     gilbert|sullivan
981 nigel 75 </pre>
982 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
983 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
984     process tries each alternative in turn, from left to right, and the first one
985     that succeeds is used. If the alternatives are within a subpattern
986 nigel 75 <a href="#subpattern">(defined below),</a>
987     "succeeds" means matching the rest of the main pattern as well as the
988     alternative in the subpattern.
989 nigel 63 </P>
990 ph10 227 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
991 nigel 63 <P>
992     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
993 ph10 243 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
994     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
995     The option letters are
996 nigel 63 <pre>
997     i for PCRE_CASELESS
998     m for PCRE_MULTILINE
999     s for PCRE_DOTALL
1000     x for PCRE_EXTENDED
1001 nigel 75 </pre>
1002 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1003     unset these options by preceding the letter with a hyphen, and a combined
1004     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1005     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1006     permitted. If a letter appears both before and after the hyphen, the option is
1007     unset.
1008     </P>
1009     <P>
1010 ph10 243 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1011     changed in the same way as the Perl-compatible options by using the characters
1012     J, U and X respectively.
1013     </P>
1014     <P>
1015 nigel 63 When an option change occurs at top level (that is, not inside subpattern
1016     parentheses), the change applies to the remainder of the pattern that follows.
1017     If the change is placed right at the start of a pattern, PCRE extracts it into
1018     the global options (and it will therefore show up in data extracted by the
1019     <b>pcre_fullinfo()</b> function).
1020     </P>
1021     <P>
1022 nigel 93 An option change within a subpattern (see below for a description of
1023     subpatterns) affects only that part of the current pattern that follows it, so
1024 nigel 63 <pre>
1025     (a(?i)b)c
1026 nigel 75 </pre>
1027 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1028     By this means, options can be made to have different settings in different
1029     parts of the pattern. Any changes made in one alternative do carry on
1030     into subsequent branches within the same subpattern. For example,
1031     <pre>
1032     (a(?i)b|c)
1033 nigel 75 </pre>
1034 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1035     branch is abandoned before the option setting. This is because the effects of
1036     option settings happen at compile time. There would be some very weird
1037     behaviour otherwise.
1038 ph10 254 </P>
1039     <P>
1040     <b>Note:</b> There are other PCRE-specific options that can be set by the
1041     application when the compile or match functions are called. In some cases the
1042     pattern can contain special leading sequences to override what the application
1043     has set or what has been defaulted. Details are given in the section entitled
1044     <a href="#newlineseq">"Newline sequences"</a>
1045 ph10 259 above.
1046 nigel 75 <a name="subpattern"></a></P>
1047 ph10 227 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1048 nigel 63 <P>
1049     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1050 nigel 75 Turning part of a pattern into a subpattern does two things:
1051     <br>
1052     <br>
1053 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1054     <pre>
1055     cat(aract|erpillar|)
1056 nigel 75 </pre>
1057 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1058 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1059 nigel 75 <br>
1060     <br>
1061     2. It sets up the subpattern as a capturing subpattern. This means that, when
1062     the whole pattern matches, that portion of the subject string that matched the
1063     subpattern is passed back to the caller via the <i>ovector</i> argument of
1064 nigel 63 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1065 nigel 75 from 1) to obtain numbers for the capturing subpatterns.
1066 nigel 63 </P>
1067     <P>
1068     For example, if the string "the red king" is matched against the pattern
1069     <pre>
1070     the ((red|white) (king|queen))
1071 nigel 75 </pre>
1072 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1073     2, and 3, respectively.
1074     </P>
1075     <P>
1076     The fact that plain parentheses fulfil two functions is not always helpful.
1077     There are often times when a grouping subpattern is required without a
1078     capturing requirement. If an opening parenthesis is followed by a question mark
1079     and a colon, the subpattern does not do any capturing, and is not counted when
1080     computing the number of any subsequent capturing subpatterns. For example, if
1081     the string "the white queen" is matched against the pattern
1082     <pre>
1083     the ((?:red|white) (king|queen))
1084 nigel 75 </pre>
1085 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1086 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1087 nigel 63 </P>
1088     <P>
1089     As a convenient shorthand, if any option settings are required at the start of
1090     a non-capturing subpattern, the option letters may appear between the "?" and
1091     the ":". Thus the two patterns
1092     <pre>
1093     (?i:saturday|sunday)
1094     (?:(?i)saturday|sunday)
1095 nigel 75 </pre>
1096 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1097     from left to right, and options are not reset until the end of the subpattern
1098     is reached, an option setting in one branch does affect subsequent branches, so
1099     the above patterns match "SUNDAY" as well as "Saturday".
1100     </P>
1101 ph10 227 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1102 nigel 63 <P>
1103 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1104     the same numbers for its capturing parentheses. Such a subpattern starts with
1105     (?| and is itself a non-capturing subpattern. For example, consider this
1106     pattern:
1107     <pre>
1108     (?|(Sat)ur|(Sun))day
1109     </pre>
1110     Because the two alternatives are inside a (?| group, both sets of capturing
1111     parentheses are numbered one. Thus, when the pattern matches, you can look
1112     at captured substring number one, whichever alternative matched. This construct
1113     is useful when you want to capture part, but not all, of one of a number of
1114     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1115     number is reset at the start of each branch. The numbers of any capturing
1116     buffers that follow the subpattern start after the highest number used in any
1117     branch. The following example is taken from the Perl documentation.
1118     The numbers underneath show in which buffer the captured content will be
1119     stored.
1120     <pre>
1121     # before ---------------branch-reset----------- after
1122     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1123     # 1 2 2 3 2 3 4
1124     </pre>
1125     A backreference or a recursive call to a numbered subpattern always refers to
1126     the first one in the pattern with the given number.
1127     </P>
1128     <P>
1129     An alternative approach to using this "branch reset" feature is to use
1130     duplicate named subpatterns, as described in the next section.
1131     </P>
1132 ph10 227 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1133 ph10 182 <P>
1134 nigel 63 Identifying capturing parentheses by number is simple, but it can be very hard
1135     to keep track of the numbers in complicated regular expressions. Furthermore,
1136 nigel 75 if an expression is modified, the numbers may change. To help with this
1137 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1138     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1139     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1140     the Perl and the Python syntax.
1141     </P>
1142     <P>
1143     In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1144     (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1145 nigel 91 parentheses from other parts of the pattern, such as
1146     <a href="#backreferences">backreferences,</a>
1147     <a href="#recursion">recursion,</a>
1148     and
1149     <a href="#conditions">conditions,</a>
1150     can be made by name as well as by number.
1151 nigel 63 </P>
1152     <P>
1153 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1154 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1155     if the names were not present. The PCRE API provides function calls for
1156     extracting the name-to-number translation table from a compiled pattern. There
1157     is also a convenience function for extracting a captured substring by name.
1158 nigel 91 </P>
1159     <P>
1160     By default, a name must be unique within a pattern, but it is possible to relax
1161     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1162     be useful for patterns where only one instance of the named parentheses can
1163     match. Suppose you want to match the name of a weekday, either as a 3-letter
1164     abbreviation or as the full name, and in both cases you want to extract the
1165     abbreviation. This pattern (ignoring the line breaks) does the job:
1166     <pre>
1167 nigel 93 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1168     (?&#60;DN&#62;Tue)(?:sday)?|
1169     (?&#60;DN&#62;Wed)(?:nesday)?|
1170     (?&#60;DN&#62;Thu)(?:rsday)?|
1171     (?&#60;DN&#62;Sat)(?:urday)?
1172 nigel 91 </pre>
1173     There are five capturing substrings, but only one is ever set after a match.
1174 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1175     subpattern, as described in the previous section.)
1176     </P>
1177     <P>
1178 nigel 91 The convenience function for extracting the data by name returns the substring
1179 nigel 93 for the first (and in this example, the only) subpattern of that name that
1180 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1181     make a reference to a non-unique named subpattern from elsewhere in the
1182     pattern, the one that corresponds to the lowest number is used. For further
1183     details of the interfaces for handling named subpatterns, see the
1184 nigel 63 <a href="pcreapi.html"><b>pcreapi</b></a>
1185     documentation.
1186     </P>
1187 ph10 227 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1188 nigel 63 <P>
1189     Repetition is specified by quantifiers, which can follow any of the following
1190     items:
1191     <pre>
1192     a literal data character
1193 nigel 93 the dot metacharacter
1194 nigel 63 the \C escape sequence
1195 nigel 75 the \X escape sequence (in UTF-8 mode with Unicode properties)
1196 nigel 93 the \R escape sequence
1197 nigel 75 an escape such as \d that matches a single character
1198 nigel 63 a character class
1199     a back reference (see next section)
1200     a parenthesized subpattern (unless it is an assertion)
1201 nigel 75 </pre>
1202 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1203     permitted matches, by giving the two numbers in curly brackets (braces),
1204     separated by a comma. The numbers must be less than 65536, and the first must
1205     be less than or equal to the second. For example:
1206     <pre>
1207     z{2,4}
1208 nigel 75 </pre>
1209 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1210     character. If the second number is omitted, but the comma is present, there is
1211     no upper limit; if the second number and the comma are both omitted, the
1212     quantifier specifies an exact number of required matches. Thus
1213     <pre>
1214     [aeiou]{3,}
1215 nigel 75 </pre>
1216 nigel 63 matches at least 3 successive vowels, but may match many more, while
1217     <pre>
1218     \d{8}
1219 nigel 75 </pre>
1220 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1221     where a quantifier is not allowed, or one that does not match the syntax of a
1222     quantifier, is taken as a literal character. For example, {,6} is not a
1223     quantifier, but a literal string of four characters.
1224     </P>
1225     <P>
1226     In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1227     bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1228 nigel 75 which is represented by a two-byte sequence. Similarly, when Unicode property
1229     support is available, \X{3} matches three Unicode extended sequences, each of
1230     which may be several bytes long (and they may be of different lengths).
1231 nigel 63 </P>
1232     <P>
1233     The quantifier {0} is permitted, causing the expression to behave as if the
1234     previous item and the quantifier were not present.
1235     </P>
1236     <P>
1237 nigel 93 For convenience, the three most common quantifiers have single-character
1238     abbreviations:
1239 nigel 63 <pre>
1240     * is equivalent to {0,}
1241     + is equivalent to {1,}
1242     ? is equivalent to {0,1}
1243 nigel 75 </pre>
1244 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1245     match no characters with a quantifier that has no upper limit, for example:
1246     <pre>
1247     (a?)*
1248 nigel 75 </pre>
1249 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1250     such patterns. However, because there are cases where this can be useful, such
1251     patterns are now accepted, but if any repetition of the subpattern does in fact
1252     match no characters, the loop is forcibly broken.
1253     </P>
1254     <P>
1255     By default, the quantifiers are "greedy", that is, they match as much as
1256     possible (up to the maximum number of permitted times), without causing the
1257     rest of the pattern to fail. The classic example of where this gives problems
1258 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1259     and within the comment, individual * and / characters may appear. An attempt to
1260     match C comments by applying the pattern
1261 nigel 63 <pre>
1262     /\*.*\*/
1263 nigel 75 </pre>
1264 nigel 63 to the string
1265     <pre>
1266 nigel 75 /* first comment */ not comment /* second comment */
1267     </pre>
1268 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1269     item.
1270     </P>
1271     <P>
1272     However, if a quantifier is followed by a question mark, it ceases to be
1273     greedy, and instead matches the minimum number of times possible, so the
1274     pattern
1275     <pre>
1276     /\*.*?\*/
1277 nigel 75 </pre>
1278 nigel 63 does the right thing with the C comments. The meaning of the various
1279     quantifiers is not otherwise changed, just the preferred number of matches.
1280     Do not confuse this use of question mark with its use as a quantifier in its
1281     own right. Because it has two uses, it can sometimes appear doubled, as in
1282     <pre>
1283     \d??\d
1284 nigel 75 </pre>
1285 nigel 63 which matches one digit by preference, but can match two if that is the only
1286     way the rest of the pattern matches.
1287     </P>
1288     <P>
1289 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1290 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1291     greedy by following them with a question mark. In other words, it inverts the
1292     default behaviour.
1293     </P>
1294     <P>
1295     When a parenthesized subpattern is quantified with a minimum repeat count that
1296 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1297 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1298     </P>
1299     <P>
1300     If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1301 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1302 nigel 63 implicitly anchored, because whatever follows will be tried against every
1303     character position in the subject string, so there is no point in retrying the
1304     overall match at any position after the first. PCRE normally treats such a
1305     pattern as though it were preceded by \A.
1306     </P>
1307     <P>
1308     In cases where it is known that the subject string contains no newlines, it is
1309     worth setting PCRE_DOTALL in order to obtain this optimization, or
1310     alternatively using ^ to indicate anchoring explicitly.
1311     </P>
1312     <P>
1313     However, there is one situation where the optimization cannot be used. When .*
1314     is inside capturing parentheses that are the subject of a backreference
1315 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1316     succeeds. Consider, for example:
1317 nigel 63 <pre>
1318     (.*)abc\1
1319 nigel 75 </pre>
1320 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1321     this reason, such a pattern is not implicitly anchored.
1322     </P>
1323     <P>
1324     When a capturing subpattern is repeated, the value captured is the substring
1325     that matched the final iteration. For example, after
1326     <pre>
1327     (tweedle[dume]{3}\s*)+
1328 nigel 75 </pre>
1329 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1330     "tweedledee". However, if there are nested capturing subpatterns, the
1331     corresponding captured values may have been set in previous iterations. For
1332     example, after
1333     <pre>
1334     /(a|(b))+/
1335 nigel 75 </pre>
1336 nigel 63 matches "aba" the value of the second captured substring is "b".
1337 nigel 75 <a name="atomicgroup"></a></P>
1338 ph10 227 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1339 nigel 63 <P>
1340 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1341     repetition, failure of what follows normally causes the repeated item to be
1342     re-evaluated to see if a different number of repeats allows the rest of the
1343     pattern to match. Sometimes it is useful to prevent this, either to change the
1344     nature of the match, or to cause it fail earlier than it otherwise might, when
1345     the author of the pattern knows there is no point in carrying on.
1346 nigel 63 </P>
1347     <P>
1348     Consider, for example, the pattern \d+foo when applied to the subject line
1349     <pre>
1350     123456bar
1351 nigel 75 </pre>
1352 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1353     action of the matcher is to try again with only 5 digits matching the \d+
1354     item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1355     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1356     that once a subpattern has matched, it is not to be re-evaluated in this way.
1357     </P>
1358     <P>
1359 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1360 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1361     special parenthesis, starting with (?&#62; as in this example:
1362     <pre>
1363 nigel 73 (?&#62;\d+)foo
1364 nigel 75 </pre>
1365 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1366     it has matched, and a failure further into the pattern is prevented from
1367     backtracking into it. Backtracking past it to previous items, however, works as
1368     normal.
1369     </P>
1370     <P>
1371     An alternative description is that a subpattern of this type matches the string
1372     of characters that an identical standalone pattern would match, if anchored at
1373     the current point in the subject string.
1374     </P>
1375     <P>
1376     Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1377     the above example can be thought of as a maximizing repeat that must swallow
1378     everything it can. So, while both \d+ and \d+? are prepared to adjust the
1379     number of digits they match in order to make the rest of the pattern match,
1380     (?&#62;\d+) can only match an entire sequence of digits.
1381     </P>
1382     <P>
1383     Atomic groups in general can of course contain arbitrarily complicated
1384     subpatterns, and can be nested. However, when the subpattern for an atomic
1385     group is just a single repeated item, as in the example above, a simpler
1386     notation, called a "possessive quantifier" can be used. This consists of an
1387     additional + character following a quantifier. Using this notation, the
1388     previous example can be rewritten as
1389     <pre>
1390 nigel 75 \d++foo
1391     </pre>
1392 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1393     example:
1394     <pre>
1395     (abc|xyz){2,3}+
1396     </pre>
1397 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1398     option is ignored. They are a convenient notation for the simpler forms of
1399 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1400     quantifier and the equivalent atomic group, though there may be a performance
1401     difference; possessive quantifiers should be slightly faster.
1402 nigel 63 </P>
1403     <P>
1404 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1405     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1406     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1407     package, and PCRE copied it from there. It ultimately found its way into Perl
1408     at release 5.10.
1409 nigel 63 </P>
1410     <P>
1411 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1412     pattern constructs. For example, the sequence A+B is treated as A++B because
1413     there is no point in backtracking into a sequence of A's when B must follow.
1414     </P>
1415     <P>
1416 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1417     be repeated an unlimited number of times, the use of an atomic group is the
1418     only way to avoid some failing matches taking a very long time indeed. The
1419     pattern
1420     <pre>
1421     (\D+|&#60;\d+&#62;)*[!?]
1422 nigel 75 </pre>
1423 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1424     digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1425     quickly. However, if it is applied to
1426     <pre>
1427     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1428 nigel 75 </pre>
1429 nigel 63 it takes a long time before reporting failure. This is because the string can
1430 nigel 75 be divided between the internal \D+ repeat and the external * repeat in a
1431     large number of ways, and all have to be tried. (The example uses [!?] rather
1432     than a single character at the end, because both PCRE and Perl have an
1433     optimization that allows for fast failure when a single character is used. They
1434     remember the last single character that is required for a match, and fail early
1435     if it is not present in the string.) If the pattern is changed so that it uses
1436     an atomic group, like this:
1437 nigel 63 <pre>
1438     ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1439 nigel 75 </pre>
1440 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1441 nigel 75 <a name="backreferences"></a></P>
1442 ph10 227 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1443 nigel 63 <P>
1444     Outside a character class, a backslash followed by a digit greater than 0 (and
1445     possibly further digits) is a back reference to a capturing subpattern earlier
1446     (that is, to its left) in the pattern, provided there have been that many
1447     previous capturing left parentheses.
1448     </P>
1449     <P>
1450     However, if the decimal number following the backslash is less than 10, it is
1451     always taken as a back reference, and causes an error only if there are not
1452     that many capturing left parentheses in the entire pattern. In other words, the
1453     parentheses that are referenced need not be to the left of the reference for
1454 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1455     when a repetition is involved and the subpattern to the right has participated
1456     in an earlier iteration.
1457     </P>
1458     <P>
1459 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1460     whose number is 10 or more using this syntax because a sequence such as \50 is
1461     interpreted as a character defined in octal. See the subsection entitled
1462 nigel 91 "Non-printing characters"
1463 nigel 75 <a href="#digitsafterbackslash">above</a>
1464 nigel 93 for further details of the handling of digits following a backslash. There is
1465     no such problem when named parentheses are used. A back reference to any
1466     subpattern is possible using named parentheses (see below).
1467 nigel 63 </P>
1468     <P>
1469 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1470     backslash is to use the \g escape sequence, which is a feature introduced in
1471 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1472     number, optionally enclosed in braces. These examples are all identical:
1473 nigel 93 <pre>
1474     (ring), \1
1475     (ring), \g1
1476     (ring), \g{1}
1477     </pre>
1478 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1479     is present in the older syntax. It is also useful when literal digits follow
1480     the reference. A negative number is a relative reference. Consider this
1481     example:
1482 nigel 93 <pre>
1483     (abc(def)ghi)\g{-1}
1484     </pre>
1485     The sequence \g{-1} is a reference to the most recently started capturing
1486     subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1487     would be equivalent to \1. The use of relative references can be helpful in
1488     long patterns, and also in patterns that are created by joining together
1489     fragments that contain references within themselves.
1490     </P>
1491     <P>
1492 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1493     the current subject string, rather than anything matching the subpattern
1494     itself (see
1495     <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1496     below for a way of doing that). So the pattern
1497     <pre>
1498     (sens|respons)e and \1ibility
1499 nigel 75 </pre>
1500 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1501     "sense and responsibility". If caseful matching is in force at the time of the
1502     back reference, the case of letters is relevant. For example,
1503     <pre>
1504     ((?i)rah)\s+\1
1505 nigel 75 </pre>
1506 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1507     capturing subpattern is matched caselessly.
1508     </P>
1509     <P>
1510 ph10 172 There are several different ways of writing back references to named
1511     subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1512     \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1513     back reference syntax, in which \g can be used for both numeric and named
1514     references, is also supported. We could rewrite the above example in any of
1515 nigel 93 the following ways:
1516 nigel 63 <pre>
1517 nigel 93 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1518 ph10 172 (?'p1'(?i)rah)\s+\k{p1}
1519 nigel 91 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1520 ph10 172 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1521 nigel 75 </pre>
1522 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1523     after the reference.
1524     </P>
1525     <P>
1526 nigel 63 There may be more than one back reference to the same subpattern. If a
1527     subpattern has not actually been used in a particular match, any back
1528     references to it always fail. For example, the pattern
1529     <pre>
1530     (a|(bc))\2
1531 nigel 75 </pre>
1532 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1533     many capturing parentheses in a pattern, all digits following the backslash are
1534     taken as part of a potential back reference number. If the pattern continues
1535     with a digit character, some delimiter must be used to terminate the back
1536     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1537 nigel 75 Otherwise an empty comment (see
1538     <a href="#comments">"Comments"</a>
1539     below) can be used.
1540 nigel 63 </P>
1541     <P>
1542     A back reference that occurs inside the parentheses to which it refers fails
1543     when the subpattern is first used, so, for example, (a\1) never matches.
1544     However, such references can be useful inside repeated subpatterns. For
1545     example, the pattern
1546     <pre>
1547     (a|b\1)+
1548 nigel 75 </pre>
1549 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1550     the subpattern, the back reference matches the character string corresponding
1551     to the previous iteration. In order for this to work, the pattern must be such
1552     that the first iteration does not need to match the back reference. This can be
1553     done using alternation, as in the example above, or by a quantifier with a
1554     minimum of zero.
1555 nigel 75 <a name="bigassertions"></a></P>
1556 ph10 227 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1557 nigel 63 <P>
1558     An assertion is a test on the characters following or preceding the current
1559     matching point that does not actually consume any characters. The simple
1560 nigel 75 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1561     <a href="#smallassertions">above.</a>
1562     </P>
1563     <P>
1564 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1565     those that look ahead of the current position in the subject string, and those
1566 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1567     except that it does not cause the current matching position to be changed.
1568 nigel 63 </P>
1569     <P>
1570 nigel 75 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1571     because it makes no sense to assert the same thing several times. If any kind
1572     of assertion contains capturing subpatterns within it, these are counted for
1573     the purposes of numbering the capturing subpatterns in the whole pattern.
1574     However, substring capturing is carried out only for positive assertions,
1575     because it does not make sense for negative assertions.
1576 nigel 63 </P>
1577 nigel 75 <br><b>
1578     Lookahead assertions
1579     </b><br>
1580 nigel 63 <P>
1581 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1582     negative assertions. For example,
1583 nigel 63 <pre>
1584     \w+(?=;)
1585 nigel 75 </pre>
1586 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1587     the match, and
1588     <pre>
1589     foo(?!bar)
1590 nigel 75 </pre>
1591 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1592     apparently similar pattern
1593     <pre>
1594     (?!foo)bar
1595 nigel 75 </pre>
1596 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1597     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1598     (?!foo) is always true when the next three characters are "bar". A
1599 nigel 75 lookbehind assertion is needed to achieve the other effect.
1600 nigel 63 </P>
1601     <P>
1602     If you want to force a matching failure at some point in a pattern, the most
1603     convenient way to do it is with (?!) because an empty string always matches, so
1604     an assertion that requires there not to be an empty string must always fail.
1605 nigel 75 <a name="lookbehind"></a></P>
1606     <br><b>
1607     Lookbehind assertions
1608     </b><br>
1609 nigel 63 <P>
1610     Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1611     negative assertions. For example,
1612     <pre>
1613     (?&#60;!foo)bar
1614 nigel 75 </pre>
1615 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1616     a lookbehind assertion are restricted such that all the strings it matches must
1617 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1618     do not all have to have the same fixed length. Thus
1619 nigel 63 <pre>
1620     (?&#60;=bullock|donkey)
1621 nigel 75 </pre>
1622 nigel 63 is permitted, but
1623     <pre>
1624     (?&#60;!dogs?|cats?)
1625 nigel 75 </pre>
1626 nigel 63 causes an error at compile time. Branches that match different length strings
1627     are permitted only at the top level of a lookbehind assertion. This is an
1628     extension compared with Perl (at least for 5.8), which requires all branches to
1629     match the same length of string. An assertion such as
1630     <pre>
1631     (?&#60;=ab(c|de))
1632 nigel 75 </pre>
1633 nigel 63 is not permitted, because its single top-level branch can match two different
1634     lengths, but it is acceptable if rewritten to use two top-level branches:
1635     <pre>
1636     (?&#60;=abc|abde)
1637 nigel 75 </pre>
1638 ph10 172 In some cases, the Perl 5.10 escape sequence \K
1639     <a href="#resetmatchstart">(see above)</a>
1640     can be used instead of a lookbehind assertion; this is not restricted to a
1641     fixed-length.
1642     </P>
1643     <P>
1644 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1645 nigel 93 temporarily move the current position back by the fixed length and then try to
1646 nigel 63 match. If there are insufficient characters before the current position, the
1647 nigel 93 assertion fails.
1648 nigel 63 </P>
1649     <P>
1650     PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1651     to appear in lookbehind assertions, because it makes it impossible to calculate
1652 nigel 93 the length of the lookbehind. The \X and \R escapes, which can match
1653     different numbers of bytes, are also not permitted.
1654 nigel 63 </P>
1655     <P>
1656 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1657     specify efficient matching at the end of the subject string. Consider a simple
1658     pattern such as
1659 nigel 63 <pre>
1660     abcd$
1661 nigel 75 </pre>
1662 nigel 63 when applied to a long string that does not match. Because matching proceeds
1663     from left to right, PCRE will look for each "a" in the subject and then see if
1664     what follows matches the rest of the pattern. If the pattern is specified as
1665     <pre>
1666     ^.*abcd$
1667 nigel 75 </pre>
1668 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1669     there is no following "a"), it backtracks to match all but the last character,
1670     then all but the last two characters, and so on. Once again the search for "a"
1671     covers the entire string, from right to left, so we are no better off. However,
1672     if the pattern is written as
1673     <pre>
1674     ^.*+(?&#60;=abcd)
1675 nigel 75 </pre>
1676 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1677 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1678     characters. If it fails, the match fails immediately. For long strings, this
1679     approach makes a significant difference to the processing time.
1680     </P>
1681 nigel 75 <br><b>
1682     Using multiple assertions
1683     </b><br>
1684 nigel 63 <P>
1685     Several assertions (of any sort) may occur in succession. For example,
1686     <pre>
1687     (?&#60;=\d{3})(?&#60;!999)foo
1688 nigel 75 </pre>
1689 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1690     the assertions is applied independently at the same point in the subject
1691     string. First there is a check that the previous three characters are all
1692     digits, and then there is a check that the same three characters are not "999".
1693     This pattern does <i>not</i> match "foo" preceded by six characters, the first
1694     of which are digits and the last three of which are not "999". For example, it
1695     doesn't match "123abcfoo". A pattern to do that is
1696     <pre>
1697     (?&#60;=\d{3}...)(?&#60;!999)foo
1698 nigel 75 </pre>
1699 nigel 63 This time the first assertion looks at the preceding six characters, checking
1700     that the first three are digits, and then the second assertion checks that the
1701     preceding three characters are not "999".
1702     </P>
1703     <P>
1704     Assertions can be nested in any combination. For example,
1705     <pre>
1706     (?&#60;=(?&#60;!foo)bar)baz
1707 nigel 75 </pre>
1708 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1709     preceded by "foo", while
1710     <pre>
1711     (?&#60;=\d{3}(?!999)...)foo
1712 nigel 75 </pre>
1713     is another pattern that matches "foo" preceded by three digits and any three
1714 nigel 63 characters that are not "999".
1715 nigel 91 <a name="conditions"></a></P>
1716 ph10 227 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1717 nigel 63 <P>
1718     It is possible to cause the matching process to obey a subpattern
1719     conditionally or to choose between two alternative subpatterns, depending on
1720     the result of an assertion, or whether a previous capturing subpattern matched
1721     or not. The two possible forms of conditional subpattern are
1722     <pre>
1723     (?(condition)yes-pattern)
1724     (?(condition)yes-pattern|no-pattern)
1725 nigel 75 </pre>
1726 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1727     no-pattern (if present) is used. If there are more than two alternatives in the
1728     subpattern, a compile-time error occurs.
1729     </P>
1730     <P>
1731 nigel 93 There are four kinds of condition: references to subpatterns, references to
1732     recursion, a pseudo-condition called DEFINE, and assertions.
1733 nigel 91 </P>
1734 nigel 93 <br><b>
1735     Checking for a used subpattern by number
1736     </b><br>
1737 nigel 91 <P>
1738 nigel 93 If the text between the parentheses consists of a sequence of digits, the
1739     condition is true if the capturing subpattern of that number has previously
1740 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1741     sign. In this case, the subpattern number is relative rather than absolute.
1742     The most recently opened parentheses can be referenced by (?(-1), the next most
1743     recent by (?(-2), and so on. In looping constructs it can also make sense to
1744     refer to subsequent groups with constructs such as (?(+2).
1745 nigel 93 </P>
1746     <P>
1747 nigel 91 Consider the following pattern, which contains non-significant white space to
1748     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1749     three parts for ease of discussion:
1750 nigel 63 <pre>
1751     ( \( )? [^()]+ (?(1) \) )
1752 nigel 75 </pre>
1753 nigel 63 The first part matches an optional opening parenthesis, and if that
1754     character is present, sets it as the first captured substring. The second part
1755     matches one or more characters that are not parentheses. The third part is a
1756     conditional subpattern that tests whether the first set of parentheses matched
1757     or not. If they did, that is, if subject started with an opening parenthesis,
1758     the condition is true, and so the yes-pattern is executed and a closing
1759     parenthesis is required. Otherwise, since no-pattern is not present, the
1760     subpattern matches nothing. In other words, this pattern matches a sequence of
1761 nigel 93 non-parentheses, optionally enclosed in parentheses.
1762     </P>
1763 ph10 172 <P>
1764     If you were embedding this pattern in a larger one, you could use a relative
1765     reference:
1766     <pre>
1767     ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
1768     </pre>
1769     This makes the fragment independent of the parentheses in the larger pattern.
1770     </P>
1771 nigel 93 <br><b>
1772     Checking for a used subpattern by name
1773     </b><br>
1774     <P>
1775     Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1776     subpattern by name. For compatibility with earlier versions of PCRE, which had
1777     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1778     there is a possible ambiguity with this syntax, because subpattern names may
1779     consist entirely of digits. PCRE looks first for a named subpattern; if it
1780     cannot find one and the name consists entirely of digits, PCRE looks for a
1781     subpattern of that number, which must be greater than zero. Using subpattern
1782     names that consist entirely of digits is not recommended.
1783     </P>
1784     <P>
1785     Rewriting the above example to use a named subpattern gives this:
1786 nigel 91 <pre>
1787 nigel 93 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
1789     </PRE>
1790     </P>
1791     <br><b>
1792     Checking for pattern recursion
1793     </b><br>
1794     <P>
1795     If the condition is the string (R), and there is no subpattern with the name R,
1796     the condition is true if a recursive call to the whole pattern or any
1797     subpattern has been made. If digits or a name preceded by ampersand follow the
1798     letter R, for example:
1799     <pre>
1800     (?(R3)...) or (?(R&name)...)
1801 nigel 91 </pre>
1802 nigel 93 the condition is true if the most recent recursion is into the subpattern whose
1803     number or name is given. This condition does not check the entire recursion
1804     stack.
1805 nigel 63 </P>
1806     <P>
1807 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1808     patterns are described below.
1809     </P>
1810     <br><b>
1811     Defining subpatterns for use by reference only
1812     </b><br>
1813     <P>
1814     If the condition is the string (DEFINE), and there is no subpattern with the
1815     name DEFINE, the condition is always false. In this case, there may be only one
1816     alternative in the subpattern. It is always skipped if control reaches this
1817     point in the pattern; the idea of DEFINE is that it can be used to define
1818     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1819     is described below.) For example, a pattern to match an IPv4 address could be
1820     written like this (ignore whitespace and line breaks):
1821     <pre>
1822     (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1823     \b (?&byte) (\.(?&byte)){3} \b
1824     </pre>
1825     The first part of the pattern is a DEFINE group inside which a another group
1826     named "byte" is defined. This matches an individual component of an IPv4
1827     address (a number less than 256). When matching takes place, this part of the
1828     pattern is skipped because DEFINE acts like a false condition.
1829     </P>
1830     <P>
1831     The rest of the pattern uses references to the named group to match the four
1832     dot-separated components of an IPv4 address, insisting on a word boundary at
1833     each end.
1834     </P>
1835     <br><b>
1836     Assertion conditions
1837     </b><br>
1838     <P>
1839     If the condition is not in any of the above formats, it must be an assertion.
1840 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1841     this pattern, again containing non-significant white space, and with the two
1842     alternatives on the second line:
1843     <pre>
1844     (?(?=[^a-z]*[a-z])
1845     \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1846 nigel 75 </pre>
1847 nigel 63 The condition is a positive lookahead assertion that matches an optional
1848     sequence of non-letters followed by a letter. In other words, it tests for the
1849     presence of at least one letter in the subject. If a letter is found, the
1850     subject is matched against the first alternative; otherwise it is matched
1851     against the second. This pattern matches strings in one of the two forms
1852     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1853 nigel 75 <a name="comments"></a></P>
1854 ph10 227 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
1855 nigel 63 <P>
1856 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1857 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1858     that make up a comment play no part in the pattern matching at all.
1859     </P>
1860     <P>
1861     If the PCRE_EXTENDED option is set, an unescaped # character outside a
1862 nigel 91 character class introduces a comment that continues to immediately after the
1863     next newline in the pattern.
1864     <a name="recursion"></a></P>
1865 ph10 227 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
1866 nigel 63 <P>
1867     Consider the problem of matching a string in parentheses, allowing for
1868     unlimited nested parentheses. Without the use of recursion, the best that can
1869     be done is to use a pattern that matches up to some fixed depth of nesting. It
1870 nigel 93 is not possible to handle an arbitrary nesting depth.
1871     </P>
1872     <P>
1873     For some time, Perl has provided a facility that allows regular expressions to
1874     recurse (amongst other things). It does this by interpolating Perl code in the
1875     expression at run time, and the code can refer to the expression itself. A Perl
1876     pattern using code interpolation to solve the parentheses problem can be
1877     created like this:
1878 nigel 63 <pre>
1879     $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1880 nigel 75 </pre>
1881 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1882 nigel 93 recursively to the pattern in which it appears.
1883 nigel 63 </P>
1884     <P>
1885 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1886     supports special syntax for recursion of the entire pattern, and also for
1887     individual subpattern recursion. After its introduction in PCRE and Python,
1888     this kind of recursion was introduced into Perl at release 5.10.
1889 nigel 63 </P>
1890     <P>
1891 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1892     closing parenthesis is a recursive call of the subpattern of the given number,
1893     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1894     call, which is described in the next section.) The special item (?R) or (?0) is
1895     a recursive call of the entire regular expression.
1896 nigel 87 </P>
1897     <P>
1898 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1899     treated as an atomic group. That is, once it has matched some of the subject
1900     string, it is never re-entered, even if it contains untried alternatives and
1901     there is a subsequent matching failure.
1902     </P>
1903     <P>
1904 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1905     PCRE_EXTENDED option is set so that white space is ignored):
1906 nigel 63 <pre>
1907     \( ( (?&#62;[^()]+) | (?R) )* \)
1908 nigel 75 </pre>
1909 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1910     substrings which can either be a sequence of non-parentheses, or a recursive
1911 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1912 nigel 63 Finally there is a closing parenthesis.
1913     </P>
1914     <P>
1915     If this were part of a larger pattern, you would not want to recurse the entire
1916     pattern, so instead you could use this:
1917     <pre>
1918     ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1919 nigel 75 </pre>
1920 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1921 ph10 172 them instead of the whole pattern.
1922     </P>
1923     <P>
1924     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1925     is made easier by the use of relative references. (A Perl 5.10 feature.)
1926     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1927     most recently opened parentheses preceding the recursion. In other words, a
1928     negative number counts capturing parentheses leftwards from the point at which
1929     it is encountered.
1930     </P>
1931     <P>
1932     It is also possible to refer to subsequently opened parentheses, by writing
1933     references such as (?+2). However, these cannot be recursive because the
1934     reference is not inside the parentheses that are referenced. They are always
1935     "subroutine" calls, as described in the next section.
1936     </P>
1937     <P>
1938     An alternative approach is to use named parentheses instead. The Perl syntax
1939     for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
1940     could rewrite the above example as follows:
1941 nigel 63 <pre>
1942 nigel 93 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
1943 nigel 75 </pre>
1944 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1945 ph10 172 used.
1946     </P>
1947     <P>
1948     This particular example pattern that we have been looking at contains nested
1949     unlimited repeats, and so the use of atomic grouping for matching strings of
1950     non-parentheses is important when applying the pattern to strings that do not
1951     match. For example, when this pattern is applied to
1952 nigel 63 <pre>
1953     (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1954 nigel 75 </pre>
1955 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1956     the match runs for a very long time indeed because there are so many different
1957     ways the + and * repeats can carve up the subject, and all have to be tested
1958     before failure can be reported.
1959     </P>
1960     <P>
1961     At the end of a match, the values set for any capturing subpatterns are those
1962     from the outermost level of the recursion at which the subpattern value is set.
1963     If you want to obtain intermediate values, a callout function can be used (see
1964 nigel 93 below and the
1965 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
1966     documentation). If the pattern above is matched against
1967     <pre>
1968     (ab(cd)ef)
1969 nigel 75 </pre>
1970 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1971     on at the top level. If additional parentheses are added, giving
1972     <pre>
1973     \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
1974     ^ ^
1975     ^ ^
1976 nigel 75 </pre>
1977 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1978     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1979     has to obtain extra memory to store data during a recursion, which it does by
1980     using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
1981     memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1982     </P>
1983     <P>
1984     Do not confuse the (?R) item with the condition (R), which tests for recursion.
1985     Consider this pattern, which matches text in angle brackets, allowing for
1986     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1987     recursing), whereas any characters are permitted at the outer level.
1988     <pre>
1989     &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
1990 nigel 75 </pre>
1991 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1992     different alternatives for the recursive and non-recursive cases. The (?R) item
1993     is the actual recursive call.
1994 nigel 75 <a name="subpatternsassubroutines"></a></P>
1995 ph10 227 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1996 nigel 63 <P>
1997     If the syntax for a recursive subpattern reference (either by number or by
1998     name) is used outside the parentheses to which it refers, it operates like a
1999 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
2000 ph10 172 before or after the reference. A numbered reference can be absolute or
2001     relative, as in these examples:
2002 nigel 63 <pre>
2003 ph10 172 (...(absolute)...)...(?2)...
2004     (...(relative)...)...(?-1)...
2005     (...(?+1)...(relative)...
2006     </pre>
2007     An earlier example pointed out that the pattern
2008     <pre>
2009 nigel 63 (sens|respons)e and \1ibility
2010 nigel 75 </pre>
2011 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2012     "sense and responsibility". If instead the pattern
2013     <pre>
2014     (sens|respons)e and (?1)ibility
2015 nigel 75 </pre>
2016 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2017 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2018 nigel 63 </P>
2019 nigel 87 <P>
2020     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2021     group. That is, once it has matched some of the subject string, it is never
2022     re-entered, even if it contains untried alternatives and there is a subsequent
2023     matching failure.
2024     </P>
2025 nigel 63 <P>
2026 nigel 93 When a subpattern is used as a subroutine, processing options such as
2027     case-independence are fixed when the subpattern is defined. They cannot be
2028     changed for different calls. For example, consider this pattern:
2029     <pre>
2030 ph10 172 (abc)(?i:(?-1))
2031 nigel 93 </pre>
2032     It matches "abcabc". It does not match "abcABC" because the change of
2033     processing option does not affect the called subpattern.
2034     </P>
2035 ph10 227 <br><a name="SEC23" href="#TOC1">CALLOUTS</a><br>
2036 nigel 93 <P>
2037 nigel 63 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2038     code to be obeyed in the middle of matching a regular expression. This makes it
2039     possible, amongst other things, to extract different substrings that match the
2040     same pair of parentheses when there is a repetition.
2041     </P>
2042     <P>
2043     PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2044     code. The feature is called "callout". The caller of PCRE provides an external
2045     function by putting its entry point in the global variable <i>pcre_callout</i>.
2046     By default, this variable contains NULL, which disables all calling out.
2047     </P>
2048     <P>
2049     Within a regular expression, (?C) indicates the points at which the external
2050     function is to be called. If you want to identify different callout points, you
2051     can put a number less than 256 after the letter C. The default value is zero.
2052     For example, this pattern has two callout points:
2053     <pre>
2054 ph10 155 (?C1)abc(?C2)def
2055 nigel 75 </pre>
2056     If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2057     automatically installed before each item in the pattern. They are all numbered
2058     255.
2059 nigel 63 </P>
2060     <P>
2061     During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2062     set), the external function is called. It is provided with the number of the
2063 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2064     originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2065     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2066     description of the interface to the callout function is given in the
2067 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2068     documentation.
2069     </P>
2070 ph10 243 <br><a name="SEC24" href="#TOC1">BACKTRACKING CONTROL</a><br>
2071 nigel 63 <P>
2072 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2073     are described in the Perl documentation as "experimental and subject to change
2074     or removal in a future version of Perl". It goes on to say: "Their usage in
2075     production code should be noted to avoid problems during upgrades." The same
2076     remarks apply to the PCRE features described in this section.
2077     </P>
2078     <P>
2079     Since these verbs are specifically related to backtracking, they can be used
2080     only when the pattern is to be matched using <b>pcre_exec()</b>, which uses a
2081     backtracking algorithm. They cause an error if encountered by
2082     <b>pcre_dfa_exec()</b>.
2083     </P>
2084     <P>
2085     The new verbs make use of what was previously invalid syntax: an opening
2086     parenthesis followed by an asterisk. In Perl, they are generally of the form
2087     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2088     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2089     are two kinds:
2090     </P>
2091     <br><b>
2092     Verbs that act immediately
2093     </b><br>
2094     <P>
2095     The following verbs act as soon as they are encountered:
2096     <pre>
2097     (*ACCEPT)
2098     </pre>
2099     This verb causes the match to end successfully, skipping the remainder of the
2100     pattern. When inside a recursion, only the innermost pattern is ended
2101     immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2102     capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2103     captured. For example:
2104     <pre>
2105     A(A|B(*ACCEPT)|C)D
2106     </pre>
2107     This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2108     captured.
2109     <pre>
2110     (*FAIL) or (*F)
2111     </pre>
2112     This verb causes the match to fail, forcing backtracking to occur. It is
2113     equivalent to (?!) but easier to read. The Perl documentation notes that it is
2114     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2115     Perl features that are not present in PCRE. The nearest equivalent is the
2116     callout feature, as for example in this pattern:
2117     <pre>
2118     a+(?C)(*FAIL)
2119     </pre>
2120     A match with the string "aaaa" always fails, but the callout is taken before
2121     each backtrack happens (in this example, 10 times).
2122     </P>
2123     <br><b>
2124     Verbs that act after backtracking
2125     </b><br>
2126     <P>
2127     The following verbs do nothing when they are encountered. Matching continues
2128     with what follows, but if there is no subsequent match, a failure is forced.
2129     The verbs differ in exactly what kind of failure occurs.
2130     <pre>
2131     (*COMMIT)
2132     </pre>
2133     This verb causes the whole match to fail outright if the rest of the pattern
2134     does not match. Even if the pattern is unanchored, no further attempts to find
2135     a match by advancing the start point take place. Once (*COMMIT) has been
2136     passed, <b>pcre_exec()</b> is committed to finding a match at the current
2137     starting point, or not at all. For example:
2138     <pre>
2139     a+(*COMMIT)b
2140     </pre>
2141     This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2142     dynamic anchor, or "I've started, so I must finish."
2143     <pre>
2144     (*PRUNE)
2145     </pre>
2146     This verb causes the match to fail at the current position if the rest of the
2147     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2148     advance to the next starting character then happens. Backtracking can occur as
2149     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2150     if there is no match to the right, backtracking cannot cross (*PRUNE).
2151     In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2152     group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2153     be expressed in any other way.
2154     <pre>
2155     (*SKIP)
2156     </pre>
2157     This verb is like (*PRUNE), except that if the pattern is unanchored, the
2158     "bumpalong" advance is not to the next character, but to the position in the
2159     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2160     was matched leading up to it cannot be part of a successful match. Consider:
2161     <pre>
2162     a+(*SKIP)b
2163     </pre>
2164     If the subject is "aaaac...", after the first match attempt fails (starting at
2165     the first character in the string), the starting point skips on to start the
2166     next attempt at "c". Note that a possessive quantifer does not have the same
2167     effect in this example; although it would suppress backtracking during the
2168     first match attempt, the second attempt would start at the second character
2169     instead of skipping on to "c".
2170     <pre>
2171     (*THEN)
2172     </pre>
2173     This verb causes a skip to the next alternation if the rest of the pattern does
2174     not match. That is, it cancels pending backtracking, but only within the
2175     current alternation. Its name comes from the observation that it can be used
2176     for a pattern-based if-then-else block:
2177     <pre>
2178     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2179     </pre>
2180     If the COND1 pattern matches, FOO is tried (and possibly further items after
2181     the end of the group if FOO succeeds); on failure the matcher skips to the
2182     second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2183     is used outside of any alternation, it acts exactly like (*PRUNE).
2184     </P>
2185 ph10 227 <br><a name="SEC25" href="#TOC1">SEE ALSO</a><br>
2186 ph10 211 <P>
2187 nigel 93 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
2188     </P>
2189 ph10 227 <br><a name="SEC26" href="#TOC1">AUTHOR</a><br>
2190 nigel 93 <P>
2191 ph10 99 Philip Hazel
2192 nigel 63 <br>
2193 ph10 99 University Computing Service
2194     <br>
2195     Cambridge CB2 3QH, England.
2196     <br>
2197     </P>
2198 ph10 227 <br><a name="SEC27" href="#TOC1">REVISION</a><br>
2199 ph10 99 <P>
2200 ph10 254 Last updated: 17 September 2007
2201 ph10 99 <br>
2202     Copyright &copy; 1997-2007 University of Cambridge.
2203     <br>
2204 nigel 75 <p>
2205     Return to the <a href="index.html">PCRE index page</a>.
2206     </p>


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

ViewVC Help
Powered by ViewVC 1.1.12