ViewVC logotype

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

Parent Directory Parent Directory | Revision Log Revision Log

Revision 454 - (hide annotations) (download) (as text)
Tue Sep 22 09:42:11 2009 UTC (5 years, 7 months ago) by ph10
File MIME type: text/html
File size: 99559 byte(s)
Allow fixed-length subroutine calls in lookbehinds.

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 ph10 345 <li><a name="TOC23" href="#SEC23">ONIGURUMA SUBROUTINE SYNTAX</a>
39     <li><a name="TOC24" href="#SEC24">CALLOUTS</a>
40     <li><a name="TOC25" href="#SEC25">BACKTRACKING CONTROL</a>
41     <li><a name="TOC26" href="#SEC26">SEE ALSO</a>
42     <li><a name="TOC27" href="#SEC27">AUTHOR</a>
43     <li><a name="TOC28" href="#SEC28">REVISION</a>
44 nigel 63 </ul>
45     <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
46     <P>
47 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE
48     are described in detail below. There is a quick-reference syntax summary in the
49     <a href="pcresyntax.html"><b>pcresyntax</b></a>
50 ph10 345 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
51     also supports some alternative regular expression syntax (which does not
52     conflict with the Perl syntax) in order to provide some compatibility with
53     regular expressions in Python, .NET, and Oniguruma.
54     </P>
55     <P>
56     Perl's regular expressions are described in its own documentation, and
57 ph10 208 regular expressions in general are covered in a number of books, some of which
58     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
59     published by O'Reilly, covers regular expressions in great detail. This
60     description of PCRE's regular expressions is intended as reference material.
61 nigel 63 </P>
62     <P>
63 nigel 75 The original operation of PCRE was on strings of one-byte characters. However,
64     there is now also support for UTF-8 character strings. To use this, you must
65     build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
66 ph10 416 the PCRE_UTF8 option. There is also a special sequence that can be given at the
67     start of a pattern:
68     <pre>
69     (*UTF8)
70     </pre>
71     Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
72     option. This feature is not Perl-compatible. How setting UTF-8 mode affects
73     pattern matching is mentioned in several places below. There is also a summary
74     of UTF-8 features in the
75 nigel 63 <a href="pcre.html#utf8support">section on UTF-8 support</a>
76     in the main
77     <a href="pcre.html"><b>pcre</b></a>
78     page.
79     </P>
80     <P>
81 nigel 77 The remainder of this document discusses the patterns that are supported by
82     PCRE when its main matching function, <b>pcre_exec()</b>, is used.
83     From release 6.0, PCRE offers a second matching function,
84     <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
85 ph10 172 Perl-compatible. Some of the features discussed below are not available when
86     <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
87     alternative function, and how it differs from the normal function, are
88     discussed in the
89 nigel 77 <a href="pcrematching.html"><b>pcrematching</b></a>
90     page.
91     </P>
92 ph10 227 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
93 nigel 77 <P>
94 ph10 227 PCRE supports five different conventions for indicating line breaks in
95     strings: a single CR (carriage return) character, a single LF (linefeed)
96     character, the two-character sequence CRLF, any of the three preceding, or any
97     Unicode newline sequence. The
98     <a href="pcreapi.html"><b>pcreapi</b></a>
99     page has
100     <a href="pcreapi.html#newlines">further discussion</a>
101     about newlines, and shows how to set the newline convention in the
102     <i>options</i> arguments for the compiling and matching functions.
103     </P>
104     <P>
105     It is also possible to specify a newline convention by starting a pattern
106     string with one of the following five sequences:
107     <pre>
108     (*CR) carriage return
109     (*LF) linefeed
110     (*CRLF) carriage return, followed by linefeed
111     (*ANYCRLF) any of the three above
112     (*ANY) all Unicode newline sequences
113     </pre>
114     These override the default and the options given to <b>pcre_compile()</b>. For
115     example, on a Unix system where LF is the default newline sequence, the pattern
116     <pre>
117     (*CR)a.b
118     </pre>
119     changes the convention to CR. That pattern matches "a\nb" because LF is no
120     longer a newline. Note that these special settings, which are not
121     Perl-compatible, are recognized only at the very start of a pattern, and that
122 ph10 231 they must be in upper case. If more than one of them is present, the last one
123     is used.
124 ph10 227 </P>
125 ph10 231 <P>
126     The newline convention does not affect what the \R escape sequence matches. By
127     default, this is any Unicode newline sequence, for Perl compatibility. However,
128     this can be changed; see the description of \R in the section entitled
129     <a href="#newlineseq">"Newline sequences"</a>
130 ph10 247 below. A change of \R setting can be combined with a change of newline
131     convention.
132 ph10 231 </P>
133 ph10 227 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
134     <P>
135 nigel 63 A regular expression is a pattern that is matched against a subject string from
136     left to right. Most characters stand for themselves in a pattern, and match the
137     corresponding characters in the subject. As a trivial example, the pattern
138     <pre>
139     The quick brown fox
140 nigel 75 </pre>
141 nigel 77 matches a portion of a subject string that is identical to itself. When
142     caseless matching is specified (the PCRE_CASELESS option), letters are matched
143     independently of case. In UTF-8 mode, PCRE always understands the concept of
144     case for characters whose values are less than 128, so caseless matching is
145     always possible. For characters with higher values, the concept of case is
146     supported if PCRE is compiled with Unicode property support, but not otherwise.
147     If you want to use caseless matching for characters 128 and above, you must
148     ensure that PCRE is compiled with Unicode property support as well as with
149     UTF-8 support.
150     </P>
151     <P>
152     The power of regular expressions comes from the ability to include alternatives
153     and repetitions in the pattern. These are encoded in the pattern by the use of
154 nigel 75 <i>metacharacters</i>, which do not stand for themselves but instead are
155 nigel 63 interpreted in some special way.
156     </P>
157     <P>
158 nigel 75 There are two different sets of metacharacters: those that are recognized
159 nigel 63 anywhere in the pattern except within square brackets, and those that are
160 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
161     are as follows:
162 nigel 63 <pre>
163     \ general escape character with several uses
164     ^ assert start of string (or line, in multiline mode)
165     $ assert end of string (or line, in multiline mode)
166     . match any character except newline (by default)
167     [ start character class definition
168     | start of alternative branch
169     ( start subpattern
170     ) end subpattern
171     ? extends the meaning of (
172     also 0 or 1 quantifier
173     also quantifier minimizer
174     * 0 or more quantifier
175     + 1 or more quantifier
176     also "possessive quantifier"
177     { start min/max quantifier
178 nigel 75 </pre>
179 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
180 nigel 75 a character class the only metacharacters are:
181 nigel 63 <pre>
182     \ general escape character
183     ^ negate the class, but only if the first character
184     - indicates character range
185 nigel 75 [ POSIX character class (only if followed by POSIX syntax)
186 nigel 63 ] terminates the character class
187 nigel 75 </pre>
188     The following sections describe the use of each of the metacharacters.
189 nigel 63 </P>
190 ph10 227 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
191 nigel 63 <P>
192     The backslash character has several uses. Firstly, if it is followed by a
193 nigel 91 non-alphanumeric character, it takes away any special meaning that character
194     may have. This use of backslash as an escape character applies both inside and
195 nigel 63 outside character classes.
196     </P>
197     <P>
198     For example, if you want to match a * character, you write \* in the pattern.
199     This escaping action applies whether or not the following character would
200 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
201     non-alphanumeric with backslash to specify that it stands for itself. In
202 nigel 63 particular, if you want to match a backslash, you write \\.
203     </P>
204     <P>
205     If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
206     pattern (other than in a character class) and characters between a # outside
207 nigel 91 a character class and the next newline are ignored. An escaping backslash can
208     be used to include a whitespace or # character as part of the pattern.
209 nigel 63 </P>
210     <P>
211     If you want to remove the special meaning from a sequence of characters, you
212     can do so by putting them between \Q and \E. This is different from Perl in
213     that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
214     Perl, $ and @ cause variable interpolation. Note the following examples:
215     <pre>
216     Pattern PCRE matches Perl matches
217 nigel 75
218     \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
219 nigel 63 \Qabc\$xyz\E abc\$xyz abc\$xyz
220     \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
221 nigel 75 </pre>
222 nigel 63 The \Q...\E sequence is recognized both inside and outside character classes.
223 nigel 75 <a name="digitsafterbackslash"></a></P>
224     <br><b>
225     Non-printing characters
226     </b><br>
227 nigel 63 <P>
228     A second use of backslash provides a way of encoding non-printing characters
229     in patterns in a visible manner. There is no restriction on the appearance of
230     non-printing characters, apart from the binary zero that terminates a pattern,
231     but when a pattern is being prepared by text editing, it is usually easier to
232     use one of the following escape sequences than the binary character it
233     represents:
234     <pre>
235     \a alarm, that is, the BEL character (hex 07)
236     \cx "control-x", where x is any character
237     \e escape (hex 1B)
238     \f formfeed (hex 0C)
239 ph10 227 \n linefeed (hex 0A)
240 nigel 63 \r carriage return (hex 0D)
241     \t tab (hex 09)
242     \ddd character with octal code ddd, or backreference
243     \xhh character with hex code hh
244 nigel 87 \x{hhh..} character with hex code hhh..
245 nigel 75 </pre>
246 nigel 63 The precise effect of \cx is as follows: if x is a lower case letter, it
247     is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
248     Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
249     7B.
250     </P>
251     <P>
252     After \x, from zero to two hexadecimal digits are read (letters can be in
253 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \x{
254     and }, but the value of the character code must be less than 256 in non-UTF-8
255 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
256     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
257     point, which is 10FFFF.
258 nigel 63 </P>
259     <P>
260 ph10 211 If characters other than hexadecimal digits appear between \x{ and }, or if
261     there is no terminating }, this form of escape is not recognized. Instead, the
262     initial \x will be interpreted as a basic hexadecimal escape, with no
263     following digits, giving a character whose value is zero.
264     </P>
265     <P>
266 nigel 63 Characters whose value is less than 256 can be defined by either of the two
267 nigel 87 syntaxes for \x. There is no difference in the way they are handled. For
268     example, \xdc is exactly the same as \x{dc}.
269 nigel 63 </P>
270     <P>
271 nigel 91 After \0 up to two further octal digits are read. If there are fewer than two
272     digits, just those that are present are used. Thus the sequence \0\x\07
273     specifies two binary zeros followed by a BEL character (code value 7). Make
274     sure you supply two digits after the initial zero if the pattern character that
275     follows is itself an octal digit.
276 nigel 63 </P>
277     <P>
278     The handling of a backslash followed by a digit other than 0 is complicated.
279     Outside a character class, PCRE reads it and any following digits as a decimal
280     number. If the number is less than 10, or if there have been at least that many
281     previous capturing left parentheses in the expression, the entire sequence is
282     taken as a <i>back reference</i>. A description of how this works is given
283 nigel 75 <a href="#backreferences">later,</a>
284     following the discussion of
285     <a href="#subpattern">parenthesized subpatterns.</a>
286 nigel 63 </P>
287     <P>
288     Inside a character class, or if the decimal number is greater than 9 and there
289     have not been that many capturing subpatterns, PCRE re-reads up to three octal
290 nigel 93 digits following the backslash, and uses them to generate a data character. Any
291 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
292     character specified in octal must be less than \400. In UTF-8 mode, values up
293     to \777 are permitted. For example:
294 nigel 63 <pre>
295     \040 is another way of writing a space
296 nigel 75 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
297 nigel 63 \7 is always a back reference
298 nigel 75 \11 might be a back reference, or another way of writing a tab
299 nigel 63 \011 is always a tab
300     \0113 is a tab followed by the character "3"
301 nigel 75 \113 might be a back reference, otherwise the character with octal code 113
302     \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
303     \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
304     </pre>
305 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
306     zero, because no more than three octal digits are ever read.
307     </P>
308     <P>
309 nigel 91 All the sequences that define a single character value can be used both inside
310     and outside character classes. In addition, inside a character class, the
311     sequence \b is interpreted as the backspace character (hex 08), and the
312 nigel 93 sequences \R and \X are interpreted as the characters "R" and "X",
313     respectively. Outside a character class, these sequences have different
314     meanings
315 nigel 75 <a href="#uniextseq">(see below).</a>
316 nigel 63 </P>
317 nigel 75 <br><b>
318 nigel 93 Absolute and relative back references
319     </b><br>
320     <P>
321 ph10 208 The sequence \g followed by an unsigned or a negative number, optionally
322     enclosed in braces, is an absolute or relative back reference. A named back
323     reference can be coded as \g{name}. Back references are discussed
324 nigel 93 <a href="#backreferences">later,</a>
325     following the discussion of
326     <a href="#subpattern">parenthesized subpatterns.</a>
327     </P>
328     <br><b>
329 ph10 345 Absolute and relative subroutine calls
330     </b><br>
331     <P>
332     For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
333     a number enclosed either in angle brackets or single quotes, is an alternative
334     syntax for referencing a subpattern as a "subroutine". Details are discussed
335     <a href="#onigurumasubroutines">later.</a>
336     Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
337 ph10 454 synonymous. The former is a back reference; the latter is a
338     <a href="#subpatternsassubroutines">subroutine</a>
339     call.
340 ph10 345 </P>
341     <br><b>
342 nigel 75 Generic character types
343     </b><br>
344 nigel 63 <P>
345 nigel 93 Another use of backslash is for specifying generic character types. The
346 nigel 75 following are always recognized:
347 nigel 63 <pre>
348     \d any decimal digit
349     \D any character that is not a decimal digit
350 ph10 182 \h any horizontal whitespace character
351     \H any character that is not a horizontal whitespace character
352 nigel 63 \s any whitespace character
353     \S any character that is not a whitespace character
354 ph10 182 \v any vertical whitespace character
355     \V any character that is not a vertical whitespace character
356 nigel 63 \w any "word" character
357     \W any "non-word" character
358 nigel 75 </pre>
359 nigel 63 Each pair of escape sequences partitions the complete set of characters into
360     two disjoint sets. Any given character matches one, and only one, of each pair.
361     </P>
362     <P>
363 nigel 75 These character type sequences can appear both inside and outside character
364     classes. They each match one character of the appropriate type. If the current
365     matching point is at the end of the subject string, all of them fail, since
366     there is no character to match.
367 nigel 63 </P>
368     <P>
369     For compatibility with Perl, \s does not match the VT character (code 11).
370     This makes it different from the the POSIX "space" class. The \s characters
371 ph10 182 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372 nigel 91 included in a Perl script, \s may match the VT character. In PCRE, it never
373 ph10 182 does.
374 nigel 63 </P>
375     <P>
376 ph10 182 In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
377     \w, and always match \D, \S, and \W. This is true even when Unicode
378     character property support is available. These sequences retain their original
379     meanings from before UTF-8 support was available, mainly for efficiency
380 ph10 406 reasons. Note that this also affects \b, because it is defined in terms of \w
381     and \W.
382 ph10 182 </P>
383     <P>
384     The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
385     other sequences, these do match certain high-valued codepoints in UTF-8 mode.
386     The horizontal space characters are:
387     <pre>
388     U+0009 Horizontal tab
389     U+0020 Space
390     U+00A0 Non-break space
391     U+1680 Ogham space mark
392     U+180E Mongolian vowel separator
393     U+2000 En quad
394     U+2001 Em quad
395     U+2002 En space
396     U+2003 Em space
397     U+2004 Three-per-em space
398     U+2005 Four-per-em space
399     U+2006 Six-per-em space
400     U+2007 Figure space
401     U+2008 Punctuation space
402     U+2009 Thin space
403     U+200A Hair space
404     U+202F Narrow no-break space
405     U+205F Medium mathematical space
406     U+3000 Ideographic space
407     </pre>
408     The vertical space characters are:
409     <pre>
410     U+000A Linefeed
411     U+000B Vertical tab
412     U+000C Formfeed
413     U+000D Carriage return
414     U+0085 Next line
415     U+2028 Line separator
416     U+2029 Paragraph separator
417     </PRE>
418     </P>
419     <P>
420 nigel 75 A "word" character is an underscore or any character less than 256 that is a
421     letter or digit. The definition of letters and digits is controlled by PCRE's
422     low-valued character tables, and may vary if locale-specific matching is taking
423     place (see
424 nigel 63 <a href="pcreapi.html#localesupport">"Locale support"</a>
425     in the
426     <a href="pcreapi.html"><b>pcreapi</b></a>
427 ph10 142 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
428     or "french" in Windows, some character codes greater than 128 are used for
429 ph10 182 accented letters, and these are matched by \w. The use of locales with Unicode
430     is discouraged.
431 ph10 231 <a name="newlineseq"></a></P>
432 nigel 93 <br><b>
433     Newline sequences
434     </b><br>
435     <P>
436 ph10 231 Outside a character class, by default, the escape sequence \R matches any
437     Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
438     equivalent to the following:
439 nigel 93 <pre>
440     (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
441     </pre>
442     This is an example of an "atomic group", details of which are given
443     <a href="#atomicgroup">below.</a>
444     This particular group matches either the two-character sequence CR followed by
445     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
446     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
447     line, U+0085). The two-character sequence is treated as a single unit that
448     cannot be split.
449     </P>
450     <P>
451     In UTF-8 mode, two additional characters whose codepoints are greater than 255
452     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
453     Unicode character property support is not needed for these characters to be
454     recognized.
455     </P>
456     <P>
457 ph10 231 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
458     complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
459 ph10 247 either at compile time or when the pattern is matched. (BSR is an abbrevation
460     for "backslash R".) This can be made the default when PCRE is built; if this is
461     the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
462     It is also possible to specify these settings by starting a pattern string with
463     one of the following sequences:
464 ph10 231 <pre>
465     (*BSR_ANYCRLF) CR, LF, or CRLF only
466     (*BSR_UNICODE) any Unicode newline sequence
467     </pre>
468     These override the default and the options given to <b>pcre_compile()</b>, but
469     they can be overridden by options given to <b>pcre_exec()</b>. Note that these
470     special settings, which are not Perl-compatible, are recognized only at the
471     very start of a pattern, and that they must be in upper case. If more than one
472 ph10 247 of them is present, the last one is used. They can be combined with a change of
473     newline convention, for example, a pattern can start with:
474     <pre>
475     (*ANY)(*BSR_ANYCRLF)
476     </pre>
477 nigel 93 Inside a character class, \R matches the letter "R".
478 nigel 75 <a name="uniextseq"></a></P>
479     <br><b>
480     Unicode character properties
481     </b><br>
482     <P>
483     When PCRE is built with Unicode character property support, three additional
484 ph10 185 escape sequences that match characters with specific properties are available.
485     When not in UTF-8 mode, these sequences are of course limited to testing
486     characters whose codepoints are less than 256, but they do work in this mode.
487     The extra escape sequences are:
488 nigel 75 <pre>
489 nigel 87 \p{<i>xx</i>} a character with the <i>xx</i> property
490     \P{<i>xx</i>} a character without the <i>xx</i> property
491     \X an extended Unicode sequence
492 nigel 75 </pre>
493 nigel 87 The property names represented by <i>xx</i> above are limited to the Unicode
494     script names, the general category properties, and "Any", which matches any
495     character (including newline). Other properties such as "InMusicalSymbols" are
496     not currently supported by PCRE. Note that \P{Any} does not match any
497     characters, so always causes a match failure.
498 nigel 63 </P>
499     <P>
500 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
501     character from one of these sets can be matched using a script name. For
502     example:
503 nigel 75 <pre>
504 nigel 87 \p{Greek}
505     \P{Han}
506     </pre>
507     Those that are not part of an identified script are lumped together as
508     "Common". The current list of scripts is:
509     </P>
510     <P>
511     Arabic,
512     Armenian,
513 nigel 93 Balinese,
514 nigel 87 Bengali,
515     Bopomofo,
516     Braille,
517     Buginese,
518     Buhid,
519     Canadian_Aboriginal,
520     Cherokee,
521     Common,
522     Coptic,
523 nigel 93 Cuneiform,
524 nigel 87 Cypriot,
525     Cyrillic,
526     Deseret,
527     Devanagari,
528     Ethiopic,
529     Georgian,
530     Glagolitic,
531     Gothic,
532     Greek,
533     Gujarati,
534     Gurmukhi,
535     Han,
536     Hangul,
537     Hanunoo,
538     Hebrew,
539     Hiragana,
540     Inherited,
541     Kannada,
542     Katakana,
543     Kharoshthi,
544     Khmer,
545     Lao,
546     Latin,
547     Limbu,
548     Linear_B,
549     Malayalam,
550     Mongolian,
551     Myanmar,
552     New_Tai_Lue,
553 nigel 93 Nko,
554 nigel 87 Ogham,
555     Old_Italic,
556     Old_Persian,
557     Oriya,
558     Osmanya,
559 nigel 93 Phags_Pa,
560     Phoenician,
561 nigel 87 Runic,
562     Shavian,
563     Sinhala,
564     Syloti_Nagri,
565     Syriac,
566     Tagalog,
567     Tagbanwa,
568     Tai_Le,
569     Tamil,
570     Telugu,
571     Thaana,
572     Thai,
573     Tibetan,
574     Tifinagh,
575     Ugaritic,
576     Yi.
577     </P>
578     <P>
579     Each character has exactly one general category property, specified by a
580     two-letter abbreviation. For compatibility with Perl, negation can be specified
581     by including a circumflex between the opening brace and the property name. For
582     example, \p{^Lu} is the same as \P{Lu}.
583     </P>
584     <P>
585     If only one letter is specified with \p or \P, it includes all the general
586     category properties that start with that letter. In this case, in the absence
587     of negation, the curly brackets in the escape sequence are optional; these two
588     examples have the same effect:
589     <pre>
590 nigel 75 \p{L}
591     \pL
592     </pre>
593 nigel 87 The following general category property codes are supported:
594 nigel 75 <pre>
595     C Other
596     Cc Control
597     Cf Format
598     Cn Unassigned
599     Co Private use
600     Cs Surrogate
602     L Letter
603     Ll Lower case letter
604     Lm Modifier letter
605     Lo Other letter
606     Lt Title case letter
607     Lu Upper case letter
609     M Mark
610     Mc Spacing mark
611     Me Enclosing mark
612     Mn Non-spacing mark
614     N Number
615     Nd Decimal number
616     Nl Letter number
617     No Other number
619     P Punctuation
620     Pc Connector punctuation
621     Pd Dash punctuation
622     Pe Close punctuation
623     Pf Final punctuation
624     Pi Initial punctuation
625     Po Other punctuation
626     Ps Open punctuation
628     S Symbol
629     Sc Currency symbol
630     Sk Modifier symbol
631     Sm Mathematical symbol
632     So Other symbol
634     Z Separator
635     Zl Line separator
636     Zp Paragraph separator
637     Zs Space separator
638     </pre>
639 nigel 87 The special property L& is also supported: it matches a character that has
640     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
641     a modifier or "other".
642 nigel 75 </P>
643     <P>
644 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
645     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
646     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
647     (see the discussion of PCRE_NO_UTF8_CHECK in the
648     <a href="pcreapi.html"><b>pcreapi</b></a>
649 ph10 453 page). Perl does not support the Cs property.
650 ph10 211 </P>
651     <P>
652 ph10 453 The long synonyms for property names that Perl supports (such as \p{Letter})
653 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
654 nigel 87 properties with "Is".
655     </P>
656     <P>
657     No character that is in the Unicode table has the Cn (unassigned) property.
658     Instead, this property is assumed for any code point that is not in the
659     Unicode table.
660     </P>
661     <P>
662 nigel 75 Specifying caseless matching does not affect these escape sequences. For
663     example, \p{Lu} always matches only upper case letters.
664     </P>
665     <P>
666     The \X escape matches any number of Unicode characters that form an extended
667     Unicode sequence. \X is equivalent to
668     <pre>
669     (?&#62;\PM\pM*)
670     </pre>
671     That is, it matches a character without the "mark" property, followed by zero
672     or more characters with the "mark" property, and treats the sequence as an
673     atomic group
674     <a href="#atomicgroup">(see below).</a>
675     Characters with the "mark" property are typically accents that affect the
676 ph10 185 preceding character. None of them have codepoints less than 256, so in
677     non-UTF-8 mode \X matches any one character.
678 nigel 75 </P>
679     <P>
680     Matching characters by Unicode property is not fast, because PCRE has to search
681     a structure that contains data for over fifteen thousand characters. That is
682     why the traditional escape sequences such as \d and \w do not use Unicode
683     properties in PCRE.
684 ph10 172 <a name="resetmatchstart"></a></P>
685     <br><b>
686     Resetting the match start
687     </b><br>
688     <P>
689     The escape sequence \K, which is a Perl 5.10 feature, causes any previously
690     matched characters not to be included in the final matched sequence. For
691     example, the pattern:
692     <pre>
693     foo\Kbar
694     </pre>
695     matches "foobar", but reports that it has matched "bar". This feature is
696     similar to a lookbehind assertion
697     <a href="#lookbehind">(described below).</a>
698     However, in this case, the part of the subject before the real match does not
699     have to be of fixed length, as lookbehind assertions do. The use of \K does
700     not interfere with the setting of
701     <a href="#subpattern">captured substrings.</a>
702     For example, when the pattern
703     <pre>
704     (foo)\Kbar
705     </pre>
706     matches "foobar", the first substring is still set to "foo".
707 nigel 75 <a name="smallassertions"></a></P>
708     <br><b>
709     Simple assertions
710     </b><br>
711     <P>
712 nigel 93 The final use of backslash is for certain simple assertions. An assertion
713 nigel 63 specifies a condition that has to be met at a particular point in a match,
714     without consuming any characters from the subject string. The use of
715 nigel 75 subpatterns for more complicated assertions is described
716     <a href="#bigassertions">below.</a>
717 nigel 91 The backslashed assertions are:
718 nigel 63 <pre>
719     \b matches at a word boundary
720     \B matches when not at a word boundary
721 nigel 93 \A matches at the start of the subject
722     \Z matches at the end of the subject
723     also matches before a newline at the end of the subject
724     \z matches only at the end of the subject
725     \G matches at the first matching position in the subject
726 nigel 75 </pre>
727 nigel 63 These assertions may not appear in character classes (but note that \b has a
728     different meaning, namely the backspace character, inside a character class).
729     </P>
730     <P>
731     A word boundary is a position in the subject string where the current character
732     and the previous character do not both match \w or \W (i.e. one matches
733     \w and the other matches \W), or the start or end of the string if the
734     first or last character matches \w, respectively.
735     </P>
736     <P>
737     The \A, \Z, and \z assertions differ from the traditional circumflex and
738 nigel 75 dollar (described in the next section) in that they only ever match at the very
739     start and end of the subject string, whatever options are set. Thus, they are
740     independent of multiline mode. These three assertions are not affected by the
741     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
742     circumflex and dollar metacharacters. However, if the <i>startoffset</i>
743     argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
744     at a point other than the beginning of the subject, \A can never match. The
745 nigel 91 difference between \Z and \z is that \Z matches before a newline at the end
746     of the string as well as at the very end, whereas \z matches only at the end.
747 nigel 63 </P>
748     <P>
749     The \G assertion is true only when the current matching position is at the
750     start point of the match, as specified by the <i>startoffset</i> argument of
751     <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
752     non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
753     arguments, you can mimic Perl's /g option, and it is in this kind of
754     implementation where \G can be useful.
755     </P>
756     <P>
757     Note, however, that PCRE's interpretation of \G, as the start of the current
758     match, is subtly different from Perl's, which defines it as the end of the
759     previous match. In Perl, these can be different when the previously matched
760     string was empty. Because PCRE does just one match at a time, it cannot
761     reproduce this behaviour.
762     </P>
763     <P>
764     If all the alternatives of a pattern begin with \G, the expression is anchored
765     to the starting match position, and the "anchored" flag is set in the compiled
766     regular expression.
767     </P>
768 ph10 227 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
769 nigel 63 <P>
770     Outside a character class, in the default matching mode, the circumflex
771 nigel 75 character is an assertion that is true only if the current matching point is
772 nigel 63 at the start of the subject string. If the <i>startoffset</i> argument of
773     <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
774     option is unset. Inside a character class, circumflex has an entirely different
775 nigel 75 meaning
776     <a href="#characterclass">(see below).</a>
777 nigel 63 </P>
778     <P>
779     Circumflex need not be the first character of the pattern if a number of
780     alternatives are involved, but it should be the first thing in each alternative
781     in which it appears if the pattern is ever to match that branch. If all
782     possible alternatives start with a circumflex, that is, if the pattern is
783     constrained to match only at the start of the subject, it is said to be an
784     "anchored" pattern. (There are also other constructs that can cause a pattern
785     to be anchored.)
786     </P>
787     <P>
788 nigel 75 A dollar character is an assertion that is true only if the current matching
789 nigel 63 point is at the end of the subject string, or immediately before a newline
790 nigel 91 at the end of the string (by default). Dollar need not be the last character of
791     the pattern if a number of alternatives are involved, but it should be the last
792     item in any branch in which it appears. Dollar has no special meaning in a
793     character class.
794 nigel 63 </P>
795     <P>
796     The meaning of dollar can be changed so that it matches only at the very end of
797     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
798     does not affect the \Z assertion.
799     </P>
800     <P>
801     The meanings of the circumflex and dollar characters are changed if the
802 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
803     immediately after internal newlines as well as at the start of the subject
804     string. It does not match after a newline that ends the string. A dollar
805     matches before any newlines in the string, as well as at the very end, when
806     PCRE_MULTILINE is set. When newline is specified as the two-character
807     sequence CRLF, isolated CR and LF characters do not indicate newlines.
808 nigel 63 </P>
809     <P>
810 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
811     \n represents a newline) in multiline mode, but not otherwise. Consequently,
812     patterns that are anchored in single line mode because all branches start with
813     ^ are not anchored in multiline mode, and a match for circumflex is possible
814     when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
815     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
816     </P>
817     <P>
818 nigel 63 Note that the sequences \A, \Z, and \z can be used to match the start and
819     end of the subject in both modes, and if all branches of a pattern start with
820 nigel 91 \A it is always anchored, whether or not PCRE_MULTILINE is set.
821 nigel 63 </P>
822 ph10 227 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
823 nigel 63 <P>
824     Outside a character class, a dot in the pattern matches any one character in
825 nigel 91 the subject string except (by default) a character that signifies the end of a
826 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
827 nigel 63 </P>
828 nigel 91 <P>
829 nigel 93 When a line ending is defined as a single character, dot never matches that
830     character; when the two-character sequence CRLF is used, dot does not match CR
831     if it is immediately followed by LF, but otherwise it matches all characters
832     (including isolated CRs and LFs). When any Unicode line endings are being
833     recognized, dot does not match CR or LF or any of the other line ending
834     characters.
835     </P>
836     <P>
837 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
838 nigel 93 option is set, a dot matches any one character, without exception. If the
839     two-character sequence CRLF is present in the subject string, it takes two dots
840     to match it.
841 nigel 91 </P>
842     <P>
843     The handling of dot is entirely independent of the handling of circumflex and
844     dollar, the only relationship being that they both involve newlines. Dot has no
845     special meaning in a character class.
846     </P>
847 ph10 227 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
848 nigel 63 <P>
849     Outside a character class, the escape sequence \C matches any one byte, both
850 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
851     characters. The feature is provided in Perl in order to match individual bytes
852     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
853     what remains in the string may be a malformed UTF-8 string. For this reason,
854     the \C escape sequence is best avoided.
855 nigel 63 </P>
856     <P>
857 nigel 75 PCRE does not allow \C to appear in lookbehind assertions
858     <a href="#lookbehind">(described below),</a>
859     because in UTF-8 mode this would make it impossible to calculate the length of
860     the lookbehind.
861     <a name="characterclass"></a></P>
862 ph10 227 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
863 nigel 63 <P>
864     An opening square bracket introduces a character class, terminated by a closing
865     square bracket. A closing square bracket on its own is not special. If a
866     closing square bracket is required as a member of the class, it should be the
867     first data character in the class (after an initial circumflex, if present) or
868     escaped with a backslash.
869     </P>
870     <P>
871     A character class matches a single character in the subject. In UTF-8 mode, the
872     character may occupy more than one byte. A matched character must be in the set
873     of characters defined by the class, unless the first character in the class
874     definition is a circumflex, in which case the subject character must not be in
875     the set defined by the class. If a circumflex is actually required as a member
876     of the class, ensure it is not the first character, or escape it with a
877     backslash.
878     </P>
879     <P>
880     For example, the character class [aeiou] matches any lower case vowel, while
881     [^aeiou] matches any character that is not a lower case vowel. Note that a
882 nigel 75 circumflex is just a convenient notation for specifying the characters that
883     are in the class by enumerating those that are not. A class that starts with a
884     circumflex is not an assertion: it still consumes a character from the subject
885     string, and therefore it fails if the current pointer is at the end of the
886     string.
887 nigel 63 </P>
888     <P>
889     In UTF-8 mode, characters with values greater than 255 can be included in a
890     class as a literal string of bytes, or by using the \x{ escaping mechanism.
891     </P>
892     <P>
893     When caseless matching is set, any letters in a class represent both their
894     upper case and lower case versions, so for example, a caseless [aeiou] matches
895     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
896 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
897     case for characters whose values are less than 128, so caseless matching is
898     always possible. For characters with higher values, the concept of case is
899     supported if PCRE is compiled with Unicode property support, but not otherwise.
900     If you want to use caseless matching for characters 128 and above, you must
901     ensure that PCRE is compiled with Unicode property support as well as with
902     UTF-8 support.
903 nigel 63 </P>
904     <P>
905 nigel 93 Characters that might indicate line breaks are never treated in any special way
906     when matching character classes, whatever line-ending sequence is in use, and
907     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
908     such as [^a] always matches one of these characters.
909 nigel 63 </P>
910     <P>
911     The minus (hyphen) character can be used to specify a range of characters in a
912     character class. For example, [d-m] matches any letter between d and m,
913     inclusive. If a minus character is required in a class, it must be escaped with
914     a backslash or appear in a position where it cannot be interpreted as
915     indicating a range, typically as the first or last character in the class.
916     </P>
917     <P>
918     It is not possible to have the literal character "]" as the end character of a
919     range. A pattern such as [W-]46] is interpreted as a class of two characters
920     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
921     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
922 nigel 75 the end of range, so [W-\]46] is interpreted as a class containing a range
923     followed by two other characters. The octal or hexadecimal representation of
924     "]" can also be used to end a range.
925 nigel 63 </P>
926     <P>
927     Ranges operate in the collating sequence of character values. They can also be
928     used for characters specified numerically, for example [\000-\037]. In UTF-8
929     mode, ranges can include characters whose values are greater than 255, for
930     example [\x{100}-\x{2ff}].
931     </P>
932     <P>
933     If a range that includes letters is used when caseless matching is set, it
934     matches the letters in either case. For example, [W-c] is equivalent to
935 nigel 75 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
936 ph10 142 tables for a French locale are in use, [\xc8-\xcb] matches accented E
937 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
938     characters with values greater than 128 only when it is compiled with Unicode
939     property support.
940 nigel 63 </P>
941     <P>
942 nigel 75 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
943     in a character class, and add the characters that they match to the class. For
944 nigel 63 example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
945     conveniently be used with the upper case character types to specify a more
946     restricted set of characters than the matching lower case type. For example,
947     the class [^\W_] matches any letter or digit, but not underscore.
948     </P>
949     <P>
950 nigel 75 The only metacharacters that are recognized in character classes are backslash,
951     hyphen (only where it can be interpreted as specifying a range), circumflex
952     (only at the start), opening square bracket (only when it can be interpreted as
953     introducing a POSIX class name - see the next section), and the terminating
954     closing square bracket. However, escaping other non-alphanumeric characters
955     does no harm.
956 nigel 63 </P>
957 ph10 227 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
958 nigel 63 <P>
959 nigel 75 Perl supports the POSIX notation for character classes. This uses names
960 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
961     this notation. For example,
962     <pre>
963     [01[:alpha:]%]
964 nigel 75 </pre>
965 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
966     are
967     <pre>
968     alnum letters and digits
969     alpha letters
970     ascii character codes 0 - 127
971     blank space or tab only
972     cntrl control characters
973     digit decimal digits (same as \d)
974     graph printing characters, excluding space
975     lower lower case letters
976     print printing characters, including space
977     punct printing characters, excluding letters and digits
978     space white space (not quite the same as \s)
979     upper upper case letters
980     word "word" characters (same as \w)
981     xdigit hexadecimal digits
982 nigel 75 </pre>
983 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
984     space (32). Notice that this list includes the VT character (code 11). This
985     makes "space" different to \s, which does not include VT (for Perl
986     compatibility).
987     </P>
988     <P>
989     The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
990     5.8. Another Perl extension is negation, which is indicated by a ^ character
991     after the colon. For example,
992     <pre>
993     [12[:^digit:]]
994 nigel 75 </pre>
995 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
996     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
997     supported, and an error is given if they are encountered.
998     </P>
999     <P>
1000 nigel 75 In UTF-8 mode, characters with values greater than 128 do not match any of
1001 nigel 63 the POSIX character classes.
1002     </P>
1003 ph10 227 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
1004 nigel 63 <P>
1005     Vertical bar characters are used to separate alternative patterns. For example,
1006     the pattern
1007     <pre>
1008     gilbert|sullivan
1009 nigel 75 </pre>
1010 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1011 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
1012     process tries each alternative in turn, from left to right, and the first one
1013     that succeeds is used. If the alternatives are within a subpattern
1014 nigel 75 <a href="#subpattern">(defined below),</a>
1015     "succeeds" means matching the rest of the main pattern as well as the
1016     alternative in the subpattern.
1017 nigel 63 </P>
1018 ph10 227 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
1019 nigel 63 <P>
1020     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1021 ph10 243 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1022     the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1023     The option letters are
1024 nigel 63 <pre>
1025     i for PCRE_CASELESS
1026     m for PCRE_MULTILINE
1027     s for PCRE_DOTALL
1028     x for PCRE_EXTENDED
1029 nigel 75 </pre>
1030 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
1031     unset these options by preceding the letter with a hyphen, and a combined
1032     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1033     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1034     permitted. If a letter appears both before and after the hyphen, the option is
1035     unset.
1036     </P>
1037     <P>
1038 ph10 243 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1039     changed in the same way as the Perl-compatible options by using the characters
1040     J, U and X respectively.
1041     </P>
1042     <P>
1043 ph10 416 When one of these option changes occurs at top level (that is, not inside
1044     subpattern parentheses), the change applies to the remainder of the pattern
1045     that follows. If the change is placed right at the start of a pattern, PCRE
1046     extracts it into the global options (and it will therefore show up in data
1047     extracted by the <b>pcre_fullinfo()</b> function).
1048 nigel 63 </P>
1049     <P>
1050 nigel 93 An option change within a subpattern (see below for a description of
1051     subpatterns) affects only that part of the current pattern that follows it, so
1052 nigel 63 <pre>
1053     (a(?i)b)c
1054 nigel 75 </pre>
1055 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1056     By this means, options can be made to have different settings in different
1057     parts of the pattern. Any changes made in one alternative do carry on
1058     into subsequent branches within the same subpattern. For example,
1059     <pre>
1060     (a(?i)b|c)
1061 nigel 75 </pre>
1062 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1063     branch is abandoned before the option setting. This is because the effects of
1064     option settings happen at compile time. There would be some very weird
1065     behaviour otherwise.
1066 ph10 254 </P>
1067     <P>
1068     <b>Note:</b> There are other PCRE-specific options that can be set by the
1069     application when the compile or match functions are called. In some cases the
1070 ph10 416 pattern can contain special leading sequences such as (*CRLF) to override what
1071     the application has set or what has been defaulted. Details are given in the
1072     section entitled
1073 ph10 254 <a href="#newlineseq">"Newline sequences"</a>
1074 ph10 416 above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1075     mode; this is equivalent to setting the PCRE_UTF8 option.
1076 nigel 75 <a name="subpattern"></a></P>
1077 ph10 227 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1078 nigel 63 <P>
1079     Subpatterns are delimited by parentheses (round brackets), which can be nested.
1080 nigel 75 Turning part of a pattern into a subpattern does two things:
1081     <br>
1082     <br>
1083 nigel 63 1. It localizes a set of alternatives. For example, the pattern
1084     <pre>
1085     cat(aract|erpillar|)
1086 nigel 75 </pre>
1087 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
1088 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
1089 nigel 75 <br>
1090     <br>
1091     2. It sets up the subpattern as a capturing subpattern. This means that, when
1092     the whole pattern matches, that portion of the subject string that matched the
1093     subpattern is passed back to the caller via the <i>ovector</i> argument of
1094 nigel 63 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1095 nigel 75 from 1) to obtain numbers for the capturing subpatterns.
1096 nigel 63 </P>
1097     <P>
1098     For example, if the string "the red king" is matched against the pattern
1099     <pre>
1100     the ((red|white) (king|queen))
1101 nigel 75 </pre>
1102 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
1103     2, and 3, respectively.
1104     </P>
1105     <P>
1106     The fact that plain parentheses fulfil two functions is not always helpful.
1107     There are often times when a grouping subpattern is required without a
1108     capturing requirement. If an opening parenthesis is followed by a question mark
1109     and a colon, the subpattern does not do any capturing, and is not counted when
1110     computing the number of any subsequent capturing subpatterns. For example, if
1111     the string "the white queen" is matched against the pattern
1112     <pre>
1113     the ((?:red|white) (king|queen))
1114 nigel 75 </pre>
1115 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1116 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1117 nigel 63 </P>
1118     <P>
1119     As a convenient shorthand, if any option settings are required at the start of
1120     a non-capturing subpattern, the option letters may appear between the "?" and
1121     the ":". Thus the two patterns
1122     <pre>
1123     (?i:saturday|sunday)
1124     (?:(?i)saturday|sunday)
1125 nigel 75 </pre>
1126 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1127     from left to right, and options are not reset until the end of the subpattern
1128     is reached, an option setting in one branch does affect subsequent branches, so
1129     the above patterns match "SUNDAY" as well as "Saturday".
1130     </P>
1131 ph10 227 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1132 nigel 63 <P>
1133 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1134     the same numbers for its capturing parentheses. Such a subpattern starts with
1135     (?| and is itself a non-capturing subpattern. For example, consider this
1136     pattern:
1137     <pre>
1138     (?|(Sat)ur|(Sun))day
1139     </pre>
1140     Because the two alternatives are inside a (?| group, both sets of capturing
1141     parentheses are numbered one. Thus, when the pattern matches, you can look
1142     at captured substring number one, whichever alternative matched. This construct
1143     is useful when you want to capture part, but not all, of one of a number of
1144     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1145     number is reset at the start of each branch. The numbers of any capturing
1146     buffers that follow the subpattern start after the highest number used in any
1147     branch. The following example is taken from the Perl documentation.
1148     The numbers underneath show in which buffer the captured content will be
1149     stored.
1150     <pre>
1151     # before ---------------branch-reset----------- after
1152     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1153     # 1 2 2 3 2 3 4
1154     </pre>
1155     A backreference or a recursive call to a numbered subpattern always refers to
1156     the first one in the pattern with the given number.
1157     </P>
1158     <P>
1159     An alternative approach to using this "branch reset" feature is to use
1160     duplicate named subpatterns, as described in the next section.
1161     </P>
1162 ph10 227 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1163 ph10 182 <P>
1164 nigel 63 Identifying capturing parentheses by number is simple, but it can be very hard
1165     to keep track of the numbers in complicated regular expressions. Furthermore,
1166 nigel 75 if an expression is modified, the numbers may change. To help with this
1167 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1168     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1169     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1170     the Perl and the Python syntax.
1171     </P>
1172     <P>
1173     In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1174     (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1175 nigel 91 parentheses from other parts of the pattern, such as
1176     <a href="#backreferences">backreferences,</a>
1177     <a href="#recursion">recursion,</a>
1178     and
1179     <a href="#conditions">conditions,</a>
1180     can be made by name as well as by number.
1181 nigel 63 </P>
1182     <P>
1183 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1184 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1185     if the names were not present. The PCRE API provides function calls for
1186     extracting the name-to-number translation table from a compiled pattern. There
1187     is also a convenience function for extracting a captured substring by name.
1188 nigel 91 </P>
1189     <P>
1190     By default, a name must be unique within a pattern, but it is possible to relax
1191     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1192     be useful for patterns where only one instance of the named parentheses can
1193     match. Suppose you want to match the name of a weekday, either as a 3-letter
1194     abbreviation or as the full name, and in both cases you want to extract the
1195     abbreviation. This pattern (ignoring the line breaks) does the job:
1196     <pre>
1197 nigel 93 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1198     (?&#60;DN&#62;Tue)(?:sday)?|
1199     (?&#60;DN&#62;Wed)(?:nesday)?|
1200     (?&#60;DN&#62;Thu)(?:rsday)?|
1201     (?&#60;DN&#62;Sat)(?:urday)?
1202 nigel 91 </pre>
1203     There are five capturing substrings, but only one is ever set after a match.
1204 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1205     subpattern, as described in the previous section.)
1206     </P>
1207     <P>
1208 nigel 91 The convenience function for extracting the data by name returns the substring
1209 nigel 93 for the first (and in this example, the only) subpattern of that name that
1210 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1211     make a reference to a non-unique named subpattern from elsewhere in the
1212     pattern, the one that corresponds to the lowest number is used. For further
1213     details of the interfaces for handling named subpatterns, see the
1214 nigel 63 <a href="pcreapi.html"><b>pcreapi</b></a>
1215     documentation.
1216     </P>
1217 ph10 392 <P>
1218     <b>Warning:</b> You cannot use different names to distinguish between two
1219     subpatterns with the same number (see the previous section) because PCRE uses
1220     only the numbers when matching.
1221     </P>
1222 ph10 227 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1223 nigel 63 <P>
1224     Repetition is specified by quantifiers, which can follow any of the following
1225     items:
1226     <pre>
1227     a literal data character
1228 nigel 93 the dot metacharacter
1229 nigel 63 the \C escape sequence
1230 nigel 75 the \X escape sequence (in UTF-8 mode with Unicode properties)
1231 nigel 93 the \R escape sequence
1232 nigel 75 an escape such as \d that matches a single character
1233 nigel 63 a character class
1234     a back reference (see next section)
1235     a parenthesized subpattern (unless it is an assertion)
1236 nigel 75 </pre>
1237 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1238     permitted matches, by giving the two numbers in curly brackets (braces),
1239     separated by a comma. The numbers must be less than 65536, and the first must
1240     be less than or equal to the second. For example:
1241     <pre>
1242     z{2,4}
1243 nigel 75 </pre>
1244 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1245     character. If the second number is omitted, but the comma is present, there is
1246     no upper limit; if the second number and the comma are both omitted, the
1247     quantifier specifies an exact number of required matches. Thus
1248     <pre>
1249     [aeiou]{3,}
1250 nigel 75 </pre>
1251 nigel 63 matches at least 3 successive vowels, but may match many more, while
1252     <pre>
1253     \d{8}
1254 nigel 75 </pre>
1255 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1256     where a quantifier is not allowed, or one that does not match the syntax of a
1257     quantifier, is taken as a literal character. For example, {,6} is not a
1258     quantifier, but a literal string of four characters.
1259     </P>
1260     <P>
1261     In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1262     bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1263 nigel 75 which is represented by a two-byte sequence. Similarly, when Unicode property
1264     support is available, \X{3} matches three Unicode extended sequences, each of
1265     which may be several bytes long (and they may be of different lengths).
1266 nigel 63 </P>
1267     <P>
1268     The quantifier {0} is permitted, causing the expression to behave as if the
1269 ph10 345 previous item and the quantifier were not present. This may be useful for
1270     subpatterns that are referenced as
1271     <a href="#subpatternsassubroutines">subroutines</a>
1272     from elsewhere in the pattern. Items other than subpatterns that have a {0}
1273     quantifier are omitted from the compiled pattern.
1274 nigel 63 </P>
1275     <P>
1276 nigel 93 For convenience, the three most common quantifiers have single-character
1277     abbreviations:
1278 nigel 63 <pre>
1279     * is equivalent to {0,}
1280     + is equivalent to {1,}
1281     ? is equivalent to {0,1}
1282 nigel 75 </pre>
1283 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1284     match no characters with a quantifier that has no upper limit, for example:
1285     <pre>
1286     (a?)*
1287 nigel 75 </pre>
1288 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1289     such patterns. However, because there are cases where this can be useful, such
1290     patterns are now accepted, but if any repetition of the subpattern does in fact
1291     match no characters, the loop is forcibly broken.
1292     </P>
1293     <P>
1294     By default, the quantifiers are "greedy", that is, they match as much as
1295     possible (up to the maximum number of permitted times), without causing the
1296     rest of the pattern to fail. The classic example of where this gives problems
1297 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1298     and within the comment, individual * and / characters may appear. An attempt to
1299     match C comments by applying the pattern
1300 nigel 63 <pre>
1301     /\*.*\*/
1302 nigel 75 </pre>
1303 nigel 63 to the string
1304     <pre>
1305 nigel 75 /* first comment */ not comment /* second comment */
1306     </pre>
1307 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1308     item.
1309     </P>
1310     <P>
1311     However, if a quantifier is followed by a question mark, it ceases to be
1312     greedy, and instead matches the minimum number of times possible, so the
1313     pattern
1314     <pre>
1315     /\*.*?\*/
1316 nigel 75 </pre>
1317 nigel 63 does the right thing with the C comments. The meaning of the various
1318     quantifiers is not otherwise changed, just the preferred number of matches.
1319     Do not confuse this use of question mark with its use as a quantifier in its
1320     own right. Because it has two uses, it can sometimes appear doubled, as in
1321     <pre>
1322     \d??\d
1323 nigel 75 </pre>
1324 nigel 63 which matches one digit by preference, but can match two if that is the only
1325     way the rest of the pattern matches.
1326     </P>
1327     <P>
1328 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1329 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1330     greedy by following them with a question mark. In other words, it inverts the
1331     default behaviour.
1332     </P>
1333     <P>
1334     When a parenthesized subpattern is quantified with a minimum repeat count that
1335 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1336 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1337     </P>
1338     <P>
1339     If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1340 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1341 nigel 63 implicitly anchored, because whatever follows will be tried against every
1342     character position in the subject string, so there is no point in retrying the
1343     overall match at any position after the first. PCRE normally treats such a
1344     pattern as though it were preceded by \A.
1345     </P>
1346     <P>
1347     In cases where it is known that the subject string contains no newlines, it is
1348     worth setting PCRE_DOTALL in order to obtain this optimization, or
1349     alternatively using ^ to indicate anchoring explicitly.
1350     </P>
1351     <P>
1352     However, there is one situation where the optimization cannot be used. When .*
1353     is inside capturing parentheses that are the subject of a backreference
1354 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1355     succeeds. Consider, for example:
1356 nigel 63 <pre>
1357     (.*)abc\1
1358 nigel 75 </pre>
1359 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1360     this reason, such a pattern is not implicitly anchored.
1361     </P>
1362     <P>
1363     When a capturing subpattern is repeated, the value captured is the substring
1364     that matched the final iteration. For example, after
1365     <pre>
1366     (tweedle[dume]{3}\s*)+
1367 nigel 75 </pre>
1368 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1369     "tweedledee". However, if there are nested capturing subpatterns, the
1370     corresponding captured values may have been set in previous iterations. For
1371     example, after
1372     <pre>
1373     /(a|(b))+/
1374 nigel 75 </pre>
1375 nigel 63 matches "aba" the value of the second captured substring is "b".
1376 nigel 75 <a name="atomicgroup"></a></P>
1377 ph10 227 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1378 nigel 63 <P>
1379 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1380     repetition, failure of what follows normally causes the repeated item to be
1381     re-evaluated to see if a different number of repeats allows the rest of the
1382     pattern to match. Sometimes it is useful to prevent this, either to change the
1383     nature of the match, or to cause it fail earlier than it otherwise might, when
1384     the author of the pattern knows there is no point in carrying on.
1385 nigel 63 </P>
1386     <P>
1387     Consider, for example, the pattern \d+foo when applied to the subject line
1388     <pre>
1389     123456bar
1390 nigel 75 </pre>
1391 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1392     action of the matcher is to try again with only 5 digits matching the \d+
1393     item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1394     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1395     that once a subpattern has matched, it is not to be re-evaluated in this way.
1396     </P>
1397     <P>
1398 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1399 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1400     special parenthesis, starting with (?&#62; as in this example:
1401     <pre>
1402 nigel 73 (?&#62;\d+)foo
1403 nigel 75 </pre>
1404 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1405     it has matched, and a failure further into the pattern is prevented from
1406     backtracking into it. Backtracking past it to previous items, however, works as
1407     normal.
1408     </P>
1409     <P>
1410     An alternative description is that a subpattern of this type matches the string
1411     of characters that an identical standalone pattern would match, if anchored at
1412     the current point in the subject string.
1413     </P>
1414     <P>
1415     Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1416     the above example can be thought of as a maximizing repeat that must swallow
1417     everything it can. So, while both \d+ and \d+? are prepared to adjust the
1418     number of digits they match in order to make the rest of the pattern match,
1419     (?&#62;\d+) can only match an entire sequence of digits.
1420     </P>
1421     <P>
1422     Atomic groups in general can of course contain arbitrarily complicated
1423     subpatterns, and can be nested. However, when the subpattern for an atomic
1424     group is just a single repeated item, as in the example above, a simpler
1425     notation, called a "possessive quantifier" can be used. This consists of an
1426     additional + character following a quantifier. Using this notation, the
1427     previous example can be rewritten as
1428     <pre>
1429 nigel 75 \d++foo
1430     </pre>
1431 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1432     example:
1433     <pre>
1434     (abc|xyz){2,3}+
1435     </pre>
1436 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1437     option is ignored. They are a convenient notation for the simpler forms of
1438 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1439     quantifier and the equivalent atomic group, though there may be a performance
1440     difference; possessive quantifiers should be slightly faster.
1441 nigel 63 </P>
1442     <P>
1443 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1444     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1445     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1446     package, and PCRE copied it from there. It ultimately found its way into Perl
1447     at release 5.10.
1448 nigel 63 </P>
1449     <P>
1450 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1451     pattern constructs. For example, the sequence A+B is treated as A++B because
1452     there is no point in backtracking into a sequence of A's when B must follow.
1453     </P>
1454     <P>
1455 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1456     be repeated an unlimited number of times, the use of an atomic group is the
1457     only way to avoid some failing matches taking a very long time indeed. The
1458     pattern
1459     <pre>
1460     (\D+|&#60;\d+&#62;)*[!?]
1461 nigel 75 </pre>
1462 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1463     digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1464     quickly. However, if it is applied to
1465     <pre>
1466     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1467 nigel 75 </pre>
1468 nigel 63 it takes a long time before reporting failure. This is because the string can
1469 nigel 75 be divided between the internal \D+ repeat and the external * repeat in a
1470     large number of ways, and all have to be tried. (The example uses [!?] rather
1471     than a single character at the end, because both PCRE and Perl have an
1472     optimization that allows for fast failure when a single character is used. They
1473     remember the last single character that is required for a match, and fail early
1474     if it is not present in the string.) If the pattern is changed so that it uses
1475     an atomic group, like this:
1476 nigel 63 <pre>
1477     ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1478 nigel 75 </pre>
1479 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1480 nigel 75 <a name="backreferences"></a></P>
1481 ph10 227 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1482 nigel 63 <P>
1483     Outside a character class, a backslash followed by a digit greater than 0 (and
1484     possibly further digits) is a back reference to a capturing subpattern earlier
1485     (that is, to its left) in the pattern, provided there have been that many
1486     previous capturing left parentheses.
1487     </P>
1488     <P>
1489     However, if the decimal number following the backslash is less than 10, it is
1490     always taken as a back reference, and causes an error only if there are not
1491     that many capturing left parentheses in the entire pattern. In other words, the
1492     parentheses that are referenced need not be to the left of the reference for
1493 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1494     when a repetition is involved and the subpattern to the right has participated
1495     in an earlier iteration.
1496     </P>
1497     <P>
1498 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1499     whose number is 10 or more using this syntax because a sequence such as \50 is
1500     interpreted as a character defined in octal. See the subsection entitled
1501 nigel 91 "Non-printing characters"
1502 nigel 75 <a href="#digitsafterbackslash">above</a>
1503 nigel 93 for further details of the handling of digits following a backslash. There is
1504     no such problem when named parentheses are used. A back reference to any
1505     subpattern is possible using named parentheses (see below).
1506 nigel 63 </P>
1507     <P>
1508 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1509     backslash is to use the \g escape sequence, which is a feature introduced in
1510 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1511     number, optionally enclosed in braces. These examples are all identical:
1512 nigel 93 <pre>
1513     (ring), \1
1514     (ring), \g1
1515     (ring), \g{1}
1516     </pre>
1517 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1518     is present in the older syntax. It is also useful when literal digits follow
1519     the reference. A negative number is a relative reference. Consider this
1520     example:
1521 nigel 93 <pre>
1522     (abc(def)ghi)\g{-1}
1523     </pre>
1524     The sequence \g{-1} is a reference to the most recently started capturing
1525     subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1526     would be equivalent to \1. The use of relative references can be helpful in
1527     long patterns, and also in patterns that are created by joining together
1528     fragments that contain references within themselves.
1529     </P>
1530     <P>
1531 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1532     the current subject string, rather than anything matching the subpattern
1533     itself (see
1534     <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1535     below for a way of doing that). So the pattern
1536     <pre>
1537     (sens|respons)e and \1ibility
1538 nigel 75 </pre>
1539 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1540     "sense and responsibility". If caseful matching is in force at the time of the
1541     back reference, the case of letters is relevant. For example,
1542     <pre>
1543     ((?i)rah)\s+\1
1544 nigel 75 </pre>
1545 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1546     capturing subpattern is matched caselessly.
1547     </P>
1548     <P>
1549 ph10 172 There are several different ways of writing back references to named
1550     subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1551     \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1552     back reference syntax, in which \g can be used for both numeric and named
1553     references, is also supported. We could rewrite the above example in any of
1554 nigel 93 the following ways:
1555 nigel 63 <pre>
1556 nigel 93 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1557 ph10 172 (?'p1'(?i)rah)\s+\k{p1}
1558 nigel 91 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1559 ph10 172 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1560 nigel 75 </pre>
1561 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1562     after the reference.
1563     </P>
1564     <P>
1565 nigel 63 There may be more than one back reference to the same subpattern. If a
1566     subpattern has not actually been used in a particular match, any back
1567     references to it always fail. For example, the pattern
1568     <pre>
1569     (a|(bc))\2
1570 nigel 75 </pre>
1571 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1572     many capturing parentheses in a pattern, all digits following the backslash are
1573     taken as part of a potential back reference number. If the pattern continues
1574     with a digit character, some delimiter must be used to terminate the back
1575     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1576 nigel 75 Otherwise an empty comment (see
1577     <a href="#comments">"Comments"</a>
1578     below) can be used.
1579 nigel 63 </P>
1580     <P>
1581     A back reference that occurs inside the parentheses to which it refers fails
1582     when the subpattern is first used, so, for example, (a\1) never matches.
1583     However, such references can be useful inside repeated subpatterns. For
1584     example, the pattern
1585     <pre>
1586     (a|b\1)+
1587 nigel 75 </pre>
1588 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1589     the subpattern, the back reference matches the character string corresponding
1590     to the previous iteration. In order for this to work, the pattern must be such
1591     that the first iteration does not need to match the back reference. This can be
1592     done using alternation, as in the example above, or by a quantifier with a
1593     minimum of zero.
1594 nigel 75 <a name="bigassertions"></a></P>
1595 ph10 227 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1596 nigel 63 <P>
1597     An assertion is a test on the characters following or preceding the current
1598     matching point that does not actually consume any characters. The simple
1599 nigel 75 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1600     <a href="#smallassertions">above.</a>
1601     </P>
1602     <P>
1603 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1604     those that look ahead of the current position in the subject string, and those
1605 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1606     except that it does not cause the current matching position to be changed.
1607 nigel 63 </P>
1608     <P>
1609 nigel 75 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1610     because it makes no sense to assert the same thing several times. If any kind
1611     of assertion contains capturing subpatterns within it, these are counted for
1612     the purposes of numbering the capturing subpatterns in the whole pattern.
1613     However, substring capturing is carried out only for positive assertions,
1614     because it does not make sense for negative assertions.
1615 nigel 63 </P>
1616 nigel 75 <br><b>
1617     Lookahead assertions
1618     </b><br>
1619 nigel 63 <P>
1620 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1621     negative assertions. For example,
1622 nigel 63 <pre>
1623     \w+(?=;)
1624 nigel 75 </pre>
1625 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1626     the match, and
1627     <pre>
1628     foo(?!bar)
1629 nigel 75 </pre>
1630 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1631     apparently similar pattern
1632     <pre>
1633     (?!foo)bar
1634 nigel 75 </pre>
1635 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1636     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1637     (?!foo) is always true when the next three characters are "bar". A
1638 nigel 75 lookbehind assertion is needed to achieve the other effect.
1639 nigel 63 </P>
1640     <P>
1641     If you want to force a matching failure at some point in a pattern, the most
1642     convenient way to do it is with (?!) because an empty string always matches, so
1643     an assertion that requires there not to be an empty string must always fail.
1644 nigel 75 <a name="lookbehind"></a></P>
1645     <br><b>
1646     Lookbehind assertions
1647     </b><br>
1648 nigel 63 <P>
1649     Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1650     negative assertions. For example,
1651     <pre>
1652     (?&#60;!foo)bar
1653 nigel 75 </pre>
1654 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1655     a lookbehind assertion are restricted such that all the strings it matches must
1656 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1657     do not all have to have the same fixed length. Thus
1658 nigel 63 <pre>
1659     (?&#60;=bullock|donkey)
1660 nigel 75 </pre>
1661 nigel 63 is permitted, but
1662     <pre>
1663     (?&#60;!dogs?|cats?)
1664 nigel 75 </pre>
1665 nigel 63 causes an error at compile time. Branches that match different length strings
1666     are permitted only at the top level of a lookbehind assertion. This is an
1667 ph10 454 extension compared with Perl (5.8 and 5.10), which requires all branches to
1668 nigel 63 match the same length of string. An assertion such as
1669     <pre>
1670     (?&#60;=ab(c|de))
1671 nigel 75 </pre>
1672 nigel 63 is not permitted, because its single top-level branch can match two different
1673 ph10 454 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1674     branches:
1675 nigel 63 <pre>
1676     (?&#60;=abc|abde)
1677 nigel 75 </pre>
1678 ph10 172 In some cases, the Perl 5.10 escape sequence \K
1679     <a href="#resetmatchstart">(see above)</a>
1680 ph10 454 can be used instead of a lookbehind assertion to get round the fixed-length
1681     restriction.
1682 ph10 172 </P>
1683     <P>
1684 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1685 nigel 93 temporarily move the current position back by the fixed length and then try to
1686 nigel 63 match. If there are insufficient characters before the current position, the
1687 nigel 93 assertion fails.
1688 nigel 63 </P>
1689     <P>
1690     PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1691     to appear in lookbehind assertions, because it makes it impossible to calculate
1692 nigel 93 the length of the lookbehind. The \X and \R escapes, which can match
1693     different numbers of bytes, are also not permitted.
1694 nigel 63 </P>
1695     <P>
1696 ph10 454 <a href="#subpatternsassubroutines">"Subroutine"</a>
1697     calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1698     as the subpattern matches a fixed-length string.
1699     <a href="#recursion">Recursion,</a>
1700     however, is not supported.
1701     </P>
1702     <P>
1703 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1704     specify efficient matching at the end of the subject string. Consider a simple
1705     pattern such as
1706 nigel 63 <pre>
1707     abcd$
1708 nigel 75 </pre>
1709 nigel 63 when applied to a long string that does not match. Because matching proceeds
1710     from left to right, PCRE will look for each "a" in the subject and then see if
1711     what follows matches the rest of the pattern. If the pattern is specified as
1712     <pre>
1713     ^.*abcd$
1714 nigel 75 </pre>
1715 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1716     there is no following "a"), it backtracks to match all but the last character,
1717     then all but the last two characters, and so on. Once again the search for "a"
1718     covers the entire string, from right to left, so we are no better off. However,
1719     if the pattern is written as
1720     <pre>
1721     ^.*+(?&#60;=abcd)
1722 nigel 75 </pre>
1723 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1724 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1725     characters. If it fails, the match fails immediately. For long strings, this
1726     approach makes a significant difference to the processing time.
1727     </P>
1728 nigel 75 <br><b>
1729     Using multiple assertions
1730     </b><br>
1731 nigel 63 <P>
1732     Several assertions (of any sort) may occur in succession. For example,
1733     <pre>
1734     (?&#60;=\d{3})(?&#60;!999)foo
1735 nigel 75 </pre>
1736 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1737     the assertions is applied independently at the same point in the subject
1738     string. First there is a check that the previous three characters are all
1739     digits, and then there is a check that the same three characters are not "999".
1740     This pattern does <i>not</i> match "foo" preceded by six characters, the first
1741     of which are digits and the last three of which are not "999". For example, it
1742     doesn't match "123abcfoo". A pattern to do that is
1743     <pre>
1744     (?&#60;=\d{3}...)(?&#60;!999)foo
1745 nigel 75 </pre>
1746 nigel 63 This time the first assertion looks at the preceding six characters, checking
1747     that the first three are digits, and then the second assertion checks that the
1748     preceding three characters are not "999".
1749     </P>
1750     <P>
1751     Assertions can be nested in any combination. For example,
1752     <pre>
1753     (?&#60;=(?&#60;!foo)bar)baz
1754 nigel 75 </pre>
1755 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1756     preceded by "foo", while
1757     <pre>
1758     (?&#60;=\d{3}(?!999)...)foo
1759 nigel 75 </pre>
1760     is another pattern that matches "foo" preceded by three digits and any three
1761 nigel 63 characters that are not "999".
1762 nigel 91 <a name="conditions"></a></P>
1763 ph10 227 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1764 nigel 63 <P>
1765     It is possible to cause the matching process to obey a subpattern
1766     conditionally or to choose between two alternative subpatterns, depending on
1767     the result of an assertion, or whether a previous capturing subpattern matched
1768     or not. The two possible forms of conditional subpattern are
1769     <pre>
1770     (?(condition)yes-pattern)
1771     (?(condition)yes-pattern|no-pattern)
1772 nigel 75 </pre>
1773 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1774     no-pattern (if present) is used. If there are more than two alternatives in the
1775     subpattern, a compile-time error occurs.
1776     </P>
1777     <P>
1778 nigel 93 There are four kinds of condition: references to subpatterns, references to
1779     recursion, a pseudo-condition called DEFINE, and assertions.
1780 nigel 91 </P>
1781 nigel 93 <br><b>
1782     Checking for a used subpattern by number
1783     </b><br>
1784 nigel 91 <P>
1785 nigel 93 If the text between the parentheses consists of a sequence of digits, the
1786     condition is true if the capturing subpattern of that number has previously
1787 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1788     sign. In this case, the subpattern number is relative rather than absolute.
1789     The most recently opened parentheses can be referenced by (?(-1), the next most
1790     recent by (?(-2), and so on. In looping constructs it can also make sense to
1791     refer to subsequent groups with constructs such as (?(+2).
1792 nigel 93 </P>
1793     <P>
1794 nigel 91 Consider the following pattern, which contains non-significant white space to
1795     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1796     three parts for ease of discussion:
1797 nigel 63 <pre>
1798     ( \( )? [^()]+ (?(1) \) )
1799 nigel 75 </pre>
1800 nigel 63 The first part matches an optional opening parenthesis, and if that
1801     character is present, sets it as the first captured substring. The second part
1802     matches one or more characters that are not parentheses. The third part is a
1803     conditional subpattern that tests whether the first set of parentheses matched
1804     or not. If they did, that is, if subject started with an opening parenthesis,
1805     the condition is true, and so the yes-pattern is executed and a closing
1806     parenthesis is required. Otherwise, since no-pattern is not present, the
1807     subpattern matches nothing. In other words, this pattern matches a sequence of
1808 nigel 93 non-parentheses, optionally enclosed in parentheses.
1809     </P>
1810 ph10 172 <P>
1811     If you were embedding this pattern in a larger one, you could use a relative
1812     reference:
1813     <pre>
1814     ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
1815     </pre>
1816     This makes the fragment independent of the parentheses in the larger pattern.
1817     </P>
1818 nigel 93 <br><b>
1819     Checking for a used subpattern by name
1820     </b><br>
1821     <P>
1822     Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1823     subpattern by name. For compatibility with earlier versions of PCRE, which had
1824     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1825     there is a possible ambiguity with this syntax, because subpattern names may
1826     consist entirely of digits. PCRE looks first for a named subpattern; if it
1827     cannot find one and the name consists entirely of digits, PCRE looks for a
1828     subpattern of that number, which must be greater than zero. Using subpattern
1829     names that consist entirely of digits is not recommended.
1830     </P>
1831     <P>
1832     Rewriting the above example to use a named subpattern gives this:
1833 nigel 91 <pre>
1834 nigel 93 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
1836     </PRE>
1837     </P>
1838     <br><b>
1839     Checking for pattern recursion
1840     </b><br>
1841     <P>
1842     If the condition is the string (R), and there is no subpattern with the name R,
1843     the condition is true if a recursive call to the whole pattern or any
1844     subpattern has been made. If digits or a name preceded by ampersand follow the
1845     letter R, for example:
1846     <pre>
1847     (?(R3)...) or (?(R&name)...)
1848 nigel 91 </pre>
1849 nigel 93 the condition is true if the most recent recursion is into the subpattern whose
1850     number or name is given. This condition does not check the entire recursion
1851     stack.
1852 nigel 63 </P>
1853     <P>
1854 ph10 454 At "top level", all these recursion test conditions are false.
1855     <a href="#recursion">Recursive patterns</a>
1856     are described below.
1857 nigel 93 </P>
1858     <br><b>
1859     Defining subpatterns for use by reference only
1860     </b><br>
1861     <P>
1862     If the condition is the string (DEFINE), and there is no subpattern with the
1863     name DEFINE, the condition is always false. In this case, there may be only one
1864     alternative in the subpattern. It is always skipped if control reaches this
1865     point in the pattern; the idea of DEFINE is that it can be used to define
1866 ph10 454 "subroutines" that can be referenced from elsewhere. (The use of
1867     <a href="#subpatternsassubroutines">"subroutines"</a>
1868 nigel 93 is described below.) For example, a pattern to match an IPv4 address could be
1869     written like this (ignore whitespace and line breaks):
1870     <pre>
1871     (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1872     \b (?&byte) (\.(?&byte)){3} \b
1873     </pre>
1874     The first part of the pattern is a DEFINE group inside which a another group
1875     named "byte" is defined. This matches an individual component of an IPv4
1876     address (a number less than 256). When matching takes place, this part of the
1877     pattern is skipped because DEFINE acts like a false condition.
1878     </P>
1879     <P>
1880     The rest of the pattern uses references to the named group to match the four
1881     dot-separated components of an IPv4 address, insisting on a word boundary at
1882     each end.
1883     </P>
1884     <br><b>
1885     Assertion conditions
1886     </b><br>
1887     <P>
1888     If the condition is not in any of the above formats, it must be an assertion.
1889 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1890     this pattern, again containing non-significant white space, and with the two
1891     alternatives on the second line:
1892     <pre>
1893     (?(?=[^a-z]*[a-z])
1894     \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1895 nigel 75 </pre>
1896 nigel 63 The condition is a positive lookahead assertion that matches an optional
1897     sequence of non-letters followed by a letter. In other words, it tests for the
1898     presence of at least one letter in the subject. If a letter is found, the
1899     subject is matched against the first alternative; otherwise it is matched
1900     against the second. This pattern matches strings in one of the two forms
1901     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1902 nigel 75 <a name="comments"></a></P>
1903 ph10 227 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
1904 nigel 63 <P>
1905 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1906 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1907     that make up a comment play no part in the pattern matching at all.
1908     </P>
1909     <P>
1910     If the PCRE_EXTENDED option is set, an unescaped # character outside a
1911 nigel 91 character class introduces a comment that continues to immediately after the
1912     next newline in the pattern.
1913     <a name="recursion"></a></P>
1914 ph10 227 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
1915 nigel 63 <P>
1916     Consider the problem of matching a string in parentheses, allowing for
1917     unlimited nested parentheses. Without the use of recursion, the best that can
1918     be done is to use a pattern that matches up to some fixed depth of nesting. It
1919 nigel 93 is not possible to handle an arbitrary nesting depth.
1920     </P>
1921     <P>
1922     For some time, Perl has provided a facility that allows regular expressions to
1923     recurse (amongst other things). It does this by interpolating Perl code in the
1924     expression at run time, and the code can refer to the expression itself. A Perl
1925     pattern using code interpolation to solve the parentheses problem can be
1926     created like this:
1927 nigel 63 <pre>
1928     $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1929 nigel 75 </pre>
1930 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1931 nigel 93 recursively to the pattern in which it appears.
1932 nigel 63 </P>
1933     <P>
1934 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1935     supports special syntax for recursion of the entire pattern, and also for
1936     individual subpattern recursion. After its introduction in PCRE and Python,
1937 ph10 453 this kind of recursion was subsequently introduced into Perl at release 5.10.
1938 nigel 63 </P>
1939     <P>
1940 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1941     closing parenthesis is a recursive call of the subpattern of the given number,
1942 ph10 454 provided that it occurs inside that subpattern. (If not, it is a
1943     <a href="#subpatternsassubroutines">"subroutine"</a>
1944 nigel 93 call, which is described in the next section.) The special item (?R) or (?0) is
1945     a recursive call of the entire regular expression.
1946 nigel 87 </P>
1947     <P>
1948     This PCRE pattern solves the nested parentheses problem (assume the
1949     PCRE_EXTENDED option is set so that white space is ignored):
1950 nigel 63 <pre>
1951     \( ( (?&#62;[^()]+) | (?R) )* \)
1952 nigel 75 </pre>
1953 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1954     substrings which can either be a sequence of non-parentheses, or a recursive
1955 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1956 nigel 63 Finally there is a closing parenthesis.
1957     </P>
1958     <P>
1959     If this were part of a larger pattern, you would not want to recurse the entire
1960     pattern, so instead you could use this:
1961     <pre>
1962     ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1963 nigel 75 </pre>
1964 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1965 ph10 172 them instead of the whole pattern.
1966     </P>
1967     <P>
1968     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1969     is made easier by the use of relative references. (A Perl 5.10 feature.)
1970     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1971     most recently opened parentheses preceding the recursion. In other words, a
1972     negative number counts capturing parentheses leftwards from the point at which
1973     it is encountered.
1974     </P>
1975     <P>
1976     It is also possible to refer to subsequently opened parentheses, by writing
1977     references such as (?+2). However, these cannot be recursive because the
1978     reference is not inside the parentheses that are referenced. They are always
1979 ph10 454 <a href="#subpatternsassubroutines">"subroutine"</a>
1980     calls, as described in the next section.
1981 ph10 172 </P>
1982     <P>
1983     An alternative approach is to use named parentheses instead. The Perl syntax
1984     for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
1985     could rewrite the above example as follows:
1986 nigel 63 <pre>
1987 nigel 93 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
1988 nigel 75 </pre>
1989 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1990 ph10 172 used.
1991     </P>
1992     <P>
1993     This particular example pattern that we have been looking at contains nested
1994     unlimited repeats, and so the use of atomic grouping for matching strings of
1995     non-parentheses is important when applying the pattern to strings that do not
1996     match. For example, when this pattern is applied to
1997 nigel 63 <pre>
1998     (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1999 nigel 75 </pre>
2000 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
2001     the match runs for a very long time indeed because there are so many different
2002     ways the + and * repeats can carve up the subject, and all have to be tested
2003     before failure can be reported.
2004     </P>
2005     <P>
2006     At the end of a match, the values set for any capturing subpatterns are those
2007     from the outermost level of the recursion at which the subpattern value is set.
2008     If you want to obtain intermediate values, a callout function can be used (see
2009 nigel 93 below and the
2010 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2011     documentation). If the pattern above is matched against
2012     <pre>
2013     (ab(cd)ef)
2014 nigel 75 </pre>
2015 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
2016     on at the top level. If additional parentheses are added, giving
2017     <pre>
2018     \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
2019     ^ ^
2020     ^ ^
2021 nigel 75 </pre>
2022 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
2023     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
2024     has to obtain extra memory to store data during a recursion, which it does by
2025     using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
2026     memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2027     </P>
2028     <P>
2029     Do not confuse the (?R) item with the condition (R), which tests for recursion.
2030     Consider this pattern, which matches text in angle brackets, allowing for
2031     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2032     recursing), whereas any characters are permitted at the outer level.
2033     <pre>
2034     &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
2035 nigel 75 </pre>
2036 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
2037     different alternatives for the recursive and non-recursive cases. The (?R) item
2038     is the actual recursive call.
2039 ph10 453 <a name="recursiondifference"></a></P>
2040     <br><b>
2041     Recursion difference from Perl
2042     </b><br>
2043     <P>
2044     In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2045     treated as an atomic group. That is, once it has matched some of the subject
2046     string, it is never re-entered, even if it contains untried alternatives and
2047     there is a subsequent matching failure. This can be illustrated by the
2048     following pattern, which purports to match a palindromic string that contains
2049     an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2050     <pre>
2051     ^(.|(.)(?1)\2)$
2052     </pre>
2053     The idea is that it either matches a single character, or two identical
2054     characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2055     it does not if the pattern is longer than three characters. Consider the
2056     subject string "abcba":
2057     </P>
2058     <P>
2059     At the top level, the first character is matched, but as it is not at the end
2060     of the string, the first alternative fails; the second alternative is taken
2061     and the recursion kicks in. The recursive call to subpattern 1 successfully
2062     matches the next character ("b"). (Note that the beginning and end of line
2063     tests are not part of the recursion).
2064     </P>
2065     <P>
2066     Back at the top level, the next character ("c") is compared with what
2067     subpattern 2 matched, which was "a". This fails. Because the recursion is
2068     treated as an atomic group, there are now no backtracking points, and so the
2069     entire match fails. (Perl is able, at this point, to re-enter the recursion and
2070     try the second alternative.) However, if the pattern is written with the
2071     alternatives in the other order, things are different:
2072     <pre>
2073     ^((.)(?1)\2|.)$
2074     </pre>
2075     This time, the recursing alternative is tried first, and continues to recurse
2076     until it runs out of characters, at which point the recursion fails. But this
2077     time we do have another alternative to try at the higher level. That is the big
2078     difference: in the previous case the remaining alternative is at a deeper
2079     recursion level, which PCRE cannot use.
2080     </P>
2081     <P>
2082     To change the pattern so that matches all palindromic strings, not just those
2083     with an odd number of characters, it is tempting to change the pattern to this:
2084     <pre>
2085     ^((.)(?1)\2|.?)$
2086     </pre>
2087     Again, this works in Perl, but not in PCRE, and for the same reason. When a
2088     deeper recursion has matched a single character, it cannot be entered again in
2089     order to match an empty string. The solution is to separate the two cases, and
2090     write out the odd and even cases as alternatives at the higher level:
2091     <pre>
2092     ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
2093     </pre>
2094     If you want to match typical palindromic phrases, the pattern has to ignore all
2095     non-word characters, which can be done like this:
2096     <pre>
2097     ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
2098     </pre>
2099     If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2100     man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2101     the use of the possessive quantifier *+ to avoid backtracking into sequences of
2102     non-word characters. Without this, PCRE takes a great deal longer (ten times or
2103     more) to match typical phrases, and Perl takes so long that you think it has
2104     gone into a loop.
2105 nigel 75 <a name="subpatternsassubroutines"></a></P>
2106 ph10 227 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
2107 nigel 63 <P>
2108     If the syntax for a recursive subpattern reference (either by number or by
2109     name) is used outside the parentheses to which it refers, it operates like a
2110 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
2111 ph10 172 before or after the reference. A numbered reference can be absolute or
2112     relative, as in these examples:
2113 nigel 63 <pre>
2114 ph10 172 (...(absolute)...)...(?2)...
2115     (...(relative)...)...(?-1)...
2116     (...(?+1)...(relative)...
2117     </pre>
2118     An earlier example pointed out that the pattern
2119     <pre>
2120 nigel 63 (sens|respons)e and \1ibility
2121 nigel 75 </pre>
2122 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
2123     "sense and responsibility". If instead the pattern
2124     <pre>
2125     (sens|respons)e and (?1)ibility
2126 nigel 75 </pre>
2127 nigel 63 is used, it does match "sense and responsibility" as well as the other two
2128 nigel 93 strings. Another example is given in the discussion of DEFINE above.
2129 nigel 63 </P>
2130 nigel 87 <P>
2131     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2132     group. That is, once it has matched some of the subject string, it is never
2133     re-entered, even if it contains untried alternatives and there is a subsequent
2134     matching failure.
2135     </P>
2136 nigel 63 <P>
2137 nigel 93 When a subpattern is used as a subroutine, processing options such as
2138     case-independence are fixed when the subpattern is defined. They cannot be
2139     changed for different calls. For example, consider this pattern:
2140     <pre>
2141 ph10 172 (abc)(?i:(?-1))
2142 nigel 93 </pre>
2143     It matches "abcabc". It does not match "abcABC" because the change of
2144     processing option does not affect the called subpattern.
2145 ph10 345 <a name="onigurumasubroutines"></a></P>
2146     <br><a name="SEC23" href="#TOC1">ONIGURUMA SUBROUTINE SYNTAX</a><br>
2147     <P>
2148     For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or
2149     a number enclosed either in angle brackets or single quotes, is an alternative
2150     syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2151     are two of the examples used above, rewritten using this syntax:
2152     <pre>
2153     (?&#60;pn&#62; \( ( (?&#62;[^()]+) | \g&#60;pn&#62; )* \) )
2154     (sens|respons)e and \g'1'ibility
2155     </pre>
2156     PCRE supports an extension to Oniguruma: if a number is preceded by a
2157     plus or a minus sign it is taken as a relative reference. For example:
2158     <pre>
2159     (abc)(?i:\g&#60;-1&#62;)
2160     </pre>
2161     Note that \g{...} (Perl syntax) and \g&#60;...&#62; (Oniguruma syntax) are <i>not</i>
2162     synonymous. The former is a back reference; the latter is a subroutine call.
2163 nigel 93 </P>
2164 ph10 345 <br><a name="SEC24" href="#TOC1">CALLOUTS</a><br>
2165 nigel 93 <P>
2166 nigel 63 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2167     code to be obeyed in the middle of matching a regular expression. This makes it
2168     possible, amongst other things, to extract different substrings that match the
2169     same pair of parentheses when there is a repetition.
2170     </P>
2171     <P>
2172     PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2173     code. The feature is called "callout". The caller of PCRE provides an external
2174     function by putting its entry point in the global variable <i>pcre_callout</i>.
2175     By default, this variable contains NULL, which disables all calling out.
2176     </P>
2177     <P>
2178     Within a regular expression, (?C) indicates the points at which the external
2179     function is to be called. If you want to identify different callout points, you
2180     can put a number less than 256 after the letter C. The default value is zero.
2181     For example, this pattern has two callout points:
2182     <pre>
2183 ph10 155 (?C1)abc(?C2)def
2184 nigel 75 </pre>
2185     If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2186     automatically installed before each item in the pattern. They are all numbered
2187     255.
2188 nigel 63 </P>
2189     <P>
2190     During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2191     set), the external function is called. It is provided with the number of the
2192 nigel 75 callout, the position in the pattern, and, optionally, one item of data
2193     originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2194     may cause matching to proceed, to backtrack, or to fail altogether. A complete
2195     description of the interface to the callout function is given in the
2196 nigel 63 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2197     documentation.
2198     </P>
2199 ph10 345 <br><a name="SEC25" href="#TOC1">BACKTRACKING CONTROL</a><br>
2200 nigel 63 <P>
2201 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2202     are described in the Perl documentation as "experimental and subject to change
2203     or removal in a future version of Perl". It goes on to say: "Their usage in
2204     production code should be noted to avoid problems during upgrades." The same
2205     remarks apply to the PCRE features described in this section.
2206     </P>
2207     <P>
2208 ph10 345 Since these verbs are specifically related to backtracking, most of them can be
2209     used only when the pattern is to be matched using <b>pcre_exec()</b>, which uses
2210     a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2211     failing negative assertion, they cause an error if encountered by
2212 ph10 211 <b>pcre_dfa_exec()</b>.
2213     </P>
2214     <P>
2215 ph10 453 If any of these verbs are used in an assertion subpattern, their effect is
2216     confined to that subpattern; it does not extend to the surrounding pattern.
2217     Note that assertion subpatterns are processed as anchored at the point where
2218     they are tested.
2219     </P>
2220     <P>
2221 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
2222     parenthesis followed by an asterisk. In Perl, they are generally of the form
2223     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2224     form is just (*VERB). Any number of these verbs may occur in a pattern. There
2225     are two kinds:
2226     </P>
2227     <br><b>
2228     Verbs that act immediately
2229     </b><br>
2230     <P>
2231     The following verbs act as soon as they are encountered:
2232     <pre>
2233     (*ACCEPT)
2234     </pre>
2235     This verb causes the match to end successfully, skipping the remainder of the
2236     pattern. When inside a recursion, only the innermost pattern is ended
2237 ph10 453 immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far
2238     is captured. (This feature was added to PCRE at release 8.00.) For example:
2239 ph10 211 <pre>
2240 ph10 453 A((?:A|B(*ACCEPT)|C)D)
2241 ph10 211 </pre>
2242 ph10 453 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2243     the outer parentheses.
2244 ph10 211 <pre>
2245     (*FAIL) or (*F)
2246     </pre>
2247     This verb causes the match to fail, forcing backtracking to occur. It is
2248     equivalent to (?!) but easier to read. The Perl documentation notes that it is
2249     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2250     Perl features that are not present in PCRE. The nearest equivalent is the
2251     callout feature, as for example in this pattern:
2252     <pre>
2253     a+(?C)(*FAIL)
2254     </pre>
2255     A match with the string "aaaa" always fails, but the callout is taken before
2256     each backtrack happens (in this example, 10 times).
2257     </P>
2258     <br><b>
2259     Verbs that act after backtracking
2260     </b><br>
2261     <P>
2262     The following verbs do nothing when they are encountered. Matching continues
2263     with what follows, but if there is no subsequent match, a failure is forced.
2264     The verbs differ in exactly what kind of failure occurs.
2265     <pre>
2266     (*COMMIT)
2267     </pre>
2268     This verb causes the whole match to fail outright if the rest of the pattern
2269     does not match. Even if the pattern is unanchored, no further attempts to find
2270     a match by advancing the start point take place. Once (*COMMIT) has been
2271     passed, <b>pcre_exec()</b> is committed to finding a match at the current
2272     starting point, or not at all. For example:
2273     <pre>
2274     a+(*COMMIT)b
2275     </pre>
2276     This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2277     dynamic anchor, or "I've started, so I must finish."
2278     <pre>
2279     (*PRUNE)
2280     </pre>
2281     This verb causes the match to fail at the current position if the rest of the
2282     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2283     advance to the next starting character then happens. Backtracking can occur as
2284     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2285     if there is no match to the right, backtracking cannot cross (*PRUNE).
2286     In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2287     group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2288     be expressed in any other way.
2289     <pre>
2290     (*SKIP)
2291     </pre>
2292     This verb is like (*PRUNE), except that if the pattern is unanchored, the
2293     "bumpalong" advance is not to the next character, but to the position in the
2294     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2295     was matched leading up to it cannot be part of a successful match. Consider:
2296     <pre>
2297     a+(*SKIP)b
2298     </pre>
2299     If the subject is "aaaac...", after the first match attempt fails (starting at
2300     the first character in the string), the starting point skips on to start the
2301     next attempt at "c". Note that a possessive quantifer does not have the same
2302     effect in this example; although it would suppress backtracking during the
2303     first match attempt, the second attempt would start at the second character
2304     instead of skipping on to "c".
2305     <pre>
2306     (*THEN)
2307     </pre>
2308     This verb causes a skip to the next alternation if the rest of the pattern does
2309     not match. That is, it cancels pending backtracking, but only within the
2310     current alternation. Its name comes from the observation that it can be used
2311     for a pattern-based if-then-else block:
2312     <pre>
2313     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2314     </pre>
2315     If the COND1 pattern matches, FOO is tried (and possibly further items after
2316     the end of the group if FOO succeeds); on failure the matcher skips to the
2317     second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2318     is used outside of any alternation, it acts exactly like (*PRUNE).
2319     </P>
2320 ph10 345 <br><a name="SEC26" href="#TOC1">SEE ALSO</a><br>
2321 ph10 211 <P>
2322 nigel 93 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
2323     </P>
2324 ph10 345 <br><a name="SEC27" href="#TOC1">AUTHOR</a><br>
2325 nigel 93 <P>
2326 ph10 99 Philip Hazel
2327 nigel 63 <br>
2328 ph10 99 University Computing Service
2329     <br>
2330     Cambridge CB2 3QH, England.
2331     <br>
2332     </P>
2333 ph10 345 <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2334 ph10 99 <P>
2335 ph10 454 Last updated: 22 September 2009
2336 ph10 99 <br>
2337 ph10 392 Copyright &copy; 1997-2009 University of Cambridge.
2338 ph10 99 <br>
2339 nigel 75 <p>
2340     Return to the <a href="index.html">PCRE index page</a>.
2341     </p>


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

ViewVC Help
Powered by ViewVC 1.1.12