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# Line 3  Line 3 
3  <title>pcrepattern specification</title>  <title>pcrepattern specification</title>
4  </head>  </head>
5  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6  This HTML document has been generated automatically from the original man page.  <h1>pcrepattern man page</h1>
7  If there is any nonsense in it, please consult the man page, in case the  <p>
8  conversion went wrong.<br>  Return to the <a href="index.html">PCRE index page</a>.
9    </p>
10    <p>
11    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    <br>
15  <ul>  <ul>
16  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>  <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17  <li><a name="TOC2" href="#SEC2">BACKSLASH</a>  <li><a name="TOC2" href="#SEC2">BACKSLASH</a>
18  <li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a>  <li><a name="TOC3" href="#SEC3">CIRCUMFLEX AND DOLLAR</a>
19  <li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a>  <li><a name="TOC4" href="#SEC4">FULL STOP (PERIOD, DOT)</a>
20  <li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a>  <li><a name="TOC5" href="#SEC5">MATCHING A SINGLE BYTE</a>
21  <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS</a>  <li><a name="TOC6" href="#SEC6">SQUARE BRACKETS AND CHARACTER CLASSES</a>
22  <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>  <li><a name="TOC7" href="#SEC7">POSIX CHARACTER CLASSES</a>
23  <li><a name="TOC8" href="#SEC8">VERTICAL BAR</a>  <li><a name="TOC8" href="#SEC8">VERTICAL BAR</a>
24  <li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a>  <li><a name="TOC9" href="#SEC9">INTERNAL OPTION SETTING</a>
# Line 32  conversion went wrong.<br> Line 38  conversion went wrong.<br>
38  <P>  <P>
39  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions supported by PCRE are
40  described below. Regular expressions are also described in the Perl  described below. Regular expressions are also described in the Perl
41  documentation and in a number of other books, some of which have copious  documentation and in a number of books, some of which have copious examples.
42  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
43  O'Reilly, covers them in great detail. The description here is intended as  regular expressions in great detail. This description of PCRE's regular
44  reference documentation.  expressions is intended as reference material.
45  </P>  </P>
46  <P>  <P>
47  The basic operation of PCRE is on strings of bytes. However, there is also  The original operation of PCRE was on strings of one-byte characters. However,
48  support for UTF-8 character strings. To use this support you must build PCRE to  there is now also support for UTF-8 character strings. To use this, you must
49  include UTF-8 support, and then call <b>pcre_compile()</b> with the PCRE_UTF8  build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
50  option. How this affects the pattern matching is mentioned in several places  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
51  below. There is also a summary of UTF-8 features in the  places below. There is also a summary of UTF-8 features in the
52  <a href="pcre.html#utf8support">section on UTF-8 support</a>  <a href="pcre.html#utf8support">section on UTF-8 support</a>
53  in the main  in the main
54  <a href="pcre.html"><b>pcre</b></a>  <a href="pcre.html"><b>pcre</b></a>
# Line 52  page. Line 58  page.
58  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
59  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
60  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
 </P>  
 <P>  
61  <pre>  <pre>
62    The quick brown fox    The quick brown fox
63  </PRE>  </pre>
 </P>  
 <P>  
64  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. The power of
65  regular expressions comes from the ability to include alternatives and  regular expressions comes from the ability to include alternatives and
66  repetitions in the pattern. These are encoded in the pattern by the use of  repetitions in the pattern. These are encoded in the pattern by the use of
67  <i>meta-characters</i>, which do not stand for themselves but instead are  <i>metacharacters</i>, which do not stand for themselves but instead are
68  interpreted in some special way.  interpreted in some special way.
69  </P>  </P>
70  <P>  <P>
71  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
72  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
73  recognized in square brackets. Outside square brackets, the meta-characters are  recognized in square brackets. Outside square brackets, the metacharacters are
74  as follows:  as follows:
 </P>  
 <P>  
75  <pre>  <pre>
76    \      general escape character with several uses    \      general escape character with several uses
77    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 88  as follows: Line 88  as follows:
88    +      1 or more quantifier    +      1 or more quantifier
89           also "possessive quantifier"           also "possessive quantifier"
90    {      start min/max quantifier    {      start min/max quantifier
91  </PRE>  </pre>
 </P>  
 <P>  
92  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
93  a character class the only meta-characters are:  a character class the only metacharacters are:
 </P>  
 <P>  
94  <pre>  <pre>
95    \      general escape character    \      general escape character
96    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
97    -      indicates character range    -      indicates character range
98    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX syntax)
            syntax)  
99    ]      terminates the character class    ]      terminates the character class
100  </PRE>  </pre>
101  </P>  The following sections describe the use of each of the metacharacters.
 <P>  
 The following sections describe the use of each of the meta-characters.  
102  </P>  </P>
103  <br><a name="SEC2" href="#TOC1">BACKSLASH</a><br>  <br><a name="SEC2" href="#TOC1">BACKSLASH</a><br>
104  <P>  <P>
105  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
106  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character may
107  have. This use of backslash as an escape character applies both inside and  have. This use of backslash as an escape character applies both inside and
108  outside character classes.  outside character classes.
109  </P>  </P>
110  <P>  <P>
111  For example, if you want to match a * character, you write \* in the pattern.  For example, if you want to match a * character, you write \* in the pattern.
112  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
113  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
114  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
115  particular, if you want to match a backslash, you write \\.  particular, if you want to match a backslash, you write \\.
116  </P>  </P>
117  <P>  <P>
# Line 133  If you want to remove the special meanin Line 126  If you want to remove the special meanin
126  can do so by putting them between \Q and \E. This is different from Perl in  can do so by putting them between \Q and \E. This is different from Perl in
127  that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
128  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
 </P>  
 <P>  
129  <pre>  <pre>
130    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
131  </PRE>  
132  </P>    \Qabc$xyz\E        abc$xyz        abc followed by the contents of $xyz
 <P>  
 <pre>  
   \Qabc$xyz\E        abc$xyz        abc followed by the  
                                       contents of $xyz  
133    \Qabc\$xyz\E       abc\$xyz       abc\$xyz    \Qabc\$xyz\E       abc\$xyz       abc\$xyz
134    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
135  </PRE>  </pre>
 </P>  
 <P>  
136  The \Q...\E sequence is recognized both inside and outside character classes.  The \Q...\E sequence is recognized both inside and outside character classes.
137  </P>  <a name="digitsafterbackslash"></a></P>
138    <br><b>
139    Non-printing characters
140    </b><br>
141  <P>  <P>
142  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
143  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
# Line 157  non-printing characters, apart from the Line 145  non-printing characters, apart from the
145  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is usually easier to
146  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
147  represents:  represents:
 </P>  
 <P>  
148  <pre>  <pre>
149    \a        alarm, that is, the BEL character (hex 07)    \a        alarm, that is, the BEL character (hex 07)
150    \cx       "control-x", where x is any character    \cx       "control-x", where x is any character
# Line 170  represents: Line 156  represents:
156    \ddd      character with octal code ddd, or backreference    \ddd      character with octal code ddd, or backreference
157    \xhh      character with hex code hh    \xhh      character with hex code hh
158    \x{hhh..} character with hex code hhh... (UTF-8 mode only)    \x{hhh..} character with hex code hhh... (UTF-8 mode only)
159  </PRE>  </pre>
 </P>  
 <P>  
160  The precise effect of \cx is as follows: if x is a lower case letter, it  The precise effect of \cx is as follows: if x is a lower case letter, it
161  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
162  Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex  Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
# Line 186  than 2**31 (that is, the maximum hexadec Line 170  than 2**31 (that is, the maximum hexadec
170  other than hexadecimal digits appear between \x{ and }, or if there is no  other than hexadecimal digits appear between \x{ and }, or if there is no
171  terminating }, this form of escape is not recognized. Instead, the initial  terminating }, this form of escape is not recognized. Instead, the initial
172  \x will be interpreted as a basic hexadecimal escape, with no following  \x will be interpreted as a basic hexadecimal escape, with no following
173  digits, giving a byte whose value is zero.  digits, giving a character whose value is zero.
174  </P>  </P>
175  <P>  <P>
176  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
# Line 198  After \0 up to two further octal digits Line 182  After \0 up to two further octal digits
182  are fewer than two digits, just those that are present are used. Thus the  are fewer than two digits, just those that are present are used. Thus the
183  sequence \0\x\07 specifies two binary zeros followed by a BEL character  sequence \0\x\07 specifies two binary zeros followed by a BEL character
184  (code value 7). Make sure you supply two digits after the initial zero if the  (code value 7). Make sure you supply two digits after the initial zero if the
185  character that follows is itself an octal digit.  pattern character that follows is itself an octal digit.
186  </P>  </P>
187  <P>  <P>
188  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
# Line 206  Outside a character class, PCRE reads it Line 190  Outside a character class, PCRE reads it
190  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
191  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
192  taken as a <i>back reference</i>. A description of how this works is given  taken as a <i>back reference</i>. A description of how this works is given
193  later, following the discussion of parenthesized subpatterns.  <a href="#backreferences">later,</a>
194    following the discussion of
195    <a href="#subpattern">parenthesized subpatterns.</a>
196  </P>  </P>
197  <P>  <P>
198  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
# Line 214  have not been that many capturing subpat Line 200  have not been that many capturing subpat
200  digits following the backslash, and generates a single byte from the least  digits following the backslash, and generates a single byte from the least
201  significant 8 bits of the value. Any subsequent digits stand for themselves.  significant 8 bits of the value. Any subsequent digits stand for themselves.
202  For example:  For example:
 </P>  
 <P>  
203  <pre>  <pre>
204    \040   is another way of writing a space    \040   is another way of writing a space
205    \40    is the same, provided there are fewer than 40    \40    is the same, provided there are fewer than 40 previous capturing subpatterns
             previous capturing subpatterns  
206    \7     is always a back reference    \7     is always a back reference
207    \11    might be a back reference, or another way of    \11    might be a back reference, or another way of writing a tab
             writing a tab  
208    \011   is always a tab    \011   is always a tab
209    \0113  is a tab followed by the character "3"    \0113  is a tab followed by the character "3"
210    \113   might be a back reference, otherwise the    \113   might be a back reference, otherwise the character with octal code 113
211              character with octal code 113    \377   might be a back reference, otherwise the byte consisting entirely of 1 bits
212    \377   might be a back reference, otherwise    \81    is either a back reference, or a binary zero followed by the two characters "8" and "1"
213              the byte consisting entirely of 1 bits  </pre>
   \81    is either a back reference, or a binary zero  
             followed by the two characters "8" and "1"  
 </PRE>  
 </P>  
 <P>  
214  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
215  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
216  </P>  </P>
# Line 241  zero, because no more than three octal d Line 218  zero, because no more than three octal d
218  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single byte value or a single UTF-8 character
219  (in UTF-8 mode) can be used both inside and outside character classes. In  (in UTF-8 mode) can be used both inside and outside character classes. In
220  addition, inside a character class, the sequence \b is interpreted as the  addition, inside a character class, the sequence \b is interpreted as the
221  backspace character (hex 08). Outside a character class it has a different  backspace character (hex 08), and the sequence \X is interpreted as the
222  meaning (see below).  character "X". Outside a character class, these sequences have different
223  </P>  meanings
224  <P>  <a href="#uniextseq">(see below).</a>
225  The third use of backslash is for specifying generic character types:  </P>
226  </P>  <br><b>
227    Generic character types
228    </b><br>
229  <P>  <P>
230    The third use of backslash is for specifying generic character types. The
231    following are always recognized:
232  <pre>  <pre>
233    \d     any decimal digit    \d     any decimal digit
234    \D     any character that is not a decimal digit    \D     any character that is not a decimal digit
# Line 255  The third use of backslash is for specif Line 236  The third use of backslash is for specif
236    \S     any character that is not a whitespace character    \S     any character that is not a whitespace character
237    \w     any "word" character    \w     any "word" character
238    \W     any "non-word" character    \W     any "non-word" character
239  </PRE>  </pre>
 </P>  
 <P>  
240  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
241  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
242  </P>  </P>
243  <P>  <P>
244  In UTF-8 mode, characters with values greater than 255 never match \d, \s, or  These character type sequences can appear both inside and outside character
245  \w, and always match \D, \S, and \W.  classes. They each match one character of the appropriate type. If the current
246    matching point is at the end of the subject string, all of them fail, since
247    there is no character to match.
248  </P>  </P>
249  <P>  <P>
250  For compatibility with Perl, \s does not match the VT character (code 11).  For compatibility with Perl, \s does not match the VT character (code 11).
# Line 271  This makes it different from the the POS Line 252  This makes it different from the the POS
252  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32).
253  </P>  </P>
254  <P>  <P>
255  A "word" character is any letter or digit or the underscore character, that is,  A "word" character is an underscore or any character less than 256 that is a
256  any character which can be part of a Perl "word". The definition of letters and  letter or digit. The definition of letters and digits is controlled by PCRE's
257  digits is controlled by PCRE's character tables, and may vary if locale-  low-valued character tables, and may vary if locale-specific matching is taking
258  specific matching is taking place (see  place (see
259  <a href="pcreapi.html#localesupport">"Locale support"</a>  <a href="pcreapi.html#localesupport">"Locale support"</a>
260  in the  in the
261  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
262  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in the "fr_FR" (French) locale, some character codes
263  than 128 are used for accented letters, and these are matched by \w.  greater than 128 are used for accented letters, and these are matched by \w.
264  </P>  </P>
265  <P>  <P>
266  These character type sequences can appear both inside and outside character  In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
267  classes. They each match one character of the appropriate type. If the current  \w, and always match \D, \S, and \W. This is true even when Unicode
268  matching point is at the end of the subject string, all of them fail, since  character property support is available.
269  there is no character to match.  <a name="uniextseq"></a></P>
270  </P>  <br><b>
271    Unicode character properties
272    </b><br>
273    <P>
274    When PCRE is built with Unicode character property support, three additional
275    escape sequences to match generic character types are available when UTF-8 mode
276    is selected. They are:
277    <pre>
278     \p{<i>xx</i>}   a character with the <i>xx</i> property
279     \P{<i>xx</i>}   a character without the <i>xx</i> property
280     \X       an extended Unicode sequence
281    </pre>
282    The property names represented by <i>xx</i> above are limited to the
283    Unicode general category properties. Each character has exactly one such
284    property, specified by a two-letter abbreviation. For compatibility with Perl,
285    negation can be specified by including a circumflex between the opening brace
286    and the property name. For example, \p{^Lu} is the same as \P{Lu}.
287    </P>
288    <P>
289    If only one letter is specified with \p or \P, it includes all the properties
290    that start with that letter. In this case, in the absence of negation, the
291    curly brackets in the escape sequence are optional; these two examples have
292    the same effect:
293    <pre>
294      \p{L}
295      \pL
296    </pre>
297    The following property codes are supported:
298    <pre>
299      C     Other
300      Cc    Control
301      Cf    Format
302      Cn    Unassigned
303      Co    Private use
304      Cs    Surrogate
305    
306      L     Letter
307      Ll    Lower case letter
308      Lm    Modifier letter
309      Lo    Other letter
310      Lt    Title case letter
311      Lu    Upper case letter
312    
313      M     Mark
314      Mc    Spacing mark
315      Me    Enclosing mark
316      Mn    Non-spacing mark
317    
318      N     Number
319      Nd    Decimal number
320      Nl    Letter number
321      No    Other number
322    
323      P     Punctuation
324      Pc    Connector punctuation
325      Pd    Dash punctuation
326      Pe    Close punctuation
327      Pf    Final punctuation
328      Pi    Initial punctuation
329      Po    Other punctuation
330      Ps    Open punctuation
331    
332      S     Symbol
333      Sc    Currency symbol
334      Sk    Modifier symbol
335      Sm    Mathematical symbol
336      So    Other symbol
337    
338      Z     Separator
339      Zl    Line separator
340      Zp    Paragraph separator
341      Zs    Space separator
342    </pre>
343    Extended properties such as "Greek" or "InMusicalSymbols" are not supported by
344    PCRE.
345    </P>
346    <P>
347    Specifying caseless matching does not affect these escape sequences. For
348    example, \p{Lu} always matches only upper case letters.
349    </P>
350    <P>
351    The \X escape matches any number of Unicode characters that form an extended
352    Unicode sequence. \X is equivalent to
353    <pre>
354      (?&#62;\PM\pM*)
355    </pre>
356    That is, it matches a character without the "mark" property, followed by zero
357    or more characters with the "mark" property, and treats the sequence as an
358    atomic group
359    <a href="#atomicgroup">(see below).</a>
360    Characters with the "mark" property are typically accents that affect the
361    preceding character.
362    </P>
363    <P>
364    Matching characters by Unicode property is not fast, because PCRE has to search
365    a structure that contains data for over fifteen thousand characters. That is
366    why the traditional escape sequences such as \d and \w do not use Unicode
367    properties in PCRE.
368    <a name="smallassertions"></a></P>
369    <br><b>
370    Simple assertions
371    </b><br>
372  <P>  <P>
373  The fourth use of backslash is for certain simple assertions. An assertion  The fourth use of backslash is for certain simple assertions. An assertion
374  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
375  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
376  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
377  assertions are  <a href="#bigassertions">below.</a>
378  </P>  The backslashed
379  <P>  assertions are:
380  <pre>  <pre>
381    \b     matches at a word boundary    \b     matches at a word boundary
382    \B     matches when not at a word boundary    \B     matches when not at a word boundary
# Line 302  assertions are Line 384  assertions are
384    \Z     matches at end of subject or before newline at end    \Z     matches at end of subject or before newline at end
385    \z     matches at end of subject    \z     matches at end of subject
386    \G     matches at first matching position in subject    \G     matches at first matching position in subject
387  </PRE>  </pre>
 </P>  
 <P>  
388  These assertions may not appear in character classes (but note that \b has a  These assertions may not appear in character classes (but note that \b has a
389  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
390  </P>  </P>
# Line 316  first or last character matches \w, resp Line 396  first or last character matches \w, resp
396  </P>  </P>
397  <P>  <P>
398  The \A, \Z, and \z assertions differ from the traditional circumflex and  The \A, \Z, and \z assertions differ from the traditional circumflex and
399  dollar (described below) in that they only ever match at the very start and end  dollar (described in the next section) in that they only ever match at the very
400  of the subject string, whatever options are set. Thus, they are independent of  start and end of the subject string, whatever options are set. Thus, they are
401  multiline mode.  independent of multiline mode. These three assertions are not affected by the
402  </P>  PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
403  <P>  circumflex and dollar metacharacters. However, if the <i>startoffset</i>
404  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
405  <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero, indicating that  at a point other than the beginning of the subject, \A can never match. The
406  matching is to start at a point other than the beginning of the subject, \A  difference between \Z and \z is that \Z matches before a newline that is the
407  can never match. The difference between \Z and \z is that \Z matches before  last character of the string as well as at the end of the string, whereas \z
408  a newline that is the last character of the string as well as at the end of the  matches only at the end.
 string, whereas \z matches only at the end.  
409  </P>  </P>
410  <P>  <P>
411  The \G assertion is true only when the current matching position is at the  The \G assertion is true only when the current matching position is at the
# Line 351  regular expression. Line 430  regular expression.
430  <br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>  <br><a name="SEC3" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
431  <P>  <P>
432  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
433  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is
434  at the start of the subject string. If the <i>startoffset</i> argument of  at the start of the subject string. If the <i>startoffset</i> argument of
435  <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE  <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
436  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
437  meaning (see below).  meaning
438    <a href="#characterclass">(see below).</a>
439  </P>  </P>
440  <P>  <P>
441  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
# Line 367  constrained to match only at the start o Line 447  constrained to match only at the start o
447  to be anchored.)  to be anchored.)
448  </P>  </P>
449  <P>  <P>
450  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
451  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline
452  character that is the last character in the string (by default). Dollar need  character that is the last character in the string (by default). Dollar need
453  not be the last character of the pattern if a number of alternatives are  not be the last character of the pattern if a number of alternatives are
# Line 384  The meanings of the circumflex and dolla Line 464  The meanings of the circumflex and dolla
464  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, they match immediately
465  after and immediately before an internal newline character, respectively, in  after and immediately before an internal newline character, respectively, in
466  addition to matching at the start and end of the subject string. For example,  addition to matching at the start and end of the subject string. For example,
467  the pattern /^abc$/ matches the subject string "def\nabc" in multiline mode,  the pattern /^abc$/ matches the subject string "def\nabc" (where \n
468  but not otherwise. Consequently, patterns that are anchored in single line mode  represents a newline character) in multiline mode, but not otherwise.
469  because all branches start with ^ are not anchored in multiline mode, and a  Consequently, patterns that are anchored in single line mode because all
470  match for circumflex is possible when the <i>startoffset</i> argument of  branches start with ^ are not anchored in multiline mode, and a match for
471  <b>pcre_exec()</b> is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  circumflex is possible when the <i>startoffset</i> argument of <b>pcre_exec()</b>
472  PCRE_MULTILINE is set.  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is
473    set.
474  </P>  </P>
475  <P>  <P>
476  Note that the sequences \A, \Z, and \z can be used to match the start and  Note that the sequences \A, \Z, and \z can be used to match the start and
# Line 401  end of the subject in both modes, and if Line 482  end of the subject in both modes, and if
482  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
483  the subject, including a non-printing character, but not (by default) newline.  the subject, including a non-printing character, but not (by default) newline.
484  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one
485  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  byte long, except (by default) newline. If the PCRE_DOTALL option is set,
486  dots match newlines as well. The handling of dot is entirely independent of the  dots match newlines as well. The handling of dot is entirely independent of the
487  handling of circumflex and dollar, the only relationship being that they both  handling of circumflex and dollar, the only relationship being that they both
488  involve newline characters. Dot has no special meaning in a character class.  involve newline characters. Dot has no special meaning in a character class.
# Line 409  involve newline characters. Dot has no s Line 490  involve newline characters. Dot has no s
490  <br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br>  <br><a name="SEC5" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
491  <P>  <P>
492  Outside a character class, the escape sequence \C matches any one byte, both  Outside a character class, the escape sequence \C matches any one byte, both
493  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is
494  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  provided in Perl in order to match individual bytes in UTF-8 mode. Because it
495  Because it breaks up UTF-8 characters into individual bytes, what remains in  breaks up UTF-8 characters into individual bytes, what remains in the string
496  the string may be a malformed UTF-8 string. For this reason it is best avoided.  may be a malformed UTF-8 string. For this reason, the \C escape sequence is
497    best avoided.
498  </P>  </P>
499  <P>  <P>
500  PCRE does not allow \C to appear in lookbehind assertions (see below), because  PCRE does not allow \C to appear in lookbehind assertions
501  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  <a href="#lookbehind">(described below),</a>
502  </P>  because in UTF-8 mode this would make it impossible to calculate the length of
503  <br><a name="SEC6" href="#TOC1">SQUARE BRACKETS</a><br>  the lookbehind.
504    <a name="characterclass"></a></P>
505    <br><a name="SEC6" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
506  <P>  <P>
507  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
508  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special. If a
# Line 438  backslash. Line 522  backslash.
522  <P>  <P>
523  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
524  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
525  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
526  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
527  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion: it still consumes a character from the subject
528  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
529    string.
530  </P>  </P>
531  <P>  <P>
532  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 mode, characters with values greater than 255 can be included in a
# Line 451  class as a literal string of bytes, or b Line 536  class as a literal string of bytes, or b
536  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
537  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
538  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
539  caseful version would. PCRE does not support the concept of case for characters  caseful version would. When running in UTF-8 mode, PCRE supports the concept of
540  with values greater than 255.  case for characters with values greater than 128 only when it is compiled with
541    Unicode property support.
542  </P>  </P>
543  <P>  <P>
544  The newline character is never treated in any special way in character classes,  The newline character is never treated in any special way in character classes,
# Line 471  It is not possible to have the literal c Line 557  It is not possible to have the literal c
557  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
558  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
559  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
560  the end of range, so [W-\]46] is interpreted as a single class containing a  the end of range, so [W-\]46] is interpreted as a class containing a range
561  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
562  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
563  </P>  </P>
564  <P>  <P>
565  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
# Line 484  example [\x{100}-\x{2ff}]. Line 570  example [\x{100}-\x{2ff}].
570  <P>  <P>
571  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
572  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
573  [][\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
574  locale are in use, [\xc8-\xcb] matches accented E characters in both cases.  tables for the "fr_FR" locale are in use, [\xc8-\xcb] matches accented E
575    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
576    characters with values greater than 128 only when it is compiled with Unicode
577    property support.
578  </P>  </P>
579  <P>  <P>
580  The character types \d, \D, \s, \S, \w, and \W may also appear in a  The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
581  character class, and add the characters that they match to the class. For  in a character class, and add the characters that they match to the class. For
582  example, [\dABCDEF] matches any hexadecimal digit. A circumflex can  example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
583  conveniently be used with the upper case character types to specify a more  conveniently be used with the upper case character types to specify a more
584  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
585  the class [^\W_] matches any letter or digit, but not underscore.  the class [^\W_] matches any letter or digit, but not underscore.
586  </P>  </P>
587  <P>  <P>
588  All non-alphameric characters other than \, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
589  terminating ] are non-special in character classes, but it does no harm if they  hyphen (only where it can be interpreted as specifying a range), circumflex
590  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
591    introducing a POSIX class name - see the next section), and the terminating
592    closing square bracket. However, escaping other non-alphanumeric characters
593    does no harm.
594  </P>  </P>
595  <br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br>  <br><a name="SEC7" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
596  <P>  <P>
597  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
598  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
599  this notation. For example,  this notation. For example,
 </P>  
 <P>  
600  <pre>  <pre>
601    [01[:alpha:]%]    [01[:alpha:]%]
602  </PRE>  </pre>
 </P>  
 <P>  
603  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
604  are  are
 </P>  
 <P>  
605  <pre>  <pre>
606    alnum    letters and digits    alnum    letters and digits
607    alpha    letters    alpha    letters
# Line 531  are Line 617  are
617    upper    upper case letters    upper    upper case letters
618    word     "word" characters (same as \w)    word     "word" characters (same as \w)
619    xdigit   hexadecimal digits    xdigit   hexadecimal digits
620  </PRE>  </pre>
 </P>  
 <P>  
621  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
622  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
623  makes "space" different to \s, which does not include VT (for Perl  makes "space" different to \s, which does not include VT (for Perl
# Line 543  compatibility). Line 627  compatibility).
627  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
628  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
629  after the colon. For example,  after the colon. For example,
 </P>  
 <P>  
630  <pre>  <pre>
631    [12[:^digit:]]    [12[:^digit:]]
632  </PRE>  </pre>
 </P>  
 <P>  
633  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
634  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
635  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
636  </P>  </P>
637  <P>  <P>
638  In UTF-8 mode, characters with values greater than 255 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
639  the POSIX character classes.  the POSIX character classes.
640  </P>  </P>
641  <br><a name="SEC8" href="#TOC1">VERTICAL BAR</a><br>  <br><a name="SEC8" href="#TOC1">VERTICAL BAR</a><br>
642  <P>  <P>
643  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
644  the pattern  the pattern
 </P>  
 <P>  
645  <pre>  <pre>
646    gilbert|sullivan    gilbert|sullivan
647  </PRE>  </pre>
 </P>  
 <P>  
648  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
649  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string).
650  The matching process tries each alternative in turn, from left to right,  The matching process tries each alternative in turn, from left to right,
651  and the first one that succeeds is used. If the alternatives are within a  and the first one that succeeds is used. If the alternatives are within a
652  subpattern (defined below), "succeeds" means matching the rest of the main  subpattern
653  pattern as well as the alternative in the subpattern.  <a href="#subpattern">(defined below),</a>
654    "succeeds" means matching the rest of the main pattern as well as the
655    alternative in the subpattern.
656  </P>  </P>
657  <br><a name="SEC9" href="#TOC1">INTERNAL OPTION SETTING</a><br>  <br><a name="SEC9" href="#TOC1">INTERNAL OPTION SETTING</a><br>
658  <P>  <P>
659  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
660  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
661  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
 </P>  
 <P>  
662  <pre>  <pre>
663    i  for PCRE_CASELESS    i  for PCRE_CASELESS
664    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
665    s  for PCRE_DOTALL    s  for PCRE_DOTALL
666    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
667  </PRE>  </pre>
 </P>  
 <P>  
668  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
669  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
670  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
# Line 608  the global options (and it will therefor Line 682  the global options (and it will therefor
682  <P>  <P>
683  An option change within a subpattern affects only that part of the current  An option change within a subpattern affects only that part of the current
684  pattern that follows it, so  pattern that follows it, so
 </P>  
 <P>  
685  <pre>  <pre>
686    (a(?i)b)c    (a(?i)b)c
687  </PRE>  </pre>
 </P>  
 <P>  
688  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
689  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
690  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
691  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
 </P>  
 <P>  
692  <pre>  <pre>
693    (a(?i)b|c)    (a(?i)b|c)
694  </PRE>  </pre>
 </P>  
 <P>  
695  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
696  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
697  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
# Line 636  The PCRE-specific options PCRE_UNGREEDY Line 702  The PCRE-specific options PCRE_UNGREEDY
702  same way as the Perl-compatible options by using the characters U and X  same way as the Perl-compatible options by using the characters U and X
703  respectively. The (?X) flag setting is special in that it must always occur  respectively. The (?X) flag setting is special in that it must always occur
704  earlier in the pattern than any of the additional features it turns on, even  earlier in the pattern than any of the additional features it turns on, even
705  when it is at top level. It is best put at the start.  when it is at top level. It is best to put it at the start.
706  </P>  <a name="subpattern"></a></P>
707  <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>  <br><a name="SEC10" href="#TOC1">SUBPATTERNS</a><br>
708  <P>  <P>
709  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
710  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
711  </P>  <br>
712  <P>  <br>
713  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
 </P>  
 <P>  
714  <pre>  <pre>
715    cat(aract|erpillar|)    cat(aract|erpillar|)
716  </PRE>  </pre>
 </P>  
 <P>  
717  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
718  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or the empty string.
719  </P>  <br>
720  <P>  <br>
721  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
722  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
723  the subpattern is passed back to the caller via the <i>ovector</i> argument of  subpattern is passed back to the caller via the <i>ovector</i> argument of
724  <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting  <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
725  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
726  </P>  </P>
727  <P>  <P>
728  For example, if the string "the red king" is matched against the pattern  For example, if the string "the red king" is matched against the pattern
 </P>  
 <P>  
729  <pre>  <pre>
730    the ((red|white) (king|queen))    the ((red|white) (king|queen))
731  </PRE>  </pre>
 </P>  
 <P>  
732  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
733  2, and 3, respectively.  2, and 3, respectively.
734  </P>  </P>
# Line 681  capturing requirement. If an opening par Line 739  capturing requirement. If an opening par
739  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
740  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
741  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
 </P>  
 <P>  
742  <pre>  <pre>
743    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
744  </PRE>  </pre>
 </P>  
 <P>  
745  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
746  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535, and the maximum depth
747  of nesting of all subpatterns, both capturing and non-capturing, is 200.  of nesting of all subpatterns, both capturing and non-capturing, is 200.
# Line 696  of nesting of all subpatterns, both capt Line 750  of nesting of all subpatterns, both capt
750  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
751  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
752  the ":". Thus the two patterns  the ":". Thus the two patterns
 </P>  
 <P>  
753  <pre>  <pre>
754    (?i:saturday|sunday)    (?i:saturday|sunday)
755    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
756  </PRE>  </pre>
 </P>  
 <P>  
757  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
758  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
759  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
# Line 713  the above patterns match "SUNDAY" as wel Line 763  the above patterns match "SUNDAY" as wel
763  <P>  <P>
764  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
765  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
766  if an expression is modified, the numbers may change. To help with the  if an expression is modified, the numbers may change. To help with this
767  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns, something that Perl does
768  not provide. The Python syntax (?P&#60;name&#62;...) is used. Names consist of  not provide. The Python syntax (?P&#60;name&#62;...) is used. Names consist of
769  alphanumeric characters and underscores, and must be unique within a pattern.  alphanumeric characters and underscores, and must be unique within a pattern.
# Line 721  alphanumeric characters and underscores, Line 771  alphanumeric characters and underscores,
771  <P>  <P>
772  Named capturing parentheses are still allocated numbers as well as names. The  Named capturing parentheses are still allocated numbers as well as names. The
773  PCRE API provides function calls for extracting the name-to-number translation  PCRE API provides function calls for extracting the name-to-number translation
774  table from a compiled pattern. For further details see the  table from a compiled pattern. There is also a convenience function for
775    extracting a captured substring by name. For further details see the
776  <a href="pcreapi.html"><b>pcreapi</b></a>  <a href="pcreapi.html"><b>pcreapi</b></a>
777  documentation.  documentation.
778  </P>  </P>
# Line 729  documentation. Line 780  documentation.
780  <P>  <P>
781  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
782  items:  items:
 </P>  
 <P>  
783  <pre>  <pre>
784    a literal data character    a literal data character
785    the . metacharacter    the . metacharacter
786    the \C escape sequence    the \C escape sequence
787    escapes such as \d that match single characters    the \X escape sequence (in UTF-8 mode with Unicode properties)
788      an escape such as \d that matches a single character
789    a character class    a character class
790    a back reference (see next section)    a back reference (see next section)
791    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
792  </PRE>  </pre>
 </P>  
 <P>  
793  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
794  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
795  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
796  be less than or equal to the second. For example:  be less than or equal to the second. For example:
 </P>  
 <P>  
797  <pre>  <pre>
798    z{2,4}    z{2,4}
799  </PRE>  </pre>
 </P>  
 <P>  
800  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
801  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
802  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
803  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
 </P>  
 <P>  
804  <pre>  <pre>
805    [aeiou]{3,}    [aeiou]{3,}
806  </PRE>  </pre>
 </P>  
 <P>  
807  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
 </P>  
 <P>  
808  <pre>  <pre>
809    \d{8}    \d{8}
810  </PRE>  </pre>
 </P>  
 <P>  
811  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
812  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
813  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
# Line 780  quantifier, but a literal string of four Line 816  quantifier, but a literal string of four
816  <P>  <P>
817  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
818  bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of  bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
819  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
820    support is available, \X{3} matches three Unicode extended sequences, each of
821    which may be several bytes long (and they may be of different lengths).
822  </P>  </P>
823  <P>  <P>
824  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
# Line 789  previous item and the quantifier were no Line 827  previous item and the quantifier were no
827  <P>  <P>
828  For convenience (and historical compatibility) the three most common  For convenience (and historical compatibility) the three most common
829  quantifiers have single-character abbreviations:  quantifiers have single-character abbreviations:
 </P>  
 <P>  
830  <pre>  <pre>
831    *    is equivalent to {0,}    *    is equivalent to {0,}
832    +    is equivalent to {1,}    +    is equivalent to {1,}
833    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
834  </PRE>  </pre>
 </P>  
 <P>  
835  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
836  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
 </P>  
 <P>  
837  <pre>  <pre>
838    (a?)*    (a?)*
839  </PRE>  </pre>
 </P>  
 <P>  
840  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
841  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
842  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
# Line 816  match no characters, the loop is forcibl Line 846  match no characters, the loop is forcibl
846  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
847  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
848  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
849  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
850  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
851  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
 </P>  
 <P>  
852  <pre>  <pre>
853    /\*.*\*/    /\*.*\*/
854  </PRE>  </pre>
 </P>  
 <P>  
855  to the string  to the string
 </P>  
 <P>  
856  <pre>  <pre>
857    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
858  </PRE>  </pre>
 </P>  
 <P>  
859  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
860  item.  item.
861  </P>  </P>
# Line 841  item. Line 863  item.
863  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
864  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
865  pattern  pattern
 </P>  
 <P>  
866  <pre>  <pre>
867    /\*.*?\*/    /\*.*?\*/
868  </PRE>  </pre>
 </P>  
 <P>  
869  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
870  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
871  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
872  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
 </P>  
 <P>  
873  <pre>  <pre>
874    \d??\d    \d??\d
875  </PRE>  </pre>
 </P>  
 <P>  
876  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
877  way the rest of the pattern matches.  way the rest of the pattern matches.
878  </P>  </P>
# Line 870  default behaviour. Line 884  default behaviour.
884  </P>  </P>
885  <P>  <P>
886  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
887  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
888  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
889  </P>  </P>
890  <P>  <P>
# Line 891  However, there is one situation where th Line 905  However, there is one situation where th
905  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
906  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail, and a later one
907  succeed. Consider, for example:  succeed. Consider, for example:
 </P>  
 <P>  
908  <pre>  <pre>
909    (.*)abc\1    (.*)abc\1
910  </PRE>  </pre>
 </P>  
 <P>  
911  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
912  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
913  </P>  </P>
914  <P>  <P>
915  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
916  that matched the final iteration. For example, after  that matched the final iteration. For example, after
 </P>  
 <P>  
917  <pre>  <pre>
918    (tweedle[dume]{3}\s*)+    (tweedle[dume]{3}\s*)+
919  </PRE>  </pre>
 </P>  
 <P>  
920  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
921  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
922  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
923  example, after  example, after
 </P>  
 <P>  
924  <pre>  <pre>
925    /(a|(b))+/    /(a|(b))+/
926  </PRE>  </pre>
 </P>  
 <P>  
927  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
928  </P>  <a name="atomicgroup"></a></P>
929  <br><a name="SEC13" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>  <br><a name="SEC13" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
930  <P>  <P>
931  With both maximizing and minimizing repetition, failure of what follows  With both maximizing and minimizing repetition, failure of what follows
# Line 935  there is no point in carrying on. Line 937  there is no point in carrying on.
937  </P>  </P>
938  <P>  <P>
939  Consider, for example, the pattern \d+foo when applied to the subject line  Consider, for example, the pattern \d+foo when applied to the subject line
 </P>  
 <P>  
940  <pre>  <pre>
941    123456bar    123456bar
942  </PRE>  </pre>
 </P>  
 <P>  
943  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
944  action of the matcher is to try again with only 5 digits matching the \d+  action of the matcher is to try again with only 5 digits matching the \d+
945  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
# Line 952  that once a subpattern has matched, it i Line 950  that once a subpattern has matched, it i
950  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher would give up
951  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
952  special parenthesis, starting with (?&#62; as in this example:  special parenthesis, starting with (?&#62; as in this example:
 </P>  
 <P>  
953  <pre>  <pre>
954    (?&#62;\d+)foo    (?&#62;\d+)foo
955  </PRE>  </pre>
 </P>  
 <P>  
956  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
957  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
958  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
# Line 983  group is just a single repeated item, as Line 977  group is just a single repeated item, as
977  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
978  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
979  previous example can be rewritten as  previous example can be rewritten as
 </P>  
 <P>  
980  <pre>  <pre>
981    \d++bar    \d++foo
982  </PRE>  </pre>
 </P>  
 <P>  
983  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
984  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
985  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning or processing of a
# Line 1004  When a pattern contains an unlimited rep Line 994  When a pattern contains an unlimited rep
994  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
995  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
996  pattern  pattern
 </P>  
 <P>  
997  <pre>  <pre>
998    (\D+|&#60;\d+&#62;)*[!?]    (\D+|&#60;\d+&#62;)*[!?]
999  </PRE>  </pre>
 </P>  
 <P>  
1000  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1001  digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs  digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1002  quickly. However, if it is applied to  quickly. However, if it is applied to
 </P>  
 <P>  
1003  <pre>  <pre>
1004    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1005  </PRE>  </pre>
 </P>  
 <P>  
1006  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1007  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \D+ repeat and the external * repeat in a
1008  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1009  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1010  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1011  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1012  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1013  </P>  an atomic group, like this:
 <P>  
1014  <pre>  <pre>
1015    ((?&#62;\D+)|&#60;\d+&#62;)*[!?]    ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1016  </PRE>  </pre>
 </P>  
 <P>  
1017  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1018  </P>  <a name="backreferences"></a></P>
1019  <br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br>  <br><a name="SEC14" href="#TOC1">BACK REFERENCES</a><br>
1020  <P>  <P>
1021  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
# Line 1049  However, if the decimal number following Line 1028  However, if the decimal number following
1028  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1029  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1030  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1031  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. See the subsection entitled "Non-printing characters"
1032  details of the handling of digits following a backslash.  <a href="#digitsafterbackslash">above</a>
1033    for further details of the handling of digits following a backslash.
1034  </P>  </P>
1035  <P>  <P>
1036  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
# Line 1058  the current subject string, rather than Line 1038  the current subject string, rather than
1038  itself (see  itself (see
1039  <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>  <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1040  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
 </P>  
 <P>  
1041  <pre>  <pre>
1042    (sens|respons)e and \1ibility    (sens|respons)e and \1ibility
1043  </PRE>  </pre>
 </P>  
 <P>  
1044  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1045  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1046  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
 </P>  
 <P>  
1047  <pre>  <pre>
1048    ((?i)rah)\s+\1    ((?i)rah)\s+\1
1049  </PRE>  </pre>
 </P>  
 <P>  
1050  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1051  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1052  </P>  </P>
1053  <P>  <P>
1054  Back references to named subpatterns use the Python syntax (?P=name). We could  Back references to named subpatterns use the Python syntax (?P=name). We could
1055  rewrite the above example as follows:  rewrite the above example as follows:
 </P>  
 <P>  
1056  <pre>  <pre>
1057    (?&#60;p1&#62;(?i)rah)\s+(?P=p1)    (?&#60;p1&#62;(?i)rah)\s+(?P=p1)
1058  </PRE>  </pre>
 </P>  
 <P>  
1059  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1060  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1061  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
 </P>  
 <P>  
1062  <pre>  <pre>
1063    (a|(bc))\2    (a|(bc))\2
1064  </PRE>  </pre>
 </P>  
 <P>  
1065  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". Because there may be
1066  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1067  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1068  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1069  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1070  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1071    <a href="#comments">"Comments"</a>
1072    below) can be used.
1073  </P>  </P>
1074  <P>  <P>
1075  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1076  when the subpattern is first used, so, for example, (a\1) never matches.  when the subpattern is first used, so, for example, (a\1) never matches.
1077  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1078  example, the pattern  example, the pattern
 </P>  
 <P>  
1079  <pre>  <pre>
1080    (a|b\1)+    (a|b\1)+
1081  </PRE>  </pre>
 </P>  
 <P>  
1082  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1083  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1084  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1085  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1086  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1087  minimum of zero.  minimum of zero.
1088  </P>  <a name="bigassertions"></a></P>
1089  <br><a name="SEC15" href="#TOC1">ASSERTIONS</a><br>  <br><a name="SEC15" href="#TOC1">ASSERTIONS</a><br>
1090  <P>  <P>
1091  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1092  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1093  assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described above.  assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1094    <a href="#smallassertions">above.</a>
1095    </P>
1096    <P>
1097  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1098  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1099  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1100    except that it does not cause the current matching position to be changed.
1101  </P>  </P>
1102  <P>  <P>
1103  An assertion subpattern is matched in the normal way, except that it does not  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1104  cause the current matching position to be changed. Lookahead assertions start  because it makes no sense to assert the same thing several times. If any kind
1105  with (?= for positive assertions and (?! for negative assertions. For example,  of assertion contains capturing subpatterns within it, these are counted for
1106    the purposes of numbering the capturing subpatterns in the whole pattern.
1107    However, substring capturing is carried out only for positive assertions,
1108    because it does not make sense for negative assertions.
1109  </P>  </P>
1110    <br><b>
1111    Lookahead assertions
1112    </b><br>
1113  <P>  <P>
1114    Lookahead assertions start
1115    with (?= for positive assertions and (?! for negative assertions. For example,
1116  <pre>  <pre>
1117    \w+(?=;)    \w+(?=;)
1118  </PRE>  </pre>
 </P>  
 <P>  
1119  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
1120  the match, and  the match, and
 </P>  
 <P>  
1121  <pre>  <pre>
1122    foo(?!bar)    foo(?!bar)
1123  </PRE>  </pre>
 </P>  
 <P>  
1124  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
1125  apparently similar pattern  apparently similar pattern
 </P>  
 <P>  
1126  <pre>  <pre>
1127    (?!foo)bar    (?!foo)bar
1128  </PRE>  </pre>
 </P>  
 <P>  
1129  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
1130  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1131  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1132  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1133  </P>  </P>
1134  <P>  <P>
1135  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1136  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1137  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1138  </P>  <a name="lookbehind"></a></P>
1139    <br><b>
1140    Lookbehind assertions
1141    </b><br>
1142  <P>  <P>
1143  Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for  Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1144  negative assertions. For example,  negative assertions. For example,
 </P>  
 <P>  
1145  <pre>  <pre>
1146    (?&#60;!foo)bar    (?&#60;!foo)bar
1147  </PRE>  </pre>
 </P>  
 <P>  
1148  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1149  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1150  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several alternatives, they do not
1151  all have to have the same fixed length. Thus  all have to have the same fixed length. Thus
 </P>  
 <P>  
1152  <pre>  <pre>
1153    (?&#60;=bullock|donkey)    (?&#60;=bullock|donkey)
1154  </PRE>  </pre>
 </P>  
 <P>  
1155  is permitted, but  is permitted, but
 </P>  
 <P>  
1156  <pre>  <pre>
1157    (?&#60;!dogs?|cats?)    (?&#60;!dogs?|cats?)
1158  </PRE>  </pre>
 </P>  
 <P>  
1159  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1160  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1161  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1162  match the same length of string. An assertion such as  match the same length of string. An assertion such as
 </P>  
 <P>  
1163  <pre>  <pre>
1164    (?&#60;=ab(c|de))    (?&#60;=ab(c|de))
1165  </PRE>  </pre>
 </P>  
 <P>  
1166  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1167  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
 </P>  
 <P>  
1168  <pre>  <pre>
1169    (?&#60;=abc|abde)    (?&#60;=abc|abde)
1170  </PRE>  </pre>
 </P>  
 <P>  
1171  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1172  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed width and then try to
1173  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1229  match is deemed to fail. Line 1176  match is deemed to fail.
1176  <P>  <P>
1177  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1178  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1179  the length of the lookbehind.  the length of the lookbehind. The \X escape, which can match different numbers
1180    of bytes, is also not permitted.
1181  </P>  </P>
1182  <P>  <P>
1183  Atomic groups can be used in conjunction with lookbehind assertions to specify  Atomic groups can be used in conjunction with lookbehind assertions to specify
1184  efficient matching at the end of the subject string. Consider a simple pattern  efficient matching at the end of the subject string. Consider a simple pattern
1185  such as  such as
 </P>  
 <P>  
1186  <pre>  <pre>
1187    abcd$    abcd$
1188  </PRE>  </pre>
 </P>  
 <P>  
1189  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
1190  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
1191  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
 </P>  
 <P>  
1192  <pre>  <pre>
1193    ^.*abcd$    ^.*abcd$
1194  </PRE>  </pre>
 </P>  
 <P>  
1195  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
1196  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
1197  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
1198  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1199  if the pattern is written as  if the pattern is written as
 </P>  
 <P>  
1200  <pre>  <pre>
1201    ^(?&#62;.*)(?&#60;=abcd)    ^(?&#62;.*)(?&#60;=abcd)
1202  </PRE>  </pre>
1203  </P>  or, equivalently, using the possessive quantifier syntax,
 <P>  
 or, equivalently,  
 </P>  
 <P>  
1204  <pre>  <pre>
1205    ^.*+(?&#60;=abcd)    ^.*+(?&#60;=abcd)
1206  </PRE>  </pre>
 </P>  
 <P>  
1207  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .* item; it can match only the entire
1208  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1209  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1210  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1211  </P>  </P>
1212    <br><b>
1213    Using multiple assertions
1214    </b><br>
1215  <P>  <P>
1216  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
 </P>  
 <P>  
1217  <pre>  <pre>
1218    (?&#60;=\d{3})(?&#60;!999)foo    (?&#60;=\d{3})(?&#60;!999)foo
1219  </PRE>  </pre>
 </P>  
 <P>  
1220  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
1221  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1222  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
# Line 1293  digits, and then there is a check that t Line 1224  digits, and then there is a check that t
1224  This pattern does <i>not</i> match "foo" preceded by six characters, the first  This pattern does <i>not</i> match "foo" preceded by six characters, the first
1225  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
1226  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
 </P>  
 <P>  
1227  <pre>  <pre>
1228    (?&#60;=\d{3}...)(?&#60;!999)foo    (?&#60;=\d{3}...)(?&#60;!999)foo
1229  </PRE>  </pre>
 </P>  
 <P>  
1230  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1231  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1232  preceding three characters are not "999".  preceding three characters are not "999".
1233  </P>  </P>
1234  <P>  <P>
1235  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
 </P>  
 <P>  
1236  <pre>  <pre>
1237    (?&#60;=(?&#60;!foo)bar)baz    (?&#60;=(?&#60;!foo)bar)baz
1238  </PRE>  </pre>
 </P>  
 <P>  
1239  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1240  preceded by "foo", while  preceded by "foo", while
 </P>  
 <P>  
1241  <pre>  <pre>
1242    (?&#60;=\d{3}(?!999)...)foo    (?&#60;=\d{3}(?!999)...)foo
1243  </PRE>  </pre>
1244  </P>  is another pattern that matches "foo" preceded by three digits and any three
 <P>  
 is another pattern which matches "foo" preceded by three digits and any three  
1245  characters that are not "999".  characters that are not "999".
1246  </P>  </P>
 <P>  
 Assertion subpatterns are not capturing subpatterns, and may not be repeated,  
 because it makes no sense to assert the same thing several times. If any kind  
 of assertion contains capturing subpatterns within it, these are counted for  
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
 </P>  
1247  <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>  <br><a name="SEC16" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1248  <P>  <P>
1249  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1250  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1251  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1252  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
 </P>  
 <P>  
1253  <pre>  <pre>
1254    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1255    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1256  </PRE>  </pre>
 </P>  
 <P>  
1257  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1258  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1259  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
# Line 1358  subpattern of that number has previously Line 1265  subpattern of that number has previously
1265  than zero. Consider the following pattern, which contains non-significant white  than zero. Consider the following pattern, which contains non-significant white
1266  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  space to make it more readable (assume the PCRE_EXTENDED option) and to divide
1267  it into three parts for ease of discussion:  it into three parts for ease of discussion:
 </P>  
 <P>  
1268  <pre>  <pre>
1269    ( \( )?    [^()]+    (?(1) \) )    ( \( )?    [^()]+    (?(1) \) )
1270  </PRE>  </pre>
 </P>  
 <P>  
1271  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1272  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
1273  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
# Line 1385  If the condition is not a sequence of di Line 1288  If the condition is not a sequence of di
1288  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1289  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1290  alternatives on the second line:  alternatives on the second line:
 </P>  
 <P>  
1291  <pre>  <pre>
1292    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1293    \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )    \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
1294  </PRE>  </pre>
 </P>  
 <P>  
1295  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1296  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
1297  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
1298  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1299  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
1300  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1301  </P>  <a name="comments"></a></P>
1302  <br><a name="SEC17" href="#TOC1">COMMENTS</a><br>  <br><a name="SEC17" href="#TOC1">COMMENTS</a><br>
1303  <P>  <P>
1304  The sequence (?# marks the start of a comment which continues up to the next  The sequence (?# marks the start of a comment that continues up to the next
1305  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1306  that make up a comment play no part in the pattern matching at all.  that make up a comment play no part in the pattern matching at all.
1307  </P>  </P>
# Line 1416  character in the pattern. Line 1315  character in the pattern.
1315  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1316  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1317  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1318  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth. Perl provides a facility
1319  experimental facility that allows regular expressions to recurse (amongst other  that allows regular expressions to recurse (amongst other things). It does this
1320  things). It does this by interpolating Perl code in the expression at run time,  by interpolating Perl code in the expression at run time, and the code can
1321  and the code can refer to the expression itself. A Perl pattern to solve the  refer to the expression itself. A Perl pattern to solve the parentheses problem
1322  parentheses problem can be created like this:  can be created like this:
 </P>  
 <P>  
1323  <pre>  <pre>
1324    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;    $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1325  </PRE>  </pre>
 </P>  
 <P>  
1326  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
1327  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears. Obviously, PCRE cannot support
1328  the interpolation of Perl code. Instead, it supports some special syntax for  the interpolation of Perl code. Instead, it supports some special syntax for
# Line 1443  number, provided that it occurs inside t Line 1338  number, provided that it occurs inside t
1338  <P>  <P>
1339  For example, this PCRE pattern solves the nested parentheses problem (assume  For example, this PCRE pattern solves the nested parentheses problem (assume
1340  the PCRE_EXTENDED option is set so that white space is ignored):  the PCRE_EXTENDED option is set so that white space is ignored):
 </P>  
 <P>  
1341  <pre>  <pre>
1342    \( ( (?&#62;[^()]+) | (?R) )* \)    \( ( (?&#62;[^()]+) | (?R) )* \)
1343  </PRE>  </pre>
 </P>  
 <P>  
1344  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1345  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
1346  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is a correctly parenthesized substring).
# Line 1458  Finally there is a closing parenthesis. Line 1349  Finally there is a closing parenthesis.
1349  <P>  <P>
1350  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
1351  pattern, so instead you could use this:  pattern, so instead you could use this:
 </P>  
 <P>  
1352  <pre>  <pre>
1353    ( \( ( (?&#62;[^()]+) | (?1) )* \) )    ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1354  </PRE>  </pre>
 </P>  
 <P>  
1355  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
1356  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern. In a larger pattern, keeping track of
1357  parenthesis numbers can be tricky. It may be more convenient to use named  parenthesis numbers can be tricky. It may be more convenient to use named
1358  parentheses instead. For this, PCRE uses (?P&#62;name), which is an extension to  parentheses instead. For this, PCRE uses (?P&#62;name), which is an extension to
1359  the Python syntax that PCRE uses for named parentheses (Perl does not provide  the Python syntax that PCRE uses for named parentheses (Perl does not provide
1360  named parentheses). We could rewrite the above example as follows:  named parentheses). We could rewrite the above example as follows:
 </P>  
 <P>  
1361  <pre>  <pre>
1362    (?P&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )    (?P&#60;pn&#62; \( ( (?&#62;[^()]+) | (?P&#62;pn) )* \) )
1363  </PRE>  </pre>
 </P>  
 <P>  
1364  This particular example pattern contains nested unlimited repeats, and so the  This particular example pattern contains nested unlimited repeats, and so the
1365  use of atomic grouping for matching strings of non-parentheses is important  use of atomic grouping for matching strings of non-parentheses is important
1366  when applying the pattern to strings that do not match. For example, when this  when applying the pattern to strings that do not match. For example, when this
1367  pattern is applied to  pattern is applied to
 </P>  
 <P>  
1368  <pre>  <pre>
1369    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1370  </PRE>  </pre>
 </P>  
 <P>  
1371  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1372  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
1373  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
# Line 1498  before failure can be reported. Line 1377  before failure can be reported.
1377  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1378  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1379  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1380  below and the  the next section and the
1381  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
1382  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
 </P>  
 <P>  
1383  <pre>  <pre>
1384    (ab(cd)ef)    (ab(cd)ef)
1385  </PRE>  </pre>
 </P>  
 <P>  
1386  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
1387  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
 </P>  
 <P>  
1388  <pre>  <pre>
1389    \( ( ( (?&#62;[^()]+) | (?R) )* ) \)    \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
1390       ^                        ^       ^                        ^
1391       ^                        ^       ^                        ^
1392  </PRE>  </pre>
 </P>  
 <P>  
1393  the string they capture is "ab(cd)ef", the contents of the top level  the string they capture is "ab(cd)ef", the contents of the top level
1394  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1395  has to obtain extra memory to store data during a recursion, which it does by  has to obtain extra memory to store data during a recursion, which it does by
# Line 1530  Do not confuse the (?R) item with the co Line 1401  Do not confuse the (?R) item with the co
1401  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1402  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1403  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
 </P>  
 <P>  
1404  <pre>  <pre>
1405    &#60; (?: (?(R) \d++  | [^&#60;&#62;]*+) | (?R)) * &#62;    &#60; (?: (?(R) \d++  | [^&#60;&#62;]*+) | (?R)) * &#62;
1406  </PRE>  </pre>
 </P>  
 <P>  
1407  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
1408  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1409  is the actual recursive call.  is the actual recursive call.
1410  </P>  <a name="subpatternsassubroutines"></a></P>
1411  <a name="subpatternsassubroutines"></a><br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>  <br><a name="SEC19" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1412  <P>  <P>
1413  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
1414  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
1415  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. An earlier example pointed out that the
1416  pattern  pattern
 </P>  
 <P>  
1417  <pre>  <pre>
1418    (sens|respons)e and \1ibility    (sens|respons)e and \1ibility
1419  </PRE>  </pre>
 </P>  
 <P>  
1420  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1421  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
 </P>  
 <P>  
1422  <pre>  <pre>
1423    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1424  </PRE>  </pre>
 </P>  
 <P>  
1425  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
1426  strings. Such references must, however, follow the subpattern to which they  strings. Such references must, however, follow the subpattern to which they
1427  refer.  refer.
# Line 1585  Within a regular expression, (?C) indica Line 1444  Within a regular expression, (?C) indica
1444  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
1445  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1446  For example, this pattern has two callout points:  For example, this pattern has two callout points:
 </P>  
 <P>  
1447  <pre>  <pre>
1448    (?C1)\dabc(?C2)def    (?C1)\dabc(?C2)def
1449  </PRE>  </pre>
1450    If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
1451    automatically installed before each item in the pattern. They are all numbered
1452    255.
1453  </P>  </P>
1454  <P>  <P>
1455  During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is  During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
1456  set), the external function is called. It is provided with the number of the  set), the external function is called. It is provided with the number of the
1457  callout, and, optionally, one item of data originally supplied by the caller of  callout, the position in the pattern, and, optionally, one item of data
1458  <b>pcre_exec()</b>. The callout function may cause matching to backtrack, or to  originally supplied by the caller of <b>pcre_exec()</b>. The callout function
1459  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1460  function is given in the  description of the interface to the callout function is given in the
1461  <a href="pcrecallout.html"><b>pcrecallout</b></a>  <a href="pcrecallout.html"><b>pcrecallout</b></a>
1462  documentation.  documentation.
1463  </P>  </P>
1464  <P>  <P>
1465  Last updated: 03 February 2003  Last updated: 09 September 2004
1466  <br>  <br>
1467  Copyright &copy; 1997-2003 University of Cambridge.  Copyright &copy; 1997-2004 University of Cambridge.
1468    <p>
1469    Return to the <a href="index.html">PCRE index page</a>.
1470    </p>
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