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revision 77 by nigel, Sat Feb 24 21:40:45 2007 UTC revision 488 by ph10, Mon Jan 11 15:29:42 2010 UTC
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1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
36  .\" </a>  .\" </a>
37  section on UTF-8 support  section on UTF-8 support
# Line 30  The remainder of this document discusses Line 46  The remainder of this document discusses
46  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
48  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
50  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51    alternative function, and how it differs from the normal function, are
52    discussed in the
53  .\" HREF  .\" HREF
54  \fBpcrematching\fP  \fBpcrematching\fP
55  .\"  .\"
56  page.  page.
57    .
58    .
59    .SH "NEWLINE CONVENTIONS"
60    .rs
61    .sp
62    PCRE supports five different conventions for indicating line breaks in
63    strings: a single CR (carriage return) character, a single LF (linefeed)
64    character, the two-character sequence CRLF, any of the three preceding, or any
65    Unicode newline sequence. The
66    .\" HREF
67    \fBpcreapi\fP
68    .\"
69    page has
70    .\" HTML <a href="pcreapi.html#newlines">
71    .\" </a>
72    further discussion
73    .\"
74    about newlines, and shows how to set the newline convention in the
75    \fIoptions\fP arguments for the compiling and matching functions.
76  .P  .P
77    It is also possible to specify a newline convention by starting a pattern
78    string with one of the following five sequences:
79    .sp
80      (*CR)        carriage return
81      (*LF)        linefeed
82      (*CRLF)      carriage return, followed by linefeed
83      (*ANYCRLF)   any of the three above
84      (*ANY)       all Unicode newline sequences
85    .sp
86    These override the default and the options given to \fBpcre_compile()\fP or
87    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88    newline sequence, the pattern
89    .sp
90      (*CR)a.b
91    .sp
92    changes the convention to CR. That pattern matches "a\enb" because LF is no
93    longer a newline. Note that these special settings, which are not
94    Perl-compatible, are recognized only at the very start of a pattern, and that
95    they must be in upper case. If more than one of them is present, the last one
96    is used.
97    .P
98    The newline convention does not affect what the \eR escape sequence matches. By
99    default, this is any Unicode newline sequence, for Perl compatibility. However,
100    this can be changed; see the description of \eR in the section entitled
101    .\" HTML <a href="#newlineseq">
102    .\" </a>
103    "Newline sequences"
104    .\"
105    below. A change of \eR setting can be combined with a change of newline
106    convention.
107    .
108    .
109    .SH "CHARACTERS AND METACHARACTERS"
110    .rs
111    .sp
112  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
113  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
114  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 132  interpreted in some special way.
132  .P  .P
133  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
134  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
135  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
136  as follows:  are as follows:
137  .sp  .sp
138    \e      general escape character with several uses    \e      general escape character with several uses
139    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 164  a character class the only metacharacter
164  .sp  .sp
165  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
166  .  .
167    .
168  .SH BACKSLASH  .SH BACKSLASH
169  .rs  .rs
170  .sp  .sp
171  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
172  non-alphanumeric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
173  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
174  outside character classes.  outside character classes.
175  .P  .P
176  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
# Line 108  particular, if you want to match a backs Line 181  particular, if you want to match a backs
181  .P  .P
182  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
183  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
184  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
185  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
 pattern.  
186  .P  .P
187  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
188  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 135  The \eQ...\eE sequence is recognized bot Line 207  The \eQ...\eE sequence is recognized bot
207  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
208  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
209  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
210  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 often easier to use
211  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
212  .sp  .sp
213    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
214    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
215    \ee        escape (hex 1B)    \ee        escape (hex 1B)
216    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
217    \en        newline (hex 0A)    \en        linefeed (hex 0A)
218    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
219    \et        tab (hex 09)    \et        tab (hex 09)
220    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
221    \exhh      character with hex code hh    \exhh      character with hex code hh
222    \ex{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
223  .sp  .sp
224  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
225  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.
# Line 156  Thus \ecz becomes hex 1A, but \ec{ becom Line 227  Thus \ecz becomes hex 1A, but \ec{ becom
227  7B.  7B.
228  .P  .P
229  After \ex, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
230  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
231  appear between \ex{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
232  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233  other than hexadecimal digits appear between \ex{ and }, or if there is no  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234  terminating }, this form of escape is not recognized. Instead, the initial  point, which is 10FFFF.
235  \ex will be interpreted as a basic hexadecimal escape, with no following  .P
236  digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
237    there is no terminating }, this form of escape is not recognized. Instead, the
238    initial \ex will be interpreted as a basic hexadecimal escape, with no
239    following digits, giving a character whose value is zero.
240  .P  .P
241  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
242  syntaxes for \ex when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
243  way they are handled. For example, \exdc is exactly the same as \ex{dc}.  example, \exdc is exactly the same as \ex{dc}.
244  .P  .P
245  After \e0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. If there are fewer than two
246  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
247  sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
248  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
249  pattern character that follows is itself an octal digit.  follows is itself an octal digit.
250  .P  .P
251  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.
252  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
# Line 191  parenthesized subpatterns. Line 265  parenthesized subpatterns.
265  .P  .P
266  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
267  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
268  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
269  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
270  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
271    to \e777 are permitted. For example:
272  .sp  .sp
273    \e040   is another way of writing a space    \e040   is another way of writing a space
274  .\" JOIN  .\" JOIN
# Line 218  For example: Line 293  For example:
293  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
294  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
295  .P  .P
296  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
297  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
298  addition, inside a character class, the sequence \eb is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
299  backspace character (hex 08), and the sequence \eX is interpreted as the  sequences \eR and \eX are interpreted as the characters "R" and "X",
300  character "X". Outside a character class, these sequences have different  respectively. Outside a character class, these sequences have different
301  meanings  meanings
302  .\" HTML <a href="#uniextseq">  .\" HTML <a href="#uniextseq">
303  .\" </a>  .\" </a>
# Line 230  meanings Line 305  meanings
305  .\"  .\"
306  .  .
307  .  .
308    .SS "Absolute and relative back references"
309    .rs
310    .sp
311    The sequence \eg followed by an unsigned or a negative number, optionally
312    enclosed in braces, is an absolute or relative back reference. A named back
313    reference can be coded as \eg{name}. Back references are discussed
314    .\" HTML <a href="#backreferences">
315    .\" </a>
316    later,
317    .\"
318    following the discussion of
319    .\" HTML <a href="#subpattern">
320    .\" </a>
321    parenthesized subpatterns.
322    .\"
323    .
324    .
325    .SS "Absolute and relative subroutine calls"
326    .rs
327    .sp
328    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329    a number enclosed either in angle brackets or single quotes, is an alternative
330    syntax for referencing a subpattern as a "subroutine". Details are discussed
331    .\" HTML <a href="#onigurumasubroutines">
332    .\" </a>
333    later.
334    .\"
335    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336    synonymous. The former is a back reference; the latter is a
337    .\" HTML <a href="#subpatternsassubroutines">
338    .\" </a>
339    subroutine
340    .\"
341    call.
342    .
343    .
344  .SS "Generic character types"  .SS "Generic character types"
345  .rs  .rs
346  .sp  .sp
347  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
348  following are always recognized:  following are always recognized:
349  .sp  .sp
350    \ed     any decimal digit    \ed     any decimal digit
351    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
352      \eh     any horizontal whitespace character
353      \eH     any character that is not a horizontal whitespace character
354    \es     any whitespace character    \es     any whitespace character
355    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
356      \ev     any vertical whitespace character
357      \eV     any character that is not a vertical whitespace character
358    \ew     any "word" character    \ew     any "word" character
359    \eW     any "non-word" character    \eW     any "non-word" character
360  .sp  .sp
# Line 253  there is no character to match. Line 368  there is no character to match.
368  .P  .P
369  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
370  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
371  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372    included in a Perl script, \es may match the VT character. In PCRE, it never
373    does.
374    .P
375    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377    character property support is available. These sequences retain their original
378    meanings from before UTF-8 support was available, mainly for efficiency
379    reasons. Note that this also affects \eb, because it is defined in terms of \ew
380    and \eW.
381    .P
382    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384    The horizontal space characters are:
385    .sp
386      U+0009     Horizontal tab
387      U+0020     Space
388      U+00A0     Non-break space
389      U+1680     Ogham space mark
390      U+180E     Mongolian vowel separator
391      U+2000     En quad
392      U+2001     Em quad
393      U+2002     En space
394      U+2003     Em space
395      U+2004     Three-per-em space
396      U+2005     Four-per-em space
397      U+2006     Six-per-em space
398      U+2007     Figure space
399      U+2008     Punctuation space
400      U+2009     Thin space
401      U+200A     Hair space
402      U+202F     Narrow no-break space
403      U+205F     Medium mathematical space
404      U+3000     Ideographic space
405    .sp
406    The vertical space characters are:
407    .sp
408      U+000A     Linefeed
409      U+000B     Vertical tab
410      U+000C     Formfeed
411      U+000D     Carriage return
412      U+0085     Next line
413      U+2028     Line separator
414      U+2029     Paragraph separator
415  .P  .P
416  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character less than 256 that is a
417  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 267  in the Line 425  in the
425  .\" HREF  .\" HREF
426  \fBpcreapi\fP  \fBpcreapi\fP
427  .\"  .\"
428  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
429  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
430  .P  accented letters, and these are matched by \ew. The use of locales with Unicode
431  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  is discouraged.
432  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .
433  character property support is available.  .
434    .\" HTML <a name="newlineseq"></a>
435    .SS "Newline sequences"
436    .rs
437    .sp
438    Outside a character class, by default, the escape sequence \eR matches any
439    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440    equivalent to the following:
441    .sp
442      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443    .sp
444    This is an example of an "atomic group", details of which are given
445    .\" HTML <a href="#atomicgroup">
446    .\" </a>
447    below.
448    .\"
449    This particular group matches either the two-character sequence CR followed by
450    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
451    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
452    line, U+0085). The two-character sequence is treated as a single unit that
453    cannot be split.
454    .P
455    In UTF-8 mode, two additional characters whose codepoints are greater than 255
456    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
457    Unicode character property support is not needed for these characters to be
458    recognized.
459    .P
460    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462    either at compile time or when the pattern is matched. (BSR is an abbrevation
463    for "backslash R".) This can be made the default when PCRE is built; if this is
464    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465    It is also possible to specify these settings by starting a pattern string with
466    one of the following sequences:
467    .sp
468      (*BSR_ANYCRLF)   CR, LF, or CRLF only
469      (*BSR_UNICODE)   any Unicode newline sequence
470    .sp
471    These override the default and the options given to \fBpcre_compile()\fP or
472    \fBpcre_compile2()\fP, but they can be overridden by options given to
473    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474    which are not Perl-compatible, are recognized only at the very start of a
475    pattern, and that they must be in upper case. If more than one of them is
476    present, the last one is used. They can be combined with a change of newline
477    convention, for example, a pattern can start with:
478    .sp
479      (*ANY)(*BSR_ANYCRLF)
480    .sp
481    Inside a character class, \eR matches the letter "R".
482  .  .
483  .  .
484  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 280  character property support is available. Line 486  character property support is available.
486  .rs  .rs
487  .sp  .sp
488  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
489  escape sequences to match generic character types are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
490  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
491  .sp  characters whose codepoints are less than 256, but they do work in this mode.
492   \ep{\fIxx\fP}   a character with the \fIxx\fP property  The extra escape sequences are:
493   \eP{\fIxx\fP}   a character without the \fIxx\fP property  .sp
494   \eX       an extended Unicode sequence    \ep{\fIxx\fP}   a character with the \fIxx\fP property
495  .sp    \eP{\fIxx\fP}   a character without the \fIxx\fP property
496  The property names represented by \fIxx\fP above are limited to the    \eX       an extended Unicode sequence
497  Unicode general category properties. Each character has exactly one such  .sp
498  property, specified by a two-letter abbreviation. For compatibility with Perl,  The property names represented by \fIxx\fP above are limited to the Unicode
499  negation can be specified by including a circumflex between the opening brace  script names, the general category properties, and "Any", which matches any
500  and the property name. For example, \ep{^Lu} is the same as \eP{Lu}.  character (including newline). Other properties such as "InMusicalSymbols" are
501  .P  not currently supported by PCRE. Note that \eP{Any} does not match any
502  If only one letter is specified with \ep or \eP, it includes all the properties  characters, so always causes a match failure.
503  that start with that letter. In this case, in the absence of negation, the  .P
504  curly brackets in the escape sequence are optional; these two examples have  Sets of Unicode characters are defined as belonging to certain scripts. A
505  the same effect:  character from one of these sets can be matched using a script name. For
506    example:
507    .sp
508      \ep{Greek}
509      \eP{Han}
510    .sp
511    Those that are not part of an identified script are lumped together as
512    "Common". The current list of scripts is:
513    .P
514    Arabic,
515    Armenian,
516    Balinese,
517    Bengali,
518    Bopomofo,
519    Braille,
520    Buginese,
521    Buhid,
522    Canadian_Aboriginal,
523    Cherokee,
524    Common,
525    Coptic,
526    Cuneiform,
527    Cypriot,
528    Cyrillic,
529    Deseret,
530    Devanagari,
531    Ethiopic,
532    Georgian,
533    Glagolitic,
534    Gothic,
535    Greek,
536    Gujarati,
537    Gurmukhi,
538    Han,
539    Hangul,
540    Hanunoo,
541    Hebrew,
542    Hiragana,
543    Inherited,
544    Kannada,
545    Katakana,
546    Kharoshthi,
547    Khmer,
548    Lao,
549    Latin,
550    Limbu,
551    Linear_B,
552    Malayalam,
553    Mongolian,
554    Myanmar,
555    New_Tai_Lue,
556    Nko,
557    Ogham,
558    Old_Italic,
559    Old_Persian,
560    Oriya,
561    Osmanya,
562    Phags_Pa,
563    Phoenician,
564    Runic,
565    Shavian,
566    Sinhala,
567    Syloti_Nagri,
568    Syriac,
569    Tagalog,
570    Tagbanwa,
571    Tai_Le,
572    Tamil,
573    Telugu,
574    Thaana,
575    Thai,
576    Tibetan,
577    Tifinagh,
578    Ugaritic,
579    Yi.
580    .P
581    Each character has exactly one general category property, specified by a
582    two-letter abbreviation. For compatibility with Perl, negation can be specified
583    by including a circumflex between the opening brace and the property name. For
584    example, \ep{^Lu} is the same as \eP{Lu}.
585    .P
586    If only one letter is specified with \ep or \eP, it includes all the general
587    category properties that start with that letter. In this case, in the absence
588    of negation, the curly brackets in the escape sequence are optional; these two
589    examples have the same effect:
590  .sp  .sp
591    \ep{L}    \ep{L}
592    \epL    \epL
593  .sp  .sp
594  The following property codes are supported:  The following general category property codes are supported:
595  .sp  .sp
596    C     Other    C     Other
597    Cc    Control    Cc    Control
# Line 347  The following property codes are support Line 637  The following property codes are support
637    Zp    Paragraph separator    Zp    Paragraph separator
638    Zs    Space separator    Zs    Space separator
639  .sp  .sp
640  Extended properties such as "Greek" or "InMusicalSymbols" are not supported by  The special property L& is also supported: it matches a character that has
641  PCRE.  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
642    a modifier or "other".
643    .P
644    The Cs (Surrogate) property applies only to characters in the range U+D800 to
645    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
646    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
647    (see the discussion of PCRE_NO_UTF8_CHECK in the
648    .\" HREF
649    \fBpcreapi\fP
650    .\"
651    page). Perl does not support the Cs property.
652    .P
653    The long synonyms for property names that Perl supports (such as \ep{Letter})
654    are not supported by PCRE, nor is it permitted to prefix any of these
655    properties with "Is".
656    .P
657    No character that is in the Unicode table has the Cn (unassigned) property.
658    Instead, this property is assumed for any code point that is not in the
659    Unicode table.
660  .P  .P
661  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
662  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters.
# Line 366  atomic group Line 674  atomic group
674  (see below).  (see below).
675  .\"  .\"
676  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
677  preceding character.  preceding character. None of them have codepoints less than 256, so in
678    non-UTF-8 mode \eX matches any one character.
679  .P  .P
680  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
681  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
# Line 374  why the traditional escape sequences suc Line 683  why the traditional escape sequences suc
683  properties in PCRE.  properties in PCRE.
684  .  .
685  .  .
686    .\" HTML <a name="resetmatchstart"></a>
687    .SS "Resetting the match start"
688    .rs
689    .sp
690    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
691    matched characters not to be included in the final matched sequence. For
692    example, the pattern:
693    .sp
694      foo\eKbar
695    .sp
696    matches "foobar", but reports that it has matched "bar". This feature is
697    similar to a lookbehind assertion
698    .\" HTML <a href="#lookbehind">
699    .\" </a>
700    (described below).
701    .\"
702    However, in this case, the part of the subject before the real match does not
703    have to be of fixed length, as lookbehind assertions do. The use of \eK does
704    not interfere with the setting of
705    .\" HTML <a href="#subpattern">
706    .\" </a>
707    captured substrings.
708    .\"
709    For example, when the pattern
710    .sp
711      (foo)\eKbar
712    .sp
713    matches "foobar", the first substring is still set to "foo".
714    .
715    .
716  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
717  .SS "Simple assertions"  .SS "Simple assertions"
718  .rs  .rs
719  .sp  .sp
720  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
721  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,
722  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
723  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 386  subpatterns for more complicated asserti Line 725  subpatterns for more complicated asserti
725  .\" </a>  .\" </a>
726  below.  below.
727  .\"  .\"
728  The backslashed  The backslashed assertions are:
 assertions are:  
729  .sp  .sp
730    \eb     matches at a word boundary    \eb     matches at a word boundary
731    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
732    \eA     matches at start of subject    \eA     matches at the start of the subject
733    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
734    \ez     matches at end of subject            also matches before a newline at the end of the subject
735    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
736      \eG     matches at the first matching position in the subject
737  .sp  .sp
738  These assertions may not appear in character classes (but note that \eb has a  These assertions may not appear in character classes (but note that \eb has a
739  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
# Line 402  different meaning, namely the backspace Line 741  different meaning, namely the backspace
741  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
742  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
743  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
744  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. Neither PCRE nor Perl has a
745    separte "start of word" or "end of word" metasequence. However, whatever
746    follows \eb normally determines which it is. For example, the fragment
747    \eba matches "a" at the start of a word.
748  .P  .P
749  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
750  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 412  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 754  PCRE_NOTBOL or PCRE_NOTEOL options, whic
754  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
755  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
756  at a point other than the beginning of the subject, \eA can never match. The  at a point other than the beginning of the subject, \eA can never match. The
757  difference between \eZ and \ez is that \eZ matches before a newline that is the  difference between \eZ and \ez is that \eZ matches before a newline at the end
758  last character of the string as well as at the end of the string, whereas \ez  of the string as well as at the very end, whereas \ez matches only at the end.
 matches only at the end.  
759  .P  .P
760  The \eG assertion is true only when the current matching position is at the  The \eG assertion is true only when the current matching position is at the
761  start point of the match, as specified by the \fIstartoffset\fP argument of  start point of the match, as specified by the \fIstartoffset\fP argument of
# Line 458  to be anchored.) Line 799  to be anchored.)
799  .P  .P
800  A dollar character is an assertion that is true only if the current matching  A dollar character is an assertion that is true only if the current matching
801  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
802  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
803  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
804  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
805  Dollar has no special meaning in a character class.  character class.
806  .P  .P
807  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
808  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
809  does not affect the \eZ assertion.  does not affect the \eZ assertion.
810  .P  .P
811  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
812  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
813  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
814  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
815  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
816  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
817  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
818  branches start with ^ are not anchored in multiline mode, and a match for  .P
819  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
820  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is  \en represents a newline) in multiline mode, but not otherwise. Consequently,
821  set.  patterns that are anchored in single line mode because all branches start with
822    ^ are not anchored in multiline mode, and a match for circumflex is possible
823    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
824    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
825  .P  .P
826  Note that the sequences \eA, \eZ, and \ez can be used to match the start and  Note that the sequences \eA, \eZ, and \ez can be used to match the start and
827  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
828  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
829  .  .
830  .  .
831  .SH "FULL STOP (PERIOD, DOT)"  .SH "FULL STOP (PERIOD, DOT)"
832  .rs  .rs
833  .sp  .sp
834  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
835  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
836  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
837  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
838  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
839  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
840  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
841    (including isolated CRs and LFs). When any Unicode line endings are being
842    recognized, dot does not match CR or LF or any of the other line ending
843    characters.
844    .P
845    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
846    option is set, a dot matches any one character, without exception. If the
847    two-character sequence CRLF is present in the subject string, it takes two dots
848    to match it.
849    .P
850    The handling of dot is entirely independent of the handling of circumflex and
851    dollar, the only relationship being that they both involve newlines. Dot has no
852    special meaning in a character class.
853  .  .
854  .  .
855  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
856  .rs  .rs
857  .sp  .sp
858  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
859  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
860  provided in Perl in order to match individual bytes in UTF-8 mode. Because it  characters. The feature is provided in Perl in order to match individual bytes
861  breaks up UTF-8 characters into individual bytes, what remains in the string  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
862  may be a malformed UTF-8 string. For this reason, the \eC escape sequence is  what remains in the string may be a malformed UTF-8 string. For this reason,
863  best avoided.  the \eC escape sequence is best avoided.
864  .P  .P
865  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
866  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 520  the lookbehind. Line 876  the lookbehind.
876  .rs  .rs
877  .sp  .sp
878  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
879  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 by default.
880  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
881  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
882  escaped with a backslash.  a member of the class, it should be the first data character in the class
883    (after an initial circumflex, if present) or escaped with a backslash.
884  .P  .P
885  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
886  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
887  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
888  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
889  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
890  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
# Line 537  For example, the character class [aeiou] Line 894  For example, the character class [aeiou]
894  [^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
895  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
896  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
897  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
898  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
899  string.  string.
900  .P  .P
# Line 551  caseful version would. In UTF-8 mode, PC Line 908  caseful version would. In UTF-8 mode, PC
908  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
909  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
910  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
911  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
912  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
913  UTF-8 support.  with UTF-8 support.
914  .P  .P
915  The newline character is never treated in any special way in character classes,  Characters that might indicate line breaks are never treated in any special way
916  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  when matching character classes, whatever line-ending sequence is in use, and
917  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
918    such as [^a] always matches one of these characters.
919  .P  .P
920  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
921  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
# Line 581  example [\ex{100}-\ex{2ff}]. Line 939  example [\ex{100}-\ex{2ff}].
939  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
940  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
941  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
942  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
943  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
944  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
945  property support.  property support.
# Line 656  the pattern Line 1014  the pattern
1014    gilbert|sullivan    gilbert|sullivan
1015  .sp  .sp
1016  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1017  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1018  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1019  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
 subpattern  
1020  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
1021  .\" </a>  .\" </a>
1022  (defined below),  (defined below),
# Line 672  alternative in the subpattern. Line 1029  alternative in the subpattern.
1029  .rs  .rs
1030  .sp  .sp
1031  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1032  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1033  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1034    The option letters are
1035  .sp  .sp
1036    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1037    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 687  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1045  PCRE_MULTILINE while unsetting PCRE_DOTA
1045  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1046  unset.  unset.
1047  .P  .P
1048  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1049  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1050  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1051  the global options (and it will therefore show up in data extracted by the  .P
1052  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1053    subpattern parentheses), the change applies to the remainder of the pattern
1054    that follows. If the change is placed right at the start of a pattern, PCRE
1055    extracts it into the global options (and it will therefore show up in data
1056    extracted by the \fBpcre_fullinfo()\fP function).
1057  .P  .P
1058  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1059  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1060  .sp  .sp
1061    (a(?i)b)c    (a(?i)b)c
1062  .sp  .sp
# Line 710  branch is abandoned before the option se Line 1072  branch is abandoned before the option se
1072  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1073  behaviour otherwise.  behaviour otherwise.
1074  .P  .P
1075  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1076  same way as the Perl-compatible options by using the characters U and X  application when the compile or match functions are called. In some cases the
1077  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences such as (*CRLF) to override what
1078  earlier in the pattern than any of the additional features it turns on, even  the application has set or what has been defaulted. Details are given in the
1079  when it is at top level. It is best to put it at the start.  section entitled
1080    .\" HTML <a href="#newlineseq">
1081    .\" </a>
1082    "Newline sequences"
1083    .\"
1084    above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1085    mode; this is equivalent to setting the PCRE_UTF8 option.
1086  .  .
1087  .  .
1088  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 729  Turning part of a pattern into a subpatt Line 1097  Turning part of a pattern into a subpatt
1097    cat(aract|erpillar|)    cat(aract|erpillar|)
1098  .sp  .sp
1099  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1100  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1101  .sp  .sp
1102  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1103  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
# Line 754  the string "the white queen" is matched Line 1122  the string "the white queen" is matched
1122    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1123  .sp  .sp
1124  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
1125  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1126  .P  .P
1127  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
1128  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 770  is reached, an option setting in one bra Line 1137  is reached, an option setting in one bra
1137  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1138  .  .
1139  .  .
1140    .\" HTML <a name="dupsubpatternnumber"></a>
1141    .SH "DUPLICATE SUBPATTERN NUMBERS"
1142    .rs
1143    .sp
1144    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1145    the same numbers for its capturing parentheses. Such a subpattern starts with
1146    (?| and is itself a non-capturing subpattern. For example, consider this
1147    pattern:
1148    .sp
1149      (?|(Sat)ur|(Sun))day
1150    .sp
1151    Because the two alternatives are inside a (?| group, both sets of capturing
1152    parentheses are numbered one. Thus, when the pattern matches, you can look
1153    at captured substring number one, whichever alternative matched. This construct
1154    is useful when you want to capture part, but not all, of one of a number of
1155    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1156    number is reset at the start of each branch. The numbers of any capturing
1157    buffers that follow the subpattern start after the highest number used in any
1158    branch. The following example is taken from the Perl documentation.
1159    The numbers underneath show in which buffer the captured content will be
1160    stored.
1161    .sp
1162      # before  ---------------branch-reset----------- after
1163      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164      # 1            2         2  3        2     3     4
1165    .sp
1166    A back reference to a numbered subpattern uses the most recent value that is
1167    set for that number by any subpattern. The following pattern matches "abcabc"
1168    or "defdef":
1169    .sp
1170      /(?|(abc)|(def))\e1/
1171    .sp
1172    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1173    refers to the first one in the pattern with the given number. The following
1174    pattern matches "abcabc" or "defabc":
1175    .sp
1176      /(?|(abc)|(def))(?1)/
1177    .sp
1178    If a
1179    .\" HTML <a href="#conditions">
1180    .\" </a>
1181    condition test
1182    .\"
1183    for a subpattern's having matched refers to a non-unique number, the test is
1184    true if any of the subpatterns of that number have matched.
1185    .P
1186    An alternative approach to using this "branch reset" feature is to use
1187    duplicate named subpatterns, as described in the next section.
1188    .
1189    .
1190  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1191  .rs  .rs
1192  .sp  .sp
1193  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
1194  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1195  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1196  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1197  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1198  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1199  .P  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1200  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1201  PCRE API provides function calls for extracting the name-to-number translation  .P
1202  table from a compiled pattern. There is also a convenience function for  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1203  extracting a captured substring by name. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1204    parentheses from other parts of the pattern, such as
1205    .\" HTML <a href="#backreferences">
1206    .\" </a>
1207    back references,
1208    .\"
1209    .\" HTML <a href="#recursion">
1210    .\" </a>
1211    recursion,
1212    .\"
1213    and
1214    .\" HTML <a href="#conditions">
1215    .\" </a>
1216    conditions,
1217    .\"
1218    can be made by name as well as by number.
1219    .P
1220    Names consist of up to 32 alphanumeric characters and underscores. Named
1221    capturing parentheses are still allocated numbers as well as names, exactly as
1222    if the names were not present. The PCRE API provides function calls for
1223    extracting the name-to-number translation table from a compiled pattern. There
1224    is also a convenience function for extracting a captured substring by name.
1225    .P
1226    By default, a name must be unique within a pattern, but it is possible to relax
1227    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1228    names are also always permitted for subpatterns with the same number, set up as
1229    described in the previous section.) Duplicate names can be useful for patterns
1230    where only one instance of the named parentheses can match. Suppose you want to
1231    match the name of a weekday, either as a 3-letter abbreviation or as the full
1232    name, and in both cases you want to extract the abbreviation. This pattern
1233    (ignoring the line breaks) does the job:
1234    .sp
1235      (?<DN>Mon|Fri|Sun)(?:day)?|
1236      (?<DN>Tue)(?:sday)?|
1237      (?<DN>Wed)(?:nesday)?|
1238      (?<DN>Thu)(?:rsday)?|
1239      (?<DN>Sat)(?:urday)?
1240    .sp
1241    There are five capturing substrings, but only one is ever set after a match.
1242    (An alternative way of solving this problem is to use a "branch reset"
1243    subpattern, as described in the previous section.)
1244    .P
1245    The convenience function for extracting the data by name returns the substring
1246    for the first (and in this example, the only) subpattern of that name that
1247    matched. This saves searching to find which numbered subpattern it was.
1248    .P
1249    If you make a back reference to a non-unique named subpattern from elsewhere in
1250    the pattern, the one that corresponds to the first occurrence of the name is
1251    used. In the absence of duplicate numbers (see the previous section) this is
1252    the one with the lowest number. If you use a named reference in a condition
1253    test (see the
1254    .\"
1255    .\" HTML <a href="#conditions">
1256    .\" </a>
1257    section about conditions
1258    .\"
1259    below), either to check whether a subpattern has matched, or to check for
1260    recursion, all subpatterns with the same name are tested. If the condition is
1261    true for any one of them, the overall condition is true. This is the same
1262    behaviour as testing by number. For further details of the interfaces for
1263    handling named subpatterns, see the
1264  .\" HREF  .\" HREF
1265  \fBpcreapi\fP  \fBpcreapi\fP
1266  .\"  .\"
1267  documentation.  documentation.
1268    .P
1269    \fBWarning:\fP You cannot use different names to distinguish between two
1270    subpatterns with the same number because PCRE uses only the numbers when
1271    matching. For this reason, an error is given at compile time if different names
1272    are given to subpatterns with the same number. However, you can give the same
1273    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1274  .  .
1275  .  .
1276  .SH REPETITION  .SH REPETITION
# Line 797  Repetition is specified by quantifiers, Line 1280  Repetition is specified by quantifiers,
1280  items:  items:
1281  .sp  .sp
1282    a literal data character    a literal data character
1283    the . metacharacter    the dot metacharacter
1284    the \eC escape sequence    the \eC escape sequence
1285    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1286      the \eR escape sequence
1287    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1288    a character class    a character class
1289    a back reference (see next section)    a back reference (see next section)
1290    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1291      a recursive or "subroutine" call to a subpattern
1292  .sp  .sp
1293  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1294  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 835  support is available, \eX{3} matches thr Line 1320  support is available, \eX{3} matches thr
1320  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1321  .P  .P
1322  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
1323  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1324    subpatterns that are referenced as
1325    .\" HTML <a href="#subpatternsassubroutines">
1326    .\" </a>
1327    subroutines
1328    .\"
1329    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1330    quantifier are omitted from the compiled pattern.
1331  .P  .P
1332  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1333  quantifiers have single-character abbreviations:  abbreviations:
1334  .sp  .sp
1335    *    is equivalent to {0,}    *    is equivalent to {0,}
1336    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 886  own right. Because it has two uses, it c Line 1378  own right. Because it has two uses, it c
1378  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
1379  way the rest of the pattern matches.  way the rest of the pattern matches.
1380  .P  .P
1381  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1382  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1383  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1384  default behaviour.  default behaviour.
# Line 896  is greater than 1 or with a limited maxi Line 1388  is greater than 1 or with a limited maxi
1388  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1389  .P  .P
1390  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1391  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1392  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1393  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1394  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
# Line 907  worth setting PCRE_DOTALL in order to ob Line 1399  worth setting PCRE_DOTALL in order to ob
1399  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1400  .P  .P
1401  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1402  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1403  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 where a later one
1404  succeed. Consider, for example:  succeeds. Consider, for example:
1405  .sp  .sp
1406    (.*)abc\e1    (.*)abc\e1
1407  .sp  .sp
# Line 935  matches "aba" the value of the second ca Line 1427  matches "aba" the value of the second ca
1427  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1428  .rs  .rs
1429  .sp  .sp
1430  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1431  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1432  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1433  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1434  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1435  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1436  .P  .P
1437  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1438  .sp  .sp
# Line 952  item, and then with 4, and so on, before Line 1444  item, and then with 4, and so on, before
1444  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1445  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1446  .P  .P
1447  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 gives up
1448  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
1449  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1450  .sp  .sp
# Line 982  previous example can be rewritten as Line 1474  previous example can be rewritten as
1474  .sp  .sp
1475    \ed++foo    \ed++foo
1476  .sp  .sp
1477    Note that a possessive quantifier can be used with an entire group, for
1478    example:
1479    .sp
1480      (abc|xyz){2,3}+
1481    .sp
1482  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1483  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
1484  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1485  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1486  .P  difference; possessive quantifiers should be slightly faster.
1487  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1488  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1489    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1490    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1491    package, and PCRE copied it from there. It ultimately found its way into Perl
1492    at release 5.10.
1493    .P
1494    PCRE has an optimization that automatically "possessifies" certain simple
1495    pattern constructs. For example, the sequence A+B is treated as A++B because
1496    there is no point in backtracking into a sequence of A's when B must follow.
1497  .P  .P
1498  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1499  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
# Line 1030  However, if the decimal number following Line 1535  However, if the decimal number following
1535  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
1536  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
1537  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
1538  numbers less than 10. See the subsection entitled "Non-printing characters"  numbers less than 10. A "forward back reference" of this type can make sense
1539    when a repetition is involved and the subpattern to the right has participated
1540    in an earlier iteration.
1541    .P
1542    It is not possible to have a numerical "forward back reference" to a subpattern
1543    whose number is 10 or more using this syntax because a sequence such as \e50 is
1544    interpreted as a character defined in octal. See the subsection entitled
1545    "Non-printing characters"
1546  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1547  .\" </a>  .\" </a>
1548  above  above
1549  .\"  .\"
1550  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1551    no such problem when named parentheses are used. A back reference to any
1552    subpattern is possible using named parentheses (see below).
1553    .P
1554    Another way of avoiding the ambiguity inherent in the use of digits following a
1555    backslash is to use the \eg escape sequence, which is a feature introduced in
1556    Perl 5.10. This escape must be followed by an unsigned number or a negative
1557    number, optionally enclosed in braces. These examples are all identical:
1558    .sp
1559      (ring), \e1
1560      (ring), \eg1
1561      (ring), \eg{1}
1562    .sp
1563    An unsigned number specifies an absolute reference without the ambiguity that
1564    is present in the older syntax. It is also useful when literal digits follow
1565    the reference. A negative number is a relative reference. Consider this
1566    example:
1567    .sp
1568      (abc(def)ghi)\eg{-1}
1569    .sp
1570    The sequence \eg{-1} is a reference to the most recently started capturing
1571    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1572    would be equivalent to \e1. The use of relative references can be helpful in
1573    long patterns, and also in patterns that are created by joining together
1574    fragments that contain references within themselves.
1575  .P  .P
1576  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1577  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1057  back reference, the case of letters is r Line 1593  back reference, the case of letters is r
1593  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
1594  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1595  .P  .P
1596  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1597  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1598  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1599    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1600    references, is also supported. We could rewrite the above example in any of
1601    the following ways:
1602    .sp
1603      (?<p1>(?i)rah)\es+\ek<p1>
1604      (?'p1'(?i)rah)\es+\ek{p1}
1605      (?P<p1>(?i)rah)\es+(?P=p1)
1606      (?<p1>(?i)rah)\es+\eg{p1}
1607  .sp  .sp
1608    A subpattern that is referenced by name may appear in the pattern before or
1609    after the reference.
1610    .P
1611  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
1612  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1613  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1614  .sp  .sp
1615    (a|(bc))\e2    (a|(bc))\e2
1616  .sp  .sp
1617  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". However, if the
1618  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1619  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1620  with a digit character, some delimiter must be used to terminate the back  .P
1621  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1622  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1623    If the pattern continues with a digit character, some delimiter must be used to
1624    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1625    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1626  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1627  .\" </a>  .\" </a>
1628  "Comments"  "Comments"
1629  .\"  .\"
1630  below) can be used.  below) can be used.
1631  .P  .
1632    .SS "Recursive back references"
1633    .rs
1634    .sp
1635  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
1636  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1637  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1093  to the previous iteration. In order for Line 1645  to the previous iteration. In order for
1645  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
1646  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
1647  minimum of zero.  minimum of zero.
1648    .P
1649    Back references of this type cause the group that they reference to be treated
1650    as an
1651    .\" HTML <a href="#atomicgroup">
1652    .\" </a>
1653    atomic group.
1654    .\"
1655    Once the whole group has been matched, a subsequent matching failure cannot
1656    cause backtracking into the middle of the group.
1657  .  .
1658  .  .
1659  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1123  because it does not make sense for negat Line 1684  because it does not make sense for negat
1684  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1685  .rs  .rs
1686  .sp  .sp
1687  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1688  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1689  .sp  .sp
1690    \ew+(?=;)    \ew+(?=;)
1691  .sp  .sp
# Line 1146  lookbehind assertion is needed to achiev Line 1707  lookbehind assertion is needed to achiev
1707  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
1708  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
1709  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.
1710    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1711    synonym for (?!).
1712  .  .
1713  .  .
1714  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1159  negative assertions. For example, Line 1722  negative assertions. For example,
1722  .sp  .sp
1723  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
1724  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
1725  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1726  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1727  .sp  .sp
1728    (?<=bullock|donkey)    (?<=bullock|donkey)
1729  .sp  .sp
# Line 1170  is permitted, but Line 1733  is permitted, but
1733  .sp  .sp
1734  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1735  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
1736  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (5.8 and 5.10), which requires all branches to
1737  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1738  .sp  .sp
1739    (?<=ab(c|de))    (?<=ab(c|de))
1740  .sp  .sp
1741  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
1742  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1743    branches:
1744  .sp  .sp
1745    (?<=abc|abde)    (?<=abc|abde)
1746  .sp  .sp
1747    In some cases, the Perl 5.10 escape sequence \eK
1748    .\" HTML <a href="#resetmatchstart">
1749    .\" </a>
1750    (see above)
1751    .\"
1752    can be used instead of a lookbehind assertion to get round the fixed-length
1753    restriction.
1754    .P
1755  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1756  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1757  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1758  match is deemed to fail.  assertion fails.
1759  .P  .P
1760  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1761  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1762  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1763  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1764    .P
1765    .\" HTML <a href="#subpatternsassubroutines">
1766    .\" </a>
1767    "Subroutine"
1768    .\"
1769    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1770    as the subpattern matches a fixed-length string.
1771    .\" HTML <a href="#recursion">
1772    .\" </a>
1773    Recursion,
1774    .\"
1775    however, is not supported.
1776  .P  .P
1777  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1778  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching of fixed-length strings at the end of subject
1779  such as  strings. Consider a simple pattern such as
1780  .sp  .sp
1781    abcd$    abcd$
1782  .sp  .sp
# Line 1208  then all but the last two characters, an Line 1792  then all but the last two characters, an
1792  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,
1793  if the pattern is written as  if the pattern is written as
1794  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1795    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1796  .sp  .sp
1797  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
1798  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
1799  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1800  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1254  is another pattern that matches "foo" pr Line 1834  is another pattern that matches "foo" pr
1834  characters that are not "999".  characters that are not "999".
1835  .  .
1836  .  .
1837    .\" HTML <a name="conditions"></a>
1838  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
1839  .rs  .rs
1840  .sp  .sp
1841  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1842  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1843  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1844  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1845  .sp  .sp
1846    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1847    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1269  If the condition is satisfied, the yes-p Line 1850  If the condition is satisfied, the yes-p
1850  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
1851  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1852  .P  .P
1853  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1854  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1855  subpattern of that number has previously matched. The number must be greater  .
1856  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1857  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1858  it into three parts for ease of discussion:  .sp
1859    If the text between the parentheses consists of a sequence of digits, the
1860    condition is true if a capturing subpattern of that number has previously
1861    matched. If there is more than one capturing subpattern with the same number
1862    (see the earlier
1863    .\"
1864    .\" HTML <a href="#recursion">
1865    .\" </a>
1866    section about duplicate subpattern numbers),
1867    .\"
1868    the condition is true if any of them have been set. An alternative notation is
1869    to precede the digits with a plus or minus sign. In this case, the subpattern
1870    number is relative rather than absolute. The most recently opened parentheses
1871    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1872    looping constructs it can also make sense to refer to subsequent groups with
1873    constructs such as (?(+2).
1874    .P
1875    Consider the following pattern, which contains non-significant white space to
1876    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1877    three parts for ease of discussion:
1878  .sp  .sp
1879    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
1880  .sp  .sp
# Line 1288  parenthesis is required. Otherwise, sinc Line 1888  parenthesis is required. Otherwise, sinc
1888  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1889  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1890  .P  .P
1891  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
1892  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1893  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1894      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1895    .sp
1896    This makes the fragment independent of the parentheses in the larger pattern.
1897    .
1898    .SS "Checking for a used subpattern by name"
1899    .rs
1900    .sp
1901    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1902    subpattern by name. For compatibility with earlier versions of PCRE, which had
1903    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1904    there is a possible ambiguity with this syntax, because subpattern names may
1905    consist entirely of digits. PCRE looks first for a named subpattern; if it
1906    cannot find one and the name consists entirely of digits, PCRE looks for a
1907    subpattern of that number, which must be greater than zero. Using subpattern
1908    names that consist entirely of digits is not recommended.
1909    .P
1910    Rewriting the above example to use a named subpattern gives this:
1911    .sp
1912      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1913    .sp
1914    If the name used in a condition of this kind is a duplicate, the test is
1915    applied to all subpatterns of the same name, and is true if any one of them has
1916    matched.
1917    .
1918    .SS "Checking for pattern recursion"
1919    .rs
1920    .sp
1921    If the condition is the string (R), and there is no subpattern with the name R,
1922    the condition is true if a recursive call to the whole pattern or any
1923    subpattern has been made. If digits or a name preceded by ampersand follow the
1924    letter R, for example:
1925    .sp
1926      (?(R3)...) or (?(R&name)...)
1927    .sp
1928    the condition is true if the most recent recursion is into a subpattern whose
1929    number or name is given. This condition does not check the entire recursion
1930    stack. If the name used in a condition of this kind is a duplicate, the test is
1931    applied to all subpatterns of the same name, and is true if any one of them is
1932    the most recent recursion.
1933  .P  .P
1934  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
1935    .\" HTML <a href="#recursion">
1936    .\" </a>
1937    The syntax for recursive patterns
1938    .\"
1939    is described below.
1940    .
1941    .SS "Defining subpatterns for use by reference only"
1942    .rs
1943    .sp
1944    If the condition is the string (DEFINE), and there is no subpattern with the
1945    name DEFINE, the condition is always false. In this case, there may be only one
1946    alternative in the subpattern. It is always skipped if control reaches this
1947    point in the pattern; the idea of DEFINE is that it can be used to define
1948    "subroutines" that can be referenced from elsewhere. (The use of
1949    .\" HTML <a href="#subpatternsassubroutines">
1950    .\" </a>
1951    "subroutines"
1952    .\"
1953    is described below.) For example, a pattern to match an IPv4 address could be
1954    written like this (ignore whitespace and line breaks):
1955    .sp
1956      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1957      \eb (?&byte) (\e.(?&byte)){3} \eb
1958    .sp
1959    The first part of the pattern is a DEFINE group inside which a another group
1960    named "byte" is defined. This matches an individual component of an IPv4
1961    address (a number less than 256). When matching takes place, this part of the
1962    pattern is skipped because DEFINE acts like a false condition. The rest of the
1963    pattern uses references to the named group to match the four dot-separated
1964    components of an IPv4 address, insisting on a word boundary at each end.
1965    .
1966    .SS "Assertion conditions"
1967    .rs
1968    .sp
1969    If the condition is not in any of the above formats, it must be an assertion.
1970  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1971  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1972  alternatives on the second line:  alternatives on the second line:
# Line 1317  closing parenthesis. Nested parentheses Line 1991  closing parenthesis. Nested parentheses
1991  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.
1992  .P  .P
1993  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1994  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1995  character in the pattern.  next newline in the pattern.
1996  .  .
1997  .  .
1998    .\" HTML <a name="recursion"></a>
1999  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
2000  .rs  .rs
2001  .sp  .sp
2002  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2003  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2004  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
2005  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2006  that allows regular expressions to recurse (amongst other things). It does this  .P
2007  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
2008  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
2009  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2010    pattern using code interpolation to solve the parentheses problem can be
2011    created like this:
2012  .sp  .sp
2013    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2014  .sp  .sp
2015  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
2016  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2017  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2018  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2019  .P  supports special syntax for recursion of the entire pattern, and also for
2020  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2021  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2022  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2023  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2024  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2025    provided that it occurs inside that subpattern. (If not, it is a
2026    .\" HTML <a href="#subpatternsassubroutines">
2027    .\" </a>
2028    "subroutine"
2029    .\"
2030    call, which is described in the next section.) The special item (?R) or (?0) is
2031    a recursive call of the entire regular expression.
2032  .P  .P
2033  For example, this PCRE pattern solves the nested parentheses problem (assume  This PCRE pattern solves the nested parentheses problem (assume the
2034  the PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2035  .sp  .sp
2036    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2037  .sp  .sp
2038  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2039  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
2040  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2041  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2042    to avoid backtracking into sequences of non-parentheses.
2043  .P  .P
2044  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
2045  pattern, so instead you could use this:  pattern, so instead you could use this:
2046  .sp  .sp
2047    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2048  .sp  .sp
2049  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
2050  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2051  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2052  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2053  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references (a Perl 5.10 feature).
2054  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2055  .sp  most recently opened parentheses preceding the recursion. In other words, a
2056    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
2057  .sp  it is encountered.
2058  This particular example pattern contains nested unlimited repeats, and so the  .P
2059  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2060  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
2061  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2062    .\" HTML <a href="#subpatternsassubroutines">
2063    .\" </a>
2064    "subroutine"
2065    .\"
2066    calls, as described in the next section.
2067    .P
2068    An alternative approach is to use named parentheses instead. The Perl syntax
2069    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2070    could rewrite the above example as follows:
2071    .sp
2072      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2073    .sp
2074    If there is more than one subpattern with the same name, the earliest one is
2075    used.
2076    .P
2077    This particular example pattern that we have been looking at contains nested
2078    unlimited repeats, and so the use of a possessive quantifier for matching
2079    strings of non-parentheses is important when applying the pattern to strings
2080    that do not match. For example, when this pattern is applied to
2081  .sp  .sp
2082    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2083  .sp  .sp
2084  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2085  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
2086  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
2087  before failure can be reported.  before failure can be reported.
2088  .P  .P
2089  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2090  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2091  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2092  .\" HREF  .\" HREF
2093  \fBpcrecallout\fP  \fBpcrecallout\fP
2094  .\"  .\"
# Line 1393  documentation). If the pattern above is Line 2096  documentation). If the pattern above is
2096  .sp  .sp
2097    (ab(cd)ef)    (ab(cd)ef)
2098  .sp  .sp
2099  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2100  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2101  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2102    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2103       ^                        ^  .P
2104       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2105  .sp  obtain extra memory to store data during a recursion, which it does by using
2106  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2107  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2108  .P  .P
2109  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2110  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1418  different alternatives for the recursive Line 2118  different alternatives for the recursive
2118  is the actual recursive call.  is the actual recursive call.
2119  .  .
2120  .  .
2121    .\" HTML <a name="recursiondifference"></a>
2122    .SS "Recursion difference from Perl"
2123    .rs
2124    .sp
2125    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2126    treated as an atomic group. That is, once it has matched some of the subject
2127    string, it is never re-entered, even if it contains untried alternatives and
2128    there is a subsequent matching failure. This can be illustrated by the
2129    following pattern, which purports to match a palindromic string that contains
2130    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2131    .sp
2132      ^(.|(.)(?1)\e2)$
2133    .sp
2134    The idea is that it either matches a single character, or two identical
2135    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2136    it does not if the pattern is longer than three characters. Consider the
2137    subject string "abcba":
2138    .P
2139    At the top level, the first character is matched, but as it is not at the end
2140    of the string, the first alternative fails; the second alternative is taken
2141    and the recursion kicks in. The recursive call to subpattern 1 successfully
2142    matches the next character ("b"). (Note that the beginning and end of line
2143    tests are not part of the recursion).
2144    .P
2145    Back at the top level, the next character ("c") is compared with what
2146    subpattern 2 matched, which was "a". This fails. Because the recursion is
2147    treated as an atomic group, there are now no backtracking points, and so the
2148    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2149    try the second alternative.) However, if the pattern is written with the
2150    alternatives in the other order, things are different:
2151    .sp
2152      ^((.)(?1)\e2|.)$
2153    .sp
2154    This time, the recursing alternative is tried first, and continues to recurse
2155    until it runs out of characters, at which point the recursion fails. But this
2156    time we do have another alternative to try at the higher level. That is the big
2157    difference: in the previous case the remaining alternative is at a deeper
2158    recursion level, which PCRE cannot use.
2159    .P
2160    To change the pattern so that matches all palindromic strings, not just those
2161    with an odd number of characters, it is tempting to change the pattern to this:
2162    .sp
2163      ^((.)(?1)\e2|.?)$
2164    .sp
2165    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2166    deeper recursion has matched a single character, it cannot be entered again in
2167    order to match an empty string. The solution is to separate the two cases, and
2168    write out the odd and even cases as alternatives at the higher level:
2169    .sp
2170      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2171    .sp
2172    If you want to match typical palindromic phrases, the pattern has to ignore all
2173    non-word characters, which can be done like this:
2174    .sp
2175      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2176    .sp
2177    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2178    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2179    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2180    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2181    more) to match typical phrases, and Perl takes so long that you think it has
2182    gone into a loop.
2183    .P
2184    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2185    string does not start with a palindrome that is shorter than the entire string.
2186    For example, although "abcba" is correctly matched, if the subject is "ababa",
2187    PCRE finds the palindrome "aba" at the start, then fails at top level because
2188    the end of the string does not follow. Once again, it cannot jump back into the
2189    recursion to try other alternatives, so the entire match fails.
2190    .
2191    .
2192  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2193  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2194  .rs  .rs
2195  .sp  .sp
2196  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
2197  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
2198  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2199  pattern  before or after the reference. A numbered reference can be absolute or
2200    relative, as in these examples:
2201    .sp
2202      (...(absolute)...)...(?2)...
2203      (...(relative)...)...(?-1)...
2204      (...(?+1)...(relative)...
2205    .sp
2206    An earlier example pointed out that the pattern
2207  .sp  .sp
2208    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2209  .sp  .sp
# Line 1435  matches "sense and sensibility" and "res Line 2213  matches "sense and sensibility" and "res
2213    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2214  .sp  .sp
2215  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
2216  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2217  refer.  .P
2218    Like recursive subpatterns, a subroutine call is always treated as an atomic
2219    group. That is, once it has matched some of the subject string, it is never
2220    re-entered, even if it contains untried alternatives and there is a subsequent
2221    matching failure. Any capturing parentheses that are set during the subroutine
2222    call revert to their previous values afterwards.
2223    .P
2224    When a subpattern is used as a subroutine, processing options such as
2225    case-independence are fixed when the subpattern is defined. They cannot be
2226    changed for different calls. For example, consider this pattern:
2227    .sp
2228      (abc)(?i:(?-1))
2229    .sp
2230    It matches "abcabc". It does not match "abcABC" because the change of
2231    processing option does not affect the called subpattern.
2232    .
2233    .
2234    .\" HTML <a name="onigurumasubroutines"></a>
2235    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2236    .rs
2237    .sp
2238    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2239    a number enclosed either in angle brackets or single quotes, is an alternative
2240    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2241    are two of the examples used above, rewritten using this syntax:
2242    .sp
2243      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2244      (sens|respons)e and \eg'1'ibility
2245    .sp
2246    PCRE supports an extension to Oniguruma: if a number is preceded by a
2247    plus or a minus sign it is taken as a relative reference. For example:
2248    .sp
2249      (abc)(?i:\eg<-1>)
2250    .sp
2251    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2252    synonymous. The former is a back reference; the latter is a subroutine call.
2253  .  .
2254  .  .
2255  .SH CALLOUTS  .SH CALLOUTS
# Line 1457  function is to be called. If you want to Line 2270  function is to be called. If you want to
2270  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.
2271  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2272  .sp  .sp
2273    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2274  .sp  .sp
2275  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2276  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1473  description of the interface to the call Line 2286  description of the interface to the call
2286  \fBpcrecallout\fP  \fBpcrecallout\fP
2287  .\"  .\"
2288  documentation.  documentation.
2289    .
2290    .
2291    .SH "BACKTRACKING CONTROL"
2292    .rs
2293    .sp
2294    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2295    are described in the Perl documentation as "experimental and subject to change
2296    or removal in a future version of Perl". It goes on to say: "Their usage in
2297    production code should be noted to avoid problems during upgrades." The same
2298    remarks apply to the PCRE features described in this section.
2299    .P
2300    Since these verbs are specifically related to backtracking, most of them can be
2301    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2302    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2303    failing negative assertion, they cause an error if encountered by
2304    \fBpcre_dfa_exec()\fP.
2305  .P  .P
2306  .in 0  If any of these verbs are used in an assertion or subroutine subpattern
2307  Last updated: 28 February 2005  (including recursive subpatterns), their effect is confined to that subpattern;
2308  .br  it does not extend to the surrounding pattern. Note that such subpatterns are
2309  Copyright (c) 1997-2005 University of Cambridge.  processed as anchored at the point where they are tested.
2310    .P
2311    The new verbs make use of what was previously invalid syntax: an opening
2312    parenthesis followed by an asterisk. In Perl, they are generally of the form
2313    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2314    form is just (*VERB). Any number of these verbs may occur in a pattern. There
2315    are two kinds:
2316    .
2317    .SS "Verbs that act immediately"
2318    .rs
2319    .sp
2320    The following verbs act as soon as they are encountered:
2321    .sp
2322       (*ACCEPT)
2323    .sp
2324    This verb causes the match to end successfully, skipping the remainder of the
2325    pattern. When inside a recursion, only the innermost pattern is ended
2326    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2327    captured. (This feature was added to PCRE at release 8.00.) For example:
2328    .sp
2329      A((?:A|B(*ACCEPT)|C)D)
2330    .sp
2331    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2332    the outer parentheses.
2333    .sp
2334      (*FAIL) or (*F)
2335    .sp
2336    This verb causes the match to fail, forcing backtracking to occur. It is
2337    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2338    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2339    Perl features that are not present in PCRE. The nearest equivalent is the
2340    callout feature, as for example in this pattern:
2341    .sp
2342      a+(?C)(*FAIL)
2343    .sp
2344    A match with the string "aaaa" always fails, but the callout is taken before
2345    each backtrack happens (in this example, 10 times).
2346    .
2347    .SS "Verbs that act after backtracking"
2348    .rs
2349    .sp
2350    The following verbs do nothing when they are encountered. Matching continues
2351    with what follows, but if there is no subsequent match, a failure is forced.
2352    The verbs differ in exactly what kind of failure occurs.
2353    .sp
2354      (*COMMIT)
2355    .sp
2356    This verb causes the whole match to fail outright if the rest of the pattern
2357    does not match. Even if the pattern is unanchored, no further attempts to find
2358    a match by advancing the starting point take place. Once (*COMMIT) has been
2359    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2360    starting point, or not at all. For example:
2361    .sp
2362      a+(*COMMIT)b
2363    .sp
2364    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2365    dynamic anchor, or "I've started, so I must finish."
2366    .sp
2367      (*PRUNE)
2368    .sp
2369    This verb causes the match to fail at the current position if the rest of the
2370    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2371    advance to the next starting character then happens. Backtracking can occur as
2372    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2373    if there is no match to the right, backtracking cannot cross (*PRUNE).
2374    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2375    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2376    be expressed in any other way.
2377    .sp
2378      (*SKIP)
2379    .sp
2380    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2381    "bumpalong" advance is not to the next character, but to the position in the
2382    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2383    was matched leading up to it cannot be part of a successful match. Consider:
2384    .sp
2385      a+(*SKIP)b
2386    .sp
2387    If the subject is "aaaac...", after the first match attempt fails (starting at
2388    the first character in the string), the starting point skips on to start the
2389    next attempt at "c". Note that a possessive quantifer does not have the same
2390    effect as this example; although it would suppress backtracking during the
2391    first match attempt, the second attempt would start at the second character
2392    instead of skipping on to "c".
2393    .sp
2394      (*THEN)
2395    .sp
2396    This verb causes a skip to the next alternation if the rest of the pattern does
2397    not match. That is, it cancels pending backtracking, but only within the
2398    current alternation. Its name comes from the observation that it can be used
2399    for a pattern-based if-then-else block:
2400    .sp
2401      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2402    .sp
2403    If the COND1 pattern matches, FOO is tried (and possibly further items after
2404    the end of the group if FOO succeeds); on failure the matcher skips to the
2405    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2406    is used outside of any alternation, it acts exactly like (*PRUNE).
2407    .
2408    .
2409    .SH "SEE ALSO"
2410    .rs
2411    .sp
2412    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2413    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2414    .
2415    .
2416    .SH AUTHOR
2417    .rs
2418    .sp
2419    .nf
2420    Philip Hazel
2421    University Computing Service
2422    Cambridge CB2 3QH, England.
2423    .fi
2424    .
2425    .
2426    .SH REVISION
2427    .rs
2428    .sp
2429    .nf
2430    Last updated: 11 January 2010
2431    Copyright (c) 1997-2010 University of Cambridge.
2432    .fi

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