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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 555 by ph10, Tue Oct 26 08:26:20 2010 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
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 26  in the main Line 42  in the main
42  .\"  .\"
43  page.  page.
44  .P  .P
45    Another special sequence that may appear at the start of a pattern or in
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
58  \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
59  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
60  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61    alternative function, and how it differs from the normal function, are
62    discussed in the
63  .\" HREF  .\" HREF
64  \fBpcrematching\fP  \fBpcrematching\fP
65  .\"  .\"
66  page.  page.
67    .
68    .
69    .SH "NEWLINE CONVENTIONS"
70    .rs
71    .sp
72    PCRE supports five different conventions for indicating line breaks in
73    strings: a single CR (carriage return) character, a single LF (linefeed)
74    character, the two-character sequence CRLF, any of the three preceding, or any
75    Unicode newline sequence. The
76    .\" HREF
77    \fBpcreapi\fP
78    .\"
79    page has
80    .\" HTML <a href="pcreapi.html#newlines">
81    .\" </a>
82    further discussion
83    .\"
84    about newlines, and shows how to set the newline convention in the
85    \fIoptions\fP arguments for the compiling and matching functions.
86  .P  .P
87    It is also possible to specify a newline convention by starting a pattern
88    string with one of the following five sequences:
89    .sp
90      (*CR)        carriage return
91      (*LF)        linefeed
92      (*CRLF)      carriage return, followed by linefeed
93      (*ANYCRLF)   any of the three above
94      (*ANY)       all Unicode newline sequences
95    .sp
96    These override the default and the options given to \fBpcre_compile()\fP or
97    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
98    newline sequence, the pattern
99    .sp
100      (*CR)a.b
101    .sp
102    changes the convention to CR. That pattern matches "a\enb" because LF is no
103    longer a newline. Note that these special settings, which are not
104    Perl-compatible, are recognized only at the very start of a pattern, and that
105    they must be in upper case. If more than one of them is present, the last one
106    is used.
107    .P
108    The newline convention affects the interpretation of the dot metacharacter when
109    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
110    affect what the \eR escape sequence matches. By default, this is any Unicode
111    newline sequence, for Perl compatibility. However, this can be changed; see the
112    description of \eR in the section entitled
113    .\" HTML <a href="#newlineseq">
114    .\" </a>
115    "Newline sequences"
116    .\"
117    below. A change of \eR setting can be combined with a change of newline
118    convention.
119    .
120    .
121    .SH "CHARACTERS AND METACHARACTERS"
122    .rs
123    .sp
124  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
125  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
126  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 144  interpreted in some special way.
144  .P  .P
145  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
146  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
147  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
148  as follows:  are as follows:
149  .sp  .sp
150    \e      general escape character with several uses    \e      general escape character with several uses
151    ^      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 176  a character class the only metacharacter
176  .sp  .sp
177  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
178  .  .
179    .
180  .SH BACKSLASH  .SH BACKSLASH
181  .rs  .rs
182  .sp  .sp
# Line 125  Perl, $ and @ cause variable interpolati Line 210  Perl, $ and @ cause variable interpolati
210    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
211  .sp  .sp
212  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
213    An isolated \eE that is not preceded by \eQ is ignored.
214  .  .
215  .  .
216  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 134  The \eQ...\eE sequence is recognized bot Line 220  The \eQ...\eE sequence is recognized bot
220  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
221  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
222  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
223  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
224  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:  
225  .sp  .sp
226    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
227    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
228    \ee        escape (hex 1B)    \ee        escape (hex 1B)
229    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
230    \en        newline (hex 0A)    \en        linefeed (hex 0A)
231    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
232    \et        tab (hex 09)    \et        tab (hex 09)
233    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
234    \exhh      character with hex code hh    \exhh      character with hex code hh
235    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
236  .sp  .sp
# Line 157  Thus \ecz becomes hex 1A, but \ec{ becom Line 242  Thus \ecz becomes hex 1A, but \ec{ becom
242  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
243  upper or lower case). Any number of hexadecimal digits may appear between \ex{  upper or lower case). Any number of hexadecimal digits may appear between \ex{
244  and }, but the value of the character code must be less than 256 in non-UTF-8  and }, but the value of the character code must be less than 256 in non-UTF-8
245  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
246  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
247  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
248  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
249  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
250    there is no terminating }, this form of escape is not recognized. Instead, the
251    initial \ex will be interpreted as a basic hexadecimal escape, with no
252    following digits, giving a character whose value is zero.
253  .P  .P
254  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
255  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex. There is no difference in the way they are handled. For
# Line 190  parenthesized subpatterns. Line 278  parenthesized subpatterns.
278  .P  .P
279  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
280  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
281  digits following the backslash, ane uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
282  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
283  character specified in octal must be less than \e400. In UTF-8 mode, values up  character specified in octal must be less than \e400. In UTF-8 mode, values up
284  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 220  zero, because no more than three octal d Line 308  zero, because no more than three octal d
308  .P  .P
309  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
310  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
311  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
312  sequence \eX is interpreted as the character "X". Outside a character class,  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
313  these sequences have different meanings  unrecognized escape sequences, they are treated as the literal characters "B",
314  .\" HTML <a href="#uniextseq">  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
315    set. Outside a character class, these sequences have different meanings.
316    .
317    .
318    .SS "Absolute and relative back references"
319    .rs
320    .sp
321    The sequence \eg followed by an unsigned or a negative number, optionally
322    enclosed in braces, is an absolute or relative back reference. A named back
323    reference can be coded as \eg{name}. Back references are discussed
324    .\" HTML <a href="#backreferences">
325  .\" </a>  .\" </a>
326  (see below).  later,
327    .\"
328    following the discussion of
329    .\" HTML <a href="#subpattern">
330    .\" </a>
331    parenthesized subpatterns.
332    .\"
333    .
334    .
335    .SS "Absolute and relative subroutine calls"
336    .rs
337    .sp
338    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
339    a number enclosed either in angle brackets or single quotes, is an alternative
340    syntax for referencing a subpattern as a "subroutine". Details are discussed
341    .\" HTML <a href="#onigurumasubroutines">
342    .\" </a>
343    later.
344    .\"
345    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
346    synonymous. The former is a back reference; the latter is a
347    .\" HTML <a href="#subpatternsassubroutines">
348    .\" </a>
349    subroutine
350  .\"  .\"
351    call.
352  .  .
353  .  .
354    .\" HTML <a name="genericchartypes"></a>
355  .SS "Generic character types"  .SS "Generic character types"
356  .rs  .rs
357  .sp  .sp
358  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
359  .sp  .sp
360    \ed     any decimal digit    \ed     any decimal digit
361    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
362      \eh     any horizontal whitespace character
363      \eH     any character that is not a horizontal whitespace character
364    \es     any whitespace character    \es     any whitespace character
365    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
366      \ev     any vertical whitespace character
367      \eV     any character that is not a vertical whitespace character
368    \ew     any "word" character    \ew     any "word" character
369    \eW     any "non-word" character    \eW     any "non-word" character
370  .sp  .sp
371  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
372  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
373    .\" HTML <a href="#fullstopdot">
374    .\" </a>
375    the "." metacharacter
376    .\"
377    when PCRE_DOTALL is not set.
378  .P  .P
379  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
380    of characters into two disjoint sets. Any given character matches one, and only
381    one, of each pair. The sequences can appear both inside and outside character
382  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
383  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
384  there is no character to match.  there is no character to match.
385  .P  .P
386  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).
387  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
388  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
389  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
390  does.)  does.
391  .P  .P
392  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
393  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
394  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
395  place (see  place (see
396  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 268  in the Line 401  in the
401  .\" HREF  .\" HREF
402  \fBpcreapi\fP  \fBpcreapi\fP
403  .\"  .\"
404  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,
405  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
406  .P  accented letters, and these are then matched by \ew. The use of locales with
407  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Unicode is discouraged.
408  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
409  character property support is available. The use of locales with Unicode is  By default, in UTF-8 mode, characters with values greater than 128 never match
410  discouraged.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
411    their original meanings from before UTF-8 support was available, mainly for
412    efficiency reasons. However, if PCRE is compiled with Unicode property support,
413    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
414    properties are used to determine character types, as follows:
415    .sp
416      \ed  any character that \ep{Nd} matches (decimal digit)
417      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
418      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
419    .sp
420    The upper case escapes match the inverse sets of characters. Note that \ed
421    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
422    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
423    \eB because they are defined in terms of \ew and \eW. Matching these sequences
424    is noticeably slower when PCRE_UCP is set.
425    .P
426    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
427    other sequences, which match only ASCII characters by default, these always
428    match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
429    set. The horizontal space characters are:
430    .sp
431      U+0009     Horizontal tab
432      U+0020     Space
433      U+00A0     Non-break space
434      U+1680     Ogham space mark
435      U+180E     Mongolian vowel separator
436      U+2000     En quad
437      U+2001     Em quad
438      U+2002     En space
439      U+2003     Em space
440      U+2004     Three-per-em space
441      U+2005     Four-per-em space
442      U+2006     Six-per-em space
443      U+2007     Figure space
444      U+2008     Punctuation space
445      U+2009     Thin space
446      U+200A     Hair space
447      U+202F     Narrow no-break space
448      U+205F     Medium mathematical space
449      U+3000     Ideographic space
450    .sp
451    The vertical space characters are:
452    .sp
453      U+000A     Linefeed
454      U+000B     Vertical tab
455      U+000C     Formfeed
456      U+000D     Carriage return
457      U+0085     Next line
458      U+2028     Line separator
459      U+2029     Paragraph separator
460    .
461    .
462    .\" HTML <a name="newlineseq"></a>
463    .SS "Newline sequences"
464    .rs
465    .sp
466    Outside a character class, by default, the escape sequence \eR matches any
467    Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
468    equivalent to the following:
469    .sp
470      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
471    .sp
472    This is an example of an "atomic group", details of which are given
473    .\" HTML <a href="#atomicgroup">
474    .\" </a>
475    below.
476    .\"
477    This particular group matches either the two-character sequence CR followed by
478    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
479    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
480    line, U+0085). The two-character sequence is treated as a single unit that
481    cannot be split.
482    .P
483    In UTF-8 mode, two additional characters whose codepoints are greater than 255
484    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
485    Unicode character property support is not needed for these characters to be
486    recognized.
487    .P
488    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
489    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
490    either at compile time or when the pattern is matched. (BSR is an abbrevation
491    for "backslash R".) This can be made the default when PCRE is built; if this is
492    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
493    It is also possible to specify these settings by starting a pattern string with
494    one of the following sequences:
495    .sp
496      (*BSR_ANYCRLF)   CR, LF, or CRLF only
497      (*BSR_UNICODE)   any Unicode newline sequence
498    .sp
499    These override the default and the options given to \fBpcre_compile()\fP or
500    \fBpcre_compile2()\fP, but they can be overridden by options given to
501    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
502    which are not Perl-compatible, are recognized only at the very start of a
503    pattern, and that they must be in upper case. If more than one of them is
504    present, the last one is used. They can be combined with a change of newline
505    convention; for example, a pattern can start with:
506    .sp
507      (*ANY)(*BSR_ANYCRLF)
508    .sp
509    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
510    a character class, \eR is treated as an unrecognized escape sequence, and so
511    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
512  .  .
513  .  .
514  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 516  discouraged.
516  .rs  .rs
517  .sp  .sp
518  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
519  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
520  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
521    characters whose codepoints are less than 256, but they do work in this mode.
522    The extra escape sequences are:
523  .sp  .sp
524    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
525    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
526    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
527  .sp  .sp
528  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
529  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
530  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
531  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
532  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
533    .\" </a>
534    next section).
535    .\"
536    Other Perl properties such as "InMusicalSymbols" are not currently supported by
537    PCRE. Note that \eP{Any} does not match any characters, so always causes a
538    match failure.
539  .P  .P
540  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
541  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 307  Those that are not part of an identified Line 549  Those that are not part of an identified
549  .P  .P
550  Arabic,  Arabic,
551  Armenian,  Armenian,
552    Avestan,
553    Balinese,
554    Bamum,
555  Bengali,  Bengali,
556  Bopomofo,  Bopomofo,
557  Braille,  Braille,
558  Buginese,  Buginese,
559  Buhid,  Buhid,
560  Canadian_Aboriginal,  Canadian_Aboriginal,
561    Carian,
562    Cham,
563  Cherokee,  Cherokee,
564  Common,  Common,
565  Coptic,  Coptic,
566    Cuneiform,
567  Cypriot,  Cypriot,
568  Cyrillic,  Cyrillic,
569  Deseret,  Deseret,
570  Devanagari,  Devanagari,
571    Egyptian_Hieroglyphs,
572  Ethiopic,  Ethiopic,
573  Georgian,  Georgian,
574  Glagolitic,  Glagolitic,
# Line 332  Hangul, Line 581  Hangul,
581  Hanunoo,  Hanunoo,
582  Hebrew,  Hebrew,
583  Hiragana,  Hiragana,
584    Imperial_Aramaic,
585  Inherited,  Inherited,
586    Inscriptional_Pahlavi,
587    Inscriptional_Parthian,
588    Javanese,
589    Kaithi,
590  Kannada,  Kannada,
591  Katakana,  Katakana,
592    Kayah_Li,
593  Kharoshthi,  Kharoshthi,
594  Khmer,  Khmer,
595  Lao,  Lao,
596  Latin,  Latin,
597    Lepcha,
598  Limbu,  Limbu,
599  Linear_B,  Linear_B,
600    Lisu,
601    Lycian,
602    Lydian,
603  Malayalam,  Malayalam,
604    Meetei_Mayek,
605  Mongolian,  Mongolian,
606  Myanmar,  Myanmar,
607  New_Tai_Lue,  New_Tai_Lue,
608    Nko,
609  Ogham,  Ogham,
610  Old_Italic,  Old_Italic,
611  Old_Persian,  Old_Persian,
612    Old_South_Arabian,
613    Old_Turkic,
614    Ol_Chiki,
615  Oriya,  Oriya,
616  Osmanya,  Osmanya,
617    Phags_Pa,
618    Phoenician,
619    Rejang,
620  Runic,  Runic,
621    Samaritan,
622    Saurashtra,
623  Shavian,  Shavian,
624  Sinhala,  Sinhala,
625    Sundanese,
626  Syloti_Nagri,  Syloti_Nagri,
627  Syriac,  Syriac,
628  Tagalog,  Tagalog,
629  Tagbanwa,  Tagbanwa,
630  Tai_Le,  Tai_Le,
631    Tai_Tham,
632    Tai_Viet,
633  Tamil,  Tamil,
634  Telugu,  Telugu,
635  Thaana,  Thaana,
# Line 365  Thai, Line 637  Thai,
637  Tibetan,  Tibetan,
638  Tifinagh,  Tifinagh,
639  Ugaritic,  Ugaritic,
640    Vai,
641  Yi.  Yi.
642  .P  .P
643  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
644  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
645  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
646  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
647  .P  .P
648  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
649  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 430  The special property L& is also supporte Line 703  The special property L& is also supporte
703  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
704  a modifier or "other".  a modifier or "other".
705  .P  .P
706  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
707    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
708    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
709    (see the discussion of PCRE_NO_UTF8_CHECK in the
710    .\" HREF
711    \fBpcreapi\fP
712    .\"
713    page). Perl does not support the Cs property.
714    .P
715    The long synonyms for property names that Perl supports (such as \ep{Letter})
716  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
717  properties with "Is".  properties with "Is".
718  .P  .P
# Line 454  atomic group Line 736  atomic group
736  (see below).  (see below).
737  .\"  .\"
738  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
739  preceding character.  preceding character. None of them have codepoints less than 256, so in
740    non-UTF-8 mode \eX matches any one character.
741  .P  .P
742  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
743  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
744  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
745  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
746    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
747    (*UCP).
748    .
749    .
750    .\" HTML <a name="extraprops"></a>
751    .SS PCRE's additional properties
752    .rs
753    .sp
754    As well as the standard Unicode properties described in the previous
755    section, PCRE supports four more that make it possible to convert traditional
756    escape sequences such as \ew and \es and POSIX character classes to use Unicode
757    properties. PCRE uses these non-standard, non-Perl properties internally when
758    PCRE_UCP is set. They are:
759    .sp
760      Xan   Any alphanumeric character
761      Xps   Any POSIX space character
762      Xsp   Any Perl space character
763      Xwd   Any Perl "word" character
764    .sp
765    Xan matches characters that have either the L (letter) or the N (number)
766    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
767    carriage return, and any other character that has the Z (separator) property.
768    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
769    same characters as Xan, plus underscore.
770    .
771    .
772    .\" HTML <a name="resetmatchstart"></a>
773    .SS "Resetting the match start"
774    .rs
775    .sp
776    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
777    matched characters not to be included in the final matched sequence. For
778    example, the pattern:
779    .sp
780      foo\eKbar
781    .sp
782    matches "foobar", but reports that it has matched "bar". This feature is
783    similar to a lookbehind assertion
784    .\" HTML <a href="#lookbehind">
785    .\" </a>
786    (described below).
787    .\"
788    However, in this case, the part of the subject before the real match does not
789    have to be of fixed length, as lookbehind assertions do. The use of \eK does
790    not interfere with the setting of
791    .\" HTML <a href="#subpattern">
792    .\" </a>
793    captured substrings.
794    .\"
795    For example, when the pattern
796    .sp
797      (foo)\eKbar
798    .sp
799    matches "foobar", the first substring is still set to "foo".
800    .P
801    Perl documents that the use of \eK within assertions is "not well defined". In
802    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
803    ignored in negative assertions.
804  .  .
805  .  .
806  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
807  .SS "Simple assertions"  .SS "Simple assertions"
808  .rs  .rs
809  .sp  .sp
810  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
811  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,
812  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
813  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 819  The backslashed assertions are:
819  .sp  .sp
820    \eb     matches at a word boundary    \eb     matches at a word boundary
821    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
822    \eA     matches at start of subject    \eA     matches at the start of the subject
823    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
824    \ez     matches at end of subject            also matches before a newline at the end of the subject
825    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
826  .sp    \eG     matches at the first matching position in the subject
827  These assertions may not appear in character classes (but note that \eb has a  .sp
828  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
829    character. If any other of these assertions appears in a character class, by
830    default it matches the corresponding literal character (for example, \eB
831    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
832    escape sequence" error is generated instead.
833  .P  .P
834  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
835  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
836  \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
837  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
838    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
839    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
840    of word" or "end of word" metasequence. However, whatever follows \eb normally
841    determines which it is. For example, the fragment \eba matches "a" at the start
842    of a word.
843  .P  .P
844  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
845  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 573  end of the subject in both modes, and if Line 923  end of the subject in both modes, and if
923  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
924  .  .
925  .  .
926  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
927    .SH "FULL STOP (PERIOD, DOT) AND \eN"
928  .rs  .rs
929  .sp  .sp
930  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
931  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
932  line. In UTF-8 mode, the matched character may be more than one byte long. When  line. In UTF-8 mode, the matched character may be more than one byte long.
933  a line ending is defined as a single character (CR or LF), dot never matches  .P
934  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
935  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
936  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
937    (including isolated CRs and LFs). When any Unicode line endings are being
938    recognized, dot does not match CR or LF or any of the other line ending
939    characters.
940  .P  .P
941  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
942  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
943  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
944    to match it.
945  .P  .P
946  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
947  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
948  special meaning in a character class.  special meaning in a character class.
949    .P
950    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
951    set. In other words, it matches any one character except one that signifies the
952    end of a line.
953  .  .
954  .  .
955  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
956  .rs  .rs
957  .sp  .sp
958  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
959  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
960  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  characters. The feature is provided in Perl in order to match individual bytes
961  Because it breaks up UTF-8 characters into individual bytes, what remains in  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
962  the string may be a malformed UTF-8 string. For this reason, the \eC escape  what remains in the string may be a malformed UTF-8 string. For this reason,
963  sequence is best avoided.  the \eC escape sequence is best avoided.
964  .P  .P
965  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
966  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 617  the lookbehind. Line 976  the lookbehind.
976  .rs  .rs
977  .sp  .sp
978  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
979  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.
980  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
981  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
982  escaped with a backslash.  a member of the class, it should be the first data character in the class
983    (after an initial circumflex, if present) or escaped with a backslash.
984  .P  .P
985  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
986  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
987  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
988  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
989  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
990  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 634  For example, the character class [aeiou] Line 994  For example, the character class [aeiou]
994  [^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
995  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
996  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
997  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
998  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
999  string.  string.
1000  .P  .P
# Line 648  caseful version would. In UTF-8 mode, PC Line 1008  caseful version would. In UTF-8 mode, PC
1008  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
1009  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1010  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1011  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,
1012  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
1013  UTF-8 support.  with UTF-8 support.
1014  .P  .P
1015  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
1016  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
1017  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1018  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
1019  .P  .P
1020  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
1021  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 679  example [\ex{100}-\ex{2ff}]. Line 1039  example [\ex{100}-\ex{2ff}].
1039  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
1040  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
1041  [][\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
1042  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
1043  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
1044  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
1045  property support.  property support.
1046  .P  .P
1047  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1048  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1049  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1050  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1051  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1052  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1053  .P  .P
1054  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1055  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 709  this notation. For example, Line 1069  this notation. For example,
1069    [01[:alpha:]%]    [01[:alpha:]%]
1070  .sp  .sp
1071  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1072  are  are:
1073  .sp  .sp
1074    alnum    letters and digits    alnum    letters and digits
1075    alpha    letters    alpha    letters
# Line 720  are Line 1080  are
1080    graph    printing characters, excluding space    graph    printing characters, excluding space
1081    lower    lower case letters    lower    lower case letters
1082    print    printing characters, including space    print    printing characters, including space
1083    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1084    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1085    upper    upper case letters    upper    upper case letters
1086    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 741  matches "1", "2", or any non-digit. PCRE Line 1101  matches "1", "2", or any non-digit. PCRE
1101  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1102  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1103  .P  .P
1104  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1105  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1106    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1107    character properties are used. This is achieved by replacing the POSIX classes
1108    by other sequences, as follows:
1109    .sp
1110      [:alnum:]  becomes  \ep{Xan}
1111      [:alpha:]  becomes  \ep{L}
1112      [:blank:]  becomes  \eh
1113      [:digit:]  becomes  \ep{Nd}
1114      [:lower:]  becomes  \ep{Ll}
1115      [:space:]  becomes  \ep{Xps}
1116      [:upper:]  becomes  \ep{Lu}
1117      [:word:]   becomes  \ep{Xwd}
1118    .sp
1119    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1120    classes are unchanged, and match only characters with code points less than
1121    128.
1122  .  .
1123  .  .
1124  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 769  alternative in the subpattern. Line 1145  alternative in the subpattern.
1145  .rs  .rs
1146  .sp  .sp
1147  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1148  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
1149  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1150    The option letters are
1151  .sp  .sp
1152    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1153    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1161  PCRE_MULTILINE while unsetting PCRE_DOTA
1161  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
1162  unset.  unset.
1163  .P  .P
1164  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
1165  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
1166  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1167  the global options (and it will therefore show up in data extracted by the  .P
1168  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1169    subpattern parentheses), the change applies to the remainder of the pattern
1170    that follows. If the change is placed right at the start of a pattern, PCRE
1171    extracts it into the global options (and it will therefore show up in data
1172    extracted by the \fBpcre_fullinfo()\fP function).
1173  .P  .P
1174  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
1175  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
1176  .sp  .sp
1177    (a(?i)b)c    (a(?i)b)c
1178  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1188  branch is abandoned before the option se
1188  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1189  behaviour otherwise.  behaviour otherwise.
1190  .P  .P
1191  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1192  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1193  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1194    the application has set or what has been defaulted. Details are given in the
1195    section entitled
1196    .\" HTML <a href="#newlineseq">
1197    .\" </a>
1198    "Newline sequences"
1199    .\"
1200    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1201    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1202    PCRE_UTF8 and the PCRE_UCP options, respectively.
1203  .  .
1204  .  .
1205  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 824  Turning part of a pattern into a subpatt Line 1214  Turning part of a pattern into a subpatt
1214    cat(aract|erpillar|)    cat(aract|erpillar|)
1215  .sp  .sp
1216  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
1217  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
1218  .sp  .sp
1219  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
1220  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 849  the string "the white queen" is matched Line 1239  the string "the white queen" is matched
1239    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1240  .sp  .sp
1241  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
1242  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.  
1243  .P  .P
1244  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
1245  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 865  is reached, an option setting in one bra Line 1254  is reached, an option setting in one bra
1254  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1255  .  .
1256  .  .
1257    .\" HTML <a name="dupsubpatternnumber"></a>
1258    .SH "DUPLICATE SUBPATTERN NUMBERS"
1259    .rs
1260    .sp
1261    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1262    the same numbers for its capturing parentheses. Such a subpattern starts with
1263    (?| and is itself a non-capturing subpattern. For example, consider this
1264    pattern:
1265    .sp
1266      (?|(Sat)ur|(Sun))day
1267    .sp
1268    Because the two alternatives are inside a (?| group, both sets of capturing
1269    parentheses are numbered one. Thus, when the pattern matches, you can look
1270    at captured substring number one, whichever alternative matched. This construct
1271    is useful when you want to capture part, but not all, of one of a number of
1272    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1273    number is reset at the start of each branch. The numbers of any capturing
1274    buffers that follow the subpattern start after the highest number used in any
1275    branch. The following example is taken from the Perl documentation.
1276    The numbers underneath show in which buffer the captured content will be
1277    stored.
1278    .sp
1279      # before  ---------------branch-reset----------- after
1280      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1281      # 1            2         2  3        2     3     4
1282    .sp
1283    A back reference to a numbered subpattern uses the most recent value that is
1284    set for that number by any subpattern. The following pattern matches "abcabc"
1285    or "defdef":
1286    .sp
1287      /(?|(abc)|(def))\e1/
1288    .sp
1289    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1290    refers to the first one in the pattern with the given number. The following
1291    pattern matches "abcabc" or "defabc":
1292    .sp
1293      /(?|(abc)|(def))(?1)/
1294    .sp
1295    If a
1296    .\" HTML <a href="#conditions">
1297    .\" </a>
1298    condition test
1299    .\"
1300    for a subpattern's having matched refers to a non-unique number, the test is
1301    true if any of the subpatterns of that number have matched.
1302    .P
1303    An alternative approach to using this "branch reset" feature is to use
1304    duplicate named subpatterns, as described in the next section.
1305    .
1306    .
1307  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1308  .rs  .rs
1309  .sp  .sp
1310  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
1311  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1312  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
1313  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1314  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1315    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1316    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1317    have different names, but PCRE does not.
1318    .P
1319    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1320    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1321  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1322  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1323  .\" </a>  .\" </a>
1324  backreferences,  back references,
1325  .\"  .\"
1326  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1327  .\" </a>  .\" </a>
# Line 890  conditions, Line 1335  conditions,
1335  can be made by name as well as by number.  can be made by name as well as by number.
1336  .P  .P
1337  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1338  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1339  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
1340  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
1341  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1342  .P  .P
1343  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1344  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1345  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1346  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1347  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1348  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1349  .sp  name, and in both cases you want to extract the abbreviation. This pattern
1350    (?P<DN>Mon|Fri|Sun)(?:day)?|  (ignoring the line breaks) does the job:
1351    (?P<DN>Tue)(?:sday)?|  .sp
1352    (?P<DN>Wed)(?:nesday)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1353    (?P<DN>Thu)(?:rsday)?|    (?<DN>Tue)(?:sday)?|
1354    (?P<DN>Sat)(?:urday)?    (?<DN>Wed)(?:nesday)?|
1355      (?<DN>Thu)(?:rsday)?|
1356      (?<DN>Sat)(?:urday)?
1357  .sp  .sp
1358  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1359    (An alternative way of solving this problem is to use a "branch reset"
1360    subpattern, as described in the previous section.)
1361    .P
1362  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1363  for the first, and in this example, the only, subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1364  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1365  make a reference to a non-unique named subpattern from elsewhere in the  .P
1366  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1367  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1368    used. In the absence of duplicate numbers (see the previous section) this is
1369    the one with the lowest number. If you use a named reference in a condition
1370    test (see the
1371    .\"
1372    .\" HTML <a href="#conditions">
1373    .\" </a>
1374    section about conditions
1375    .\"
1376    below), either to check whether a subpattern has matched, or to check for
1377    recursion, all subpatterns with the same name are tested. If the condition is
1378    true for any one of them, the overall condition is true. This is the same
1379    behaviour as testing by number. For further details of the interfaces for
1380    handling named subpatterns, see the
1381  .\" HREF  .\" HREF
1382  \fBpcreapi\fP  \fBpcreapi\fP
1383  .\"  .\"
1384  documentation.  documentation.
1385    .P
1386    \fBWarning:\fP You cannot use different names to distinguish between two
1387    subpatterns with the same number because PCRE uses only the numbers when
1388    matching. For this reason, an error is given at compile time if different names
1389    are given to subpatterns with the same number. However, you can give the same
1390    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1391  .  .
1392  .  .
1393  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1397  Repetition is specified by quantifiers,
1397  items:  items:
1398  .sp  .sp
1399    a literal data character    a literal data character
1400    the . metacharacter    the dot metacharacter
1401    the \eC escape sequence    the \eC escape sequence
1402    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1403      the \eR escape sequence
1404    an escape such as \ed that matches a single character    an escape such as \ed that matches a single character
1405    a character class    a character class
1406    a back reference (see next section)    a back reference (see next section)
1407    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1408      a recursive or "subroutine" call to a subpattern
1409  .sp  .sp
1410  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1411  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 966  support is available, \eX{3} matches thr Line 1437  support is available, \eX{3} matches thr
1437  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).
1438  .P  .P
1439  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
1440  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1441    subpatterns that are referenced as
1442    .\" HTML <a href="#subpatternsassubroutines">
1443    .\" </a>
1444    subroutines
1445    .\"
1446    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1447    quantifier are omitted from the compiled pattern.
1448  .P  .P
1449  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1450  quantifiers have single-character abbreviations:  abbreviations:
1451  .sp  .sp
1452    *    is equivalent to {0,}    *    is equivalent to {0,}
1453    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1495  own right. Because it has two uses, it c
1495  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
1496  way the rest of the pattern matches.  way the rest of the pattern matches.
1497  .P  .P
1498  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),
1499  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
1500  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
1501  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1505  is greater than 1 or with a limited maxi
1505  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1506  .P  .P
1507  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
1508  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
1509  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1510  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
1511  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 1038  worth setting PCRE_DOTALL in order to ob Line 1516  worth setting PCRE_DOTALL in order to ob
1516  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1517  .P  .P
1518  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1519  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1520  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
1521  succeed. Consider, for example:  succeeds. Consider, for example:
1522  .sp  .sp
1523    (.*)abc\e1    (.*)abc\e1
1524  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1544  matches "aba" the value of the second ca
1544  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1545  .rs  .rs
1546  .sp  .sp
1547  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1548  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
1549  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
1550  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
1551  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
1552  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1553  .P  .P
1554  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
1555  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1561  item, and then with 4, and so on, before
1561  (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
1562  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.
1563  .P  .P
1564  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
1565  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
1566  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1567  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1591  previous example can be rewritten as
1591  .sp  .sp
1592    \ed++foo    \ed++foo
1593  .sp  .sp
1594    Note that a possessive quantifier can be used with an entire group, for
1595    example:
1596    .sp
1597      (abc|xyz){2,3}+
1598    .sp
1599  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1600  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
1601  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
1602  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1603  .P  difference; possessive quantifiers should be slightly faster.
1604  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1605  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1606  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1607  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1608    package, and PCRE copied it from there. It ultimately found its way into Perl
1609    at release 5.10.
1610    .P
1611    PCRE has an optimization that automatically "possessifies" certain simple
1612    pattern constructs. For example, the sequence A+B is treated as A++B because
1613    there is no point in backtracking into a sequence of A's when B must follow.
1614  .P  .P
1615  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
1616  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 1167  numbers less than 10. A "forward back re Line 1656  numbers less than 10. A "forward back re
1656  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1657  in an earlier iteration.  in an earlier iteration.
1658  .P  .P
1659  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1660  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1661  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1662  "Non-printing characters"  "Non-printing characters"
1663  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1664  .\" </a>  .\" </a>
1665  above  above
1666  .\"  .\"
1667  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1668    no such problem when named parentheses are used. A back reference to any
1669    subpattern is possible using named parentheses (see below).
1670    .P
1671    Another way of avoiding the ambiguity inherent in the use of digits following a
1672    backslash is to use the \eg escape sequence, which is a feature introduced in
1673    Perl 5.10. This escape must be followed by an unsigned number or a negative
1674    number, optionally enclosed in braces. These examples are all identical:
1675    .sp
1676      (ring), \e1
1677      (ring), \eg1
1678      (ring), \eg{1}
1679    .sp
1680    An unsigned number specifies an absolute reference without the ambiguity that
1681    is present in the older syntax. It is also useful when literal digits follow
1682    the reference. A negative number is a relative reference. Consider this
1683    example:
1684    .sp
1685      (abc(def)ghi)\eg{-1}
1686    .sp
1687    The sequence \eg{-1} is a reference to the most recently started capturing
1688    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1689    would be equivalent to \e1. The use of relative references can be helpful in
1690    long patterns, and also in patterns that are created by joining together
1691    fragments that contain references within themselves.
1692  .P  .P
1693  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1694  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1710  back reference, the case of letters is r
1710  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
1711  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1712  .P  .P
1713  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1714  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1715    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1716    back reference syntax, in which \eg can be used for both numeric and named
1717    references, is also supported. We could rewrite the above example in any of
1718    the following ways:
1719  .sp  .sp
1720      (?<p1>(?i)rah)\es+\ek<p1>
1721      (?'p1'(?i)rah)\es+\ek{p1}
1722    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1723      (?<p1>(?i)rah)\es+\eg{p1}
1724  .sp  .sp
1725  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1726  after the reference.  after the reference.
1727  .P  .P
1728  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
1729  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1730  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1731  .sp  .sp
1732    (a|(bc))\e2    (a|(bc))\e2
1733  .sp  .sp
1734  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
1735  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
1736  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1737  with a digit character, some delimiter must be used to terminate the back  .P
1738  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1739  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1740    If the pattern continues with a digit character, some delimiter must be used to
1741    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1742    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1743  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1744  .\" </a>  .\" </a>
1745  "Comments"  "Comments"
1746  .\"  .\"
1747  below) can be used.  below) can be used.
1748  .P  .
1749    .SS "Recursive back references"
1750    .rs
1751    .sp
1752  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
1753  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.
1754  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1236  to the previous iteration. In order for Line 1762  to the previous iteration. In order for
1762  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
1763  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
1764  minimum of zero.  minimum of zero.
1765    .P
1766    Back references of this type cause the group that they reference to be treated
1767    as an
1768    .\" HTML <a href="#atomicgroup">
1769    .\" </a>
1770    atomic group.
1771    .\"
1772    Once the whole group has been matched, a subsequent matching failure cannot
1773    cause backtracking into the middle of the group.
1774  .  .
1775  .  .
1776  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1289  lookbehind assertion is needed to achiev Line 1824  lookbehind assertion is needed to achiev
1824  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
1825  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
1826  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.
1827    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1828    synonym for (?!).
1829  .  .
1830  .  .
1831  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 1850  is permitted, but
1850  .sp  .sp
1851  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1852  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
1853  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
1854  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1855  .sp  .sp
1856    (?<=ab(c|de))    (?<=ab(c|de))
1857  .sp  .sp
1858  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
1859  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
1860    branches:
1861  .sp  .sp
1862    (?<=abc|abde)    (?<=abc|abde)
1863  .sp  .sp
1864    In some cases, the Perl 5.10 escape sequence \eK
1865    .\" HTML <a href="#resetmatchstart">
1866    .\" </a>
1867    (see above)
1868    .\"
1869    can be used instead of a lookbehind assertion to get round the fixed-length
1870    restriction.
1871    .P
1872  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1873  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
1874  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1875  match is deemed to fail.  assertion fails.
1876  .P  .P
1877  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)
1878  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1879  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
1880  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1881  .P  .P
1882  Atomic groups can be used in conjunction with lookbehind assertions to specify  .\" HTML <a href="#subpatternsassubroutines">
1883  efficient matching at the end of the subject string. Consider a simple pattern  .\" </a>
1884  such as  "Subroutine"
1885    .\"
1886    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1887    as the subpattern matches a fixed-length string.
1888    .\" HTML <a href="#recursion">
1889    .\" </a>
1890    Recursion,
1891    .\"
1892    however, is not supported.
1893    .P
1894    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1895    specify efficient matching of fixed-length strings at the end of subject
1896    strings. Consider a simple pattern such as
1897  .sp  .sp
1898    abcd$    abcd$
1899  .sp  .sp
# Line 1351  then all but the last two characters, an Line 1909  then all but the last two characters, an
1909  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,
1910  if the pattern is written as  if the pattern is written as
1911  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
1912    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1913  .sp  .sp
1914  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
1915  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
1916  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1917  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1403  characters that are not "999". Line 1957  characters that are not "999".
1957  .sp  .sp
1958  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1959  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1960  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1961  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1962  .sp  .sp
1963    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1964    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
# Line 1413  If the condition is satisfied, the yes-p Line 1967  If the condition is satisfied, the yes-p
1967  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
1968  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1969  .P  .P
1970  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1971  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
1972  underscores, the condition is satisfied if the capturing subpattern of that  .
1973  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
1974  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
1975  named subpattern; if it cannot find one and the text consists entirely of  .sp
1976  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
1977  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
1978  recommended.  matched. If there is more than one capturing subpattern with the same number
1979    (see the earlier
1980    .\"
1981    .\" HTML <a href="#recursion">
1982    .\" </a>
1983    section about duplicate subpattern numbers),
1984    .\"
1985    the condition is true if any of them have been set. An alternative notation is
1986    to precede the digits with a plus or minus sign. In this case, the subpattern
1987    number is relative rather than absolute. The most recently opened parentheses
1988    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1989    looping constructs it can also make sense to refer to subsequent groups with
1990    constructs such as (?(+2).
1991  .P  .P
1992  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1993  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1437  or not. If they did, that is, if subject Line 2003  or not. If they did, that is, if subject
2003  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2004  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2005  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2006  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
2007  named subpattern gives this:  .P
2008    If you were embedding this pattern in a larger one, you could use a relative
2009    reference:
2010  .sp  .sp
2011    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2012    .sp
2013    This makes the fragment independent of the parentheses in the larger pattern.
2014    .
2015    .SS "Checking for a used subpattern by name"
2016    .rs
2017    .sp
2018    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2019    subpattern by name. For compatibility with earlier versions of PCRE, which had
2020    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2021    there is a possible ambiguity with this syntax, because subpattern names may
2022    consist entirely of digits. PCRE looks first for a named subpattern; if it
2023    cannot find one and the name consists entirely of digits, PCRE looks for a
2024    subpattern of that number, which must be greater than zero. Using subpattern
2025    names that consist entirely of digits is not recommended.
2026    .P
2027    Rewriting the above example to use a named subpattern gives this:
2028    .sp
2029      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2030    .sp
2031    If the name used in a condition of this kind is a duplicate, the test is
2032    applied to all subpatterns of the same name, and is true if any one of them has
2033    matched.
2034    .
2035    .SS "Checking for pattern recursion"
2036    .rs
2037  .sp  .sp
2038  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
2039  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
2040  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
2041  Recursive patterns are described in the next section.  letter R, for example:
2042    .sp
2043      (?(R3)...) or (?(R&name)...)
2044    .sp
2045    the condition is true if the most recent recursion is into a subpattern whose
2046    number or name is given. This condition does not check the entire recursion
2047    stack. If the name used in a condition of this kind is a duplicate, the test is
2048    applied to all subpatterns of the same name, and is true if any one of them is
2049    the most recent recursion.
2050  .P  .P
2051  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.
2052    .\" HTML <a href="#recursion">
2053    .\" </a>
2054    The syntax for recursive patterns
2055    .\"
2056    is described below.
2057    .
2058    .SS "Defining subpatterns for use by reference only"
2059    .rs
2060    .sp
2061    If the condition is the string (DEFINE), and there is no subpattern with the
2062    name DEFINE, the condition is always false. In this case, there may be only one
2063    alternative in the subpattern. It is always skipped if control reaches this
2064    point in the pattern; the idea of DEFINE is that it can be used to define
2065    "subroutines" that can be referenced from elsewhere. (The use of
2066    .\" HTML <a href="#subpatternsassubroutines">
2067    .\" </a>
2068    "subroutines"
2069    .\"
2070    is described below.) For example, a pattern to match an IPv4 address could be
2071    written like this (ignore whitespace and line breaks):
2072    .sp
2073      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2074      \eb (?&byte) (\e.(?&byte)){3} \eb
2075    .sp
2076    The first part of the pattern is a DEFINE group inside which a another group
2077    named "byte" is defined. This matches an individual component of an IPv4
2078    address (a number less than 256). When matching takes place, this part of the
2079    pattern is skipped because DEFINE acts like a false condition. The rest of the
2080    pattern uses references to the named group to match the four dot-separated
2081    components of an IPv4 address, insisting on a word boundary at each end.
2082    .
2083    .SS "Assertion conditions"
2084    .rs
2085    .sp
2086    If the condition is not in any of the above formats, it must be an assertion.
2087  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2088  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2089  alternatives on the second line:  alternatives on the second line:
# Line 1483  next newline in the pattern. Line 2119  next newline in the pattern.
2119  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2120  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2121  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
2122  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2123  that allows regular expressions to recurse (amongst other things). It does this  .P
2124  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
2125  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
2126  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2127    pattern using code interpolation to solve the parentheses problem can be
2128    created like this:
2129  .sp  .sp
2130    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2131  .sp  .sp
2132  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
2133  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2134  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2135  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2136  .P  supports special syntax for recursion of the entire pattern, and also for
2137  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2138  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.
2139  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2140  "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
2141  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2142  .P  provided that it occurs inside that subpattern. (If not, it is a
2143  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2144  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2145  it contains untried alternatives and there is a subsequent matching failure.  "subroutine"
2146    .\"
2147    call, which is described in the next section.) The special item (?R) or (?0) is
2148    a recursive call of the entire regular expression.
2149  .P  .P
2150  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2151  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2152  .sp  .sp
2153    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2154  .sp  .sp
2155  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2156  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
2157  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2158  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2159    to avoid backtracking into sequences of non-parentheses.
2160  .P  .P
2161  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
2162  pattern, so instead you could use this:  pattern, so instead you could use this:
2163  .sp  .sp
2164    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2165  .sp  .sp
2166  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
2167  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2168  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2169  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
2170  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).
2171  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
2172  .sp  most recently opened parentheses preceding the recursion. In other words, a
2173    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  negative number counts capturing parentheses leftwards from the point at which
2174  .sp  it is encountered.
2175  This particular example pattern contains nested unlimited repeats, and so the  .P
2176  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
2177  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
2178  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
2179    .\" HTML <a href="#subpatternsassubroutines">
2180    .\" </a>
2181    "subroutine"
2182    .\"
2183    calls, as described in the next section.
2184    .P
2185    An alternative approach is to use named parentheses instead. The Perl syntax
2186    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2187    could rewrite the above example as follows:
2188    .sp
2189      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2190    .sp
2191    If there is more than one subpattern with the same name, the earliest one is
2192    used.
2193    .P
2194    This particular example pattern that we have been looking at contains nested
2195    unlimited repeats, and so the use of a possessive quantifier for matching
2196    strings of non-parentheses is important when applying the pattern to strings
2197    that do not match. For example, when this pattern is applied to
2198  .sp  .sp
2199    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2200  .sp  .sp
2201  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,
2202  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
2203  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
2204  before failure can be reported.  before failure can be reported.
2205  .P  .P
2206  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
2207  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
2208  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  
2209  .\" HREF  .\" HREF
2210  \fBpcrecallout\fP  \fBpcrecallout\fP
2211  .\"  .\"
# Line 1553  documentation). If the pattern above is Line 2213  documentation). If the pattern above is
2213  .sp  .sp
2214    (ab(cd)ef)    (ab(cd)ef)
2215  .sp  .sp
2216  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
2217  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
2218  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2219    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2220       ^                        ^  .P
2221       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2222  .sp  obtain extra memory to store data during a recursion, which it does by using
2223  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
2224  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.  
2225  .P  .P
2226  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.
2227  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1578  different alternatives for the recursive Line 2235  different alternatives for the recursive
2235  is the actual recursive call.  is the actual recursive call.
2236  .  .
2237  .  .
2238    .\" HTML <a name="recursiondifference"></a>
2239    .SS "Recursion difference from Perl"
2240    .rs
2241    .sp
2242    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2243    treated as an atomic group. That is, once it has matched some of the subject
2244    string, it is never re-entered, even if it contains untried alternatives and
2245    there is a subsequent matching failure. This can be illustrated by the
2246    following pattern, which purports to match a palindromic string that contains
2247    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2248    .sp
2249      ^(.|(.)(?1)\e2)$
2250    .sp
2251    The idea is that it either matches a single character, or two identical
2252    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2253    it does not if the pattern is longer than three characters. Consider the
2254    subject string "abcba":
2255    .P
2256    At the top level, the first character is matched, but as it is not at the end
2257    of the string, the first alternative fails; the second alternative is taken
2258    and the recursion kicks in. The recursive call to subpattern 1 successfully
2259    matches the next character ("b"). (Note that the beginning and end of line
2260    tests are not part of the recursion).
2261    .P
2262    Back at the top level, the next character ("c") is compared with what
2263    subpattern 2 matched, which was "a". This fails. Because the recursion is
2264    treated as an atomic group, there are now no backtracking points, and so the
2265    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2266    try the second alternative.) However, if the pattern is written with the
2267    alternatives in the other order, things are different:
2268    .sp
2269      ^((.)(?1)\e2|.)$
2270    .sp
2271    This time, the recursing alternative is tried first, and continues to recurse
2272    until it runs out of characters, at which point the recursion fails. But this
2273    time we do have another alternative to try at the higher level. That is the big
2274    difference: in the previous case the remaining alternative is at a deeper
2275    recursion level, which PCRE cannot use.
2276    .P
2277    To change the pattern so that matches all palindromic strings, not just those
2278    with an odd number of characters, it is tempting to change the pattern to this:
2279    .sp
2280      ^((.)(?1)\e2|.?)$
2281    .sp
2282    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2283    deeper recursion has matched a single character, it cannot be entered again in
2284    order to match an empty string. The solution is to separate the two cases, and
2285    write out the odd and even cases as alternatives at the higher level:
2286    .sp
2287      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2288    .sp
2289    If you want to match typical palindromic phrases, the pattern has to ignore all
2290    non-word characters, which can be done like this:
2291    .sp
2292      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2293    .sp
2294    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2295    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2296    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2297    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2298    more) to match typical phrases, and Perl takes so long that you think it has
2299    gone into a loop.
2300    .P
2301    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2302    string does not start with a palindrome that is shorter than the entire string.
2303    For example, although "abcba" is correctly matched, if the subject is "ababa",
2304    PCRE finds the palindrome "aba" at the start, then fails at top level because
2305    the end of the string does not follow. Once again, it cannot jump back into the
2306    recursion to try other alternatives, so the entire match fails.
2307    .
2308    .
2309  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2310  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2311  .rs  .rs
2312  .sp  .sp
2313  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
2314  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
2315  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2316  pattern  before or after the reference. A numbered reference can be absolute or
2317    relative, as in these examples:
2318    .sp
2319      (...(absolute)...)...(?2)...
2320      (...(relative)...)...(?-1)...
2321      (...(?+1)...(relative)...
2322    .sp
2323    An earlier example pointed out that the pattern
2324  .sp  .sp
2325    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2326  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2330  matches "sense and sensibility" and "res
2330    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2331  .sp  .sp
2332  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
2333  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
 which they refer. However, named references can refer to later subpatterns.  
2334  .P  .P
2335  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2336  group. That is, once it has matched some of the subject string, it is never  group. That is, once it has matched some of the subject string, it is never
2337  re-entered, even if it contains untried alternatives and there is a subsequent  re-entered, even if it contains untried alternatives and there is a subsequent
2338  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2339    call revert to their previous values afterwards.
2340    .P
2341    When a subpattern is used as a subroutine, processing options such as
2342    case-independence are fixed when the subpattern is defined. They cannot be
2343    changed for different calls. For example, consider this pattern:
2344    .sp
2345      (abc)(?i:(?-1))
2346    .sp
2347    It matches "abcabc". It does not match "abcABC" because the change of
2348    processing option does not affect the called subpattern.
2349    .
2350    .
2351    .\" HTML <a name="onigurumasubroutines"></a>
2352    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2353    .rs
2354    .sp
2355    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2356    a number enclosed either in angle brackets or single quotes, is an alternative
2357    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2358    are two of the examples used above, rewritten using this syntax:
2359    .sp
2360      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2361      (sens|respons)e and \eg'1'ibility
2362    .sp
2363    PCRE supports an extension to Oniguruma: if a number is preceded by a
2364    plus or a minus sign it is taken as a relative reference. For example:
2365    .sp
2366      (abc)(?i:\eg<-1>)
2367    .sp
2368    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2369    synonymous. The former is a back reference; the latter is a subroutine call.
2370  .  .
2371  .  .
2372  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2387  function is to be called. If you want to
2387  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.
2388  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2389  .sp  .sp
2390    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2391  .sp  .sp
2392  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
2393  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 1638  description of the interface to the call Line 2403  description of the interface to the call
2403  \fBpcrecallout\fP  \fBpcrecallout\fP
2404  .\"  .\"
2405  documentation.  documentation.
2406    .
2407    .
2408    .\" HTML <a name="backtrackcontrol"></a>
2409    .SH "BACKTRACKING CONTROL"
2410    .rs
2411    .sp
2412    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2413    are described in the Perl documentation as "experimental and subject to change
2414    or removal in a future version of Perl". It goes on to say: "Their usage in
2415    production code should be noted to avoid problems during upgrades." The same
2416    remarks apply to the PCRE features described in this section.
2417    .P
2418    Since these verbs are specifically related to backtracking, most of them can be
2419    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2420    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2421    failing negative assertion, they cause an error if encountered by
2422    \fBpcre_dfa_exec()\fP.
2423    .P
2424    If any of these verbs are used in an assertion or subroutine subpattern
2425    (including recursive subpatterns), their effect is confined to that subpattern;
2426    it does not extend to the surrounding pattern. Note that such subpatterns are
2427    processed as anchored at the point where they are tested.
2428    .P
2429    The new verbs make use of what was previously invalid syntax: an opening
2430    parenthesis followed by an asterisk. They are generally of the form
2431    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2432    depending on whether or not an argument is present. An name is a sequence of
2433    letters, digits, and underscores. If the name is empty, that is, if the closing
2434    parenthesis immediately follows the colon, the effect is as if the colon were
2435    not there. Any number of these verbs may occur in a pattern.
2436    .P
2437    PCRE contains some optimizations that are used to speed up matching by running
2438    some checks at the start of each match attempt. For example, it may know the
2439    minimum length of matching subject, or that a particular character must be
2440    present. When one of these optimizations suppresses the running of a match, any
2441    included backtracking verbs will not, of course, be processed. You can suppress
2442    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2443    when calling \fBpcre_exec()\fP.
2444    .
2445    .
2446    .SS "Verbs that act immediately"
2447    .rs
2448    .sp
2449    The following verbs act as soon as they are encountered. They may not be
2450    followed by a name.
2451    .sp
2452       (*ACCEPT)
2453    .sp
2454    This verb causes the match to end successfully, skipping the remainder of the
2455    pattern. When inside a recursion, only the innermost pattern is ended
2456    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2457    captured. (This feature was added to PCRE at release 8.00.) For example:
2458    .sp
2459      A((?:A|B(*ACCEPT)|C)D)
2460    .sp
2461    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2462    the outer parentheses.
2463    .sp
2464      (*FAIL) or (*F)
2465    .sp
2466    This verb causes the match to fail, forcing backtracking to occur. It is
2467    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2468    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2469    Perl features that are not present in PCRE. The nearest equivalent is the
2470    callout feature, as for example in this pattern:
2471    .sp
2472      a+(?C)(*FAIL)
2473    .sp
2474    A match with the string "aaaa" always fails, but the callout is taken before
2475    each backtrack happens (in this example, 10 times).
2476    .
2477    .
2478    .SS "Recording which path was taken"
2479    .rs
2480    .sp
2481    There is one verb whose main purpose is to track how a match was arrived at,
2482    though it also has a secondary use in conjunction with advancing the match
2483    starting point (see (*SKIP) below).
2484    .sp
2485      (*MARK:NAME) or (*:NAME)
2486    .sp
2487    A name is always required with this verb. There may be as many instances of
2488    (*MARK) as you like in a pattern, and their names do not have to be unique.
2489    .P
2490    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2491    to the caller via the \fIpcre_extra\fP data structure, as described in the
2492    .\" HTML <a href="pcreapi.html#extradata">
2493    .\" </a>
2494    section on \fIpcre_extra\fP
2495    .\"
2496    in the
2497    .\" HREF
2498    \fBpcreapi\fP
2499    .\"
2500    documentation. No data is returned for a partial match. Here is an example of
2501    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2502    outputting of (*MARK) data:
2503    .sp
2504      /X(*MARK:A)Y|X(*MARK:B)Z/K
2505      XY
2506       0: XY
2507      MK: A
2508      XZ
2509       0: XZ
2510      MK: B
2511    .sp
2512    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2513    indicates which of the two alternatives matched. This is a more efficient way
2514    of obtaining this information than putting each alternative in its own
2515    capturing parentheses.
2516    .P
2517    A name may also be returned after a failed match if the final path through the
2518    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2519    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2520    starting point for matching is advanced, the final check is often with an empty
2521    string, causing a failure before (*MARK) is reached. For example:
2522    .sp
2523      /X(*MARK:A)Y|X(*MARK:B)Z/K
2524      XP
2525      No match
2526    .sp
2527    There are three potential starting points for this match (starting with X,
2528    starting with P, and with an empty string). If the pattern is anchored, the
2529    result is different:
2530    .sp
2531      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2532      XP
2533      No match, mark = B
2534    .sp
2535    PCRE's start-of-match optimizations can also interfere with this. For example,
2536    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2537    subject length for a match, a shorter subject will not be scanned at all.
2538    .P
2539    Note that similar anomalies (though different in detail) exist in Perl, no
2540    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2541    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2542    .
2543    .
2544    .SS "Verbs that act after backtracking"
2545    .rs
2546    .sp
2547    The following verbs do nothing when they are encountered. Matching continues
2548    with what follows, but if there is no subsequent match, causing a backtrack to
2549    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2550    the verb. However, when one of these verbs appears inside an atomic group, its
2551    effect is confined to that group, because once the group has been matched,
2552    there is never any backtracking into it. In this situation, backtracking can
2553    "jump back" to the left of the entire atomic group. (Remember also, as stated
2554    above, that this localization also applies in subroutine calls and assertions.)
2555  .P  .P
2556  .in 0  These verbs differ in exactly what kind of failure occurs when backtracking
2557  Last updated: 06 June 2006  reaches them.
2558  .br  .sp
2559  Copyright (c) 1997-2006 University of Cambridge.    (*COMMIT)
2560    .sp
2561    This verb, which may not be followed by a name, causes the whole match to fail
2562    outright if the rest of the pattern does not match. Even if the pattern is
2563    unanchored, no further attempts to find a match by advancing the starting point
2564    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2565    finding a match at the current starting point, or not at all. For example:
2566    .sp
2567      a+(*COMMIT)b
2568    .sp
2569    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2570    dynamic anchor, or "I've started, so I must finish." The name of the most
2571    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2572    match failure.
2573    .P
2574    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2575    unless PCRE's start-of-match optimizations are turned off, as shown in this
2576    \fBpcretest\fP example:
2577    .sp
2578      /(*COMMIT)abc/
2579      xyzabc
2580       0: abc
2581      xyzabc\eY
2582      No match
2583    .sp
2584    PCRE knows that any match must start with "a", so the optimization skips along
2585    the subject to "a" before running the first match attempt, which succeeds. When
2586    the optimization is disabled by the \eY escape in the second subject, the match
2587    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2588    starting points.
2589    .sp
2590      (*PRUNE) or (*PRUNE:NAME)
2591    .sp
2592    This verb causes the match to fail at the current starting position in the
2593    subject if the rest of the pattern does not match. If the pattern is
2594    unanchored, the normal "bumpalong" advance to the next starting character then
2595    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2596    reached, or when matching to the right of (*PRUNE), but if there is no match to
2597    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2598    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2599    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2600    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2601    match fails completely; the name is passed back if this is the final attempt.
2602    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2603    pattern (*PRUNE) has the same effect as (*COMMIT).
2604    .sp
2605      (*SKIP)
2606    .sp
2607    This verb, when given without a name, is like (*PRUNE), except that if the
2608    pattern is unanchored, the "bumpalong" advance is not to the next character,
2609    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2610    signifies that whatever text was matched leading up to it cannot be part of a
2611    successful match. Consider:
2612    .sp
2613      a+(*SKIP)b
2614    .sp
2615    If the subject is "aaaac...", after the first match attempt fails (starting at
2616    the first character in the string), the starting point skips on to start the
2617    next attempt at "c". Note that a possessive quantifer does not have the same
2618    effect as this example; although it would suppress backtracking during the
2619    first match attempt, the second attempt would start at the second character
2620    instead of skipping on to "c".
2621    .sp
2622      (*SKIP:NAME)
2623    .sp
2624    When (*SKIP) has an associated name, its behaviour is modified. If the
2625    following pattern fails to match, the previous path through the pattern is
2626    searched for the most recent (*MARK) that has the same name. If one is found,
2627    the "bumpalong" advance is to the subject position that corresponds to that
2628    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2629    matching name is found, normal "bumpalong" of one character happens (the
2630    (*SKIP) is ignored).
2631    .sp
2632      (*THEN) or (*THEN:NAME)
2633    .sp
2634    This verb causes a skip to the next alternation in the innermost enclosing
2635    group if the rest of the pattern does not match. That is, it cancels pending
2636    backtracking, but only within the current alternation. Its name comes from the
2637    observation that it can be used for a pattern-based if-then-else block:
2638    .sp
2639      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2640    .sp
2641    If the COND1 pattern matches, FOO is tried (and possibly further items after
2642    the end of the group if FOO succeeds); on failure the matcher skips to the
2643    second alternative and tries COND2, without backtracking into COND1. The
2644    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2645    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2646    like (*PRUNE).
2647    .
2648    .P
2649    The above verbs provide four different "strengths" of control when subsequent
2650    matching fails. (*THEN) is the weakest, carrying on the match at the next
2651    alternation. (*PRUNE) comes next, failing the match at the current starting
2652    position, but allowing an advance to the next character (for an unanchored
2653    pattern). (*SKIP) is similar, except that the advance may be more than one
2654    character. (*COMMIT) is the strongest, causing the entire match to fail.
2655    .P
2656    If more than one is present in a pattern, the "stongest" one wins. For example,
2657    consider this pattern, where A, B, etc. are complex pattern fragments:
2658    .sp
2659      (A(*COMMIT)B(*THEN)C|D)
2660    .sp
2661    Once A has matched, PCRE is committed to this match, at the current starting
2662    position. If subsequently B matches, but C does not, the normal (*THEN) action
2663    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2664    overrides.
2665    .
2666    .
2667    .SH "SEE ALSO"
2668    .rs
2669    .sp
2670    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2671    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2672    .
2673    .
2674    .SH AUTHOR
2675    .rs
2676    .sp
2677    .nf
2678    Philip Hazel
2679    University Computing Service
2680    Cambridge CB2 3QH, England.
2681    .fi
2682    .
2683    .
2684    .SH REVISION
2685    .rs
2686    .sp
2687    .nf
2688    Last updated: 26 October 2010
2689    Copyright (c) 1997-2010 University of Cambridge.
2690    .fi

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