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# 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,
# Line 40  discussed in the Line 66  discussed in the
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70    .SH "NEWLINE CONVENTIONS"
71    .rs
72    .sp
73    PCRE supports five different conventions for indicating line breaks in
74    strings: a single CR (carriage return) character, a single LF (linefeed)
75    character, the two-character sequence CRLF, any of the three preceding, or any
76    Unicode newline sequence. The
77    .\" HREF
78    \fBpcreapi\fP
79    .\"
80    page has
81    .\" HTML <a href="pcreapi.html#newlines">
82    .\" </a>
83    further discussion
84    .\"
85    about newlines, and shows how to set the newline convention in the
86    \fIoptions\fP arguments for the compiling and matching functions.
87    .P
88    It is also possible to specify a newline convention by starting a pattern
89    string with one of the following five sequences:
90    .sp
91      (*CR)        carriage return
92      (*LF)        linefeed
93      (*CRLF)      carriage return, followed by linefeed
94      (*ANYCRLF)   any of the three above
95      (*ANY)       all Unicode newline sequences
96    .sp
97    These override the default and the options given to \fBpcre_compile()\fP or
98    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99    newline sequence, the pattern
100    .sp
101      (*CR)a.b
102    .sp
103    changes the convention to CR. That pattern matches "a\enb" because LF is no
104    longer a newline. Note that these special settings, which are not
105    Perl-compatible, are recognized only at the very start of a pattern, and that
106    they must be in upper case. If more than one of them is present, the last one
107    is used.
108    .P
109    The newline convention affects the interpretation of the dot metacharacter when
110    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111    affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114    .\" HTML <a href="#newlineseq">
115    .\" </a>
116    "Newline sequences"
117    .\"
118    below. A change of \eR setting can be combined with a change of newline
119    convention.
120    .
121    .
122  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
123  .rs  .rs
124  .sp  .sp
# Line 132  Perl, $ and @ cause variable interpolati Line 211  Perl, $ and @ cause variable interpolati
211    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
212  .sp  .sp
213  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
214    An isolated \eE that is not preceded by \eQ is ignored.
215  .  .
216  .  .
217  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 141  The \eQ...\eE sequence is recognized bot Line 221  The \eQ...\eE sequence is recognized bot
221  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
222  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
223  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
224  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
225  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:  
226  .sp  .sp
227    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
228    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any character
229    \ee        escape (hex 1B)    \ee        escape (hex 1B)
230    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
231    \en        newline (hex 0A)    \en        linefeed (hex 0A)
232    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
233    \et        tab (hex 09)    \et        tab (hex 09)
234    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
235    \exhh      character with hex code hh    \exhh      character with hex code hh
236    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh..
237  .sp  .sp
# Line 164  Thus \ecz becomes hex 1A, but \ec{ becom Line 243  Thus \ecz becomes hex 1A, but \ec{ becom
243  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
244  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{
245  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
246  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
247  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
248  and }, or if there is no terminating }, this form of escape is not recognized.  point, which is 10FFFF.
249  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  .P
250  with no following digits, giving a character whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
251    there is no terminating }, this form of escape is not recognized. Instead, the
252    initial \ex will be interpreted as a basic hexadecimal escape, with no
253    following digits, giving a character whose value is zero.
254  .P  .P
255  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
256  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 227  zero, because no more than three octal d Line 309  zero, because no more than three octal d
309  .P  .P
310  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
311  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
312  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
313  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
314  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
315  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
316  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
317  .  .
318  .  .
319  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
320  .rs  .rs
321  .sp  .sp
322  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
323  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
324  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
325  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
326  .\" </a>  .\" </a>
327  later,  later,
# Line 254  parenthesized subpatterns. Line 333  parenthesized subpatterns.
333  .\"  .\"
334  .  .
335  .  .
336    .SS "Absolute and relative subroutine calls"
337    .rs
338    .sp
339    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
340    a number enclosed either in angle brackets or single quotes, is an alternative
341    syntax for referencing a subpattern as a "subroutine". Details are discussed
342    .\" HTML <a href="#onigurumasubroutines">
343    .\" </a>
344    later.
345    .\"
346    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
347    synonymous. The former is a back reference; the latter is a
348    .\" HTML <a href="#subpatternsassubroutines">
349    .\" </a>
350    subroutine
351    .\"
352    call.
353    .
354    .
355    .\" HTML <a name="genericchartypes"></a>
356  .SS "Generic character types"  .SS "Generic character types"
357  .rs  .rs
358  .sp  .sp
359  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
360  .sp  .sp
361    \ed     any decimal digit    \ed     any decimal digit
362    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
363    \eh     any horizontal whitespace character    \eh     any horizontal whitespace character
364    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal whitespace character
365    \es     any whitespace character    \es     any whitespace character
366    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
367    \ev     any vertical whitespace character    \ev     any vertical whitespace character
368    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical whitespace character
369    \ew     any "word" character    \ew     any "word" character
370    \eW     any "non-word" character    \eW     any "non-word" character
371  .sp  .sp
372  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.
373  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
374    .\" HTML <a href="#fullstopdot">
375    .\" </a>
376    the "." metacharacter
377    .\"
378    when PCRE_DOTALL is not set.
379  .P  .P
380  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
381    of characters into two disjoint sets. Any given character matches one, and only
382    one, of each pair. The sequences can appear both inside and outside character
383  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
384  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
385  there is no character to match.  there is no character to match.
386  .P  .P
387  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).
# Line 285  are HT (9), LF (10), FF (12), CR (13), a Line 390  are HT (9), LF (10), FF (12), CR (13), a
390  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
391  does.  does.
392  .P  .P
393  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
394  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
395  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
396  meanings from before UTF-8 support was available, mainly for efficiency  place (see
397  reasons.  .\" HTML <a href="pcreapi.html#localesupport">
398  .P  .\" </a>
399  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  "Locale support"
400  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  .\"
401  The horizontal space characters are:  in the
402    .\" HREF
403    \fBpcreapi\fP
404    .\"
405    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
406    or "french" in Windows, some character codes greater than 128 are used for
407    accented letters, and these are then matched by \ew. The use of locales with
408    Unicode is discouraged.
409    .P
410    By default, in UTF-8 mode, characters with values greater than 128 never match
411    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
412    their original meanings from before UTF-8 support was available, mainly for
413    efficiency reasons. However, if PCRE is compiled with Unicode property support,
414    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
415    properties are used to determine character types, as follows:
416    .sp
417      \ed  any character that \ep{Nd} matches (decimal digit)
418      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
419      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
420    .sp
421    The upper case escapes match the inverse sets of characters. Note that \ed
422    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
423    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
424    \eB because they are defined in terms of \ew and \eW. Matching these sequences
425    is noticeably slower when PCRE_UCP is set.
426    .P
427    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
428    other sequences, which match only ASCII characters by default, these always
429    match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
430    set. The horizontal space characters are:
431  .sp  .sp
432    U+0009     Horizontal tab    U+0009     Horizontal tab
433    U+0020     Space    U+0020     Space
# Line 324  The vertical space characters are: Line 458  The vertical space characters are:
458    U+0085     Next line    U+0085     Next line
459    U+2028     Line separator    U+2028     Line separator
460    U+2029     Paragraph separator    U+2029     Paragraph separator
 .P  
 A "word" character is an underscore or any character less than 256 that is a  
 letter or digit. The definition of letters and digits is controlled by PCRE's  
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
461  .  .
462  .  .
463    .\" HTML <a name="newlineseq"></a>
464  .SS "Newline sequences"  .SS "Newline sequences"
465  .rs  .rs
466  .sp  .sp
467  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
468  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
469  the following:  equivalent to the following:
470  .sp  .sp
471    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
472  .sp  .sp
# Line 368  are added: LS (line separator, U+2028) a Line 486  are added: LS (line separator, U+2028) a
486  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
487  recognized.  recognized.
488  .P  .P
489  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
490    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
491    either at compile time or when the pattern is matched. (BSR is an abbrevation
492    for "backslash R".) This can be made the default when PCRE is built; if this is
493    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
494    It is also possible to specify these settings by starting a pattern string with
495    one of the following sequences:
496    .sp
497      (*BSR_ANYCRLF)   CR, LF, or CRLF only
498      (*BSR_UNICODE)   any Unicode newline sequence
499    .sp
500    These override the default and the options given to \fBpcre_compile()\fP or
501    \fBpcre_compile2()\fP, but they can be overridden by options given to
502    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
503    which are not Perl-compatible, are recognized only at the very start of a
504    pattern, and that they must be in upper case. If more than one of them is
505    present, the last one is used. They can be combined with a change of newline
506    convention; for example, a pattern can start with:
507    .sp
508      (*ANY)(*BSR_ANYCRLF)
509    .sp
510    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
511    a character class, \eR is treated as an unrecognized escape sequence, and so
512    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
513  .  .
514  .  .
515  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 376  Inside a character class, \eR matches th Line 517  Inside a character class, \eR matches th
517  .rs  .rs
518  .sp  .sp
519  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
520  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
521  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
522    characters whose codepoints are less than 256, but they do work in this mode.
523    The extra escape sequences are:
524  .sp  .sp
525    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
526    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
527    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
528  .sp  .sp
529  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
530  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
531  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
532  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
533  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
534    .\" </a>
535    next section).
536    .\"
537    Other Perl properties such as "InMusicalSymbols" are not currently supported by
538    PCRE. Note that \eP{Any} does not match any characters, so always causes a
539    match failure.
540  .P  .P
541  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
542  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 401  Those that are not part of an identified Line 550  Those that are not part of an identified
550  .P  .P
551  Arabic,  Arabic,
552  Armenian,  Armenian,
553    Avestan,
554  Balinese,  Balinese,
555    Bamum,
556  Bengali,  Bengali,
557  Bopomofo,  Bopomofo,
558  Braille,  Braille,
559  Buginese,  Buginese,
560  Buhid,  Buhid,
561  Canadian_Aboriginal,  Canadian_Aboriginal,
562    Carian,
563    Cham,
564  Cherokee,  Cherokee,
565  Common,  Common,
566  Coptic,  Coptic,
# Line 416  Cypriot, Line 569  Cypriot,
569  Cyrillic,  Cyrillic,
570  Deseret,  Deseret,
571  Devanagari,  Devanagari,
572    Egyptian_Hieroglyphs,
573  Ethiopic,  Ethiopic,
574  Georgian,  Georgian,
575  Glagolitic,  Glagolitic,
# Line 428  Hangul, Line 582  Hangul,
582  Hanunoo,  Hanunoo,
583  Hebrew,  Hebrew,
584  Hiragana,  Hiragana,
585    Imperial_Aramaic,
586  Inherited,  Inherited,
587    Inscriptional_Pahlavi,
588    Inscriptional_Parthian,
589    Javanese,
590    Kaithi,
591  Kannada,  Kannada,
592  Katakana,  Katakana,
593    Kayah_Li,
594  Kharoshthi,  Kharoshthi,
595  Khmer,  Khmer,
596  Lao,  Lao,
597  Latin,  Latin,
598    Lepcha,
599  Limbu,  Limbu,
600  Linear_B,  Linear_B,
601    Lisu,
602    Lycian,
603    Lydian,
604  Malayalam,  Malayalam,
605    Meetei_Mayek,
606  Mongolian,  Mongolian,
607  Myanmar,  Myanmar,
608  New_Tai_Lue,  New_Tai_Lue,
# Line 445  Nko, Line 610  Nko,
610  Ogham,  Ogham,
611  Old_Italic,  Old_Italic,
612  Old_Persian,  Old_Persian,
613    Old_South_Arabian,
614    Old_Turkic,
615    Ol_Chiki,
616  Oriya,  Oriya,
617  Osmanya,  Osmanya,
618  Phags_Pa,  Phags_Pa,
619  Phoenician,  Phoenician,
620    Rejang,
621  Runic,  Runic,
622    Samaritan,
623    Saurashtra,
624  Shavian,  Shavian,
625  Sinhala,  Sinhala,
626    Sundanese,
627  Syloti_Nagri,  Syloti_Nagri,
628  Syriac,  Syriac,
629  Tagalog,  Tagalog,
630  Tagbanwa,  Tagbanwa,
631  Tai_Le,  Tai_Le,
632    Tai_Tham,
633    Tai_Viet,
634  Tamil,  Tamil,
635  Telugu,  Telugu,
636  Thaana,  Thaana,
# Line 464  Thai, Line 638  Thai,
638  Tibetan,  Tibetan,
639  Tifinagh,  Tifinagh,
640  Ugaritic,  Ugaritic,
641    Vai,
642  Yi.  Yi.
643  .P  .P
644  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
645  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
646  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
647  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
648  .P  .P
649  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
650  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 529  The special property L& is also supporte Line 704  The special property L& is also supporte
704  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
705  a modifier or "other".  a modifier or "other".
706  .P  .P
707  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
708    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
709    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
710    (see the discussion of PCRE_NO_UTF8_CHECK in the
711    .\" HREF
712    \fBpcreapi\fP
713    .\"
714    page). Perl does not support the Cs property.
715    .P
716    The long synonyms for property names that Perl supports (such as \ep{Letter})
717  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
718  properties with "Is".  properties with "Is".
719  .P  .P
# Line 553  atomic group Line 737  atomic group
737  (see below).  (see below).
738  .\"  .\"
739  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
740  preceding character.  preceding character. None of them have codepoints less than 256, so in
741    non-UTF-8 mode \eX matches any one character.
742  .P  .P
743  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
744  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
745  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
746  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
747    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
748    (*UCP).
749    .
750    .
751    .\" HTML <a name="extraprops"></a>
752    .SS PCRE's additional properties
753    .rs
754    .sp
755    As well as the standard Unicode properties described in the previous
756    section, PCRE supports four more that make it possible to convert traditional
757    escape sequences such as \ew and \es and POSIX character classes to use Unicode
758    properties. PCRE uses these non-standard, non-Perl properties internally when
759    PCRE_UCP is set. They are:
760    .sp
761      Xan   Any alphanumeric character
762      Xps   Any POSIX space character
763      Xsp   Any Perl space character
764      Xwd   Any Perl "word" character
765    .sp
766    Xan matches characters that have either the L (letter) or the N (number)
767    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
768    carriage return, and any other character that has the Z (separator) property.
769    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
770    same characters as Xan, plus underscore.
771  .  .
772  .  .
773  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 589  For example, when the pattern Line 798  For example, when the pattern
798    (foo)\eKbar    (foo)\eKbar
799  .sp  .sp
800  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
801    .P
802    Perl documents that the use of \eK within assertions is "not well defined". In
803    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
804    ignored in negative assertions.
805  .  .
806  .  .
807  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 613  The backslashed assertions are: Line 826  The backslashed assertions are:
826    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
827    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
828  .sp  .sp
829  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
830  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
831    default it matches the corresponding literal character (for example, \eB
832    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
833    escape sequence" error is generated instead.
834  .P  .P
835  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
836  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
837  \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
838  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
839    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
840    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
841    of word" or "end of word" metasequence. However, whatever follows \eb normally
842    determines which it is. For example, the fragment \eba matches "a" at the start
843    of a word.
844  .P  .P
845  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
846  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 703  end of the subject in both modes, and if Line 924  end of the subject in both modes, and if
924  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
925  .  .
926  .  .
927  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
928    .SH "FULL STOP (PERIOD, DOT) AND \eN"
929  .rs  .rs
930  .sp  .sp
931  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
# Line 725  to match it. Line 947  to match it.
947  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
948  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
949  special meaning in a character class.  special meaning in a character class.
950    .P
951    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
952    set. In other words, it matches any one character except one that signifies the
953    end of a line.
954  .  .
955  .  .
956  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 751  the lookbehind. Line 977  the lookbehind.
977  .rs  .rs
978  .sp  .sp
979  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
980  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.
981  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
982  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
983  escaped with a backslash.  a member of the class, it should be the first data character in the class
984    (after an initial circumflex, if present) or escaped with a backslash.
985  .P  .P
986  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
987  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
988  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
989  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
990  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
991  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 768  For example, the character class [aeiou] Line 995  For example, the character class [aeiou]
995  [^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
996  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
997  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
998  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
999  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
1000  string.  string.
1001  .P  .P
# Line 782  caseful version would. In UTF-8 mode, PC Line 1009  caseful version would. In UTF-8 mode, PC
1009  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
1010  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1011  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1012  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,
1013  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
1014  UTF-8 support.  with UTF-8 support.
1015  .P  .P
1016  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1017  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 818  characters in both cases. In UTF-8 mode, Line 1045  characters in both cases. In UTF-8 mode,
1045  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
1046  property support.  property support.
1047  .P  .P
1048  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
1049  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
1050  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1051  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
1052  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.
1053  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1054  .P  .P
1055  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1056  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 843  this notation. For example, Line 1070  this notation. For example,
1070    [01[:alpha:]%]    [01[:alpha:]%]
1071  .sp  .sp
1072  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1073  are  are:
1074  .sp  .sp
1075    alnum    letters and digits    alnum    letters and digits
1076    alpha    letters    alpha    letters
# Line 854  are Line 1081  are
1081    graph    printing characters, excluding space    graph    printing characters, excluding space
1082    lower    lower case letters    lower    lower case letters
1083    print    printing characters, including space    print    printing characters, including space
1084    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1085    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1086    upper    upper case letters    upper    upper case letters
1087    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 875  matches "1", "2", or any non-digit. PCRE Line 1102  matches "1", "2", or any non-digit. PCRE
1102  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
1103  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1104  .P  .P
1105  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
1106  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1107    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1108    character properties are used. This is achieved by replacing the POSIX classes
1109    by other sequences, as follows:
1110    .sp
1111      [:alnum:]  becomes  \ep{Xan}
1112      [:alpha:]  becomes  \ep{L}
1113      [:blank:]  becomes  \eh
1114      [:digit:]  becomes  \ep{Nd}
1115      [:lower:]  becomes  \ep{Ll}
1116      [:space:]  becomes  \ep{Xps}
1117      [:upper:]  becomes  \ep{Lu}
1118      [:word:]   becomes  \ep{Xwd}
1119    .sp
1120    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1121    classes are unchanged, and match only characters with code points less than
1122    128.
1123  .  .
1124  .  .
1125  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 903  alternative in the subpattern. Line 1146  alternative in the subpattern.
1146  .rs  .rs
1147  .sp  .sp
1148  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1149  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
1150  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1151    The option letters are
1152  .sp  .sp
1153    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1154    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 918  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1162  PCRE_MULTILINE while unsetting PCRE_DOTA
1162  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
1163  unset.  unset.
1164  .P  .P
1165  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
1166  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
1167  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1168  the global options (and it will therefore show up in data extracted by the  .P
1169  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1170    subpattern parentheses), the change applies to the remainder of the pattern
1171    that follows. If the change is placed right at the start of a pattern, PCRE
1172    extracts it into the global options (and it will therefore show up in data
1173    extracted by the \fBpcre_fullinfo()\fP function).
1174  .P  .P
1175  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1176  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
# Line 941  branch is abandoned before the option se Line 1189  branch is abandoned before the option se
1189  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1190  behaviour otherwise.  behaviour otherwise.
1191  .P  .P
1192  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
1193  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
1194  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1195    the application has set or what has been defaulted. Details are given in the
1196    section entitled
1197    .\" HTML <a href="#newlineseq">
1198    .\" </a>
1199    "Newline sequences"
1200    .\"
1201    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1202    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1203    PCRE_UTF8 and the PCRE_UCP options, respectively.
1204  .  .
1205  .  .
1206  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 998  is reached, an option setting in one bra Line 1255  is reached, an option setting in one bra
1255  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1256  .  .
1257  .  .
1258    .\" HTML <a name="dupsubpatternnumber"></a>
1259  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1260  .rs  .rs
1261  .sp  .sp
1262  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses  Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1263  the same numbers for its capturing parentheses. Such a subpattern starts with  the same numbers for its capturing parentheses. Such a subpattern starts with
1264  (?| and is itself a non-capturing subpattern. For example, consider this  (?| and is itself a non-capturing subpattern. For example, consider this
1265  pattern:  pattern:
1266  .sp  .sp
1267    (?|(Sat)ur|(Sun))day    (?|(Sat)ur|(Sun))day
1268  .sp  .sp
1269  Because the two alternatives are inside a (?| group, both sets of capturing  Because the two alternatives are inside a (?| group, both sets of capturing
1270  parentheses are numbered one. Thus, when the pattern matches, you can look  parentheses are numbered one. Thus, when the pattern matches, you can look
1271  at captured substring number one, whichever alternative matched. This construct  at captured substring number one, whichever alternative matched. This construct
1272  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1273  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1274  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1275  buffers that follow the subpattern start after the highest number used in any  buffers that follow the subpattern start after the highest number used in any
1276  branch. The following example is taken from the Perl documentation.  branch. The following example is taken from the Perl documentation.
1277  The numbers underneath show in which buffer the captured content will be  The numbers underneath show in which buffer the captured content will be
1278  stored.  stored.
1279  .sp  .sp
1280    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1281    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1282    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1283  .sp  .sp
1284  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1285  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1286    or "defdef":
1287    .sp
1288      /(?|(abc)|(def))\e1/
1289    .sp
1290    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1291    refers to the first one in the pattern with the given number. The following
1292    pattern matches "abcabc" or "defabc":
1293    .sp
1294      /(?|(abc)|(def))(?1)/
1295    .sp
1296    If a
1297    .\" HTML <a href="#conditions">
1298    .\" </a>
1299    condition test
1300    .\"
1301    for a subpattern's having matched refers to a non-unique number, the test is
1302    true if any of the subpatterns of that number have matched.
1303  .P  .P
1304  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1305  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1039  if an expression is modified, the number Line 1314  if an expression is modified, the number
1314  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1315  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1316  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1317  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1318    have different names, but PCRE does not.
1319  .P  .P
1320  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1321  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1322  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1323  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1324  .\" </a>  .\" </a>
1325  backreferences,  back references,
1326  .\"  .\"
1327  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1328  .\" </a>  .\" </a>
# Line 1066  extracting the name-to-number translatio Line 1342  extracting the name-to-number translatio
1342  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1343  .P  .P
1344  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
1345  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
1346  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
1347  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
1348  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
1349  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
1350    name, and in both cases you want to extract the abbreviation. This pattern
1351    (ignoring the line breaks) does the job:
1352  .sp  .sp
1353    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1354    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1079  abbreviation. This pattern (ignoring the Line 1357  abbreviation. This pattern (ignoring the
1357    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1358  .sp  .sp
1359  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.
1360  (An alternative way of solving this problem is to use a "branch reset"  (An alternative way of solving this problem is to use a "branch reset"
1361  subpattern, as described in the previous section.)  subpattern, as described in the previous section.)
1362  .P  .P
1363  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1364  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
1365  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1366  make a reference to a non-unique named subpattern from elsewhere in the  .P
1367  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
1368  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1369    used. In the absence of duplicate numbers (see the previous section) this is
1370    the one with the lowest number. If you use a named reference in a condition
1371    test (see the
1372    .\"
1373    .\" HTML <a href="#conditions">
1374    .\" </a>
1375    section about conditions
1376    .\"
1377    below), either to check whether a subpattern has matched, or to check for
1378    recursion, all subpatterns with the same name are tested. If the condition is
1379    true for any one of them, the overall condition is true. This is the same
1380    behaviour as testing by number. For further details of the interfaces for
1381    handling named subpatterns, see the
1382  .\" HREF  .\" HREF
1383  \fBpcreapi\fP  \fBpcreapi\fP
1384  .\"  .\"
1385  documentation.  documentation.
1386    .P
1387    \fBWarning:\fP You cannot use different names to distinguish between two
1388    subpatterns with the same number because PCRE uses only the numbers when
1389    matching. For this reason, an error is given at compile time if different names
1390    are given to subpatterns with the same number. However, you can give the same
1391    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1392  .  .
1393  .  .
1394  .SH REPETITION  .SH REPETITION
# Line 1109  items: Line 1406  items:
1406    a character class    a character class
1407    a back reference (see next section)    a back reference (see next section)
1408    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1409      a recursive or "subroutine" call to a subpattern
1410  .sp  .sp
1411  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1412  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1140  support is available, \eX{3} matches thr Line 1438  support is available, \eX{3} matches thr
1438  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).
1439  .P  .P
1440  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
1441  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1442    subpatterns that are referenced as
1443    .\" HTML <a href="#subpatternsassubroutines">
1444    .\" </a>
1445    subroutines
1446    .\"
1447    from elsewhere in the pattern. Items other than subpatterns that have a {0}
1448    quantifier are omitted from the compiled pattern.
1449  .P  .P
1450  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1451  abbreviations:  abbreviations:
# Line 1212  worth setting PCRE_DOTALL in order to ob Line 1517  worth setting PCRE_DOTALL in order to ob
1517  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1518  .P  .P
1519  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1520  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1521  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1522  succeeds. Consider, for example:  succeeds. Consider, for example:
1523  .sp  .sp
# Line 1287  previous example can be rewritten as Line 1592  previous example can be rewritten as
1592  .sp  .sp
1593    \ed++foo    \ed++foo
1594  .sp  .sp
1595    Note that a possessive quantifier can be used with an entire group, for
1596    example:
1597    .sp
1598      (abc|xyz){2,3}+
1599    .sp
1600  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1601  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
1602  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1361  subpattern is possible using named paren Line 1671  subpattern is possible using named paren
1671  .P  .P
1672  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1673  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence, which is a feature introduced in
1674  Perl 5.10. This escape must be followed by a positive or a negative number,  Perl 5.10. This escape must be followed by an unsigned number or a negative
1675  optionally enclosed in braces. These examples are all identical:  number, optionally enclosed in braces. These examples are all identical:
1676  .sp  .sp
1677    (ring), \e1    (ring), \e1
1678    (ring), \eg1    (ring), \eg1
1679    (ring), \eg{1}    (ring), \eg{1}
1680  .sp  .sp
1681  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1682  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1683  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1684    example:
1685  .sp  .sp
1686    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1687  .sp  .sp
# Line 1417  after the reference. Line 1728  after the reference.
1728  .P  .P
1729  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
1730  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1731  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1732  .sp  .sp
1733    (a|(bc))\e2    (a|(bc))\e2
1734  .sp  .sp
1735  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
1736  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
1737  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1738  with a digit character, some delimiter must be used to terminate the back  .P
1739  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1740  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1741    If the pattern continues with a digit character, some delimiter must be used to
1742    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1743    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1744  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1745  .\" </a>  .\" </a>
1746  "Comments"  "Comments"
1747  .\"  .\"
1748  below) can be used.  below) can be used.
1749  .P  .
1750    .SS "Recursive back references"
1751    .rs
1752    .sp
1753  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
1754  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.
1755  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1446  to the previous iteration. In order for Line 1763  to the previous iteration. In order for
1763  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
1764  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
1765  minimum of zero.  minimum of zero.
1766    .P
1767    Back references of this type cause the group that they reference to be treated
1768    as an
1769    .\" HTML <a href="#atomicgroup">
1770    .\" </a>
1771    atomic group.
1772    .\"
1773    Once the whole group has been matched, a subsequent matching failure cannot
1774    cause backtracking into the middle of the group.
1775  .  .
1776  .  .
1777  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1499  lookbehind assertion is needed to achiev Line 1825  lookbehind assertion is needed to achiev
1825  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
1826  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
1827  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.
1828    The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1829    synonym for (?!).
1830  .  .
1831  .  .
1832  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1523  is permitted, but Line 1851  is permitted, but
1851  .sp  .sp
1852  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1853  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
1854  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
1855  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1856  .sp  .sp
1857    (?<=ab(c|de))    (?<=ab(c|de))
1858  .sp  .sp
1859  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
1860  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
1861    branches:
1862  .sp  .sp
1863    (?<=abc|abde)    (?<=abc|abde)
1864  .sp  .sp
# Line 1538  In some cases, the Perl 5.10 escape sequ Line 1867  In some cases, the Perl 5.10 escape sequ
1867  .\" </a>  .\" </a>
1868  (see above)  (see above)
1869  .\"  .\"
1870  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1871  fixed-length.  restriction.
1872  .P  .P
1873  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1874  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
# Line 1551  to appear in lookbehind assertions, beca Line 1880  to appear in lookbehind assertions, beca
1880  the length of the lookbehind. The \eX and \eR escapes, which can match  the length of the lookbehind. The \eX and \eR escapes, which can match
1881  different numbers of bytes, are also not permitted.  different numbers of bytes, are also not permitted.
1882  .P  .P
1883    .\" HTML <a href="#subpatternsassubroutines">
1884    .\" </a>
1885    "Subroutine"
1886    .\"
1887    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1888    as the subpattern matches a fixed-length string.
1889    .\" HTML <a href="#recursion">
1890    .\" </a>
1891    Recursion,
1892    .\"
1893    however, is not supported.
1894    .P
1895  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1896  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
1897  pattern such as  strings. Consider a simple pattern such as
1898  .sp  .sp
1899    abcd$    abcd$
1900  .sp  .sp
# Line 1617  characters that are not "999". Line 1958  characters that are not "999".
1958  .sp  .sp
1959  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1960  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1961  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
1962  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
1963  .sp  .sp
1964    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1965    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1966  .sp  .sp
1967  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1968  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
1969  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
1970    itself contain nested subpatterns of any form, including conditional
1971    subpatterns; the restriction to two alternatives applies only at the level of
1972    the condition. This pattern fragment is an example where the alternatives are
1973    complex:
1974    .sp
1975      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1976    .sp
1977  .P  .P
1978  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
1979  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1634  recursion, a pseudo-condition called DEF Line 1982  recursion, a pseudo-condition called DEF
1982  .rs  .rs
1983  .sp  .sp
1984  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1985  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
1986  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
1987  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
1988  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
1989  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
1990  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
1991    section about duplicate subpattern numbers),
1992    .\"
1993    the condition is true if any of them have been set. An alternative notation is
1994    to precede the digits with a plus or minus sign. In this case, the subpattern
1995    number is relative rather than absolute. The most recently opened parentheses
1996    can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1997    looping constructs it can also make sense to refer to subsequent groups with
1998    constructs such as (?(+2).
1999  .P  .P
2000  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2001  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 1680  Rewriting the above example to use a nam Line 2036  Rewriting the above example to use a nam
2036  .sp  .sp
2037    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2038  .sp  .sp
2039    If the name used in a condition of this kind is a duplicate, the test is
2040    applied to all subpatterns of the same name, and is true if any one of them has
2041    matched.
2042  .  .
2043  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2044  .rs  .rs
# Line 1691  letter R, for example: Line 2050  letter R, for example:
2050  .sp  .sp
2051    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2052  .sp  .sp
2053  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2054  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2055  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2056    applied to all subpatterns of the same name, and is true if any one of them is
2057    the most recent recursion.
2058  .P  .P
2059  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2060  patterns are described below.  .\" HTML <a href="#recursion">
2061    .\" </a>
2062    The syntax for recursive patterns
2063    .\"
2064    is described below.
2065  .  .
2066  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2067  .rs  .rs
# Line 1705  If the condition is the string (DEFINE), Line 2070  If the condition is the string (DEFINE),
2070  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2071  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2072  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2073  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  "subroutines" that can be referenced from elsewhere. (The use of
2074    .\" HTML <a href="#subpatternsassubroutines">
2075    .\" </a>
2076    "subroutines"
2077    .\"
2078  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address could be
2079  written like this (ignore whitespace and line breaks):  written like this (ignore whitespace and line breaks):
2080  .sp  .sp
# Line 1715  written like this (ignore whitespace and Line 2084  written like this (ignore whitespace and
2084  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2085  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2086  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2087  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2088  .P  pattern uses references to the named group to match the four dot-separated
2089  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2090  .  .
2091  .SS "Assertion conditions"  .SS "Assertion conditions"
2092  .rs  .rs
# Line 1750  that make up a comment play no part in t Line 2117  that make up a comment play no part in t
2117  .P  .P
2118  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
2119  character class introduces a comment that continues to immediately after the  character class introduces a comment that continues to immediately after the
2120  next newline in the pattern.  next newline character or character sequence in the pattern. Which characters
2121    are interpreted as newlines is controlled by the options passed to
2122    \fBpcre_compile()\fP or by a special sequence at the start of the pattern, as
2123    described in the section entitled
2124    .\" HTML <a href="#recursion">
2125    .\" </a>
2126    "Newline conventions"
2127    .\"
2128    above. Note that end of a comment is a literal newline sequence in the pattern;
2129    escape sequences that happen to represent a newline do not terminate a comment.
2130    For example, consider this pattern when PCRE_EXTENDED is set, and the default
2131    newline convention is in force:
2132    .sp
2133      abc #comment \en still comment
2134    .sp
2135    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2136    a newline in the pattern. The sequence \en is still literal at this stage, so
2137    it does not terminate the comment. Only an actual character with the code value
2138    0x0a does so.
2139  .  .
2140  .  .
2141  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1776  recursively to the pattern in which it a Line 2161  recursively to the pattern in which it a
2161  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2162  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2163  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2164  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2165  .P  .P
2166  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2167  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive call of the subpattern of the given number,
2168  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  provided that it occurs inside that subpattern. (If not, it is a
2169    .\" HTML <a href="#subpatternsassubroutines">
2170    .\" </a>
2171    "subroutine"
2172    .\"
2173  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2174  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2175  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2176  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2177  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2178  .sp  .sp
2179    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2180  .sp  .sp
2181  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2182  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
2183  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2184  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2185    to avoid backtracking into sequences of non-parentheses.
2186  .P  .P
2187  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
2188  pattern, so instead you could use this:  pattern, so instead you could use this:
2189  .sp  .sp
2190    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2191  .sp  .sp
2192  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
2193  them instead of the whole pattern.  them instead of the whole pattern.
2194  .P  .P
2195  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2196  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references (a Perl 5.10 feature).
2197  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2198  most recently opened parentheses preceding the recursion. In other words, a  most recently opened parentheses preceding the recursion. In other words, a
2199  negative number counts capturing parentheses leftwards from the point at which  negative number counts capturing parentheses leftwards from the point at which
# Line 1817  it is encountered. Line 2202  it is encountered.
2202  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2203  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2204  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2205  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2206    .\" </a>
2207    "subroutine"
2208    .\"
2209    calls, as described in the next section.
2210  .P  .P
2211  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2212  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2213  could rewrite the above example as follows:  could rewrite the above example as follows:
2214  .sp  .sp
2215    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2216  .sp  .sp
2217  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2218  used.  used.
2219  .P  .P
2220  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2221  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2222  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2223  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2224  .sp  .sp
2225    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2226  .sp  .sp
2227  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,
2228  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
2229  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
2230  before failure can be reported.  before failure can be reported.
2231  .P  .P
2232  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
2233  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
2234  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2235  .\" HREF  .\" HREF
2236  \fBpcrecallout\fP  \fBpcrecallout\fP
2237  .\"  .\"
# Line 1851  documentation). If the pattern above is Line 2239  documentation). If the pattern above is
2239  .sp  .sp
2240    (ab(cd)ef)    (ab(cd)ef)
2241  .sp  .sp
2242  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
2243  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
2244  .sp  matched at the top level, its final value is unset, even if it is (temporarily)
2245    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  set at a deeper level.
2246       ^                        ^  .P
2247       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2248  .sp  obtain extra memory to store data during a recursion, which it does by using
2249  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
2250  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.  
2251  .P  .P
2252  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.
2253  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1876  different alternatives for the recursive Line 2261  different alternatives for the recursive
2261  is the actual recursive call.  is the actual recursive call.
2262  .  .
2263  .  .
2264    .\" HTML <a name="recursiondifference"></a>
2265    .SS "Recursion difference from Perl"
2266    .rs
2267    .sp
2268    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2269    treated as an atomic group. That is, once it has matched some of the subject
2270    string, it is never re-entered, even if it contains untried alternatives and
2271    there is a subsequent matching failure. This can be illustrated by the
2272    following pattern, which purports to match a palindromic string that contains
2273    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2274    .sp
2275      ^(.|(.)(?1)\e2)$
2276    .sp
2277    The idea is that it either matches a single character, or two identical
2278    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2279    it does not if the pattern is longer than three characters. Consider the
2280    subject string "abcba":
2281    .P
2282    At the top level, the first character is matched, but as it is not at the end
2283    of the string, the first alternative fails; the second alternative is taken
2284    and the recursion kicks in. The recursive call to subpattern 1 successfully
2285    matches the next character ("b"). (Note that the beginning and end of line
2286    tests are not part of the recursion).
2287    .P
2288    Back at the top level, the next character ("c") is compared with what
2289    subpattern 2 matched, which was "a". This fails. Because the recursion is
2290    treated as an atomic group, there are now no backtracking points, and so the
2291    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2292    try the second alternative.) However, if the pattern is written with the
2293    alternatives in the other order, things are different:
2294    .sp
2295      ^((.)(?1)\e2|.)$
2296    .sp
2297    This time, the recursing alternative is tried first, and continues to recurse
2298    until it runs out of characters, at which point the recursion fails. But this
2299    time we do have another alternative to try at the higher level. That is the big
2300    difference: in the previous case the remaining alternative is at a deeper
2301    recursion level, which PCRE cannot use.
2302    .P
2303    To change the pattern so that matches all palindromic strings, not just those
2304    with an odd number of characters, it is tempting to change the pattern to this:
2305    .sp
2306      ^((.)(?1)\e2|.?)$
2307    .sp
2308    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2309    deeper recursion has matched a single character, it cannot be entered again in
2310    order to match an empty string. The solution is to separate the two cases, and
2311    write out the odd and even cases as alternatives at the higher level:
2312    .sp
2313      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2314    .sp
2315    If you want to match typical palindromic phrases, the pattern has to ignore all
2316    non-word characters, which can be done like this:
2317    .sp
2318      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2319    .sp
2320    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2321    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2322    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2323    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2324    more) to match typical phrases, and Perl takes so long that you think it has
2325    gone into a loop.
2326    .P
2327    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2328    string does not start with a palindrome that is shorter than the entire string.
2329    For example, although "abcba" is correctly matched, if the subject is "ababa",
2330    PCRE finds the palindrome "aba" at the start, then fails at top level because
2331    the end of the string does not follow. Once again, it cannot jump back into the
2332    recursion to try other alternatives, so the entire match fails.
2333    .
2334    .
2335  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2336  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2337  .rs  .rs
# Line 1902  matches "sense and sensibility" and "res Line 2358  matches "sense and sensibility" and "res
2358  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
2359  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2360  .P  .P
2361  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Like recursive subpatterns, a subroutine call is always treated as an atomic
2362  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
2363  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
2364  matching failure.  matching failure. Any capturing parentheses that are set during the subroutine
2365    call revert to their previous values afterwards.
2366  .P  .P
2367  When a subpattern is used as a subroutine, processing options such as  When a subpattern is used as a subroutine, processing options such as
2368  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
# Line 1917  It matches "abcabc". It does not match " Line 2374  It matches "abcabc". It does not match "
2374  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2375  .  .
2376  .  .
2377    .\" HTML <a name="onigurumasubroutines"></a>
2378    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2379    .rs
2380    .sp
2381    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2382    a number enclosed either in angle brackets or single quotes, is an alternative
2383    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2384    are two of the examples used above, rewritten using this syntax:
2385    .sp
2386      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2387      (sens|respons)e and \eg'1'ibility
2388    .sp
2389    PCRE supports an extension to Oniguruma: if a number is preceded by a
2390    plus or a minus sign it is taken as a relative reference. For example:
2391    .sp
2392      (abc)(?i:\eg<-1>)
2393    .sp
2394    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2395    synonymous. The former is a back reference; the latter is a subroutine call.
2396    .
2397    .
2398  .SH CALLOUTS  .SH CALLOUTS
2399  .rs  .rs
2400  .sp  .sp
# Line 1953  description of the interface to the call Line 2431  description of the interface to the call
2431  documentation.  documentation.
2432  .  .
2433  .  .
2434    .\" HTML <a name="backtrackcontrol"></a>
2435    .SH "BACKTRACKING CONTROL"
2436    .rs
2437    .sp
2438    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2439    are described in the Perl documentation as "experimental and subject to change
2440    or removal in a future version of Perl". It goes on to say: "Their usage in
2441    production code should be noted to avoid problems during upgrades." The same
2442    remarks apply to the PCRE features described in this section.
2443    .P
2444    Since these verbs are specifically related to backtracking, most of them can be
2445    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2446    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2447    failing negative assertion, they cause an error if encountered by
2448    \fBpcre_dfa_exec()\fP.
2449    .P
2450    If any of these verbs are used in an assertion or subroutine subpattern
2451    (including recursive subpatterns), their effect is confined to that subpattern;
2452    it does not extend to the surrounding pattern. Note that such subpatterns are
2453    processed as anchored at the point where they are tested.
2454    .P
2455    The new verbs make use of what was previously invalid syntax: an opening
2456    parenthesis followed by an asterisk. They are generally of the form
2457    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2458    depending on whether or not an argument is present. An name is a sequence of
2459    letters, digits, and underscores. If the name is empty, that is, if the closing
2460    parenthesis immediately follows the colon, the effect is as if the colon were
2461    not there. Any number of these verbs may occur in a pattern.
2462    .P
2463    PCRE contains some optimizations that are used to speed up matching by running
2464    some checks at the start of each match attempt. For example, it may know the
2465    minimum length of matching subject, or that a particular character must be
2466    present. When one of these optimizations suppresses the running of a match, any
2467    included backtracking verbs will not, of course, be processed. You can suppress
2468    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2469    when calling \fBpcre_exec()\fP.
2470    .
2471    .
2472    .SS "Verbs that act immediately"
2473    .rs
2474    .sp
2475    The following verbs act as soon as they are encountered. They may not be
2476    followed by a name.
2477    .sp
2478       (*ACCEPT)
2479    .sp
2480    This verb causes the match to end successfully, skipping the remainder of the
2481    pattern. When inside a recursion, only the innermost pattern is ended
2482    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2483    captured. (This feature was added to PCRE at release 8.00.) For example:
2484    .sp
2485      A((?:A|B(*ACCEPT)|C)D)
2486    .sp
2487    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2488    the outer parentheses.
2489    .sp
2490      (*FAIL) or (*F)
2491    .sp
2492    This verb causes the match to fail, forcing backtracking to occur. It is
2493    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2494    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2495    Perl features that are not present in PCRE. The nearest equivalent is the
2496    callout feature, as for example in this pattern:
2497    .sp
2498      a+(?C)(*FAIL)
2499    .sp
2500    A match with the string "aaaa" always fails, but the callout is taken before
2501    each backtrack happens (in this example, 10 times).
2502    .
2503    .
2504    .SS "Recording which path was taken"
2505    .rs
2506    .sp
2507    There is one verb whose main purpose is to track how a match was arrived at,
2508    though it also has a secondary use in conjunction with advancing the match
2509    starting point (see (*SKIP) below).
2510    .sp
2511      (*MARK:NAME) or (*:NAME)
2512    .sp
2513    A name is always required with this verb. There may be as many instances of
2514    (*MARK) as you like in a pattern, and their names do not have to be unique.
2515    .P
2516    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2517    to the caller via the \fIpcre_extra\fP data structure, as described in the
2518    .\" HTML <a href="pcreapi.html#extradata">
2519    .\" </a>
2520    section on \fIpcre_extra\fP
2521    .\"
2522    in the
2523    .\" HREF
2524    \fBpcreapi\fP
2525    .\"
2526    documentation. No data is returned for a partial match. Here is an example of
2527    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2528    outputting of (*MARK) data:
2529    .sp
2530      /X(*MARK:A)Y|X(*MARK:B)Z/K
2531      XY
2532       0: XY
2533      MK: A
2534      XZ
2535       0: XZ
2536      MK: B
2537    .sp
2538    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2539    indicates which of the two alternatives matched. This is a more efficient way
2540    of obtaining this information than putting each alternative in its own
2541    capturing parentheses.
2542    .P
2543    A name may also be returned after a failed match if the final path through the
2544    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2545    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2546    starting point for matching is advanced, the final check is often with an empty
2547    string, causing a failure before (*MARK) is reached. For example:
2548    .sp
2549      /X(*MARK:A)Y|X(*MARK:B)Z/K
2550      XP
2551      No match
2552    .sp
2553    There are three potential starting points for this match (starting with X,
2554    starting with P, and with an empty string). If the pattern is anchored, the
2555    result is different:
2556    .sp
2557      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2558      XP
2559      No match, mark = B
2560    .sp
2561    PCRE's start-of-match optimizations can also interfere with this. For example,
2562    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2563    subject length for a match, a shorter subject will not be scanned at all.
2564    .P
2565    Note that similar anomalies (though different in detail) exist in Perl, no
2566    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2567    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2568    .
2569    .
2570    .SS "Verbs that act after backtracking"
2571    .rs
2572    .sp
2573    The following verbs do nothing when they are encountered. Matching continues
2574    with what follows, but if there is no subsequent match, causing a backtrack to
2575    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2576    the verb. However, when one of these verbs appears inside an atomic group, its
2577    effect is confined to that group, because once the group has been matched,
2578    there is never any backtracking into it. In this situation, backtracking can
2579    "jump back" to the left of the entire atomic group. (Remember also, as stated
2580    above, that this localization also applies in subroutine calls and assertions.)
2581    .P
2582    These verbs differ in exactly what kind of failure occurs when backtracking
2583    reaches them.
2584    .sp
2585      (*COMMIT)
2586    .sp
2587    This verb, which may not be followed by a name, causes the whole match to fail
2588    outright if the rest of the pattern does not match. Even if the pattern is
2589    unanchored, no further attempts to find a match by advancing the starting point
2590    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2591    finding a match at the current starting point, or not at all. For example:
2592    .sp
2593      a+(*COMMIT)b
2594    .sp
2595    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2596    dynamic anchor, or "I've started, so I must finish." The name of the most
2597    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2598    match failure.
2599    .P
2600    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2601    unless PCRE's start-of-match optimizations are turned off, as shown in this
2602    \fBpcretest\fP example:
2603    .sp
2604      /(*COMMIT)abc/
2605      xyzabc
2606       0: abc
2607      xyzabc\eY
2608      No match
2609    .sp
2610    PCRE knows that any match must start with "a", so the optimization skips along
2611    the subject to "a" before running the first match attempt, which succeeds. When
2612    the optimization is disabled by the \eY escape in the second subject, the match
2613    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2614    starting points.
2615    .sp
2616      (*PRUNE) or (*PRUNE:NAME)
2617    .sp
2618    This verb causes the match to fail at the current starting position in the
2619    subject if the rest of the pattern does not match. If the pattern is
2620    unanchored, the normal "bumpalong" advance to the next starting character then
2621    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2622    reached, or when matching to the right of (*PRUNE), but if there is no match to
2623    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2624    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2625    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2626    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2627    match fails completely; the name is passed back if this is the final attempt.
2628    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2629    pattern (*PRUNE) has the same effect as (*COMMIT).
2630    .sp
2631      (*SKIP)
2632    .sp
2633    This verb, when given without a name, is like (*PRUNE), except that if the
2634    pattern is unanchored, the "bumpalong" advance is not to the next character,
2635    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2636    signifies that whatever text was matched leading up to it cannot be part of a
2637    successful match. Consider:
2638    .sp
2639      a+(*SKIP)b
2640    .sp
2641    If the subject is "aaaac...", after the first match attempt fails (starting at
2642    the first character in the string), the starting point skips on to start the
2643    next attempt at "c". Note that a possessive quantifer does not have the same
2644    effect as this example; although it would suppress backtracking during the
2645    first match attempt, the second attempt would start at the second character
2646    instead of skipping on to "c".
2647    .sp
2648      (*SKIP:NAME)
2649    .sp
2650    When (*SKIP) has an associated name, its behaviour is modified. If the
2651    following pattern fails to match, the previous path through the pattern is
2652    searched for the most recent (*MARK) that has the same name. If one is found,
2653    the "bumpalong" advance is to the subject position that corresponds to that
2654    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2655    matching name is found, normal "bumpalong" of one character happens (the
2656    (*SKIP) is ignored).
2657    .sp
2658      (*THEN) or (*THEN:NAME)
2659    .sp
2660    This verb causes a skip to the next alternation in the innermost enclosing
2661    group if the rest of the pattern does not match. That is, it cancels pending
2662    backtracking, but only within the current alternation. Its name comes from the
2663    observation that it can be used for a pattern-based if-then-else block:
2664    .sp
2665      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2666    .sp
2667    If the COND1 pattern matches, FOO is tried (and possibly further items after
2668    the end of the group if FOO succeeds); on failure the matcher skips to the
2669    second alternative and tries COND2, without backtracking into COND1. The
2670    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2671    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2672    like (*PRUNE).
2673    .
2674    .P
2675    The above verbs provide four different "strengths" of control when subsequent
2676    matching fails. (*THEN) is the weakest, carrying on the match at the next
2677    alternation. (*PRUNE) comes next, failing the match at the current starting
2678    position, but allowing an advance to the next character (for an unanchored
2679    pattern). (*SKIP) is similar, except that the advance may be more than one
2680    character. (*COMMIT) is the strongest, causing the entire match to fail.
2681    .P
2682    If more than one is present in a pattern, the "stongest" one wins. For example,
2683    consider this pattern, where A, B, etc. are complex pattern fragments:
2684    .sp
2685      (A(*COMMIT)B(*THEN)C|D)
2686    .sp
2687    Once A has matched, PCRE is committed to this match, at the current starting
2688    position. If subsequently B matches, but C does not, the normal (*THEN) action
2689    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2690    overrides.
2691    .
2692    .
2693  .SH "SEE ALSO"  .SH "SEE ALSO"
2694  .rs  .rs
2695  .sp  .sp
2696  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2697    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2698  .  .
2699  .  .
2700  .SH AUTHOR  .SH AUTHOR
# Line 1973  Cambridge CB2 3QH, England. Line 2711  Cambridge CB2 3QH, England.
2711  .rs  .rs
2712  .sp  .sp
2713  .nf  .nf
2714  Last updated: 13 June 2007  Last updated: 26 October 2010
2715  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2716  .fi  .fi

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