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revision 518 by ph10, Tue May 18 15:47:01 2010 UTC revision 738 by ph10, Fri Oct 21 09:04:01 2011 UTC
# Line 32  Starting a pattern with this sequence is Line 32  Starting a pattern with this sequence is
32  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  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  pattern matching is mentioned in several places below. There is also a summary
34  of UTF-8 features in the  of UTF-8 features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
35  .\" HREF  .\" HREF
36  \fBpcre\fP  \fBpcreunicode\fP
37  .\"  .\"
38  page.  page.
39  .P  .P
40  Another special sequence that may appear at the start of a pattern or in  Another special sequence that may appear at the start of a pattern or in
41  combination with (*UTF8) is:  combination with (*UTF8) is:
42  .sp  .sp
43    (*UCP)    (*UCP)
44  .sp  .sp
45  This has the same effect as setting the PCRE_UCP option: it causes sequences  This has the same effect as setting the PCRE_UCP option: it causes sequences
46  such as \ed and \ew to use Unicode properties to determine character types,  such as \ed and \ew to use Unicode properties to determine character types,
47  instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
48  table.  table.
49  .P  .P
50    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52    also some more of these special sequences that are concerned with the handling
53    of newlines; they are described below.
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 66  discussed in the Line 66  discussed in the
66  page.  page.
67  .  .
68  .  .
69    .\" HTML <a name="newlines"></a>
70  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
71  .rs  .rs
72  .sp  .sp
# Line 181  The following sections describe the use Line 182  The following sections describe the use
182  .rs  .rs
183  .sp  .sp
184  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
185  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
186  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 191  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 210  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219    by \eE later in the pattern, the literal interpretation continues to the end of
220    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221    a character class, this causes an error, because the character class is not
222    terminated.
223  .  .
224  .  .
225  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 223  but when a pattern is being prepared by Line 233  but when a pattern is being prepared by
233  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
234  .sp  .sp
235    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
236    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
237    \ee        escape (hex 1B)    \ee        escape (hex 1B)
238    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
239    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
# Line 235  one of the following escape sequences th Line 245  one of the following escape sequences th
245  .sp  .sp
246  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
247  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
248  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
249  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
250    than 127, a compile-time error occurs. This locks out non-ASCII characters in
251    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
252    values are valid. A lower case letter is converted to upper case, and then the
253    0xc0 bits are flipped.)
254  .P  .P
255  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
256  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{
# Line 367  Another use of backslash is for specifyi Line 381  Another use of backslash is for specifyi
381    \ew     any "word" character    \ew     any "word" character
382    \eW     any "non-word" character    \eW     any "non-word" character
383  .sp  .sp
384  There is also the single sequence \eN, which matches a non-newline character.  There is also the single sequence \eN, which matches a non-newline character.
385  This is the same as  This is the same as
386  .\" HTML <a href="#fullstopdot">  .\" HTML <a href="#fullstopdot">
387  .\" </a>  .\" </a>
388  the "." metacharacter  the "." metacharacter
389  .\"  .\"
390  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set.
391  .P  .P
# Line 408  Unicode is discouraged. Line 422  Unicode is discouraged.
422  By default, in UTF-8 mode, characters with values greater than 128 never match  By default, in UTF-8 mode, characters with values greater than 128 never match
423  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
424  their original meanings from before UTF-8 support was available, mainly for  their original meanings from before UTF-8 support was available, mainly for
425  efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
426  and the PCRE_UCP option is set, the behaviour is changed so that Unicode  and the PCRE_UCP option is set, the behaviour is changed so that Unicode
427  properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
428  .sp  .sp
# Line 417  properties are used to determine charact Line 431  properties are used to determine charact
431    \ew  any character that \ep{L} or \ep{N} matches, plus underscore    \ew  any character that \ep{L} or \ep{N} matches, plus underscore
432  .sp  .sp
433  The upper case escapes match the inverse sets of characters. Note that \ed  The upper case escapes match the inverse sets of characters. Note that \ed
434  matches only decimal digits, whereas \ew matches any Unicode digit, as well as  matches only decimal digits, whereas \ew matches any Unicode digit, as well as
435  any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and  any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
436  \eB because they are defined in terms of \ew and \eW. Matching these sequences  \eB because they are defined in terms of \ew and \eW. Matching these sequences
437  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
438  .P  .P
439  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
440  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
441  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is  characters by default, these always match certain high-valued codepoints in
442  set. The horizontal space characters are:  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
443    are:
444  .sp  .sp
445    U+0009     Horizontal tab    U+0009     Horizontal tab
446    U+0020     Space    U+0020     Space
# Line 463  The vertical space characters are: Line 478  The vertical space characters are:
478  .rs  .rs
479  .sp  .sp
480  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
481  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
 equivalent to the following:  
482  .sp  .sp
483    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
484  .sp  .sp
# Line 527  The extra escape sequences are: Line 541  The extra escape sequences are:
541  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
542  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
543  character (including newline), and some special PCRE properties (described  character (including newline), and some special PCRE properties (described
544  in the  in the
545  .\" HTML <a href="#extraprops">  .\" HTML <a href="#extraprops">
546  .\" </a>  .\" </a>
547  next section).  next section).
548  .\"  .\"
549  Other Perl properties such as "InMusicalSymbols" are not currently supported by  Other Perl properties such as "InMusicalSymbols" are not currently supported by
550  PCRE. Note that \eP{Any} does not match any characters, so always causes a  PCRE. Note that \eP{Any} does not match any characters, so always causes a
# Line 738  Characters with the "mark" property are Line 752  Characters with the "mark" property are
752  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
753  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
754  .P  .P
755    Note that recent versions of Perl have changed \eX to match what Unicode calls
756    an "extended grapheme cluster", which has a more complicated definition.
757    .P
758  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
759  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
760  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
761  properties in PCRE by default, though you can make them do so by setting the  properties in PCRE by default, though you can make them do so by setting the
762  PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with  PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
763  (*UCP).  (*UCP).
764  .  .
# Line 750  PCRE_UCP option for \fBpcre_compile()\fP Line 767  PCRE_UCP option for \fBpcre_compile()\fP
767  .SS PCRE's additional properties  .SS PCRE's additional properties
768  .rs  .rs
769  .sp  .sp
770  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described in the previous
771  section, PCRE supports four more that make it possible to convert traditional  section, PCRE supports four more that make it possible to convert traditional
772  escape sequences such as \ew and \es and POSIX character classes to use Unicode  escape sequences such as \ew and \es and POSIX character classes to use Unicode
773  properties. PCRE uses these non-standard, non-Perl properties internally when  properties. PCRE uses these non-standard, non-Perl properties internally when
774  PCRE_UCP is set. They are:  PCRE_UCP is set. They are:
# Line 761  PCRE_UCP is set. They are: Line 778  PCRE_UCP is set. They are:
778    Xsp   Any Perl space character    Xsp   Any Perl space character
779    Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
780  .sp  .sp
781  Xan matches characters that have either the L (letter) or the N (number)  Xan matches characters that have either the L (letter) or the N (number)
782  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
783  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
784  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
785  same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
786  .  .
787  .  .
# Line 772  same characters as Xan, plus underscore. Line 789  same characters as Xan, plus underscore.
789  .SS "Resetting the match start"  .SS "Resetting the match start"
790  .rs  .rs
791  .sp  .sp
792  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
793  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
794  .sp  .sp
795    foo\eKbar    foo\eKbar
796  .sp  .sp
# Line 825  The backslashed assertions are: Line 841  The backslashed assertions are:
841    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
842  .sp  .sp
843  Inside a character class, \eb has a different meaning; it matches the backspace  Inside a character class, \eb has a different meaning; it matches the backspace
844  character. If any other of these assertions appears in a character class, by  character. If any other of these assertions appears in a character class, by
845  default it matches the corresponding literal character (for example, \eB  default it matches the corresponding literal character (for example, \eB
846  matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid  matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
847  escape sequence" error is generated instead.  escape sequence" error is generated instead.
# Line 946  The handling of dot is entirely independ Line 962  The handling of dot is entirely independ
962  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
963  special meaning in a character class.  special meaning in a character class.
964  .P  .P
965  The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not  The escape sequence \eN behaves like a dot, except that it is not affected by
966  set. In other words, it matches any one character except one that signifies the  the PCRE_DOTALL option. In other words, it matches any character except one
967  end of a line.  that signifies the end of a line.
968  .  .
969  .  .
970  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
971  .rs  .rs
972  .sp  .sp
973  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
974  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
975  characters. The feature is provided in Perl in order to match individual bytes  characters. The feature is provided in Perl in order to match individual bytes
976  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
977  what remains in the string may be a malformed UTF-8 string. For this reason,  breaks up characters into individual bytes, matching one byte with \eC in UTF-8
978  the \eC escape sequence is best avoided.  mode means that the rest of the string may start with a malformed UTF-8
979    character. This has undefined results, because PCRE assumes that it is dealing
980    with valid UTF-8 strings (and by default it checks this at the start of
981    processing unless the PCRE_NO_UTF8_CHECK option is used).
982  .P  .P
983  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
984  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 968  PCRE does not allow \eC to appear in loo Line 987  PCRE does not allow \eC to appear in loo
987  .\"  .\"
988  because in UTF-8 mode this would make it impossible to calculate the length of  because in UTF-8 mode this would make it impossible to calculate the length of
989  the lookbehind.  the lookbehind.
990    .P
991    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
992    way of using it that avoids the problem of malformed UTF-8 characters is to
993    use a lookahead to check the length of the next character, as in this pattern
994    (ignore white space and line breaks):
995    .sp
996      (?| (?=[\ex00-\ex7f])(\eC) |
997          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
998          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
999          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1000    .sp
1001    A group that starts with (?| resets the capturing parentheses numbers in each
1002    alternative (see
1003    .\" HTML <a href="#dupsubpatternnumber">
1004    .\" </a>
1005    "Duplicate Subpattern Numbers"
1006    .\"
1007    below). The assertions at the start of each branch check the next UTF-8
1008    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1009    character's individual bytes are then captured by the appropriate number of
1010    groups.
1011  .  .
1012  .  .
1013  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 1043  characters in both cases. In UTF-8 mode, Line 1083  characters in both cases. In UTF-8 mode,
1083  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
1084  property support.  property support.
1085  .P  .P
1086  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1087  \eW may also appear in a character class, and add the characters that they  \eV, \ew, and \eW may appear in a character class, and add the characters that
1088  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1089  circumflex can conveniently be used with the upper case character types to  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1090    and their upper case partners, just as it does when they appear outside a
1091    character class, as described in the section entitled
1092    .\" HTML <a href="#genericchartypes">
1093    .\" </a>
1094    "Generic character types"
1095    .\"
1096    above. The escape sequence \eb has a different meaning inside a character
1097    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1098    are not special inside a character class. Like any other unrecognized escape
1099    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1100    default, but cause an error if the PCRE_EXTRA option is set.
1101    .P
1102    A circumflex can conveniently be used with the upper case character types to
1103  specify a more restricted set of characters than the matching lower case type.  specify a more restricted set of characters than the matching lower case type.
1104  For example, the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore,
1105    whereas [\ew] includes underscore. A positive character class should be read as
1106    "something OR something OR ..." and a negative class as "NOT something AND NOT
1107    something AND NOT ...".
1108  .P  .P
1109  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1110  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 1102  supported, and an error is given if they Line 1158  supported, and an error is given if they
1158  .P  .P
1159  By default, in UTF-8 mode, characters with values greater than 128 do not match  By default, in UTF-8 mode, characters with values greater than 128 do not match
1160  any of the POSIX character classes. However, if the PCRE_UCP option is passed  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1161  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1162  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
1163  by other sequences, as follows:  by other sequences, as follows:
1164  .sp  .sp
1165    [:alnum:]  becomes  \ep{Xan}    [:alnum:]  becomes  \ep{Xan}
1166    [:alpha:]  becomes  \ep{L}    [:alpha:]  becomes  \ep{L}
1167    [:blank:]  becomes  \eh    [:blank:]  becomes  \eh
1168    [:digit:]  becomes  \ep{Nd}    [:digit:]  becomes  \ep{Nd}
1169    [:lower:]  becomes  \ep{Ll}    [:lower:]  becomes  \ep{Ll}
1170    [:space:]  becomes  \ep{Xps}    [:space:]  becomes  \ep{Xps}
1171    [:upper:]  becomes  \ep{Lu}    [:upper:]  becomes  \ep{Lu}
1172    [:word:]   becomes  \ep{Xwd}    [:word:]   becomes  \ep{Xwd}
1173  .sp  .sp
# Line 1171  extracts it into the global options (and Line 1227  extracts it into the global options (and
1227  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1228  .P  .P
1229  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1230  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1231  .sp  .sp
1232    (a(?i)b)c    (a(?i)b)c
1233  .sp  .sp
# Line 1212  Turning part of a pattern into a subpatt Line 1268  Turning part of a pattern into a subpatt
1268  .sp  .sp
1269    cat(aract|erpillar|)    cat(aract|erpillar|)
1270  .sp  .sp
1271  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1272  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1273  .sp  .sp
1274  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1275  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1276  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1277  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1278  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1279  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1280  .sp  .sp
1281    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1282  .sp  .sp
# Line 1270  at captured substring number one, whiche Line 1325  at captured substring number one, whiche
1325  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
1326  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1327  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
1328  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1329  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1330  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1331  .sp  .sp
1332    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1333    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# Line 1285  or "defdef": Line 1339  or "defdef":
1339  .sp  .sp
1340    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1341  .sp  .sp
1342  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1343  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1344  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1345  .sp  .sp
1346    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1347  .sp  .sp
# Line 1400  items: Line 1454  items:
1454    the \eC escape sequence    the \eC escape sequence
1455    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1456    the \eR escape sequence    the \eR escape sequence
1457    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1458    a character class    a character class
1459    a back reference (see next section)    a back reference (see next section)
1460    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1461    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1462  .sp  .sp
1463  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1464  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1442  subpatterns that are referenced as Line 1496  subpatterns that are referenced as
1496  .\" </a>  .\" </a>
1497  subroutines  subroutines
1498  .\"  .\"
1499  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1500  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1501    .\" </a>
1502    "Defining subpatterns for use by reference only"
1503    .\"
1504    below). Items other than subpatterns that have a {0} quantifier are omitted
1505    from the compiled pattern.
1506  .P  .P
1507  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1508  abbreviations:  abbreviations:
# Line 1668  no such problem when named parentheses a Line 1727  no such problem when named parentheses a
1727  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1728  .P  .P
1729  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
1730  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1731  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1732  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1733  .sp  .sp
1734    (ring), \e1    (ring), \e1
1735    (ring), \eg1    (ring), \eg1
# Line 1684  example: Line 1743  example:
1743    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1744  .sp  .sp
1745  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1746  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1747  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1748  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1749  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1750  .P  .P
1751  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1752  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1789  those that look ahead of the current pos Line 1848  those that look ahead of the current pos
1848  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1849  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1850  .P  .P
1851  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1852  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1853  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1854  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1855  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1856  because it does not make sense for negative assertions.  .P
1857    For compatibility with Perl, assertion subpatterns may be repeated; though
1858    it makes no sense to assert the same thing several times, the side effect of
1859    capturing parentheses may occasionally be useful. In practice, there only three
1860    cases:
1861    .sp
1862    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1863    However, it may contain internal capturing parenthesized groups that are called
1864    from elsewhere via the
1865    .\" HTML <a href="#subpatternsassubroutines">
1866    .\" </a>
1867    subroutine mechanism.
1868    .\"
1869    .sp
1870    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1871    were {0,1}. At run time, the rest of the pattern match is tried with and
1872    without the assertion, the order depending on the greediness of the quantifier.
1873    .sp
1874    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1875    The assertion is obeyed just once when encountered during matching.
1876  .  .
1877  .  .
1878  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1823  lookbehind assertion is needed to achiev Line 1901  lookbehind assertion is needed to achiev
1901  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
1902  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
1903  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.
1904  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
1905  .  .
1906  .  .
1907  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1849  is permitted, but Line 1926  is permitted, but
1926  .sp  .sp
1927  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1928  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
1929  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1930  match the same length of string. An assertion such as  length of string. An assertion such as
1931  .sp  .sp
1932    (?<=ab(c|de))    (?<=ab(c|de))
1933  .sp  .sp
# Line 1860  branches: Line 1937  branches:
1937  .sp  .sp
1938    (?<=abc|abde)    (?<=abc|abde)
1939  .sp  .sp
1940  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1941  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1942  .\" </a>  .\" </a>
1943  (see above)  (see above)
# Line 1964  already been matched. The two possible f Line 2041  already been matched. The two possible f
2041  .sp  .sp
2042  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2043  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
2044  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2045    itself contain nested subpatterns of any form, including conditional
2046    subpatterns; the restriction to two alternatives applies only at the level of
2047    the condition. This pattern fragment is an example where the alternatives are
2048    complex:
2049    .sp
2050      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2051    .sp
2052  .P  .P
2053  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2054  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1981  matched. If there is more than one captu Line 2065  matched. If there is more than one captu
2065  .\" </a>  .\" </a>
2066  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2067  .\"  .\"
2068  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2069  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2070  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2071  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2072  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2073  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2074    zero in any of these forms is not used; it provokes a compile-time error.)
2075  .P  .P
2076  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2077  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 1997  three parts for ease of discussion: Line 2082  three parts for ease of discussion:
2082  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2083  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
2084  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
2085  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2086  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2087  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2088  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2089  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 2054  The syntax for recursive patterns Line 2139  The syntax for recursive patterns
2139  .\"  .\"
2140  is described below.  is described below.
2141  .  .
2142    .\" HTML <a name="subdefine"></a>
2143  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2144  .rs  .rs
2145  .sp  .sp
# Line 2061  If the condition is the string (DEFINE), Line 2147  If the condition is the string (DEFINE),
2147  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
2148  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2149  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
2150  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2151  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2152  .\" </a>  .\" </a>
2153  "subroutines"  subroutines
2154  .\"  .\"
2155  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 such as
2156  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2157    breaks):
2158  .sp  .sp
2159    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2160    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2102  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2189  dd-aaa-dd or dd-dd-dd, where aaa are let
2189  .SH COMMENTS  .SH COMMENTS
2190  .rs  .rs
2191  .sp  .sp
2192  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2193  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2194  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2195    subpattern name or number. The characters that make up a comment play no part
2196    in the pattern matching.
2197  .P  .P
2198  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2199  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2200  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2201    this case continues to immediately after the next newline character or
2202    character sequence in the pattern. Which characters are interpreted as newlines
2203    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2204    sequence at the start of the pattern, as described in the section entitled
2205    .\" HTML <a href="#newlines">
2206    .\" </a>
2207    "Newline conventions"
2208    .\"
2209    above. Note that the end of this type of comment is a literal newline sequence
2210    in the pattern; escape sequences that happen to represent a newline do not
2211    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2212    default newline convention is in force:
2213    .sp
2214      abc #comment \en still comment
2215    .sp
2216    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2217    a newline in the pattern. The sequence \en is still literal at this stage, so
2218    it does not terminate the comment. Only an actual character with the code value
2219    0x0a (the default newline) does so.
2220  .  .
2221  .  .
2222  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2137  individual subpattern recursion. After i Line 2245  individual subpattern recursion. After i
2245  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2246  .P  .P
2247  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
2248  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2249  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2250  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2251  .\" </a>  .\" </a>
2252  "subroutine"  non-recursive subroutine
2253  .\"  .\"
2254  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
2255  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2166  We have put the pattern into parentheses Line 2274  We have put the pattern into parentheses
2274  them instead of the whole pattern.  them instead of the whole pattern.
2275  .P  .P
2276  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
2277  is made easier by the use of relative references (a Perl 5.10 feature).  is made easier by the use of relative references. Instead of (?1) in the
2278  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2279  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2280  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2281  .P  .P
2282  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2283  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2284  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2285  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2286  .\" </a>  .\" </a>
2287  "subroutine"  non-recursive subroutine
2288  .\"  .\"
2289  calls, as described in the next section.  calls, as described in the next section.
2290  .P  .P
# Line 2214  documentation). If the pattern above is Line 2321  documentation). If the pattern above is
2321  .sp  .sp
2322  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2323  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2324  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2325  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2326  .P  .P
2327  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2328  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2235  is the actual recursive call. Line 2342  is the actual recursive call.
2342  .  .
2343  .  .
2344  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2345  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2346  .rs  .rs
2347  .sp  .sp
2348  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2349  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2350  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2351  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2352  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2353  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2354    characters (for example, "a", "aba", "abcba", "abcdcba"):
2355  .sp  .sp
2356    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2357  .sp  .sp
# Line 2273  time we do have another alternative to t Line 2381  time we do have another alternative to t
2381  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2382  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2383  .P  .P
2384  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2385  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2386    this:
2387  .sp  .sp
2388    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2389  .sp  .sp
# Line 2303  For example, although "abcba" is correct Line 2412  For example, although "abcba" is correct
2412  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2413  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2414  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2415    .P
2416    The second way in which PCRE and Perl differ in their recursion processing is
2417    in the handling of captured values. In Perl, when a subpattern is called
2418    recursively or as a subpattern (see the next section), it has no access to any
2419    values that were captured outside the recursion, whereas in PCRE these values
2420    can be referenced. Consider this pattern:
2421    .sp
2422      ^(.)(\e1|a(?2))
2423    .sp
2424    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2425    then in the second group, when the back reference \e1 fails to match "b", the
2426    second alternative matches "a" and then recurses. In the recursion, \e1 does
2427    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2428    match because inside the recursive call \e1 cannot access the externally set
2429    value.
2430  .  .
2431  .  .
2432  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2433  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2434  .rs  .rs
2435  .sp  .sp
2436  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2437  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
2438  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2439  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2440  relative, as in these examples:  relative, as in these examples:
2441  .sp  .sp
# Line 2331  matches "sense and sensibility" and "res Line 2455  matches "sense and sensibility" and "res
2455  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
2456  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2457  .P  .P
2458  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2459  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2460  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2461  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2462  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2463  .P  .P
2464  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2465  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2466  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2467  .sp  .sp
2468    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2469  .sp  .sp
# Line 2420  a backtracking algorithm. With the excep Line 2544  a backtracking algorithm. With the excep
2544  failing negative assertion, they cause an error if encountered by  failing negative assertion, they cause an error if encountered by
2545  \fBpcre_dfa_exec()\fP.  \fBpcre_dfa_exec()\fP.
2546  .P  .P
2547  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or in a subpattern that is
2548  (including recursive subpatterns), their effect is confined to that subpattern;  called as a subroutine (whether or not recursively), their effect is confined
2549  it does not extend to the surrounding pattern. Note that such subpatterns are  to that subpattern; it does not extend to the surrounding pattern, with one
2550  processed as anchored at the point where they are tested.  exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2551    back (compare capturing parentheses in assertions). Note that such subpatterns
2552    are processed as anchored at the point where they are tested. Note also that
2553    Perl's treatment of subroutines is different in some cases.
2554  .P  .P
2555  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2556  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2557  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2558  depending on whether or not an argument is present. An name is a sequence of  depending on whether or not an argument is present. A name is any sequence of
2559  letters, digits, and underscores. If the name is empty, that is, if the closing  characters that does not include a closing parenthesis. If the name is empty,
2560  parenthesis immediately follows the colon, the effect is as if the colon were  that is, if the closing parenthesis immediately follows the colon, the effect
2561  not there. Any number of these verbs may occur in a pattern.  is as if the colon were not there. Any number of these verbs may occur in a
2562    pattern.
2563  .P  .P
2564  PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
2565  some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
# Line 2439  minimum length of matching subject, or t Line 2567  minimum length of matching subject, or t
2567  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations suppresses the running of a match, any
2568  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2569  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2570  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2571    pattern with (*NO_START_OPT).
2572  .  .
2573  .  .
2574  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2451  followed by a name. Line 2580  followed by a name.
2580     (*ACCEPT)     (*ACCEPT)
2581  .sp  .sp
2582  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2583  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2584  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2585  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2586    far is captured. For example:
2587  .sp  .sp
2588    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2589  .sp  .sp
# Line 2462  the outer parentheses. Line 2592  the outer parentheses.
2592  .sp  .sp
2593    (*FAIL) or (*F)    (*FAIL) or (*F)
2594  .sp  .sp
2595  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2596  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2597  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2598  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2513  indicates which of the two alternatives Line 2643  indicates which of the two alternatives
2643  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2644  capturing parentheses.  capturing parentheses.
2645  .P  .P
2646    If (*MARK) is encountered in a positive assertion, its name is recorded and
2647    passed back if it is the last-encountered. This does not happen for negative
2648    assertions.
2649    .P
2650  A name may also be returned after a failed match if the final path through the  A name may also be returned after a failed match if the final path through the
2651  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2652  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
# Line 2625  following pattern fails to match, the pr Line 2759  following pattern fails to match, the pr
2759  searched for the most recent (*MARK) that has the same name. If one is found,  searched for the most recent (*MARK) that has the same name. If one is found,
2760  the "bumpalong" advance is to the subject position that corresponds to that  the "bumpalong" advance is to the subject position that corresponds to that
2761  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2762  matching name is found, normal "bumpalong" of one character happens (the  matching name is found, normal "bumpalong" of one character happens (that is,
2763  (*SKIP) is ignored).  the (*SKIP) is ignored).
2764  .sp  .sp
2765    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2766  .sp  .sp
2767  This verb causes a skip to the next alternation if the rest of the pattern does  This verb causes a skip to the next innermost alternative if the rest of the
2768  not match. That is, it cancels pending backtracking, but only within the  pattern does not match. That is, it cancels pending backtracking, but only
2769  current alternation. Its name comes from the observation that it can be used  within the current alternative. Its name comes from the observation that it can
2770  for a pattern-based if-then-else block:  be used for a pattern-based if-then-else block:
2771  .sp  .sp
2772    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2773  .sp  .sp
2774  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2775  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2776  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. The
2777  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2778  overall match fails. If (*THEN) is not directly inside an alternation, it acts  overall match fails. If (*THEN) is not inside an alternation, it acts like
2779  like (*PRUNE).  (*PRUNE).
2780    .P
2781    Note that a subpattern that does not contain a | character is just a part of
2782    the enclosing alternative; it is not a nested alternation with only one
2783    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2784    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2785    pattern fragments that do not contain any | characters at this level:
2786    .sp
2787      A (B(*THEN)C) | D
2788    .sp
2789    If A and B are matched, but there is a failure in C, matching does not
2790    backtrack into A; instead it moves to the next alternative, that is, D.
2791    However, if the subpattern containing (*THEN) is given an alternative, it
2792    behaves differently:
2793    .sp
2794      A (B(*THEN)C | (*FAIL)) | D
2795    .sp
2796    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2797    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2798    because there are no more alternatives to try. In this case, matching does now
2799    backtrack into A.
2800    .P
2801    Note also that a conditional subpattern is not considered as having two
2802    alternatives, because only one is ever used. In other words, the | character in
2803    a conditional subpattern has a different meaning. Ignoring white space,
2804    consider:
2805    .sp
2806      ^.*? (?(?=a) a | b(*THEN)c )
2807    .sp
2808    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2809    it initially matches zero characters. The condition (?=a) then fails, the
2810    character "b" is matched, but "c" is not. At this point, matching does not
2811    backtrack to .*? as might perhaps be expected from the presence of the |
2812    character. The conditional subpattern is part of the single alternative that
2813    comprises the whole pattern, and so the match fails. (If there was a backtrack
2814    into .*?, allowing it to match "b", the match would succeed.)
2815    .P
2816    The verbs just described provide four different "strengths" of control when
2817    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2818    next alternative. (*PRUNE) comes next, failing the match at the current
2819    starting position, but allowing an advance to the next character (for an
2820    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2821    than one character. (*COMMIT) is the strongest, causing the entire match to
2822    fail.
2823    .P
2824    If more than one such verb is present in a pattern, the "strongest" one wins.
2825    For example, consider this pattern, where A, B, etc. are complex pattern
2826    fragments:
2827    .sp
2828      (A(*COMMIT)B(*THEN)C|D)
2829    .sp
2830    Once A has matched, PCRE is committed to this match, at the current starting
2831    position. If subsequently B matches, but C does not, the normal (*THEN) action
2832    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2833    overrides.
2834  .  .
2835  .  .
2836  .SH "SEE ALSO"  .SH "SEE ALSO"
# Line 2666  Cambridge CB2 3QH, England. Line 2854  Cambridge CB2 3QH, England.
2854  .rs  .rs
2855  .sp  .sp
2856  .nf  .nf
2857  Last updated: 18 May 2010  Last updated: 19 October 2011
2858  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2859  .fi  .fi

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