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revision 518 by ph10, Tue May 18 15:47:01 2010 UTC revision 654 by ph10, Tue Aug 2 11:00:40 2011 UTC
# Line 42  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  Another special sequence that may appear at the start of a pattern or in
46  combination with (*UTF8) is:  combination with (*UTF8) is:
47  .sp  .sp
48    (*UCP)    (*UCP)
49  .sp  .sp
50  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
51  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,
52  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
53  table.  table.
54  .P  .P
55    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
56    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
57    also some more of these special sequences that are concerned with the handling
58    of newlines; they are described below.
59    .P
60  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
61  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
62  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 71  discussed in the
71  page.  page.
72  .  .
73  .  .
74    .\" HTML <a name="newlines"></a>
75  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
76  .rs  .rs
77  .sp  .sp
# Line 181  The following sections describe the use Line 187  The following sections describe the use
187  .rs  .rs
188  .sp  .sp
189  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
190  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
191  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
192  outside character classes.  both inside and outside character classes.
193  .P  .P
194  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.
195  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 197  otherwise be interpreted as a metacharac
197  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
198  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
199  .P  .P
200    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
201    backslash. All other characters (in particular, those whose codepoints are
202    greater than 127) are treated as literals.
203    .P
204  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
205  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
206  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 220  Perl, $ and @ cause variable interpolati
220    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
221  .sp  .sp
222  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
223    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
224    by \eE later in the pattern, the literal interpretation continues to the end of
225    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
226    a character class, this causes an error, because the character class is not
227    terminated.
228  .  .
229  .  .
230  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 223  but when a pattern is being prepared by Line 238  but when a pattern is being prepared by
238  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
239  .sp  .sp
240    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
241    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
242    \ee        escape (hex 1B)    \ee        escape (hex 1B)
243    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
244    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
# Line 235  one of the following escape sequences th Line 250  one of the following escape sequences th
250  .sp  .sp
251  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
252  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.
253  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
254  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
255    than 127, a compile-time error occurs. This locks out non-ASCII characters in
256    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
257    values are valid. A lower case letter is converted to upper case, and then the
258    0xc0 bits are flipped.)
259  .P  .P
260  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
261  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 386  Another use of backslash is for specifyi
386    \ew     any "word" character    \ew     any "word" character
387    \eW     any "non-word" character    \eW     any "non-word" character
388  .sp  .sp
389  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.
390  This is the same as  This is the same as
391  .\" HTML <a href="#fullstopdot">  .\" HTML <a href="#fullstopdot">
392  .\" </a>  .\" </a>
393  the "." metacharacter  the "." metacharacter
394  .\"  .\"
395  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set.
396  .P  .P
# Line 408  Unicode is discouraged. Line 427  Unicode is discouraged.
427  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
428  \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
429  their original meanings from before UTF-8 support was available, mainly for  their original meanings from before UTF-8 support was available, mainly for
430  efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
431  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
432  properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
433  .sp  .sp
# Line 417  properties are used to determine charact Line 436  properties are used to determine charact
436    \ew  any character that \ep{L} or \ep{N} matches, plus underscore    \ew  any character that \ep{L} or \ep{N} matches, plus underscore
437  .sp  .sp
438  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
439  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
440  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
441  \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
442  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
443  .P  .P
444  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
445  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
446  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
447  set. The horizontal space characters are:  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
448    are:
449  .sp  .sp
450    U+0009     Horizontal tab    U+0009     Horizontal tab
451    U+0020     Space    U+0020     Space
# Line 463  The vertical space characters are: Line 483  The vertical space characters are:
483  .rs  .rs
484  .sp  .sp
485  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
486  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:  
487  .sp  .sp
488    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
489  .sp  .sp
# Line 527  The extra escape sequences are: Line 546  The extra escape sequences are:
546  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
547  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
548  character (including newline), and some special PCRE properties (described  character (including newline), and some special PCRE properties (described
549  in the  in the
550  .\" HTML <a href="#extraprops">  .\" HTML <a href="#extraprops">
551  .\" </a>  .\" </a>
552  next section).  next section).
553  .\"  .\"
554  Other Perl properties such as "InMusicalSymbols" are not currently supported by  Other Perl properties such as "InMusicalSymbols" are not currently supported by
555  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 757  Characters with the "mark" property are
757  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
758  non-UTF-8 mode \eX matches any one character.  non-UTF-8 mode \eX matches any one character.
759  .P  .P
760    Note that recent versions of Perl have changed \eX to match what Unicode calls
761    an "extended grapheme cluster", which has a more complicated definition.
762    .P
763  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
764  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
765  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
766  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
767  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
768  (*UCP).  (*UCP).
769  .  .
# Line 750  PCRE_UCP option for \fBpcre_compile()\fP Line 772  PCRE_UCP option for \fBpcre_compile()\fP
772  .SS PCRE's additional properties  .SS PCRE's additional properties
773  .rs  .rs
774  .sp  .sp
775  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described in the previous
776  section, PCRE supports four more that make it possible to convert traditional  section, PCRE supports four more that make it possible to convert traditional
777  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
778  properties. PCRE uses these non-standard, non-Perl properties internally when  properties. PCRE uses these non-standard, non-Perl properties internally when
779  PCRE_UCP is set. They are:  PCRE_UCP is set. They are:
# Line 761  PCRE_UCP is set. They are: Line 783  PCRE_UCP is set. They are:
783    Xsp   Any Perl space character    Xsp   Any Perl space character
784    Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
785  .sp  .sp
786  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)
787  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
788  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
789  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
790  same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
791  .  .
792  .  .
# Line 772  same characters as Xan, plus underscore. Line 794  same characters as Xan, plus underscore.
794  .SS "Resetting the match start"  .SS "Resetting the match start"
795  .rs  .rs
796  .sp  .sp
797  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
798  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:  
799  .sp  .sp
800    foo\eKbar    foo\eKbar
801  .sp  .sp
# Line 825  The backslashed assertions are: Line 846  The backslashed assertions are:
846    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
847  .sp  .sp
848  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
849  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
850  default it matches the corresponding literal character (for example, \eB  default it matches the corresponding literal character (for example, \eB
851  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
852  escape sequence" error is generated instead.  escape sequence" error is generated instead.
# Line 946  The handling of dot is entirely independ Line 967  The handling of dot is entirely independ
967  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
968  special meaning in a character class.  special meaning in a character class.
969  .P  .P
970  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
971  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
972  end of a line.  that signifies the end of a line.
973  .  .
974  .  .
975  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 957  end of a line. Line 978  end of a line.
978  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
979  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 any line-ending
980  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
981  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
982  what remains in the string may be a malformed UTF-8 string. For this reason,  rest of the string may start with a malformed UTF-8 character. For this reason,
983  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
984  .P  .P
985  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
# Line 1043  characters in both cases. In UTF-8 mode, Line 1064  characters in both cases. In UTF-8 mode,
1064  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
1065  property support.  property support.
1066  .P  .P
1067  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,
1068  \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
1069  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1070  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
1071    and their upper case partners, just as it does when they appear outside a
1072    character class, as described in the section entitled
1073    .\" HTML <a href="#genericchartypes">
1074    .\" </a>
1075    "Generic character types"
1076    .\"
1077    above. The escape sequence \eb has a different meaning inside a character
1078    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1079    are not special inside a character class. Like any other unrecognized escape
1080    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1081    default, but cause an error if the PCRE_EXTRA option is set.
1082    .P
1083    A circumflex can conveniently be used with the upper case character types to
1084  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.
1085  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,
1086    whereas [\ew] includes underscore. A positive character class should be read as
1087    "something OR something OR ..." and a negative class as "NOT something AND NOT
1088    something AND NOT ...".
1089  .P  .P
1090  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1091  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 1139  supported, and an error is given if they
1139  .P  .P
1140  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
1141  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
1142  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
1143  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
1144  by other sequences, as follows:  by other sequences, as follows:
1145  .sp  .sp
1146    [:alnum:]  becomes  \ep{Xan}    [:alnum:]  becomes  \ep{Xan}
1147    [:alpha:]  becomes  \ep{L}    [:alpha:]  becomes  \ep{L}
1148    [:blank:]  becomes  \eh    [:blank:]  becomes  \eh
1149    [:digit:]  becomes  \ep{Nd}    [:digit:]  becomes  \ep{Nd}
1150    [:lower:]  becomes  \ep{Ll}    [:lower:]  becomes  \ep{Ll}
1151    [:space:]  becomes  \ep{Xps}    [:space:]  becomes  \ep{Xps}
1152    [:upper:]  becomes  \ep{Lu}    [:upper:]  becomes  \ep{Lu}
1153    [:word:]   becomes  \ep{Xwd}    [:word:]   becomes  \ep{Xwd}
1154  .sp  .sp
# Line 1171  extracts it into the global options (and Line 1208  extracts it into the global options (and
1208  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1209  .P  .P
1210  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1211  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
1212  .sp  .sp
1213    (a(?i)b)c    (a(?i)b)c
1214  .sp  .sp
# Line 1212  Turning part of a pattern into a subpatt Line 1249  Turning part of a pattern into a subpatt
1249  .sp  .sp
1250    cat(aract|erpillar|)    cat(aract|erpillar|)
1251  .sp  .sp
1252  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1253  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1254  .sp  .sp
1255  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
1256  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
1257  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
1258  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1259  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1260  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1261  .sp  .sp
1262    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1263  .sp  .sp
# Line 1270  at captured substring number one, whiche Line 1306  at captured substring number one, whiche
1306  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
1307  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1308  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
1309  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
1310  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1311  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.  
1312  .sp  .sp
1313    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1314    / ( 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 1400  items: Line 1435  items:
1435    the \eC escape sequence    the \eC escape sequence
1436    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1437    the \eR escape sequence    the \eR escape sequence
1438    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1439    a character class    a character class
1440    a back reference (see next section)    a back reference (see next section)
1441    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1442    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1443  .sp  .sp
1444  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
# Line 1442  subpatterns that are referenced as Line 1477  subpatterns that are referenced as
1477  .\" </a>  .\" </a>
1478  subroutines  subroutines
1479  .\"  .\"
1480  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1481  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1482    .\" </a>
1483    "Defining subpatterns for use by reference only"
1484    .\"
1485    below). Items other than subpatterns that have a {0} quantifier are omitted
1486    from the compiled pattern.
1487  .P  .P
1488  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1489  abbreviations:  abbreviations:
# Line 1668  no such problem when named parentheses a Line 1708  no such problem when named parentheses a
1708  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1709  .P  .P
1710  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
1711  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
1712  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
1713  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1714  .sp  .sp
1715    (ring), \e1    (ring), \e1
1716    (ring), \eg1    (ring), \eg1
# Line 1684  example: Line 1724  example:
1724    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1725  .sp  .sp
1726  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
1727  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.
1728  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
1729  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
1730  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1731  .P  .P
1732  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1733  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 1829  those that look ahead of the current pos
1829  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,
1830  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.
1831  .P  .P
1832  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1833  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
1834  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1835  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1836  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1837  because it does not make sense for negative assertions.  .P
1838    For compatibility with Perl, assertion subpatterns may be repeated; though
1839    it makes no sense to assert the same thing several times, the side effect of
1840    capturing parentheses may occasionally be useful. In practice, there only three
1841    cases:
1842    .sp
1843    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1844    However, it may contain internal capturing parenthesized groups that are called
1845    from elsewhere via the
1846    .\" HTML <a href="#subpatternsassubroutines">
1847    .\" </a>
1848    subroutine mechanism.
1849    .\"
1850    .sp
1851    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1852    were {0,1}. At run time, the rest of the pattern match is tried with and
1853    without the assertion, the order depending on the greediness of the quantifier.
1854    .sp
1855    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1856    The assertion is obeyed just once when encountered during matching.
1857  .  .
1858  .  .
1859  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1823  lookbehind assertion is needed to achiev Line 1882  lookbehind assertion is needed to achiev
1882  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
1883  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
1884  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.
1885  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 (?!).  
1886  .  .
1887  .  .
1888  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1849  is permitted, but Line 1907  is permitted, but
1907  .sp  .sp
1908  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1909  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
1910  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
1911  match the same length of string. An assertion such as  length of string. An assertion such as
1912  .sp  .sp
1913    (?<=ab(c|de))    (?<=ab(c|de))
1914  .sp  .sp
# Line 1860  branches: Line 1918  branches:
1918  .sp  .sp
1919    (?<=abc|abde)    (?<=abc|abde)
1920  .sp  .sp
1921  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1922  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1923  .\" </a>  .\" </a>
1924  (see above)  (see above)
# Line 1964  already been matched. The two possible f Line 2022  already been matched. The two possible f
2022  .sp  .sp
2023  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2024  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
2025  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2026    itself contain nested subpatterns of any form, including conditional
2027    subpatterns; the restriction to two alternatives applies only at the level of
2028    the condition. This pattern fragment is an example where the alternatives are
2029    complex:
2030    .sp
2031      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2032    .sp
2033  .P  .P
2034  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2035  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 2046  matched. If there is more than one captu
2046  .\" </a>  .\" </a>
2047  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2048  .\"  .\"
2049  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
2050  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
2051  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2052  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
2053  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
2054  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2055    zero in any of these forms is not used; it provokes a compile-time error.)
2056  .P  .P
2057  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2058  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 2063  three parts for ease of discussion:
2063  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2064  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
2065  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
2066  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2067  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,
2068  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
2069  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2070  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 2120  The syntax for recursive patterns
2120  .\"  .\"
2121  is described below.  is described below.
2122  .  .
2123    .\" HTML <a name="subdefine"></a>
2124  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2125  .rs  .rs
2126  .sp  .sp
# Line 2066  point in the pattern; the idea of DEFINE Line 2133  point in the pattern; the idea of DEFINE
2133  .\" </a>  .\" </a>
2134  "subroutines"  "subroutines"
2135  .\"  .\"
2136  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
2137  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2138    breaks):
2139  .sp  .sp
2140    (?(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) )
2141    \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 2170  dd-aaa-dd or dd-dd-dd, where aaa are let
2170  .SH COMMENTS  .SH COMMENTS
2171  .rs  .rs
2172  .sp  .sp
2173  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
2174  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,
2175  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
2176    subpattern name or number. The characters that make up a comment play no part
2177    in the pattern matching.
2178  .P  .P
2179  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
2180  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2181  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2182    this case continues to immediately after the next newline character or
2183    character sequence in the pattern. Which characters are interpreted as newlines
2184    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2185    sequence at the start of the pattern, as described in the section entitled
2186    .\" HTML <a href="#newlines">
2187    .\" </a>
2188    "Newline conventions"
2189    .\"
2190    above. Note that the end of this type of comment is a literal newline sequence
2191    in the pattern; escape sequences that happen to represent a newline do not
2192    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2193    default newline convention is in force:
2194    .sp
2195      abc #comment \en still comment
2196    .sp
2197    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2198    a newline in the pattern. The sequence \en is still literal at this stage, so
2199    it does not terminate the comment. Only an actual character with the code value
2200    0x0a (the default newline) does so.
2201  .  .
2202  .  .
2203  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2166  We have put the pattern into parentheses Line 2255  We have put the pattern into parentheses
2255  them instead of the whole pattern.  them instead of the whole pattern.
2256  .P  .P
2257  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
2258  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
2259  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
2260  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2261  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.  
2262  .P  .P
2263  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2264  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
# Line 2273  time we do have another alternative to t Line 2361  time we do have another alternative to t
2361  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2362  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2363  .P  .P
2364  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
2365  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
2366    this:
2367  .sp  .sp
2368    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2369  .sp  .sp
# Line 2422  failing negative assertion, they cause a Line 2511  failing negative assertion, they cause a
2511  .P  .P
2512  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or subroutine subpattern
2513  (including recursive subpatterns), their effect is confined to that subpattern;  (including recursive subpatterns), their effect is confined to that subpattern;
2514  it does not extend to the surrounding pattern. Note that such subpatterns are  it does not extend to the surrounding pattern, with one exception: a *MARK that
2515  processed as anchored at the point where they are tested.  is encountered in a positive assertion \fIis\fP passed back (compare capturing
2516    parentheses in assertions). Note that such subpatterns are processed as
2517    anchored at the point where they are tested.
2518  .P  .P
2519  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
2520  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
# Line 2439  minimum length of matching subject, or t Line 2530  minimum length of matching subject, or t
2530  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
2531  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2532  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
2533  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2534    pattern with (*NO_START_OPT).
2535  .  .
2536  .  .
2537  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2513  indicates which of the two alternatives Line 2605  indicates which of the two alternatives
2605  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2606  capturing parentheses.  capturing parentheses.
2607  .P  .P
2608    If (*MARK) is encountered in a positive assertion, its name is recorded and
2609    passed back if it is the last-encountered. This does not happen for negative
2610    assetions.
2611    .P
2612  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
2613  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2614  (*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 2630  matching name is found, normal "bumpalon Line 2726  matching name is found, normal "bumpalon
2726  .sp  .sp
2727    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2728  .sp  .sp
2729  This verb causes a skip to the next alternation if the rest of the pattern does  This verb causes a skip to the next alternation in the innermost enclosing
2730  not match. That is, it cancels pending backtracking, but only within the  group if the rest of the pattern does not match. That is, it cancels pending
2731  current alternation. Its name comes from the observation that it can be used  backtracking, but only within the current alternation. Its name comes from the
2732  for a pattern-based if-then-else block:  observation that it can be used for a pattern-based if-then-else block:
2733  .sp  .sp
2734    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2735  .sp  .sp
# Line 2644  behaviour of (*THEN:NAME) is exactly the Line 2740  behaviour of (*THEN:NAME) is exactly the
2740  overall match fails. If (*THEN) is not directly inside an alternation, it acts  overall match fails. If (*THEN) is not directly inside an alternation, it acts
2741  like (*PRUNE).  like (*PRUNE).
2742  .  .
2743    .P
2744    The above verbs provide four different "strengths" of control when subsequent
2745    matching fails. (*THEN) is the weakest, carrying on the match at the next
2746    alternation. (*PRUNE) comes next, failing the match at the current starting
2747    position, but allowing an advance to the next character (for an unanchored
2748    pattern). (*SKIP) is similar, except that the advance may be more than one
2749    character. (*COMMIT) is the strongest, causing the entire match to fail.
2750    .P
2751    If more than one is present in a pattern, the "stongest" one wins. For example,
2752    consider this pattern, where A, B, etc. are complex pattern fragments:
2753    .sp
2754      (A(*COMMIT)B(*THEN)C|D)
2755    .sp
2756    Once A has matched, PCRE is committed to this match, at the current starting
2757    position. If subsequently B matches, but C does not, the normal (*THEN) action
2758    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2759    overrides.
2760    .
2761  .  .
2762  .SH "SEE ALSO"  .SH "SEE ALSO"
2763  .rs  .rs
# Line 2666  Cambridge CB2 3QH, England. Line 2780  Cambridge CB2 3QH, England.
2780  .rs  .rs
2781  .sp  .sp
2782  .nf  .nf
2783  Last updated: 18 May 2010  Last updated: 24 July 2011
2784  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2785  .fi  .fi

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