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revision 518 by ph10, Tue May 18 15:47:01 2010 UTC revision 678 by ph10, Sun Aug 28 15:23:03 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"
# Line 957  end of a line. Line 973  end of a line.
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 any 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. Because it breaks up UTF-8 characters into individual bytes, the
977  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,
978  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
979  .P  .P
980  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 1059  characters in both cases. In UTF-8 mode,
1059  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
1060  property support.  property support.
1061  .P  .P
1062  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,
1063  \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
1064  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1065  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
1066    and their upper case partners, just as it does when they appear outside a
1067    character class, as described in the section entitled
1068    .\" HTML <a href="#genericchartypes">
1069    .\" </a>
1070    "Generic character types"
1071    .\"
1072    above. The escape sequence \eb has a different meaning inside a character
1073    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1074    are not special inside a character class. Like any other unrecognized escape
1075    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1076    default, but cause an error if the PCRE_EXTRA option is set.
1077    .P
1078    A circumflex can conveniently be used with the upper case character types to
1079  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.
1080  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,
1081    whereas [\ew] includes underscore. A positive character class should be read as
1082    "something OR something OR ..." and a negative class as "NOT something AND NOT
1083    something AND NOT ...".
1084  .P  .P
1085  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1086  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 1134  supported, and an error is given if they
1134  .P  .P
1135  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
1136  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
1137  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
1138  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
1139  by other sequences, as follows:  by other sequences, as follows:
1140  .sp  .sp
1141    [:alnum:]  becomes  \ep{Xan}    [:alnum:]  becomes  \ep{Xan}
1142    [:alpha:]  becomes  \ep{L}    [:alpha:]  becomes  \ep{L}
1143    [:blank:]  becomes  \eh    [:blank:]  becomes  \eh
1144    [:digit:]  becomes  \ep{Nd}    [:digit:]  becomes  \ep{Nd}
1145    [:lower:]  becomes  \ep{Ll}    [:lower:]  becomes  \ep{Ll}
1146    [:space:]  becomes  \ep{Xps}    [:space:]  becomes  \ep{Xps}
1147    [:upper:]  becomes  \ep{Lu}    [:upper:]  becomes  \ep{Lu}
1148    [:word:]   becomes  \ep{Xwd}    [:word:]   becomes  \ep{Xwd}
1149  .sp  .sp
# Line 1171  extracts it into the global options (and Line 1203  extracts it into the global options (and
1203  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1204  .P  .P
1205  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1206  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
1207  .sp  .sp
1208    (a(?i)b)c    (a(?i)b)c
1209  .sp  .sp
# Line 1212  Turning part of a pattern into a subpatt Line 1244  Turning part of a pattern into a subpatt
1244  .sp  .sp
1245    cat(aract|erpillar|)    cat(aract|erpillar|)
1246  .sp  .sp
1247  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1248  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1249  .sp  .sp
1250  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
1251  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
1252  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
1253  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1254  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1255  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1256  .sp  .sp
1257    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1258  .sp  .sp
# Line 1270  at captured substring number one, whiche Line 1301  at captured substring number one, whiche
1301  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
1302  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1303  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
1304  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
1305  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1306  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.  
1307  .sp  .sp
1308    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1309    / ( 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 1430  items:
1430    the \eC escape sequence    the \eC escape sequence
1431    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1432    the \eR escape sequence    the \eR escape sequence
1433    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1434    a character class    a character class
1435    a back reference (see next section)    a back reference (see next section)
1436    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1437    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1438  .sp  .sp
1439  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 1472  subpatterns that are referenced as
1472  .\" </a>  .\" </a>
1473  subroutines  subroutines
1474  .\"  .\"
1475  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1476  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1477    .\" </a>
1478    "Defining subpatterns for use by reference only"
1479    .\"
1480    below). Items other than subpatterns that have a {0} quantifier are omitted
1481    from the compiled pattern.
1482  .P  .P
1483  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1484  abbreviations:  abbreviations:
# Line 1668  no such problem when named parentheses a Line 1703  no such problem when named parentheses a
1703  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1704  .P  .P
1705  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
1706  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
1707  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
1708  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1709  .sp  .sp
1710    (ring), \e1    (ring), \e1
1711    (ring), \eg1    (ring), \eg1
# Line 1684  example: Line 1719  example:
1719    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1720  .sp  .sp
1721  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
1722  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.
1723  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
1724  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
1725  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1726  .P  .P
1727  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1728  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 1824  those that look ahead of the current pos
1824  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,
1825  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.
1826  .P  .P
1827  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1828  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
1829  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1830  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1831  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1832  because it does not make sense for negative assertions.  .P
1833    For compatibility with Perl, assertion subpatterns may be repeated; though
1834    it makes no sense to assert the same thing several times, the side effect of
1835    capturing parentheses may occasionally be useful. In practice, there only three
1836    cases:
1837    .sp
1838    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1839    However, it may contain internal capturing parenthesized groups that are called
1840    from elsewhere via the
1841    .\" HTML <a href="#subpatternsassubroutines">
1842    .\" </a>
1843    subroutine mechanism.
1844    .\"
1845    .sp
1846    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1847    were {0,1}. At run time, the rest of the pattern match is tried with and
1848    without the assertion, the order depending on the greediness of the quantifier.
1849    .sp
1850    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1851    The assertion is obeyed just once when encountered during matching.
1852  .  .
1853  .  .
1854  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1823  lookbehind assertion is needed to achiev Line 1877  lookbehind assertion is needed to achiev
1877  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
1878  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
1879  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.
1880  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 (?!).  
1881  .  .
1882  .  .
1883  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1849  is permitted, but Line 1902  is permitted, but
1902  .sp  .sp
1903  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1904  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
1905  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
1906  match the same length of string. An assertion such as  length of string. An assertion such as
1907  .sp  .sp
1908    (?<=ab(c|de))    (?<=ab(c|de))
1909  .sp  .sp
# Line 1860  branches: Line 1913  branches:
1913  .sp  .sp
1914    (?<=abc|abde)    (?<=abc|abde)
1915  .sp  .sp
1916  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1917  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1918  .\" </a>  .\" </a>
1919  (see above)  (see above)
# Line 1964  already been matched. The two possible f Line 2017  already been matched. The two possible f
2017  .sp  .sp
2018  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2019  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
2020  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2021    itself contain nested subpatterns of any form, including conditional
2022    subpatterns; the restriction to two alternatives applies only at the level of
2023    the condition. This pattern fragment is an example where the alternatives are
2024    complex:
2025    .sp
2026      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2027    .sp
2028  .P  .P
2029  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2030  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 2041  matched. If there is more than one captu
2041  .\" </a>  .\" </a>
2042  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2043  .\"  .\"
2044  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
2045  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
2046  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2047  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
2048  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
2049  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2050    zero in any of these forms is not used; it provokes a compile-time error.)
2051  .P  .P
2052  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2053  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 2058  three parts for ease of discussion:
2058  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2059  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
2060  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
2061  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2062  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,
2063  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
2064  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2065  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 2115  The syntax for recursive patterns
2115  .\"  .\"
2116  is described below.  is described below.
2117  .  .
2118    .\" HTML <a name="subdefine"></a>
2119  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2120  .rs  .rs
2121  .sp  .sp
# Line 2066  point in the pattern; the idea of DEFINE Line 2128  point in the pattern; the idea of DEFINE
2128  .\" </a>  .\" </a>
2129  "subroutines"  "subroutines"
2130  .\"  .\"
2131  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
2132  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2133    breaks):
2134  .sp  .sp
2135    (?(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) )
2136    \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 2165  dd-aaa-dd or dd-dd-dd, where aaa are let
2165  .SH COMMENTS  .SH COMMENTS
2166  .rs  .rs
2167  .sp  .sp
2168  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
2169  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,
2170  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
2171    subpattern name or number. The characters that make up a comment play no part
2172    in the pattern matching.
2173  .P  .P
2174  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
2175  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2176  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2177    this case continues to immediately after the next newline character or
2178    character sequence in the pattern. Which characters are interpreted as newlines
2179    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2180    sequence at the start of the pattern, as described in the section entitled
2181    .\" HTML <a href="#newlines">
2182    .\" </a>
2183    "Newline conventions"
2184    .\"
2185    above. Note that the end of this type of comment is a literal newline sequence
2186    in the pattern; escape sequences that happen to represent a newline do not
2187    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2188    default newline convention is in force:
2189    .sp
2190      abc #comment \en still comment
2191    .sp
2192    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2193    a newline in the pattern. The sequence \en is still literal at this stage, so
2194    it does not terminate the comment. Only an actual character with the code value
2195    0x0a (the default newline) does so.
2196  .  .
2197  .  .
2198  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2166  We have put the pattern into parentheses Line 2250  We have put the pattern into parentheses
2250  them instead of the whole pattern.  them instead of the whole pattern.
2251  .P  .P
2252  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
2253  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
2254  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
2255  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2256  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.  
2257  .P  .P
2258  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2259  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 2356  time we do have another alternative to t
2356  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2357  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2358  .P  .P
2359  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
2360  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
2361    this:
2362  .sp  .sp
2363    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2364  .sp  .sp
# Line 2422  failing negative assertion, they cause a Line 2506  failing negative assertion, they cause a
2506  .P  .P
2507  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
2508  (including recursive subpatterns), their effect is confined to that subpattern;  (including recursive subpatterns), their effect is confined to that subpattern;
2509  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
2510  processed as anchored at the point where they are tested.  is encountered in a positive assertion \fIis\fP passed back (compare capturing
2511    parentheses in assertions). Note that such subpatterns are processed as
2512    anchored at the point where they are tested.
2513  .P  .P
2514  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
2515  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 2525  minimum length of matching subject, or t
2525  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
2526  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2527  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
2528  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2529    pattern with (*NO_START_OPT).
2530  .  .
2531  .  .
2532  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2513  indicates which of the two alternatives Line 2600  indicates which of the two alternatives
2600  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2601  capturing parentheses.  capturing parentheses.
2602  .P  .P
2603    If (*MARK) is encountered in a positive assertion, its name is recorded and
2604    passed back if it is the last-encountered. This does not happen for negative
2605    assetions.
2606    .P
2607  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
2608  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2609  (*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 2721  matching name is found, normal "bumpalon
2721  .sp  .sp
2722    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2723  .sp  .sp
2724  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
2725  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
2726  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
2727  for a pattern-based if-then-else block:  observation that it can be used for a pattern-based if-then-else block:
2728  .sp  .sp
2729    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2730  .sp  .sp
# Line 2644  behaviour of (*THEN:NAME) is exactly the Line 2735  behaviour of (*THEN:NAME) is exactly the
2735  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
2736  like (*PRUNE).  like (*PRUNE).
2737  .  .
2738    .P
2739    The above verbs provide four different "strengths" of control when subsequent
2740    matching fails. (*THEN) is the weakest, carrying on the match at the next
2741    alternation. (*PRUNE) comes next, failing the match at the current starting
2742    position, but allowing an advance to the next character (for an unanchored
2743    pattern). (*SKIP) is similar, except that the advance may be more than one
2744    character. (*COMMIT) is the strongest, causing the entire match to fail.
2745    .P
2746    If more than one is present in a pattern, the "stongest" one wins. For example,
2747    consider this pattern, where A, B, etc. are complex pattern fragments:
2748    .sp
2749      (A(*COMMIT)B(*THEN)C|D)
2750    .sp
2751    Once A has matched, PCRE is committed to this match, at the current starting
2752    position. If subsequently B matches, but C does not, the normal (*THEN) action
2753    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2754    overrides.
2755    .
2756  .  .
2757  .SH "SEE ALSO"  .SH "SEE ALSO"
2758  .rs  .rs
# Line 2666  Cambridge CB2 3QH, England. Line 2775  Cambridge CB2 3QH, England.
2775  .rs  .rs
2776  .sp  .sp
2777  .nf  .nf
2778  Last updated: 18 May 2010  Last updated: 24 August 2011
2779  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
2780  .fi  .fi

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