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revision 518 by ph10, Tue May 18 15:47:01 2010 UTC revision 576 by ph10, Sun Nov 21 18:45:10 2010 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.
224  .  .
225  .  .
226  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 223  but when a pattern is being prepared by Line 234  but when a pattern is being prepared by
234  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
235  .sp  .sp
236    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
237    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
238    \ee        escape (hex 1B)    \ee        escape (hex 1B)
239    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
240    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
# Line 235  one of the following escape sequences th Line 246  one of the following escape sequences th
246  .sp  .sp
247  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
248  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.
249  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
250  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
251    than 127, a compile-time error occurs. This locks out non-ASCII characters in
252    both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
253    values are valid. A lower case letter is converted to upper case, and then the
254    0xc0 bits are flipped.)
255  .P  .P
256  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
257  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 382  Another use of backslash is for specifyi
382    \ew     any "word" character    \ew     any "word" character
383    \eW     any "non-word" character    \eW     any "non-word" character
384  .sp  .sp
385  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.
386  This is the same as  This is the same as
387  .\" HTML <a href="#fullstopdot">  .\" HTML <a href="#fullstopdot">
388  .\" </a>  .\" </a>
389  the "." metacharacter  the "." metacharacter
390  .\"  .\"
391  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set.
392  .P  .P
# Line 408  Unicode is discouraged. Line 423  Unicode is discouraged.
423  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
424  \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
425  their original meanings from before UTF-8 support was available, mainly for  their original meanings from before UTF-8 support was available, mainly for
426  efficiency reasons. However, if PCRE is compiled with Unicode property support,  efficiency reasons. However, if PCRE is compiled with Unicode property support,
427  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
428  properties are used to determine character types, as follows:  properties are used to determine character types, as follows:
429  .sp  .sp
# Line 417  properties are used to determine charact Line 432  properties are used to determine charact
432    \ew  any character that \ep{L} or \ep{N} matches, plus underscore    \ew  any character that \ep{L} or \ep{N} matches, plus underscore
433  .sp  .sp
434  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
435  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
436  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
437  \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
438  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
439  .P  .P
440  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
441  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
442  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
443  set. The horizontal space characters are:  UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
444    are:
445  .sp  .sp
446    U+0009     Horizontal tab    U+0009     Horizontal tab
447    U+0020     Space    U+0020     Space
# Line 463  The vertical space characters are: Line 479  The vertical space characters are:
479  .rs  .rs
480  .sp  .sp
481  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
482  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:  
483  .sp  .sp
484    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
485  .sp  .sp
# Line 527  The extra escape sequences are: Line 542  The extra escape sequences are:
542  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
543  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
544  character (including newline), and some special PCRE properties (described  character (including newline), and some special PCRE properties (described
545  in the  in the
546  .\" HTML <a href="#extraprops">  .\" HTML <a href="#extraprops">
547  .\" </a>  .\" </a>
548  next section).  next section).
549  .\"  .\"
550  Other Perl properties such as "InMusicalSymbols" are not currently supported by  Other Perl properties such as "InMusicalSymbols" are not currently supported by
551  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 741  non-UTF-8 mode \eX matches any one chara Line 756  non-UTF-8 mode \eX matches any one chara
756  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
757  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
758  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
759  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
760  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
761  (*UCP).  (*UCP).
762  .  .
# Line 750  PCRE_UCP option for \fBpcre_compile()\fP Line 765  PCRE_UCP option for \fBpcre_compile()\fP
765  .SS PCRE's additional properties  .SS PCRE's additional properties
766  .rs  .rs
767  .sp  .sp
768  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described in the previous
769  section, PCRE supports four more that make it possible to convert traditional  section, PCRE supports four more that make it possible to convert traditional
770  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
771  properties. PCRE uses these non-standard, non-Perl properties internally when  properties. PCRE uses these non-standard, non-Perl properties internally when
772  PCRE_UCP is set. They are:  PCRE_UCP is set. They are:
# Line 761  PCRE_UCP is set. They are: Line 776  PCRE_UCP is set. They are:
776    Xsp   Any Perl space character    Xsp   Any Perl space character
777    Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
778  .sp  .sp
779  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)
780  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
781  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
782  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
783  same characters as Xan, plus underscore.  same characters as Xan, plus underscore.
784  .  .
785  .  .
# Line 772  same characters as Xan, plus underscore. Line 787  same characters as Xan, plus underscore.
787  .SS "Resetting the match start"  .SS "Resetting the match start"
788  .rs  .rs
789  .sp  .sp
790  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
791  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:  
792  .sp  .sp
793    foo\eKbar    foo\eKbar
794  .sp  .sp
# Line 825  The backslashed assertions are: Line 839  The backslashed assertions are:
839    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
840  .sp  .sp
841  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
842  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
843  default it matches the corresponding literal character (for example, \eB  default it matches the corresponding literal character (for example, \eB
844  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
845  escape sequence" error is generated instead.  escape sequence" error is generated instead.
# Line 946  The handling of dot is entirely independ Line 960  The handling of dot is entirely independ
960  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
961  special meaning in a character class.  special meaning in a character class.
962  .P  .P
963  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
964  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
965  end of a line.  that signifies the end of a line.
966  .  .
967  .  .
968  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 957  end of a line. Line 971  end of a line.
971  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
972  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
973  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
974  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
975  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,
976  the \eC escape sequence is best avoided.  the \eC escape sequence is best avoided.
977  .P  .P
978  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 1057  characters in both cases. In UTF-8 mode,
1057  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
1058  property support.  property support.
1059  .P  .P
1060  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,
1061  \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
1062  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1063  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
1064    and their upper case partners, just as it does when they appear outside a
1065    character class, as described in the section entitled
1066    .\" HTML <a href="#genericchartypes">
1067    .\" </a>
1068    "Generic character types"
1069    .\"
1070    above. The escape sequence \eb has a different meaning inside a character
1071    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1072    are not special inside a character class. Like any other unrecognized escape
1073    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1074    default, but cause an error if the PCRE_EXTRA option is set.
1075    .P
1076    A circumflex can conveniently be used with the upper case character types to
1077  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.
1078  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,
1079    whereas [\ew] includes underscore. A positive character class should be read as
1080    "something OR something OR ..." and a negative class as "NOT something AND NOT
1081    something AND NOT ...".
1082  .P  .P
1083  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1084  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 1132  supported, and an error is given if they
1132  .P  .P
1133  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
1134  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
1135  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
1136  character properties are used. This is achieved by replacing the POSIX classes  character properties are used. This is achieved by replacing the POSIX classes
1137  by other sequences, as follows:  by other sequences, as follows:
1138  .sp  .sp
1139    [:alnum:]  becomes  \ep{Xan}    [:alnum:]  becomes  \ep{Xan}
1140    [:alpha:]  becomes  \ep{L}    [:alpha:]  becomes  \ep{L}
1141    [:blank:]  becomes  \eh    [:blank:]  becomes  \eh
1142    [:digit:]  becomes  \ep{Nd}    [:digit:]  becomes  \ep{Nd}
1143    [:lower:]  becomes  \ep{Ll}    [:lower:]  becomes  \ep{Ll}
1144    [:space:]  becomes  \ep{Xps}    [:space:]  becomes  \ep{Xps}
1145    [:upper:]  becomes  \ep{Lu}    [:upper:]  becomes  \ep{Lu}
1146    [:word:]   becomes  \ep{Xwd}    [:word:]   becomes  \ep{Xwd}
1147  .sp  .sp
# Line 1171  extracts it into the global options (and Line 1201  extracts it into the global options (and
1201  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1202  .P  .P
1203  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1204  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
1205  .sp  .sp
1206    (a(?i)b)c    (a(?i)b)c
1207  .sp  .sp
# Line 1212  Turning part of a pattern into a subpatt Line 1242  Turning part of a pattern into a subpatt
1242  .sp  .sp
1243    cat(aract|erpillar|)    cat(aract|erpillar|)
1244  .sp  .sp
1245  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1246  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1247  .sp  .sp
1248  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
1249  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
1250  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
1251  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1252  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1253  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1254  .sp  .sp
1255    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1256  .sp  .sp
# Line 1270  at captured substring number one, whiche Line 1299  at captured substring number one, whiche
1299  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
1300  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1301  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
1302  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
1303  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1304  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.  
1305  .sp  .sp
1306    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1307    / ( 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 1428  items:
1428    the \eC escape sequence    the \eC escape sequence
1429    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1430    the \eR escape sequence    the \eR escape sequence
1431    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1432    a character class    a character class
1433    a back reference (see next section)    a back reference (see next section)
1434    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
# Line 1442  subpatterns that are referenced as Line 1470  subpatterns that are referenced as
1470  .\" </a>  .\" </a>
1471  subroutines  subroutines
1472  .\"  .\"
1473  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1474  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1475    .\" </a>
1476    "Defining subpatterns for use by reference only"
1477    .\"
1478    below). Items other than subpatterns that have a {0} quantifier are omitted
1479    from the compiled pattern.
1480  .P  .P
1481  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1482  abbreviations:  abbreviations:
# Line 1668  no such problem when named parentheses a Line 1701  no such problem when named parentheses a
1701  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1702  .P  .P
1703  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
1704  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
1705  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
1706  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1707  .sp  .sp
1708    (ring), \e1    (ring), \e1
1709    (ring), \eg1    (ring), \eg1
# Line 1684  example: Line 1717  example:
1717    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1718  .sp  .sp
1719  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
1720  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.
1721  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
1722  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
1723  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1724  .P  .P
1725  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1726  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1823  lookbehind assertion is needed to achiev Line 1856  lookbehind assertion is needed to achiev
1856  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
1857  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
1858  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.
1859  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 (?!).  
1860  .  .
1861  .  .
1862  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1849  is permitted, but Line 1881  is permitted, but
1881  .sp  .sp
1882  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1883  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
1884  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
1885  match the same length of string. An assertion such as  length of string. An assertion such as
1886  .sp  .sp
1887    (?<=ab(c|de))    (?<=ab(c|de))
1888  .sp  .sp
# Line 1860  branches: Line 1892  branches:
1892  .sp  .sp
1893    (?<=abc|abde)    (?<=abc|abde)
1894  .sp  .sp
1895  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1896  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1897  .\" </a>  .\" </a>
1898  (see above)  (see above)
# Line 1964  already been matched. The two possible f Line 1996  already been matched. The two possible f
1996  .sp  .sp
1997  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1998  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
1999  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2000    itself contain nested subpatterns of any form, including conditional
2001    subpatterns; the restriction to two alternatives applies only at the level of
2002    the condition. This pattern fragment is an example where the alternatives are
2003    complex:
2004    .sp
2005      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2006    .sp
2007  .P  .P
2008  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2009  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 2020  matched. If there is more than one captu
2020  .\" </a>  .\" </a>
2021  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2022  .\"  .\"
2023  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
2024  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
2025  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2026  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
2027  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
2028  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2029    zero in any of these forms is not used; it provokes a compile-time error.)
2030  .P  .P
2031  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2032  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 2037  three parts for ease of discussion:
2037  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2038  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
2039  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
2040  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2041  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,
2042  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
2043  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2044  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 2094  The syntax for recursive patterns
2094  .\"  .\"
2095  is described below.  is described below.
2096  .  .
2097    .\" HTML <a name="subdefine"></a>
2098  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2099  .rs  .rs
2100  .sp  .sp
# Line 2066  point in the pattern; the idea of DEFINE Line 2107  point in the pattern; the idea of DEFINE
2107  .\" </a>  .\" </a>
2108  "subroutines"  "subroutines"
2109  .\"  .\"
2110  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
2111  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore whitespace and line
2112    breaks):
2113  .sp  .sp
2114    (?(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) )
2115    \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 2144  dd-aaa-dd or dd-dd-dd, where aaa are let
2144  .SH COMMENTS  .SH COMMENTS
2145  .rs  .rs
2146  .sp  .sp
2147  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
2148  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,
2149  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
2150    subpattern name or number. The characters that make up a comment play no part
2151    in the pattern matching.
2152  .P  .P
2153  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
2154  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2155  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2156    this case continues to immediately after the next newline character or
2157    character sequence in the pattern. Which characters are interpreted as newlines
2158    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2159    sequence at the start of the pattern, as described in the section entitled
2160    .\" HTML <a href="#newlines">
2161    .\" </a>
2162    "Newline conventions"
2163    .\"
2164    above. Note that the end of this type of comment is a literal newline sequence
2165    in the pattern; escape sequences that happen to represent a newline do not
2166    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2167    default newline convention is in force:
2168    .sp
2169      abc #comment \en still comment
2170    .sp
2171    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2172    a newline in the pattern. The sequence \en is still literal at this stage, so
2173    it does not terminate the comment. Only an actual character with the code value
2174    0x0a (the default newline) does so.
2175  .  .
2176  .  .
2177  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2166  We have put the pattern into parentheses Line 2229  We have put the pattern into parentheses
2229  them instead of the whole pattern.  them instead of the whole pattern.
2230  .P  .P
2231  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
2232  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
2233  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
2234  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2235  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.  
2236  .P  .P
2237  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2238  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 2335  time we do have another alternative to t
2335  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2336  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2337  .P  .P
2338  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
2339  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
2340    this:
2341  .sp  .sp
2342    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2343  .sp  .sp
# Line 2630  matching name is found, normal "bumpalon Line 2693  matching name is found, normal "bumpalon
2693  .sp  .sp
2694    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
2695  .sp  .sp
2696  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
2697  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
2698  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
2699  for a pattern-based if-then-else block:  observation that it can be used for a pattern-based if-then-else block:
2700  .sp  .sp
2701    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2702  .sp  .sp
# Line 2644  behaviour of (*THEN:NAME) is exactly the Line 2707  behaviour of (*THEN:NAME) is exactly the
2707  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
2708  like (*PRUNE).  like (*PRUNE).
2709  .  .
2710    .P
2711    The above verbs provide four different "strengths" of control when subsequent
2712    matching fails. (*THEN) is the weakest, carrying on the match at the next
2713    alternation. (*PRUNE) comes next, failing the match at the current starting
2714    position, but allowing an advance to the next character (for an unanchored
2715    pattern). (*SKIP) is similar, except that the advance may be more than one
2716    character. (*COMMIT) is the strongest, causing the entire match to fail.
2717    .P
2718    If more than one is present in a pattern, the "stongest" one wins. For example,
2719    consider this pattern, where A, B, etc. are complex pattern fragments:
2720    .sp
2721      (A(*COMMIT)B(*THEN)C|D)
2722    .sp
2723    Once A has matched, PCRE is committed to this match, at the current starting
2724    position. If subsequently B matches, but C does not, the normal (*THEN) action
2725    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2726    overrides.
2727    .
2728  .  .
2729  .SH "SEE ALSO"  .SH "SEE ALSO"
2730  .rs  .rs
# Line 2666  Cambridge CB2 3QH, England. Line 2747  Cambridge CB2 3QH, England.
2747  .rs  .rs
2748  .sp  .sp
2749  .nf  .nf
2750  Last updated: 18 May 2010  Last updated: 21 November 2010
2751  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2752  .fi  .fi

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