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revision 491 by ph10, Mon Mar 1 17:45:08 2010 UTC revision 562 by ph10, Sun Oct 31 14:06:43 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
46    combination with (*UTF8) is:
47    .sp
48      (*UCP)
49    .sp
50    This has the same effect as setting the PCRE_UCP option: it causes sequences
51    such as \ed and \ew to use Unicode properties to determine character types,
52    instead of recognizing only characters with codes less than 128 via a lookup
53    table.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
# Line 56  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 95  Perl-compatible, are recognized only at Line 106  Perl-compatible, are recognized only at
106  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
107  is used.  is used.
108  .P  .P
109  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
110  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111  this can be changed; see the description of \eR in the section entitled  affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
115  .\" </a>  .\" </a>
116  "Newline sequences"  "Newline sequences"
# Line 198  Perl, $ and @ cause variable interpolati Line 211  Perl, $ and @ cause variable interpolati
211    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
212  .sp  .sp
213  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
214    An isolated \eE that is not preceded by \eQ is ignored.
215  .  .
216  .  .
217  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 295  zero, because no more than three octal d Line 309  zero, because no more than three octal d
309  .P  .P
310  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
311  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
312  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
313  sequences \eR and \eX are interpreted as the characters "R" and "X",  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
314  respectively. Outside a character class, these sequences have different  unrecognized escape sequences, they are treated as the literal characters "B",
315  meanings  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
316  .\" HTML <a href="#uniextseq">  set. Outside a character class, these sequences have different meanings.
 .\" </a>  
 (see below).  
 .\"  
317  .  .
318  .  .
319  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 341  subroutine Line 352  subroutine
352  call.  call.
353  .  .
354  .  .
355    .\" HTML <a name="genericchartypes"></a>
356  .SS "Generic character types"  .SS "Generic character types"
357  .rs  .rs
358  .sp  .sp
359  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
360  .sp  .sp
361    \ed     any decimal digit    \ed     any decimal digit
362    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
# Line 358  following are always recognized: Line 369  following are always recognized:
369    \ew     any "word" character    \ew     any "word" character
370    \eW     any "non-word" character    \eW     any "non-word" character
371  .sp  .sp
372  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
373  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
374    .\" HTML <a href="#fullstopdot">
375    .\" </a>
376    the "." metacharacter
377    .\"
378    when PCRE_DOTALL is not set.
379  .P  .P
380  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
381    of characters into two disjoint sets. Any given character matches one, and only
382    one, of each pair. The sequences can appear both inside and outside character
383  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
384  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
385  there is no character to match.  there is no character to match.
386  .P  .P
387  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
# Line 372  are HT (9), LF (10), FF (12), CR (13), a Line 390  are HT (9), LF (10), FF (12), CR (13), a
390  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
391  does.  does.
392  .P  .P
393  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
394  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
395  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
396  meanings from before UTF-8 support was available, mainly for efficiency  place (see
397  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
398  and \eW.  .\" </a>
399    "Locale support"
400    .\"
401    in the
402    .\" HREF
403    \fBpcreapi\fP
404    .\"
405    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
406    or "french" in Windows, some character codes greater than 128 are used for
407    accented letters, and these are then matched by \ew. The use of locales with
408    Unicode is discouraged.
409    .P
410    By default, in UTF-8 mode, characters with values greater than 128 never match
411    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
412    their original meanings from before UTF-8 support was available, mainly for
413    efficiency reasons. However, if PCRE is compiled with Unicode property support,
414    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
415    properties are used to determine character types, as follows:
416    .sp
417      \ed  any character that \ep{Nd} matches (decimal digit)
418      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
419      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
420    .sp
421    The upper case escapes match the inverse sets of characters. Note that \ed
422    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
423    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
424    \eB because they are defined in terms of \ew and \eW. Matching these sequences
425    is noticeably slower when PCRE_UCP is set.
426  .P  .P
427  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
428  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  other sequences, which match only ASCII characters by default, these always
429  The horizontal space characters are:  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
430    set. The horizontal space characters are:
431  .sp  .sp
432    U+0009     Horizontal tab    U+0009     Horizontal tab
433    U+0020     Space    U+0020     Space
# Line 412  The vertical space characters are: Line 458  The vertical space characters are:
458    U+0085     Next line    U+0085     Next line
459    U+2028     Line separator    U+2028     Line separator
460    U+2029     Paragraph separator    U+2029     Paragraph separator
 .P  
 A "word" character is an underscore or any character less than 256 that is a  
 letter or digit. The definition of letters and digits is controlled by PCRE's  
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
461  .  .
462  .  .
463  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 474  These override the default and the optio Line 503  These override the default and the optio
503  which are not Perl-compatible, are recognized only at the very start of a  which are not Perl-compatible, are recognized only at the very start of a
504  pattern, and that they must be in upper case. If more than one of them is  pattern, and that they must be in upper case. If more than one of them is
505  present, the last one is used. They can be combined with a change of newline  present, the last one is used. They can be combined with a change of newline
506  convention, for example, a pattern can start with:  convention; for example, a pattern can start with:
507  .sp  .sp
508    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
509  .sp  .sp
510  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
511    a character class, \eR is treated as an unrecognized escape sequence, and so
512    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
513  .  .
514  .  .
515  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 496  The extra escape sequences are: Line 527  The extra escape sequences are:
527    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
528  .sp  .sp
529  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
530  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
531  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
532  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
533  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
534    .\" </a>
535    next section).
536    .\"
537    Other Perl properties such as "InMusicalSymbols" are not currently supported by
538    PCRE. Note that \eP{Any} does not match any characters, so always causes a
539    match failure.
540  .P  .P
541  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
542  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 604  Ugaritic, Line 641  Ugaritic,
641  Vai,  Vai,
642  Yi.  Yi.
643  .P  .P
644  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
645  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
646  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
647  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
648  .P  .P
649  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
650  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 706  non-UTF-8 mode \eX matches any one chara Line 743  non-UTF-8 mode \eX matches any one chara
743  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
744  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
745  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
746  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
747    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
748    (*UCP).
749    .
750    .
751    .\" HTML <a name="extraprops"></a>
752    .SS PCRE's additional properties
753    .rs
754    .sp
755    As well as the standard Unicode properties described in the previous
756    section, PCRE supports four more that make it possible to convert traditional
757    escape sequences such as \ew and \es and POSIX character classes to use Unicode
758    properties. PCRE uses these non-standard, non-Perl properties internally when
759    PCRE_UCP is set. They are:
760    .sp
761      Xan   Any alphanumeric character
762      Xps   Any POSIX space character
763      Xsp   Any Perl space character
764      Xwd   Any Perl "word" character
765    .sp
766    Xan matches characters that have either the L (letter) or the N (number)
767    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
768    carriage return, and any other character that has the Z (separator) property.
769    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
770    same characters as Xan, plus underscore.
771  .  .
772  .  .
773  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
# Line 737  For example, when the pattern Line 798  For example, when the pattern
798    (foo)\eKbar    (foo)\eKbar
799  .sp  .sp
800  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
801    .P
802    Perl documents that the use of \eK within assertions is "not well defined". In
803    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
804    ignored in negative assertions.
805  .  .
806  .  .
807  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 761  The backslashed assertions are: Line 826  The backslashed assertions are:
826    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
827    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
828  .sp  .sp
829  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
830  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
831    default it matches the corresponding literal character (for example, \eB
832    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
833    escape sequence" error is generated instead.
834  .P  .P
835  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
836  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
837  \ew and the other matches \eW), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
838  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
839  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
840  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
841  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
842    determines which it is. For example, the fragment \eba matches "a" at the start
843    of a word.
844  .P  .P
845  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
846  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 854  end of the subject in both modes, and if Line 924  end of the subject in both modes, and if
924  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
925  .  .
926  .  .
927  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
928    .SH "FULL STOP (PERIOD, DOT) AND \eN"
929  .rs  .rs
930  .sp  .sp
931  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
# Line 876  to match it. Line 947  to match it.
947  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
948  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
949  special meaning in a character class.  special meaning in a character class.
950    .P
951    The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
952    set. In other words, it matches any one character except one that signifies the
953    end of a line.
954  .  .
955  .  .
956  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
# Line 970  characters in both cases. In UTF-8 mode, Line 1045  characters in both cases. In UTF-8 mode,
1045  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1046  property support.  property support.
1047  .P  .P
1048  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1049  in a character class, and add the characters that they match to the class. For  \eW may also appear in a character class, and add the characters that they
1050  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1051  conveniently be used with the upper case character types to specify a more  circumflex can conveniently be used with the upper case character types to
1052  restricted set of characters than the matching lower case type. For example,  specify a more restricted set of characters than the matching lower case type.
1053  the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore.
1054  .P  .P
1055  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1056  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 995  this notation. For example, Line 1070  this notation. For example,
1070    [01[:alpha:]%]    [01[:alpha:]%]
1071  .sp  .sp
1072  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1073  are  are:
1074  .sp  .sp
1075    alnum    letters and digits    alnum    letters and digits
1076    alpha    letters    alpha    letters
# Line 1006  are Line 1081  are
1081    graph    printing characters, excluding space    graph    printing characters, excluding space
1082    lower    lower case letters    lower    lower case letters
1083    print    printing characters, including space    print    printing characters, including space
1084    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1085    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1086    upper    upper case letters    upper    upper case letters
1087    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1027  matches "1", "2", or any non-digit. PCRE Line 1102  matches "1", "2", or any non-digit. PCRE
1102  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1103  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1104  .P  .P
1105  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1106  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1107    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1108    character properties are used. This is achieved by replacing the POSIX classes
1109    by other sequences, as follows:
1110    .sp
1111      [:alnum:]  becomes  \ep{Xan}
1112      [:alpha:]  becomes  \ep{L}
1113      [:blank:]  becomes  \eh
1114      [:digit:]  becomes  \ep{Nd}
1115      [:lower:]  becomes  \ep{Ll}
1116      [:space:]  becomes  \ep{Xps}
1117      [:upper:]  becomes  \ep{Lu}
1118      [:word:]   becomes  \ep{Xwd}
1119    .sp
1120    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1121    classes are unchanged, and match only characters with code points less than
1122    128.
1123  .  .
1124  .  .
1125  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1107  section entitled Line 1198  section entitled
1198  .\" </a>  .\" </a>
1199  "Newline sequences"  "Newline sequences"
1200  .\"  .\"
1201  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1202  mode; this is equivalent to setting the PCRE_UTF8 option.  to set UTF-8 and Unicode property modes; they are equivalent to setting the
1203    PCRE_UTF8 and the PCRE_UCP options, respectively.
1204  .  .
1205  .  .
1206  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1874  already been matched. The two possible f Line 1966  already been matched. The two possible f
1966  .sp  .sp
1967  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1968  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1969  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
1970    itself contain nested subpatterns of any form, including conditional
1971    subpatterns; the restriction to two alternatives applies only at the level of
1972    the condition. This pattern fragment is an example where the alternatives are
1973    complex:
1974    .sp
1975      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1976    .sp
1977  .P  .P
1978  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
1979  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 2012  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2111  dd-aaa-dd or dd-dd-dd, where aaa are let
2111  .SH COMMENTS  .SH COMMENTS
2112  .rs  .rs
2113  .sp  .sp
2114  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
2115  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,
2116  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
2117    subpattern name or number. The characters that make up a comment play no part
2118    in the pattern matching.
2119  .P  .P
2120  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
2121  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2122  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2123    this case continues to immediately after the next newline character or
2124    character sequence in the pattern. Which characters are interpreted as newlines
2125    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2126    sequence at the start of the pattern, as described in the section entitled
2127    .\" HTML <a href="#recursion">
2128    .\" </a>
2129    "Newline conventions"
2130    .\"
2131    above. Note that end of this type of comment is a literal newline sequence in
2132    the pattern; escape sequences that happen to represent a newline do not count.
2133    For example, consider this pattern when PCRE_EXTENDED is set, and the default
2134    newline convention is in force:
2135    .sp
2136      abc #comment \en still comment
2137    .sp
2138    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2139    a newline in the pattern. The sequence \en is still literal at this stage, so
2140    it does not terminate the comment. Only an actual character with the code value
2141    0x0a (the default newline) does so.
2142  .  .
2143  .  .
2144  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2314  description of the interface to the call Line 2434  description of the interface to the call
2434  documentation.  documentation.
2435  .  .
2436  .  .
2437    .\" HTML <a name="backtrackcontrol"></a>
2438  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2439  .rs  .rs
2440  .sp  .sp
# Line 2335  it does not extend to the surrounding pa Line 2456  it does not extend to the surrounding pa
2456  processed as anchored at the point where they are tested.  processed as anchored at the point where they are tested.
2457  .P  .P
2458  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
2459  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2460  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2461  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. An name is a sequence of
2462  are two kinds:  letters, digits, and underscores. If the name is empty, that is, if the closing
2463    parenthesis immediately follows the colon, the effect is as if the colon were
2464    not there. Any number of these verbs may occur in a pattern.
2465    .P
2466    PCRE contains some optimizations that are used to speed up matching by running
2467    some checks at the start of each match attempt. For example, it may know the
2468    minimum length of matching subject, or that a particular character must be
2469    present. When one of these optimizations suppresses the running of a match, any
2470    included backtracking verbs will not, of course, be processed. You can suppress
2471    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2472    when calling \fBpcre_exec()\fP.
2473    .
2474  .  .
2475  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2476  .rs  .rs
2477  .sp  .sp
2478  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2479    followed by a name.
2480  .sp  .sp
2481     (*ACCEPT)     (*ACCEPT)
2482  .sp  .sp
# Line 2370  callout feature, as for example in this Line 2503  callout feature, as for example in this
2503  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2504  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2505  .  .
2506    .
2507    .SS "Recording which path was taken"
2508    .rs
2509    .sp
2510    There is one verb whose main purpose is to track how a match was arrived at,
2511    though it also has a secondary use in conjunction with advancing the match
2512    starting point (see (*SKIP) below).
2513    .sp
2514      (*MARK:NAME) or (*:NAME)
2515    .sp
2516    A name is always required with this verb. There may be as many instances of
2517    (*MARK) as you like in a pattern, and their names do not have to be unique.
2518    .P
2519    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2520    to the caller via the \fIpcre_extra\fP data structure, as described in the
2521    .\" HTML <a href="pcreapi.html#extradata">
2522    .\" </a>
2523    section on \fIpcre_extra\fP
2524    .\"
2525    in the
2526    .\" HREF
2527    \fBpcreapi\fP
2528    .\"
2529    documentation. No data is returned for a partial match. Here is an example of
2530    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2531    outputting of (*MARK) data:
2532    .sp
2533      /X(*MARK:A)Y|X(*MARK:B)Z/K
2534      XY
2535       0: XY
2536      MK: A
2537      XZ
2538       0: XZ
2539      MK: B
2540    .sp
2541    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2542    indicates which of the two alternatives matched. This is a more efficient way
2543    of obtaining this information than putting each alternative in its own
2544    capturing parentheses.
2545    .P
2546    A name may also be returned after a failed match if the final path through the
2547    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2548    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2549    starting point for matching is advanced, the final check is often with an empty
2550    string, causing a failure before (*MARK) is reached. For example:
2551    .sp
2552      /X(*MARK:A)Y|X(*MARK:B)Z/K
2553      XP
2554      No match
2555    .sp
2556    There are three potential starting points for this match (starting with X,
2557    starting with P, and with an empty string). If the pattern is anchored, the
2558    result is different:
2559    .sp
2560      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2561      XP
2562      No match, mark = B
2563    .sp
2564    PCRE's start-of-match optimizations can also interfere with this. For example,
2565    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2566    subject length for a match, a shorter subject will not be scanned at all.
2567    .P
2568    Note that similar anomalies (though different in detail) exist in Perl, no
2569    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2570    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2571    .
2572    .
2573  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2574  .rs  .rs
2575  .sp  .sp
2576  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2577  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2578  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2579    the verb. However, when one of these verbs appears inside an atomic group, its
2580    effect is confined to that group, because once the group has been matched,
2581    there is never any backtracking into it. In this situation, backtracking can
2582    "jump back" to the left of the entire atomic group. (Remember also, as stated
2583    above, that this localization also applies in subroutine calls and assertions.)
2584    .P
2585    These verbs differ in exactly what kind of failure occurs when backtracking
2586    reaches them.
2587  .sp  .sp
2588    (*COMMIT)    (*COMMIT)
2589  .sp  .sp
2590  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2591  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2592  a match by advancing the starting point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2593  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2594  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2595  .sp  .sp
2596    a+(*COMMIT)b    a+(*COMMIT)b
2597  .sp  .sp
2598  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2599  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2600  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2601    (*PRUNE)  match failure.
2602  .sp  .P
2603  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2604  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2605  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2606  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2607  if there is no match to the right, backtracking cannot cross (*PRUNE).    /(*COMMIT)abc/
2608  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    xyzabc
2609  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2610  be expressed in any other way.    xyzabc\eY
2611      No match
2612    .sp
2613    PCRE knows that any match must start with "a", so the optimization skips along
2614    the subject to "a" before running the first match attempt, which succeeds. When
2615    the optimization is disabled by the \eY escape in the second subject, the match
2616    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2617    starting points.
2618    .sp
2619      (*PRUNE) or (*PRUNE:NAME)
2620    .sp
2621    This verb causes the match to fail at the current starting position in the
2622    subject if the rest of the pattern does not match. If the pattern is
2623    unanchored, the normal "bumpalong" advance to the next starting character then
2624    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2625    reached, or when matching to the right of (*PRUNE), but if there is no match to
2626    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2627    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2628    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2629    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2630    match fails completely; the name is passed back if this is the final attempt.
2631    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2632    pattern (*PRUNE) has the same effect as (*COMMIT).
2633  .sp  .sp
2634    (*SKIP)    (*SKIP)
2635  .sp  .sp
2636  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2637  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2638  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2639  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2640    successful match. Consider:
2641  .sp  .sp
2642    a+(*SKIP)b    a+(*SKIP)b
2643  .sp  .sp
# Line 2417  effect as this example; although it woul Line 2648  effect as this example; although it woul
2648  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2649  instead of skipping on to "c".  instead of skipping on to "c".
2650  .sp  .sp
2651    (*THEN)    (*SKIP:NAME)
2652  .sp  .sp
2653  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2654  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2655  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2656  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2657    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2658    matching name is found, normal "bumpalong" of one character happens (the
2659    (*SKIP) is ignored).
2660    .sp
2661      (*THEN) or (*THEN:NAME)
2662    .sp
2663    This verb causes a skip to the next alternation in the innermost enclosing
2664    group if the rest of the pattern does not match. That is, it cancels pending
2665    backtracking, but only within the current alternation. Its name comes from the
2666    observation that it can be used for a pattern-based if-then-else block:
2667  .sp  .sp
2668    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2669  .sp  .sp
2670  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2671  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure the matcher skips to the
2672  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2673  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2674    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2675    like (*PRUNE).
2676    .
2677    .P
2678    The above verbs provide four different "strengths" of control when subsequent
2679    matching fails. (*THEN) is the weakest, carrying on the match at the next
2680    alternation. (*PRUNE) comes next, failing the match at the current starting
2681    position, but allowing an advance to the next character (for an unanchored
2682    pattern). (*SKIP) is similar, except that the advance may be more than one
2683    character. (*COMMIT) is the strongest, causing the entire match to fail.
2684    .P
2685    If more than one is present in a pattern, the "stongest" one wins. For example,
2686    consider this pattern, where A, B, etc. are complex pattern fragments:
2687    .sp
2688      (A(*COMMIT)B(*THEN)C|D)
2689    .sp
2690    Once A has matched, PCRE is committed to this match, at the current starting
2691    position. If subsequently B matches, but C does not, the normal (*THEN) action
2692    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2693    overrides.
2694  .  .
2695  .  .
2696  .SH "SEE ALSO"  .SH "SEE ALSO"
# Line 2453  Cambridge CB2 3QH, England. Line 2714  Cambridge CB2 3QH, England.
2714  .rs  .rs
2715  .sp  .sp
2716  .nf  .nf
2717  Last updated: 01 March 2010  Last updated: 31 October 2010
2718  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2010 University of Cambridge.
2719  .fi  .fi

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