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revision 167 by ph10, Wed May 9 15:53:54 2007 UTC revision 178 by ph10, Wed Jun 13 08:44:34 2007 UTC
# Line 30  The remainder of this document discusses Line 30  The remainder of this document discusses
30  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
32  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
34  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
35    alternative function, and how it differs from the normal function, are
36    discussed in the
37  .\" HREF  .\" HREF
38  \fBpcrematching\fP  \fBpcrematching\fP
39  .\"  .\"
# Line 239  meanings Line 241  meanings
241  .rs  .rs
242  .sp  .sp
243  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by a positive or negative number, optionally enclosed
244  in braces, is an absolute or relative back reference. Back references are  in braces, is an absolute or relative back reference. A named back reference
245  discussed  can be coded as \eg{name}. Back references are discussed
246  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
247  .\" </a>  .\" </a>
248  later,  later,
# Line 258  parenthesized subpatterns. Line 260  parenthesized subpatterns.
260  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types. The
261  following are always recognized:  following are always recognized:
262  .sp  .sp
263    \ed     any decimal digit    \ed     any decimal digit
264    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
265      \eh     any horizontal whitespace character
266      \eH     any character that is not a horizontal whitespace character
267    \es     any whitespace character    \es     any whitespace character
268    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
269      \ev     any vertical whitespace character
270      \eV     any character that is not a vertical whitespace character
271    \ew     any "word" character    \ew     any "word" character
272    \eW     any "non-word" character    \eW     any "non-word" character
273  .sp  .sp
# Line 275  there is no character to match. Line 281  there is no character to match.
281  .P  .P
282  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).
283  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
284  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
285  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
286  does.)  does.
287    .P
288    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
289    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
290    character property support is available. These sequences retain their original
291    meanings from before UTF-8 support was available, mainly for efficiency
292    reasons.
293    .P
294    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
295    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
296    The horizontal space characters are:
297    .sp
298      U+0009     Horizontal tab
299      U+0020     Space
300      U+00A0     Non-break space
301      U+1680     Ogham space mark
302      U+180E     Mongolian vowel separator
303      U+2000     En quad
304      U+2001     Em quad
305      U+2002     En space
306      U+2003     Em space
307      U+2004     Three-per-em space
308      U+2005     Four-per-em space
309      U+2006     Six-per-em space
310      U+2007     Figure space
311      U+2008     Punctuation space
312      U+2009     Thin space
313      U+200A     Hair space
314      U+202F     Narrow no-break space
315      U+205F     Medium mathematical space
316      U+3000     Ideographic space
317    .sp
318    The vertical space characters are:
319    .sp
320      U+000A     Linefeed
321      U+000B     Vertical tab
322      U+000C     Formfeed
323      U+000D     Carriage return
324      U+0085     Next line
325      U+2028     Line separator
326      U+2029     Paragraph separator
327  .P  .P
328  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character less than 256 that is a
329  letter or digit. The definition of letters and digits is controlled by PCRE's  letter or digit. The definition of letters and digits is controlled by PCRE's
# Line 293  in the Line 339  in the
339  .\"  .\"
340  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
341  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
342  accented letters, and these are matched by \ew.  accented letters, and these are matched by \ew. The use of locales with Unicode
343  .P  is discouraged.
 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  
 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  
 character property support is available. The use of locales with Unicode is  
 discouraged.  
344  .  .
345  .  .
346  .SS "Newline sequences"  .SS "Newline sequences"
347  .rs  .rs
348  .sp  .sp
349  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, the escape sequence \eR matches any Unicode newline
350  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
351  the following:  the following:
352  .sp  .sp
353    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
# Line 519  why the traditional escape sequences suc Line 561  why the traditional escape sequences suc
561  properties in PCRE.  properties in PCRE.
562  .  .
563  .  .
564    .\" HTML <a name="resetmatchstart"></a>
565    .SS "Resetting the match start"
566    .rs
567    .sp
568    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
569    matched characters not to be included in the final matched sequence. For
570    example, the pattern:
571    .sp
572      foo\eKbar
573    .sp
574    matches "foobar", but reports that it has matched "bar". This feature is
575    similar to a lookbehind assertion
576    .\" HTML <a href="#lookbehind">
577    .\" </a>
578    (described below).
579    .\"
580    However, in this case, the part of the subject before the real match does not
581    have to be of fixed length, as lookbehind assertions do. The use of \eK does
582    not interfere with the setting of
583    .\" HTML <a href="#subpattern">
584    .\" </a>
585    captured substrings.
586    .\"
587    For example, when the pattern
588    .sp
589      (foo)\eKbar
590    .sp
591    matches "foobar", the first substring is still set to "foo".
592    .
593    .
594  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
595  .SS "Simple assertions"  .SS "Simple assertions"
596  .rs  .rs
# Line 926  is reached, an option setting in one bra Line 998  is reached, an option setting in one bra
998  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
999  .  .
1000  .  .
1001    .SH "DUPLICATE SUBPATTERN NUMBERS"
1002    .rs
1003    .sp
1004    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1005    the same numbers for its capturing parentheses. Such a subpattern starts with
1006    (?| and is itself a non-capturing subpattern. For example, consider this
1007    pattern:
1008    .sp
1009      (?|(Sat)ur|(Sun))day
1010    .sp
1011    Because the two alternatives are inside a (?| group, both sets of capturing
1012    parentheses are numbered one. Thus, when the pattern matches, you can look
1013    at captured substring number one, whichever alternative matched. This construct
1014    is useful when you want to capture part, but not all, of one of a number of
1015    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1016    number is reset at the start of each branch. The numbers of any capturing
1017    buffers that follow the subpattern start after the highest number used in any
1018    branch. The following example is taken from the Perl documentation.
1019    The numbers underneath show in which buffer the captured content will be
1020    stored.
1021    .sp
1022      # before  ---------------branch-reset----------- after
1023      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1024      # 1            2         2  3        2     3     4
1025    .sp
1026    A backreference or a recursive call to a numbered subpattern always refers to
1027    the first one in the pattern with the given number.
1028    .P
1029    An alternative approach to using this "branch reset" feature is to use
1030    duplicate named subpatterns, as described in the next section.
1031    .
1032    .
1033  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1034  .rs  .rs
1035  .sp  .sp
# Line 975  abbreviation. This pattern (ignoring the Line 1079  abbreviation. This pattern (ignoring the
1079    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1080  .sp  .sp
1081  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1082    (An alternative way of solving this problem is to use a "branch reset"
1083    subpattern, as described in the previous section.)
1084    .P
1085  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1086  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1087  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was. If you
# Line 1293  back reference, the case of letters is r Line 1400  back reference, the case of letters is r
1400  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1401  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1402  .P  .P
1403  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1404  or the Python syntax (?P=name). We could rewrite the above example in either of  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1405    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1406    back reference syntax, in which \eg can be used for both numeric and named
1407    references, is also supported. We could rewrite the above example in any of
1408  the following ways:  the following ways:
1409  .sp  .sp
1410    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1411      (?'p1'(?i)rah)\es+\ek{p1}
1412    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1413      (?<p1>(?i)rah)\es+\eg{p1}
1414  .sp  .sp
1415  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1416  after the reference.  after the reference.
# Line 1421  lengths, but it is acceptable if rewritt Line 1533  lengths, but it is acceptable if rewritt
1533  .sp  .sp
1534    (?<=abc|abde)    (?<=abc|abde)
1535  .sp  .sp
1536    In some cases, the Perl 5.10 escape sequence \eK
1537    .\" HTML <a href="#resetmatchstart">
1538    .\" </a>
1539    (see above)
1540    .\"
1541    can be used instead of a lookbehind assertion; this is not restricted to a
1542    fixed-length.
1543    .P
1544  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1545  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
1546  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1515  recursion, a pseudo-condition called DEF Line 1635  recursion, a pseudo-condition called DEF
1635  .sp  .sp
1636  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
1637  condition is true if the capturing subpattern of that number has previously  condition is true if the capturing subpattern of that number has previously
1638  matched. An alternative notation is to precede the digits with a plus or minus  matched. An alternative notation is to precede the digits with a plus or minus
1639  sign. In this case, the subpattern number is relative rather than absolute.  sign. In this case, the subpattern number is relative rather than absolute.
1640  The most recently opened parentheses can be referenced by (?(-1), the next most  The most recently opened parentheses can be referenced by (?(-1), the next most
1641  recent by (?(-2), and so on. In looping constructs it can also make sense to  recent by (?(-2), and so on. In looping constructs it can also make sense to
1642  refer to subsequent groups with constructs such as (?(+2).  refer to subsequent groups with constructs such as (?(+2).
1643  .P  .P
# Line 1537  parenthesis is required. Otherwise, sinc Line 1657  parenthesis is required. Otherwise, sinc
1657  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1658  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1659  .P  .P
1660  If you were embedding this pattern in a larger one, you could use a relative  If you were embedding this pattern in a larger one, you could use a relative
1661  reference:  reference:
1662  .sp  .sp
1663    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1685  pattern, so instead you could use this: Line 1805  pattern, so instead you could use this:
1805    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1806  .sp  .sp
1807  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
1808  them instead of the whole pattern.  them instead of the whole pattern.
1809  .P  .P
1810  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
1811  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. (A Perl 5.10 feature.)
# Line 1706  could rewrite the above example as follo Line 1826  could rewrite the above example as follo
1826    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1827  .sp  .sp
1828  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
1829  used.  used.
1830  .P  .P
1831  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
1832  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of atomic grouping for matching strings of
# Line 1768  relative, as in these examples: Line 1888  relative, as in these examples:
1888  .sp  .sp
1889    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
1890    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
1891    (...(?+1)...(relative)...    (...(?+1)...(relative)...
1892  .sp  .sp
1893  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
1894  .sp  .sp
# Line 1853  Cambridge CB2 3QH, England. Line 1973  Cambridge CB2 3QH, England.
1973  .rs  .rs
1974  .sp  .sp
1975  .nf  .nf
1976  Last updated: 09 May 2007  Last updated: 13 June 2007
1977  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
1978  .fi  .fi

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