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revision 96 by nigel, Fri Mar 2 13:10:43 2007 UTC revision 172 by ph10, Tue Jun 5 10:40:13 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 291  in the Line 293  in the
293  .\" HREF  .\" HREF
294  \fBpcreapi\fP  \fBpcreapi\fP
295  .\"  .\"
296  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
297  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
298    accented letters, and these are matched by \ew.
299  .P  .P
300  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
301  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
# Line 518  why the traditional escape sequences suc Line 521  why the traditional escape sequences suc
521  properties in PCRE.  properties in PCRE.
522  .  .
523  .  .
524    .\" HTML <a name="resetmatchstart"></a>
525    .SS "Resetting the match start"
526    .rs
527    .sp
528    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
529    matched characters not to be included in the final matched sequence. For
530    example, the pattern:
531    .sp
532      foo\eKbar
533    .sp
534    matches "foobar", but reports that it has matched "bar". This feature is
535    similar to a lookbehind assertion
536    .\" HTML <a href="#lookbehind">
537    .\" </a>
538    (described below).
539    .\"
540    However, in this case, the part of the subject before the real match does not
541    have to be of fixed length, as lookbehind assertions do. The use of \eK does
542    not interfere with the setting of
543    .\" HTML <a href="#subpattern">
544    .\" </a>
545    captured substrings.
546    .\"
547    For example, when the pattern
548    .sp
549      (foo)\eKbar
550    .sp
551    matches "foobar", the first substring is still set to "foo".
552    .
553    .
554  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
555  .SS "Simple assertions"  .SS "Simple assertions"
556  .rs  .rs
# Line 740  example [\ex{100}-\ex{2ff}]. Line 773  example [\ex{100}-\ex{2ff}].
773  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
774  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
775  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
776  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
777  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
778  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
779  property support.  property support.
# Line 1292  back reference, the case of letters is r Line 1325  back reference, the case of letters is r
1325  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
1326  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1327  .P  .P
1328  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
1329  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
1330    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1331    back reference syntax, in which \eg can be used for both numeric and named
1332    references, is also supported. We could rewrite the above example in any of
1333  the following ways:  the following ways:
1334  .sp  .sp
1335    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1336      (?'p1'(?i)rah)\es+\ek{p1}
1337    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1338      (?<p1>(?i)rah)\es+\eg{p1}
1339  .sp  .sp
1340  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
1341  after the reference.  after the reference.
# Line 1420  lengths, but it is acceptable if rewritt Line 1458  lengths, but it is acceptable if rewritt
1458  .sp  .sp
1459    (?<=abc|abde)    (?<=abc|abde)
1460  .sp  .sp
1461    In some cases, the Perl 5.10 escape sequence \eK
1462    .\" HTML <a href="#resetmatchstart">
1463    .\" </a>
1464    (see above)
1465    .\"
1466    can be used instead of a lookbehind assertion; this is not restricted to a
1467    fixed-length.
1468    .P
1469  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1470  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
1471  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
# Line 1514  recursion, a pseudo-condition called DEF Line 1560  recursion, a pseudo-condition called DEF
1560  .sp  .sp
1561  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
1562  condition is true if the capturing subpattern of that number has previously  condition is true if the capturing subpattern of that number has previously
1563  matched.  matched. An alternative notation is to precede the digits with a plus or minus
1564    sign. In this case, the subpattern number is relative rather than absolute.
1565    The most recently opened parentheses can be referenced by (?(-1), the next most
1566    recent by (?(-2), and so on. In looping constructs it can also make sense to
1567    refer to subsequent groups with constructs such as (?(+2).
1568  .P  .P
1569  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1570  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 1531  the condition is true, and so the yes-pa Line 1581  the condition is true, and so the yes-pa
1581  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1582  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1583  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1584    .P
1585    If you were embedding this pattern in a larger one, you could use a relative
1586    reference:
1587    .sp
1588      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1589    .sp
1590    This makes the fragment independent of the parentheses in the larger pattern.
1591  .  .
1592  .SS "Checking for a used subpattern by name"  .SS "Checking for a used subpattern by name"
1593  .rs  .rs
# Line 1673  pattern, so instead you could use this: Line 1730  pattern, so instead you could use this:
1730    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1731  .sp  .sp
1732  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
1733  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1734  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1735  parentheses instead. The Perl syntax for this is (?&name); PCRE's earlier  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1736  syntax (?P>name) is also supported. We could rewrite the above example as  is made easier by the use of relative references. (A Perl 5.10 feature.)
1737  follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1738    most recently opened parentheses preceding the recursion. In other words, a
1739    negative number counts capturing parentheses leftwards from the point at which
1740    it is encountered.
1741    .P
1742    It is also possible to refer to subsequently opened parentheses, by writing
1743    references such as (?+2). However, these cannot be recursive because the
1744    reference is not inside the parentheses that are referenced. They are always
1745    "subroutine" calls, as described in the next section.
1746    .P
1747    An alternative approach is to use named parentheses instead. The Perl syntax
1748    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1749    could rewrite the above example as follows:
1750  .sp  .sp
1751    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1752  .sp  .sp
1753  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
1754  used. This particular example pattern contains nested unlimited repeats, and so  used.
1755  the use of atomic grouping for matching strings of non-parentheses is important  .P
1756  when applying the pattern to strings that do not match. For example, when this  This particular example pattern that we have been looking at contains nested
1757  pattern is applied to  unlimited repeats, and so the use of atomic grouping for matching strings of
1758    non-parentheses is important when applying the pattern to strings that do not
1759    match. For example, when this pattern is applied to
1760  .sp  .sp
1761    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1762  .sp  .sp
# Line 1737  is the actual recursive call. Line 1808  is the actual recursive call.
1808  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
1809  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
1810  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The "called" subpattern may be defined
1811  before or after the reference. An earlier example pointed out that the pattern  before or after the reference. A numbered reference can be absolute or
1812    relative, as in these examples:
1813    .sp
1814      (...(absolute)...)...(?2)...
1815      (...(relative)...)...(?-1)...
1816      (...(?+1)...(relative)...
1817    .sp
1818    An earlier example pointed out that the pattern
1819  .sp  .sp
1820    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
1821  .sp  .sp
# Line 1758  When a subpattern is used as a subroutin Line 1836  When a subpattern is used as a subroutin
1836  case-independence are fixed when the subpattern is defined. They cannot be  case-independence are fixed when the subpattern is defined. They cannot be
1837  changed for different calls. For example, consider this pattern:  changed for different calls. For example, consider this pattern:
1838  .sp  .sp
1839    (abc)(?i:(?1))    (abc)(?i:(?-1))
1840  .sp  .sp
1841  It matches "abcabc". It does not match "abcABC" because the change of  It matches "abcabc". It does not match "abcABC" because the change of
1842  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
# Line 1782  function is to be called. If you want to Line 1860  function is to be called. If you want to
1860  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1861  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1862  .sp  .sp
1863    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1864  .sp  .sp
1865  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1866  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1804  documentation. Line 1882  documentation.
1882  .rs  .rs
1883  .sp  .sp
1884  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1885  .P  .
1886  .in 0  .
1887  Last updated: 06 December 2006  .SH AUTHOR
1888  .br  .rs
1889  Copyright (c) 1997-2006 University of Cambridge.  .sp
1890    .nf
1891    Philip Hazel
1892    University Computing Service
1893    Cambridge CB2 3QH, England.
1894    .fi
1895    .
1896    .
1897    .SH REVISION
1898    .rs
1899    .sp
1900    .nf
1901    Last updated: 29 May 2007
1902    Copyright (c) 1997-2007 University of Cambridge.
1903    .fi

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