/[pcre]/code/trunk/doc/pcrepattern.3
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revision 166 by ph10, Wed May 9 14:48:28 2007 UTC revision 168 by ph10, Tue May 29 15:18:18 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 519  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 1421  lengths, but it is acceptable if rewritt Line 1453  lengths, but it is acceptable if rewritt
1453  .sp  .sp
1454    (?<=abc|abde)    (?<=abc|abde)
1455  .sp  .sp
1456    In some cases, the Perl 5.10 escape sequence \eK
1457    .\" HTML <a href="#resetmatchstart">
1458    .\" </a>
1459    (see above)
1460    .\"
1461    can be used instead of a lookbehind assertion; this is not restricted to a
1462    fixed-length.
1463    .P
1464  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1465  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
1466  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 1555  recursion, a pseudo-condition called DEF
1555  .sp  .sp
1556  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
1557  condition is true if the capturing subpattern of that number has previously  condition is true if the capturing subpattern of that number has previously
1558  matched.  matched. An alternative notation is to precede the digits with a plus or minus
1559    sign. In this case, the subpattern number is relative rather than absolute.
1560    The most recently opened parentheses can be referenced by (?(-1), the next most
1561    recent by (?(-2), and so on. In looping constructs it can also make sense to
1562    refer to subsequent groups with constructs such as (?(+2).
1563  .P  .P
1564  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
1565  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 1532  the condition is true, and so the yes-pa Line 1576  the condition is true, and so the yes-pa
1576  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1577  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1578  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1579    .P
1580    If you were embedding this pattern in a larger one, you could use a relative
1581    reference:
1582    .sp
1583      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1584    .sp
1585    This makes the fragment independent of the parentheses in the larger pattern.
1586  .  .
1587  .SS "Checking for a used subpattern by name"  .SS "Checking for a used subpattern by name"
1588  .rs  .rs
# Line 1842  Cambridge CB2 3QH, England. Line 1893  Cambridge CB2 3QH, England.
1893  .rs  .rs
1894  .sp  .sp
1895  .nf  .nf
1896  Last updated: 06 March 2007  Last updated: 29 May 2007
1897  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2007 University of Cambridge.
1898  .fi  .fi
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