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revision 653 by ph10, Wed Nov 24 17:39:25 2010 UTC revision 654 by ph10, Tue Aug 2 11:00:40 2011 UTC
# Line 245  Perl, $ and @ cause variable interpolati Line 245  Perl, $ and @ cause variable interpolati
245    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
246  </pre>  </pre>
247  The \Q...\E sequence is recognized both inside and outside character classes.  The \Q...\E sequence is recognized both inside and outside character classes.
248  An isolated \E that is not preceded by \Q is ignored.  An isolated \E that is not preceded by \Q is ignored. If \Q is not followed
249    by \E later in the pattern, the literal interpretation continues to the end of
250    the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
251    a character class, this causes an error, because the character class is not
252    terminated.
253  <a name="digitsafterbackslash"></a></P>  <a name="digitsafterbackslash"></a></P>
254  <br><b>  <br><b>
255  Non-printing characters  Non-printing characters
# Line 752  preceding character. None of them have c Line 756  preceding character. None of them have c
756  non-UTF-8 mode \X matches any one character.  non-UTF-8 mode \X matches any one character.
757  </P>  </P>
758  <P>  <P>
759    Note that recent versions of Perl have changed \X to match what Unicode calls
760    an "extended grapheme cluster", which has a more complicated definition.
761    </P>
762    <P>
763  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
764  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
765  why the traditional escape sequences such as \d and \w do not use Unicode  why the traditional escape sequences such as \d and \w do not use Unicode
# Line 1405  items: Line 1413  items:
1413    an escape such as \d or \pL that matches a single character    an escape such as \d or \pL that matches a single character
1414    a character class    a character class
1415    a back reference (see next section)    a back reference (see next section)
1416    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1417    a recursive or "subroutine" call to a subpattern    a recursive or "subroutine" call to a subpattern
1418  </pre>  </pre>
1419  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
# Line 1796  that look behind it. An assertion subpat Line 1804  that look behind it. An assertion subpat
1804  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1805  </P>  </P>
1806  <P>  <P>
1807  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1808  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1809  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1810  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1811  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1812  because it does not make sense for negative assertions.  </P>
1813    <P>
1814    For compatibility with Perl, assertion subpatterns may be repeated; though
1815    it makes no sense to assert the same thing several times, the side effect of
1816    capturing parentheses may occasionally be useful. In practice, there only three
1817    cases:
1818    <br>
1819    <br>
1820    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1821    However, it may contain internal capturing parenthesized groups that are called
1822    from elsewhere via the
1823    <a href="#subpatternsassubroutines">subroutine mechanism.</a>
1824    <br>
1825    <br>
1826    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1827    were {0,1}. At run time, the rest of the pattern match is tried with and
1828    without the assertion, the order depending on the greediness of the quantifier.
1829    <br>
1830    <br>
1831    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1832    The assertion is obeyed just once when encountered during matching.
1833  </P>  </P>
1834  <br><b>  <br><b>
1835  Lookahead assertions  Lookahead assertions
# Line 2445  failing negative assertion, they cause a Line 2473  failing negative assertion, they cause a
2473  <P>  <P>
2474  If any of these verbs are used in an assertion or subroutine subpattern  If any of these verbs are used in an assertion or subroutine subpattern
2475  (including recursive subpatterns), their effect is confined to that subpattern;  (including recursive subpatterns), their effect is confined to that subpattern;
2476  it does not extend to the surrounding pattern. Note that such subpatterns are  it does not extend to the surrounding pattern, with one exception: a *MARK that
2477  processed as anchored at the point where they are tested.  is encountered in a positive assertion <i>is</i> passed back (compare capturing
2478    parentheses in assertions). Note that such subpatterns are processed as
2479    anchored at the point where they are tested.
2480  </P>  </P>
2481  <P>  <P>
2482  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
# Line 2536  of obtaining this information than putti Line 2566  of obtaining this information than putti
2566  capturing parentheses.  capturing parentheses.
2567  </P>  </P>
2568  <P>  <P>
2569    If (*MARK) is encountered in a positive assertion, its name is recorded and
2570    passed back if it is the last-encountered. This does not happen for negative
2571    assetions.
2572    </P>
2573    <P>
2574  A name may also be returned after a failed match if the final path through the  A name may also be returned after a failed match if the final path through the
2575  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2576  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
# Line 2705  Cambridge CB2 3QH, England. Line 2740  Cambridge CB2 3QH, England.
2740  </P>  </P>
2741  <br><a name="SEC28" href="#TOC1">REVISION</a><br>  <br><a name="SEC28" href="#TOC1">REVISION</a><br>
2742  <P>  <P>
2743  Last updated: 21 November 2010  Last updated: 24 July 2011
2744  <br>  <br>
2745  Copyright &copy; 1997-2010 University of Cambridge.  Copyright &copy; 1997-2011 University of Cambridge.
2746  <br>  <br>
2747  <p>  <p>
2748  Return to the <a href="index.html">PCRE index page</a>.  Return to the <a href="index.html">PCRE index page</a>.

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