/[pcre]/code/trunk/doc/html/pcrepattern.html

Diff of /code/trunk/doc/html/pcrepattern.html

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-revision 579 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
    \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
  </pre>
  The \Q...\E sequence is recognized both inside and outside character classes.
- 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
+ by \E later in the pattern, the literal interpretation continues to the end of
+ the pattern (that is, \E is assumed at the end). If the isolated \Q is inside
+ a character class, this causes an error, because the character class is not
+ terminated.
  <a name="digitsafterbackslash"></a></P>
  <br><b>
  Non-printing characters
-Line 752 
 preceding character. None of them have c
+Line 756 
 preceding character. None of them have c
  non-UTF-8 mode \X matches any one character.
  </P>
  <P>
+ Note that recent versions of Perl have changed \X to match what Unicode calls
+ an "extended grapheme cluster", which has a more complicated definition.
+ </P>
+ <P>
  Matching characters by Unicode property is not fast, because PCRE has to search
  a structure that contains data for over fifteen thousand characters. That is
  why the traditional escape sequences such as \d and \w do not use Unicode
-Line 1405 
 items:
+Line 1413 
 items:
    an escape such as \d or \pL that matches a single character
    a character class
    a back reference (see next section)
-   a parenthesized subpattern (unless it is an assertion)
+   a parenthesized subpattern (including assertions)
    a recursive or "subroutine" call to a subpattern
  </pre>
  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
  except that it does not cause the current matching position to be changed.
  </P>
  <P>
- Assertion subpatterns are not capturing subpatterns, and may not be repeated,
+ Assertion subpatterns are not capturing subpatterns. If such an assertion
- 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
- of assertion contains capturing subpatterns within it, these are counted for
+ numbering the capturing subpatterns in the whole pattern. However, substring
- the purposes of numbering the capturing subpatterns in the whole pattern.
+ capturing is carried out only for positive assertions, because it does not make
- However, substring capturing is carried out only for positive assertions,
+ sense for negative assertions.
- because it does not make sense for negative assertions.
+ </P>
+ <P>
+ For compatibility with Perl, assertion subpatterns may be repeated; though
+ it makes no sense to assert the same thing several times, the side effect of
+ capturing parentheses may occasionally be useful. In practice, there only three
+ cases:
+ <br>
+ <br>
+ (1) If the quantifier is {0}, the assertion is never obeyed during matching.
+ However, it may contain internal capturing parenthesized groups that are called
+ from elsewhere via the
+ <a href="#subpatternsassubroutines">subroutine mechanism.</a>
+ <br>
+ <br>
+ (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
+ were {0,1}. At run time, the rest of the pattern match is tried with and
+ without the assertion, the order depending on the greediness of the quantifier.
+ <br>
+ <br>
+ (3) If the minimum repetition is greater than zero, the quantifier is ignored.
+ The assertion is obeyed just once when encountered during matching.
  </P>
  <br><b>
  Lookahead assertions
-Line 2445 
 failing negative assertion, they cause a
+Line 2473 
 failing negative assertion, they cause a
  <P>
  If any of these verbs are used in an assertion or subroutine subpattern
  (including recursive subpatterns), their effect is confined to that subpattern;
- 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
- processed as anchored at the point where they are tested.
+ is encountered in a positive assertion <i>is</i> passed back (compare capturing
+ parentheses in assertions). Note that such subpatterns are processed as
+ anchored at the point where they are tested.
  </P>
  <P>
  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
  capturing parentheses.
  </P>
  <P>
+ If (*MARK) is encountered in a positive assertion, its name is recorded and
+ passed back if it is the last-encountered. This does not happen for negative
+ assetions.
+ </P>
+ <P>
  A name may also be returned after a failed match if the final path through the
  pattern involves (*MARK). However, unless (*MARK) used in conjunction with
  (*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.
  </P>
  <br><a name="SEC28" href="#TOC1">REVISION</a><br>
  <P>
- Last updated: 21 November 2010
+ Last updated: 24 July 2011
  <br>
- Copyright &copy; 1997-2010 University of Cambridge.
+ Copyright &copy; 1997-2011 University of Cambridge.
  <br>
  <p>
  Return to the <a href="index.html">PCRE index page</a>.

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