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version 1.1, 2012/02/21 23:05:52 version 1.1.1.5, 2014/06/15 19:46:05
Line 21  regular expressions. The differences described here ar Line 21  regular expressions. The differences described here ar
 versions 5.10 and above.  versions 5.10 and above.
 </P>  </P>
 <P>  <P>
1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what1. PCRE has only a subset of Perl's Unicode support. Details of what it does
it does have are given in thehave are given in the
 <a href="pcreunicode.html"><b>pcreunicode</b></a>  <a href="pcreunicode.html"><b>pcreunicode</b></a>
 page.  page.
 </P>  </P>
Line 36  these do not seem to have any use. Line 36  these do not seem to have any use.
 </P>  </P>
 <P>  <P>
 3. Capturing subpatterns that occur inside negative lookahead assertions are  3. Capturing subpatterns that occur inside negative lookahead assertions are
counted, but their entries in the offsets vector are never set. Perl sets itscounted, but their entries in the offsets vector are never set. Perl sometimes
numerical variables from any such patterns that are matched before the(but not always) sets its numerical variables from inside negative assertions.
assertion fails to match something (thereby succeeding), but only if the 
negative lookahead assertion contains just one branch. 
 </P>  </P>
 <P>  <P>
 4. Though binary zero characters are supported in the subject string, they are  4. Though binary zero characters are supported in the subject string, they are
Line 67  the internal representation of Unicode characters, the Line 65  the internal representation of Unicode characters, the
 implement the somewhat messy concept of surrogates."  implement the somewhat messy concept of surrogates."
 </P>  </P>
 <P>  <P>
7. PCRE implements a simpler version of \X than Perl, which changed to make7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
\X match what Unicode calls an "extended grapheme cluster". This is more 
complicated than an extended Unicode sequence, which is what PCRE matches. 
</P> 
<P> 
8. PCRE does support the \Q...\E escape for quoting substrings. Characters in 
 between are treated as literals. This is slightly different from Perl in that $  between are treated as literals. This is slightly different from Perl in that $
 and @ are also handled as literals inside the quotes. In Perl, they cause  and @ are also handled as literals inside the quotes. In Perl, they cause
 variable interpolation (but of course PCRE does not have variables). Note the  variable interpolation (but of course PCRE does not have variables). Note the
Line 87  following examples: Line 80  following examples:
 The \Q...\E sequence is recognized both inside and outside character classes.  The \Q...\E sequence is recognized both inside and outside character classes.
 </P>  </P>
 <P>  <P>
9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
 constructions. However, there is support for recursive patterns. This is not  constructions. However, there is support for recursive patterns. This is not
 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
 feature allows an external function to be called during pattern matching. See  feature allows an external function to be called during pattern matching. See
Line 96  the Line 89  the
 documentation for details.  documentation for details.
 </P>  </P>
 <P>  <P>
10. Subpatterns that are called as subroutines (whether or not recursively) are9. Subpatterns that are called as subroutines (whether or not recursively) are
 always treated as atomic groups in PCRE. This is like Python, but unlike Perl.  always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
 Captured values that are set outside a subroutine call can be reference from  Captured values that are set outside a subroutine call can be reference from
 inside in PCRE, but not in Perl. There is a discussion that explains these  inside in PCRE, but not in Perl. There is a discussion that explains these
Line 107  in the Line 100  in the
 page.  page.
 </P>  </P>
 <P>  <P>
11. If (*THEN) is present in a group that is called as a subroutine, its action10. If any of the backtracking control verbs are used in a subpattern that is
is limited to that group, even if the group does not contain any | characters.called as a subroutine (whether or not recursively), their effect is confined
 to that subpattern; it does not extend to the surrounding pattern. This is not
 always the case in Perl. In particular, if (*THEN) is present in a group that
 is called as a subroutine, its action is limited to that group, even if the
 group does not contain any | characters. Note that such subpatterns are
 processed as anchored at the point where they are tested.
 </P>  </P>
 <P>  <P>
12. There are some differences that are concerned with the settings of captured11. If a pattern contains more than one backtracking control verb, the first
 one that is backtracked onto acts. For example, in the pattern
 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
 triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
 same as PCRE, but there are examples where it differs.
 </P>
 <P>
 12. Most backtracking verbs in assertions have their normal actions. They are
 not confined to the assertion.
 </P>
 <P>
 13. There are some differences that are concerned with the settings of captured
 strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
 </P>  </P>
 <P>  <P>
13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern14. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
 names is not as general as Perl's. This is a consequence of the fact the PCRE  names is not as general as Perl's. This is a consequence of the fact the PCRE
 works internally just with numbers, using an external table to translate  works internally just with numbers, using an external table to translate
 between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),  between numbers and names. In particular, a pattern such as (?|(?&#60;a&#62;A)|(?&#60;b)B),
Line 127  names map to capturing subpattern number 1. To avoid t Line 136  names map to capturing subpattern number 1. To avoid t
 an error is given at compile time.  an error is given at compile time.
 </P>  </P>
 <P>  <P>
14. Perl recognizes comments in some places that PCRE does not, for example,15. Perl recognizes comments in some places that PCRE does not, for example,
 between the ( and ? at the start of a subpattern. If the /x modifier is set,  between the ( and ? at the start of a subpattern. If the /x modifier is set,
Perl allows whitespace between ( and ? but PCRE never does, even if thePerl allows white space between ( and ? (though current Perls warn that this is
PCRE_EXTENDED option is set.deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set.
 </P>  </P>
 <P>  <P>
15. PCRE provides some extensions to the Perl regular expression facilities.16. Perl, when in warning mode, gives warnings for character classes such as
 [A-\d] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE has no
 warning features, so it gives an error in these cases because they are almost
 certainly user mistakes.
 </P>
 <P>
 17. In PCRE, the upper/lower case character properties Lu and Ll are not
 affected when case-independent matching is specified. For example, \p{Lu}
 always matches an upper case letter. I think Perl has changed in this respect;
 in the release at the time of writing (5.16), \p{Lu} and \p{Ll} match all
 letters, regardless of case, when case independence is specified.
 </P>
 <P>
 18. PCRE provides some extensions to the Perl regular expression facilities.
 Perl 5.10 includes new features that are not in earlier versions of Perl, some  Perl 5.10 includes new features that are not in earlier versions of Perl, some
 of which (such as named parentheses) have been in PCRE for some time. This list  of which (such as named parentheses) have been in PCRE for some time. This list
 is with respect to Perl 5.10:  is with respect to Perl 5.10:
Line 181  different hosts that have the other endianness. Howeve Line 203  different hosts that have the other endianness. Howeve
 optimized data created by the just-in-time compiler.  optimized data created by the just-in-time compiler.
 <br>  <br>
 <br>  <br>
(k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a(k) The alternative matching functions (<b>pcre_dfa_exec()</b>,
different way and is not Perl-compatible.<b>pcre16_dfa_exec()</b> and <b>pcre32_dfa_exec()</b>,) match in a different way
 and are not Perl-compatible.
 <br>  <br>
 <br>  <br>
 (l) PCRE recognizes some special sequences such as (*CR) at the start of  (l) PCRE recognizes some special sequences such as (*CR) at the start of
Line 203  Cambridge CB2 3QH, England. Line 226  Cambridge CB2 3QH, England.
 REVISION  REVISION
 </b><br>  </b><br>
 <P>  <P>
Last updated: 14 November 2011Last updated: 10 November 2013
 <br>  <br>
Copyright &copy; 1997-2011 University of Cambridge.Copyright &copy; 1997-2013 University of Cambridge.
 <br>  <br>
 <p>  <p>
 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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