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version 1.1.1.1, 2012/02/21 23:05:52	version 1.1.1.4, 2013/07/22 08:25:57
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 what	1. PCRE has only a subset of Perl's Unicode support. Details of what it does
it does have are given in the	have 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 its	counted, 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 make	7. 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) are	9. 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 action	10. 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 captured	11. 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 subpattern	14. 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 (?\|(?<a>A)\|(?<b)B),	between numbers and names. In particular, a pattern such as (?\|(?<a>A)\|(?<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 the	Perl allows white space between ( and ? but PCRE never does, even if the
PCRE_EXTENDED option is set.	PCRE_EXTENDED option is set.
</P>	</P>
<P>	<P>
15. PCRE provides some extensions to the Perl regular expression facilities.	16. 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>
	17. 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 197 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 220 Cambridge CB2 3QH, England.
REVISION	REVISION
</b><br>	</b><br>
<P>	<P>
Last updated: 14 November 2011	Last updated: 19 March 2013
<br>	<br>
Copyright © 1997-2011 University of Cambridge.	Copyright © 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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