doc/html/pcrecompat.html

<html>
<head>
<title>pcrecompat specification</title>
</head>
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
<h1>pcrecompat man page</h1>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
<p>
This page is part of the PCRE HTML documentation. It was generated automatically
from the original man page. If there is any nonsense in it, please consult the
man page, in case the conversion went wrong.
<br>
<br><b>
DIFFERENCES BETWEEN PCRE AND PERL
</b><br>
<P>
This document describes the differences in the ways that PCRE and Perl handle
regular expressions. The differences described here are mainly with respect to
Perl 5.8, though PCRE versions 7.0 and later contain some features that are
expected to be in the forthcoming Perl 5.10.
</P>
<P>
1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
it does have are given in the
<a href="pcre.html#utf8support">section on UTF-8 support</a>
in the main
<a href="pcre.html"><b>pcre</b></a>
page.
</P>
<P>
2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
them, but they do not mean what you might think. For example, (?!a){3} does
not assert that the next three characters are not "a". It just asserts that the
next character is not "a" three times.
</P>
<P>
3. Capturing subpatterns that occur inside negative lookahead assertions are
counted, but their entries in the offsets vector are never set. Perl sets its
numerical variables from any such patterns that are matched before the
assertion fails to match something (thereby succeeding), but only if the
negative lookahead assertion contains just one branch.
</P>
<P>
4. Though binary zero characters are supported in the subject string, they are
not allowed in a pattern string because it is passed as a normal C string,
terminated by zero. The escape sequence \0 can be used in the pattern to
represent a binary zero.
</P>
<P>
5. The following Perl escape sequences are not supported: \l, \u, \L,
\U, and \N. In fact these are implemented by Perl's general string-handling
and are not part of its pattern matching engine. If any of these are
encountered by PCRE, an error is generated.
</P>
<P>
6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
built with Unicode character property support. The properties that can be
tested with \p and \P are limited to the general category properties such as
Lu and Nd, script names such as Greek or Han, and the derived properties Any
and L&.
</P>
<P>
7. 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 $
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
following examples:
<pre>
    Pattern            PCRE matches      Perl matches

    \Qabc$xyz\E        abc$xyz           abc followed by the contents of $xyz
    \Qabc\$xyz\E       abc\$xyz          abc\$xyz
    \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
</pre>
The \Q...\E sequence is recognized both inside and outside character classes.
</P>
<P>
8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
the
<a href="pcrecallout.html"><b>pcrecallout</b></a>
documentation for details.
</P>
<P>
9. Subpatterns that are called recursively or as "subroutines" are always
treated as atomic groups in PCRE. This is like Python, but unlike Perl.
</P>
<P>
10. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
</P>
<P>
11. PCRE provides some extensions to the Perl regular expression facilities.
Perl 5.10 will include new features that are not in earlier versions, some of
which (such as named parentheses) have been in PCRE for some time. This list is
with respect to Perl 5.10:
<br>
<br>
(a) Although lookbehind assertions must match fixed length strings, each
alternative branch of a lookbehind assertion can match a different length of
string. Perl requires them all to have the same length.
<br>
<br>
(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
meta-character matches only at the very end of the string.
<br>
<br>
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
(Perl can be made to issue a warning.)
<br>
<br>
(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
inverted, that is, by default they are not greedy, but if followed by a
question mark they are.
<br>
<br>
(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
only at the first matching position in the subject string.
<br>
<br>
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
options for <b>pcre_exec()</b> have no Perl equivalents.
<br>
<br>
(g) The callout facility is PCRE-specific.
<br>
<br>
(h) The partial matching facility is PCRE-specific.
<br>
<br>
(i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
different hosts that have the other endianness.
<br>
<br>
(j) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
different way and is not Perl-compatible.
</P>
<br><b>
AUTHOR
</b><br>
<P>
Philip Hazel
<br>
University Computing Service
<br>
Cambridge CB2 3QH, England.
<br>
</P>
<br><b>
REVISION
</b><br>
<P>
Last updated: 13 June 2007
<br>
Copyright &copy; 1997-2007 University of Cambridge.
<br>
<p>
Return to the <a href="index.html">PCRE index page</a>.
</p>
1	nigel	63	<html>
2			<head>
3			<title>pcrecompat specification</title>
4			</head>
5			<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6	nigel	75	<h1>pcrecompat man page</h1>
7			<p>
8			Return to the <a href="index.html">PCRE index page</a>.
9			</p>
10	ph10	111	<p>
11	nigel	75	This page is part of the PCRE HTML documentation. It was generated automatically
12			from the original man page. If there is any nonsense in it, please consult the
13			man page, in case the conversion went wrong.
14	ph10	111	<br>
15	nigel	75	<br><b>
16			DIFFERENCES BETWEEN PCRE AND PERL
17			</b><br>
18	nigel	63	<P>
19			This document describes the differences in the ways that PCRE and Perl handle
20	nigel	93	regular expressions. The differences described here are mainly with respect to
21	ph10	182	Perl 5.8, though PCRE versions 7.0 and later contain some features that are
22			expected to be in the forthcoming Perl 5.10.
23	nigel	63	</P>
24			<P>
25	nigel	87	1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
26			it does have are given in the
27	nigel	73	<a href="pcre.html#utf8support">section on UTF-8 support</a>
28			in the main
29			<a href="pcre.html"><b>pcre</b></a>
30			page.
31			</P>
32			<P>
33			2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
34	nigel	63	them, but they do not mean what you might think. For example, (?!a){3} does
35			not assert that the next three characters are not "a". It just asserts that the
36			next character is not "a" three times.
37			</P>
38			<P>
39	nigel	73	3. Capturing subpatterns that occur inside negative lookahead assertions are
40	nigel	63	counted, but their entries in the offsets vector are never set. Perl sets its
41			numerical variables from any such patterns that are matched before the
42			assertion fails to match something (thereby succeeding), but only if the
43			negative lookahead assertion contains just one branch.
44			</P>
45			<P>
46	nigel	73	4. Though binary zero characters are supported in the subject string, they are
47	nigel	63	not allowed in a pattern string because it is passed as a normal C string,
48	nigel	75	terminated by zero. The escape sequence \0 can be used in the pattern to
49	nigel	63	represent a binary zero.
50			</P>
51			<P>
52	nigel	73	5. The following Perl escape sequences are not supported: \l, \u, \L,
53	nigel	75	\U, and \N. In fact these are implemented by Perl's general string-handling
54			and are not part of its pattern matching engine. If any of these are
55			encountered by PCRE, an error is generated.
56	nigel	63	</P>
57			<P>
58	nigel	75	6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
59			built with Unicode character property support. The properties that can be
60			tested with \p and \P are limited to the general category properties such as
61	nigel	87	Lu and Nd, script names such as Greek or Han, and the derived properties Any
62			and L&.
63	nigel	75	</P>
64			<P>
65			7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
66	nigel	63	between are treated as literals. This is slightly different from Perl in that $
67			and @ are also handled as literals inside the quotes. In Perl, they cause
68			variable interpolation (but of course PCRE does not have variables). Note the
69			following examples:
70			<pre>
71			Pattern PCRE matches Perl matches
72	nigel	75
73			\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
74	nigel	63	\Qabc\$xyz\E abc\$xyz abc\$xyz
75			\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
76	nigel	75	</pre>
77	nigel	73	The \Q...\E sequence is recognized both inside and outside character classes.
78	nigel	63	</P>
79			<P>
80	nigel	93	8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
81			constructions. However, there is support for recursive patterns. This is not
82			available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE "callout"
83			feature allows an external function to be called during pattern matching. See
84			the
85	nigel	75	<a href="pcrecallout.html"><b>pcrecallout</b></a>
86			documentation for details.
87	nigel	63	</P>
88			<P>
89	nigel	93	9. Subpatterns that are called recursively or as "subroutines" are always
90			treated as atomic groups in PCRE. This is like Python, but unlike Perl.
91			</P>
92			<P>
93			10. There are some differences that are concerned with the settings of captured
94	nigel	63	strings when part of a pattern is repeated. For example, matching "aba" against
95			the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
96			</P>
97			<P>
98	nigel	93	11. PCRE provides some extensions to the Perl regular expression facilities.
99			Perl 5.10 will include new features that are not in earlier versions, some of
100			which (such as named parentheses) have been in PCRE for some time. This list is
101			with respect to Perl 5.10:
102	nigel	75	<br>
103			<br>
104	nigel	63	(a) Although lookbehind assertions must match fixed length strings, each
105			alternative branch of a lookbehind assertion can match a different length of
106			string. Perl requires them all to have the same length.
107	nigel	75	<br>
108			<br>
109	nigel	63	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
110			meta-character matches only at the very end of the string.
111	nigel	75	<br>
112			<br>
113			(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
114	ph10	182	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
115			(Perl can be made to issue a warning.)
116	nigel	75	<br>
117			<br>
118	nigel	63	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
119			inverted, that is, by default they are not greedy, but if followed by a
120			question mark they are.
121	nigel	75	<br>
122			<br>
123			(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
124			only at the first matching position in the subject string.
125			<br>
126			<br>
127	nigel	63	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
128			options for <b>pcre_exec()</b> have no Perl equivalents.
129	nigel	75	<br>
130			<br>
131	nigel	93	(g) The callout facility is PCRE-specific.
132	nigel	75	<br>
133			<br>
134	nigel	93	(h) The partial matching facility is PCRE-specific.
135	nigel	75	<br>
136			<br>
137	nigel	93	(i) Patterns compiled by PCRE can be saved and re-used at a later time, even on
138	nigel	75	different hosts that have the other endianness.
139	nigel	77	<br>
140			<br>
141	nigel	93	(j) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
142	nigel	77	different way and is not Perl-compatible.
143	nigel	63	</P>
144	ph10	99	<br><b>
145			AUTHOR
146			</b><br>
147	nigel	63	<P>
148	ph10	99	Philip Hazel
149	nigel	63	<br>
150	ph10	99	University Computing Service
151			<br>
152			Cambridge CB2 3QH, England.
153			<br>
154			</P>
155			<br><b>
156			REVISION
157			</b><br>
158			<P>
159	ph10	182	Last updated: 13 June 2007
160	ph10	99	<br>
161			Copyright © 1997-2007 University of Cambridge.
162			<br>
163	nigel	75	<p>
164			Return to the <a href="index.html">PCRE index page</a>.
165			</p>
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svn:keywords	"Author Date Id Revision Url"