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 with respect to Perl
5.8.
</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$xyz
</pre>
The \Q...\E sequence is recognized both inside and outside character classes.
</P>
<P>
8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
constructions. However, there is support for recursive patterns using the
non-Perl items (?R), (?number), and (?P&#62;name). Also, the PCRE "callout" feature
allows an external function to be called during pattern matching. See the
<a href="pcrecallout.html"><b>pcrecallout</b></a>
documentation for details.
</P>
<P>
9. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
</P>
<P>
10. PCRE provides some extensions to the Perl regular expression facilities:
<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 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 (?R), (?number), and (?P&#62;name) constructs allows for recursive pattern
matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
support.)
<br>
<br>
(h) PCRE supports named capturing substrings, using the Python syntax.
<br>
<br>
(i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
package.
<br>
<br>
(j) The (R) condition, for testing recursion, is a PCRE extension.
<br>
<br>
(k) The callout facility is PCRE-specific.
<br>
<br>
(l) The partial matching facility is PCRE-specific.
<br>
<br>
(m) 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>
(n) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
different way and is not Perl-compatible.
</P>
<P>
Last updated: 06 June 2006
<br>
Copyright &copy; 1997-2006 University of Cambridge.
<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			<p>
11			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			<br>
15			<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			regular expressions. The differences described here are with respect to Perl
21			5.8.
22			</P>
23			<P>
24	nigel	87	1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
25			it does have are given in the
26	nigel	73	<a href="pcre.html#utf8support">section on UTF-8 support</a>
27			in the main
28			<a href="pcre.html"><b>pcre</b></a>
29			page.
30			</P>
31			<P>
32			2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
33	nigel	63	them, but they do not mean what you might think. For example, (?!a){3} does
34			not assert that the next three characters are not "a". It just asserts that the
35			next character is not "a" three times.
36			</P>
37			<P>
38	nigel	73	3. Capturing subpatterns that occur inside negative lookahead assertions are
39	nigel	63	counted, but their entries in the offsets vector are never set. Perl sets its
40			numerical variables from any such patterns that are matched before the
41			assertion fails to match something (thereby succeeding), but only if the
42			negative lookahead assertion contains just one branch.
43			</P>
44			<P>
45	nigel	73	4. Though binary zero characters are supported in the subject string, they are
46	nigel	63	not allowed in a pattern string because it is passed as a normal C string,
47	nigel	75	terminated by zero. The escape sequence \0 can be used in the pattern to
48	nigel	63	represent a binary zero.
49			</P>
50			<P>
51	nigel	73	5. The following Perl escape sequences are not supported: \l, \u, \L,
52	nigel	75	\U, and \N. In fact these are implemented by Perl's general string-handling
53			and are not part of its pattern matching engine. If any of these are
54			encountered by PCRE, an error is generated.
55	nigel	63	</P>
56			<P>
57	nigel	75	6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
58			built with Unicode character property support. The properties that can be
59			tested with \p and \P are limited to the general category properties such as
60	nigel	87	Lu and Nd, script names such as Greek or Han, and the derived properties Any
61			and L&.
62	nigel	75	</P>
63			<P>
64			7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
65	nigel	63	between are treated as literals. This is slightly different from Perl in that $
66			and @ are also handled as literals inside the quotes. In Perl, they cause
67			variable interpolation (but of course PCRE does not have variables). Note the
68			following examples:
69			<pre>
70			Pattern PCRE matches Perl matches
71	nigel	75
72			\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
73	nigel	63	\Qabc\$xyz\E abc\$xyz abc\$xyz
74			\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
75	nigel	75	</pre>
76	nigel	73	The \Q...\E sequence is recognized both inside and outside character classes.
77	nigel	63	</P>
78			<P>
79	nigel	75	8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
80			constructions. However, there is support for recursive patterns using the
81			non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature
82			allows an external function to be called during pattern matching. See the
83			<a href="pcrecallout.html"><b>pcrecallout</b></a>
84			documentation for details.
85	nigel	63	</P>
86			<P>
87	nigel	75	9. There are some differences that are concerned with the settings of captured
88	nigel	63	strings when part of a pattern is repeated. For example, matching "aba" against
89			the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
90			</P>
91			<P>
92	nigel	75	10. PCRE provides some extensions to the Perl regular expression facilities:
93			<br>
94			<br>
95	nigel	63	(a) Although lookbehind assertions must match fixed length strings, each
96			alternative branch of a lookbehind assertion can match a different length of
97			string. Perl requires them all to have the same length.
98	nigel	75	<br>
99			<br>
100	nigel	63	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
101			meta-character matches only at the very end of the string.
102	nigel	75	<br>
103			<br>
104			(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
105	nigel	91	meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can
106			be made to issue a warning.)
107	nigel	75	<br>
108			<br>
109	nigel	63	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
110			inverted, that is, by default they are not greedy, but if followed by a
111			question mark they are.
112	nigel	75	<br>
113			<br>
114			(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
115			only at the first matching position in the subject string.
116			<br>
117			<br>
118	nigel	63	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
119			options for <b>pcre_exec()</b> have no Perl equivalents.
120	nigel	75	<br>
121			<br>
122	nigel	63	(g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern
123			matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
124			support.)
125	nigel	75	<br>
126			<br>
127	nigel	63	(h) PCRE supports named capturing substrings, using the Python syntax.
128	nigel	75	<br>
129			<br>
130	nigel	63	(i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
131			package.
132	nigel	75	<br>
133			<br>
134	nigel	63	(j) The (R) condition, for testing recursion, is a PCRE extension.
135	nigel	75	<br>
136			<br>
137	nigel	63	(k) The callout facility is PCRE-specific.
138	nigel	75	<br>
139			<br>
140			(l) The partial matching facility is PCRE-specific.
141			<br>
142			<br>
143			(m) Patterns compiled by PCRE can be saved and re-used at a later time, even on
144			different hosts that have the other endianness.
145	nigel	77	<br>
146			<br>
147			(n) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
148			different way and is not Perl-compatible.
149	nigel	63	</P>
150			<P>
151	nigel	91	Last updated: 06 June 2006
152	nigel	63	<br>
153	nigel	87	Copyright © 1997-2006 University of Cambridge.
154	nigel	75	<p>
155			Return to the <a href="index.html">PCRE index page</a>.
156			</p>