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
5.10/5.11.
</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&. PCRE does support the Cs (surrogate) property, which Perl does not; the
Perl documentation says "Because Perl hides the need for the user to understand
the internal representation of Unicode characters, there is no need to
implement the somewhat messy concept of surrogates."
</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$xyz
</pre>
The \Q...\E sequence is recognized both inside and outside character classes.
</P>
<P>
8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
the
<a href="pcrecallout.html"><b>pcrecallout</b></a>
documentation for details.
</P>
<P>
9. Subpatterns that are called recursively or as "subroutines" are always
treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
is a discussion of an example that explains this in more detail in the
<a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
in the
<a href="pcrepattern.html"><b>pcrepattern</b></a>
page.
</P>
<P>
10. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
</P>
<P>
11. PCRE'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
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),
where the two capturing parentheses have the same number but different names,
is not supported, and causes an error at compile time. If it were allowed, it
would not be possible to distinguish which parentheses matched, because both
names map to capturing subpattern number 1. To avoid this confusing situation,
an error is given at compile time.
</P>
<P>
12. 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
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 in PCRE 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, PCRE_NOTEMPTY_ATSTART, and
PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
<br>
<br>
(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
by the PCRE_BSR_ANYCRLF option.
<br>
<br>
(h) The callout facility is PCRE-specific.
<br>
<br>
(i) The partial matching facility is PCRE-specific.
<br>
<br>
(j) 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>
(k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
different way and is not Perl-compatible.
<br>
<br>
(l) PCRE recognizes some special sequences such as (*CR) at the start of
a pattern that set overall options that cannot be changed within the pattern.
</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: 12 May 2010
<br>
Copyright &copy; 1997-2010 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	ph10	461	regular expressions. The differences described here are with respect to Perl
21	ph10	535	5.10/5.11.
22	nigel	63	</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	ph10	453	and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
62			Perl documentation says "Because Perl hides the need for the user to understand
63			the internal representation of Unicode characters, there is no need to
64			implement the somewhat messy concept of surrogates."
65	nigel	75	</P>
66			<P>
67			7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
68	nigel	63	between are treated as literals. This is slightly different from Perl in that $
69			and @ are also handled as literals inside the quotes. In Perl, they cause
70			variable interpolation (but of course PCRE does not have variables). Note the
71			following examples:
72			<pre>
73			Pattern PCRE matches Perl matches
74	nigel	75
75			\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
76	nigel	63	\Qabc\$xyz\E abc\$xyz abc\$xyz
77			\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
78	nigel	75	</pre>
79	nigel	73	The \Q...\E sequence is recognized both inside and outside character classes.
80	nigel	63	</P>
81			<P>
82	nigel	93	8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
83			constructions. However, there is support for recursive patterns. This is not
84	ph10	453	available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
85	nigel	93	feature allows an external function to be called during pattern matching. See
86			the
87	nigel	75	<a href="pcrecallout.html"><b>pcrecallout</b></a>
88			documentation for details.
89	nigel	63	</P>
90			<P>
91	nigel	93	9. Subpatterns that are called recursively or as "subroutines" are always
92	ph10	461	treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
93	ph10	453	is a discussion of an example that explains this in more detail in the
94			<a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
95			in the
96	ph10	461	<a href="pcrepattern.html"><b>pcrepattern</b></a>
97	ph10	453	page.
98	nigel	93	</P>
99			<P>
100			10. There are some differences that are concerned with the settings of captured
101	nigel	63	strings when part of a pattern is repeated. For example, matching "aba" against
102			the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
103			</P>
104			<P>
105	ph10	535	11. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
106	ph10	461	names is not as general as Perl's. This is a consequence of the fact the PCRE
107			works internally just with numbers, using an external table to translate
108			between numbers and names. In particular, a pattern such as (?\|(?<a>A)\|(?<b)B),
109			where the two capturing parentheses have the same number but different names,
110			is not supported, and causes an error at compile time. If it were allowed, it
111			would not be possible to distinguish which parentheses matched, because both
112			names map to capturing subpattern number 1. To avoid this confusing situation,
113			an error is given at compile time.
114			</P>
115			<P>
116	ph10	535	12. PCRE provides some extensions to the Perl regular expression facilities.
117	ph10	461	Perl 5.10 includes new features that are not in earlier versions of Perl, some
118			of which (such as named parentheses) have been in PCRE for some time. This list
119			is with respect to Perl 5.10:
120	nigel	75	<br>
121			<br>
122	ph10	461	(a) Although lookbehind assertions in PCRE must match fixed length strings,
123			each alternative branch of a lookbehind assertion can match a different length
124			of string. Perl requires them all to have the same length.
125	nigel	75	<br>
126			<br>
127	nigel	63	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
128			meta-character matches only at the very end of the string.
129	nigel	75	<br>
130			<br>
131			(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
132	ph10	182	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
133			(Perl can be made to issue a warning.)
134	nigel	75	<br>
135			<br>
136	nigel	63	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
137			inverted, that is, by default they are not greedy, but if followed by a
138			question mark they are.
139	nigel	75	<br>
140			<br>
141			(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
142			only at the first matching position in the subject string.
143			<br>
144			<br>
145	ph10	453	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
146			PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
147	nigel	75	<br>
148			<br>
149	ph10	231	(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
150			by the PCRE_BSR_ANYCRLF option.
151	nigel	75	<br>
152			<br>
153	ph10	231	(h) The callout facility is PCRE-specific.
154	nigel	75	<br>
155			<br>
156	ph10	231	(i) The partial matching facility is PCRE-specific.
157			<br>
158			<br>
159			(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
160	nigel	75	different hosts that have the other endianness.
161	nigel	77	<br>
162			<br>
163	ph10	231	(k) The alternative matching function (<b>pcre_dfa_exec()</b>) matches in a
164	nigel	77	different way and is not Perl-compatible.
165	ph10	231	<br>
166			<br>
167			(l) PCRE recognizes some special sequences such as (*CR) at the start of
168			a pattern that set overall options that cannot be changed within the pattern.
169	nigel	63	</P>
170	ph10	99	<br><b>
171			AUTHOR
172			</b><br>
173	nigel	63	<P>
174	ph10	99	Philip Hazel
175	nigel	63	<br>
176	ph10	99	University Computing Service
177			<br>
178			Cambridge CB2 3QH, England.
179			<br>
180			</P>
181			<br><b>
182			REVISION
183			</b><br>
184			<P>
185	ph10	535	Last updated: 12 May 2010
186	ph10	99	<br>
187	ph10	535	Copyright © 1997-2010 University of Cambridge.
188	ph10	99	<br>
189	nigel	75	<p>
190			Return to the <a href="index.html">PCRE index page</a>.
191			</p>
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