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
versions 5.10 and above.
</P>
<P>
1. PCRE has only a subset of Perl's Unicode support. Details of what it does
have are given in the
<a href="pcreunicode.html"><b>pcreunicode</b></a>
page.
</P>
<P>
2. PCRE allows repeat quantifiers only on parenthesized assertions, 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 (in principle: PCRE optimizes this to run the assertion
just once). Perl allows repeat quantifiers on other assertions such as \b, but
these do not seem to have any use.
</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 when followed by a character name or Unicode value. (\N on its
own, matching a non-newline character, is supported.) 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 by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
\U and \u are interpreted as JavaScript interprets them.
</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 implements a simpler version of \X than Perl, which changed to make
\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 $
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$xyz
</pre>
The \Q...\E sequence is recognized both inside and outside character classes.
</P>
<P>
9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
the
<a href="pcrecallout.html"><b>pcrecallout</b></a>
documentation for details.
</P>
<P>
10. Subpatterns that are called as subroutines (whether or not recursively) are
always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
Captured values that are set outside a subroutine call can be reference from
inside in PCRE, but not in Perl. There is a discussion that explains these
differences in more detail in the
<a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
in the
<a href="pcrepattern.html"><b>pcrepattern</b></a>
page.
</P>
<P>
11. If any of the backtracking control verbs are used in an assertion or in a
subpattern that is called as a subroutine (whether or not recursively), their
effect is confined to that subpattern; it does not extend to the surrounding
pattern. This is not always the case in Perl. In particular, if (*THEN) is
present in a group that is called as a subroutine, its action is limited to
that group, even if the group does not contain any | characters. There is one
exception to this: the name from a *(MARK), (*PRUNE), or (*THEN) that is
encountered in a successful positive assertion <i>is</i> passed back when a
match succeeds (compare capturing parentheses in assertions). Note that such
subpatterns are processed as anchored at the point where they are tested.
</P>
<P>
12. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
</P>
<P>
13. 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>
14. 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,
Perl allows white space between ( and ? but PCRE never does, even if the
PCRE_EXTENDED option is set.
</P>
<P>
15. 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. However, this does not apply to
optimized data created by the just-in-time compiler.
<br>
<br>
(k) The alternative matching functions (<b>pcre_dfa_exec()</b> and
<b>pcre16_dfa_exec()</b>) match in a different way and are 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: 01 June 2012
<br>
Copyright &copy; 1997-2012 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	567	versions 5.10 and above.
22	nigel	63	</P>
23			<P>
24	ph10	869	1. PCRE has only a subset of Perl's Unicode support. Details of what it does
25			have are given in the
26	ph10	678	<a href="pcreunicode.html"><b>pcreunicode</b></a>
27	nigel	73	page.
28			</P>
29			<P>
30	ph10	654	2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
31			not mean what you might think. For example, (?!a){3} does not assert that the
32			next three characters are not "a". It just asserts that the next character is
33			not "a" three times (in principle: PCRE optimizes this to run the assertion
34			just once). Perl allows repeat quantifiers on other assertions such as \b, but
35			these do not seem to have any use.
36	nigel	63	</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	ph10	654	\U, and \N when followed by a character name or Unicode value. (\N on its
53			own, matching a non-newline character, is supported.) In fact these are
54			implemented by Perl's general string-handling and are not part of its pattern
55			matching engine. If any of these are encountered by PCRE, an error is
56	ph10	836	generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
57			\U and \u are interpreted as JavaScript interprets them.
58	nigel	63	</P>
59			<P>
60	nigel	75	6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
61			built with Unicode character property support. The properties that can be
62			tested with \p and \P are limited to the general category properties such as
63	nigel	87	Lu and Nd, script names such as Greek or Han, and the derived properties Any
64	ph10	453	and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
65			Perl documentation says "Because Perl hides the need for the user to understand
66			the internal representation of Unicode characters, there is no need to
67			implement the somewhat messy concept of surrogates."
68	nigel	75	</P>
69			<P>
70	ph10	654	7. PCRE implements a simpler version of \X than Perl, which changed to make
71			\X match what Unicode calls an "extended grapheme cluster". This is more
72			complicated than an extended Unicode sequence, which is what PCRE matches.
73			</P>
74			<P>
75			8. PCRE does support the \Q...\E escape for quoting substrings. Characters in
76	nigel	63	between are treated as literals. This is slightly different from Perl in that $
77			and @ are also handled as literals inside the quotes. In Perl, they cause
78			variable interpolation (but of course PCRE does not have variables). Note the
79			following examples:
80			<pre>
81			Pattern PCRE matches Perl matches
82	nigel	75
83			\Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
84	nigel	63	\Qabc\$xyz\E abc\$xyz abc\$xyz
85			\Qabc\E\$\Qxyz\E abc$xyz abc$xyz
86	nigel	75	</pre>
87	nigel	73	The \Q...\E sequence is recognized both inside and outside character classes.
88	nigel	63	</P>
89			<P>
90	ph10	654	9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
91	nigel	93	constructions. However, there is support for recursive patterns. This is not
92	ph10	453	available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
93	nigel	93	feature allows an external function to be called during pattern matching. See
94			the
95	nigel	75	<a href="pcrecallout.html"><b>pcrecallout</b></a>
96			documentation for details.
97	nigel	63	</P>
98			<P>
99	ph10	733	10. Subpatterns that are called as subroutines (whether or not recursively) are
100			always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
101			Captured values that are set outside a subroutine call can be reference from
102			inside in PCRE, but not in Perl. There is a discussion that explains these
103			differences in more detail in the
104	ph10	453	<a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
105			in the
106	ph10	461	<a href="pcrepattern.html"><b>pcrepattern</b></a>
107	ph10	453	page.
108	nigel	93	</P>
109			<P>
110	ph10	975	11. If any of the backtracking control verbs are used in an assertion or in a
111			subpattern that is called as a subroutine (whether or not recursively), their
112			effect is confined to that subpattern; it does not extend to the surrounding
113			pattern. This is not always the case in Perl. In particular, if (*THEN) is
114			present in a group that is called as a subroutine, its action is limited to
115			that group, even if the group does not contain any \| characters. There is one
116			exception to this: the name from a (MARK), (PRUNE), or (*THEN) that is
117			encountered in a successful positive assertion <i>is</i> passed back when a
118			match succeeds (compare capturing parentheses in assertions). Note that such
119			subpatterns are processed as anchored at the point where they are tested.
120	ph10	733	</P>
121			<P>
122			12. There are some differences that are concerned with the settings of captured
123	nigel	63	strings when part of a pattern is repeated. For example, matching "aba" against
124			the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
125			</P>
126			<P>
127	ph10	733	13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
128	ph10	461	names is not as general as Perl's. This is a consequence of the fact the PCRE
129			works internally just with numbers, using an external table to translate
130			between numbers and names. In particular, a pattern such as (?\|(?<a>A)\|(?<b)B),
131			where the two capturing parentheses have the same number but different names,
132			is not supported, and causes an error at compile time. If it were allowed, it
133			would not be possible to distinguish which parentheses matched, because both
134			names map to capturing subpattern number 1. To avoid this confusing situation,
135			an error is given at compile time.
136			</P>
137			<P>
138	ph10	733	14. Perl recognizes comments in some places that PCRE does not, for example,
139	ph10	654	between the ( and ? at the start of a subpattern. If the /x modifier is set,
140	ph10	975	Perl allows white space between ( and ? but PCRE never does, even if the
141	ph10	654	PCRE_EXTENDED option is set.
142	ph10	567	</P>
143			<P>
144	ph10	733	15. PCRE provides some extensions to the Perl regular expression facilities.
145	ph10	461	Perl 5.10 includes new features that are not in earlier versions of Perl, some
146			of which (such as named parentheses) have been in PCRE for some time. This list
147			is with respect to Perl 5.10:
148	nigel	75	<br>
149			<br>
150	ph10	461	(a) Although lookbehind assertions in PCRE must match fixed length strings,
151			each alternative branch of a lookbehind assertion can match a different length
152			of string. Perl requires them all to have the same length.
153	nigel	75	<br>
154			<br>
155	nigel	63	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
156			meta-character matches only at the very end of the string.
157	nigel	75	<br>
158			<br>
159			(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
160	ph10	182	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
161			(Perl can be made to issue a warning.)
162	nigel	75	<br>
163			<br>
164	nigel	63	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
165			inverted, that is, by default they are not greedy, but if followed by a
166			question mark they are.
167	nigel	75	<br>
168			<br>
169			(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
170			only at the first matching position in the subject string.
171			<br>
172			<br>
173	ph10	453	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
174			PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
175	nigel	75	<br>
176			<br>
177	ph10	231	(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
178			by the PCRE_BSR_ANYCRLF option.
179	nigel	75	<br>
180			<br>
181	ph10	231	(h) The callout facility is PCRE-specific.
182	nigel	75	<br>
183			<br>
184	ph10	231	(i) The partial matching facility is PCRE-specific.
185			<br>
186			<br>
187			(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
188	ph10	733	different hosts that have the other endianness. However, this does not apply to
189			optimized data created by the just-in-time compiler.
190	nigel	77	<br>
191			<br>
192	ph10	869	(k) The alternative matching functions (<b>pcre_dfa_exec()</b> and
193			<b>pcre16_dfa_exec()</b>) match in a different way and are not Perl-compatible.
194	ph10	231	<br>
195			<br>
196			(l) PCRE recognizes some special sequences such as (*CR) at the start of
197			a pattern that set overall options that cannot be changed within the pattern.
198	nigel	63	</P>
199	ph10	99	<br><b>
200			AUTHOR
201			</b><br>
202	nigel	63	<P>
203	ph10	99	Philip Hazel
204	nigel	63	<br>
205	ph10	99	University Computing Service
206			<br>
207			Cambridge CB2 3QH, England.
208			<br>
209			</P>
210			<br><b>
211			REVISION
212			</b><br>
213			<P>
214	ph10	975	Last updated: 01 June 2012
215	ph10	99	<br>
216	ph10	869	Copyright © 1997-2012 University of Cambridge.
217	ph10	99	<br>
218	nigel	75	<p>
219			Return to the <a href="index.html">PCRE index page</a>.
220			</p>
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"