trunk/doc/pcrecompat.3

.TH PCRECOMPAT 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "DIFFERENCES BETWEEN PCRE AND PERL"
.rs
.sp
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
1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
it does have are given in the
.\" HREF
\fBpcreunicode\fP
.\"
page.
.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 \eb, but
these do not seem to have any use.
.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
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 \e0 can be used in the pattern to
represent a binary zero.
.P
5. The following Perl escape sequences are not supported: \el, \eu, \eL,
\eU, and \eN when followed by a character name or Unicode value. (\eN 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.
.P
6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
built with Unicode character property support. The properties that can be
tested with \ep and \eP 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
7. PCRE implements a simpler version of \eX than Perl, which changed to make
\eX match what Unicode calls an "extended grapheme cluster". This is more
complicated than an extended Unicode sequence, which is what PCRE matches.
.P
8. PCRE does support the \eQ...\eE 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:
.sp
    Pattern            PCRE matches      Perl matches
.sp
.\" JOIN
    \eQabc$xyz\eE        abc$xyz           abc followed by the
                                           contents of $xyz
    \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
    \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
.sp
The \eQ...\eE sequence is recognized both inside and outside character classes.
.P
9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not
available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
feature allows an external function to be called during pattern matching. See
the
.\" HREF
\fBpcrecallout\fP
.\"
documentation for details.
.P
10. Subpatterns that are called as subroutines (whether or not recursively) are
always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
There is a discussion of an example that explains this in more detail in the
.\" HTML <a href="pcrepattern.html#recursiondifference">
.\" </a>
section on recursion differences from Perl
.\"
in the
.\" HREF
\fBpcrepattern\fP
.\"
page.
.P
11. If (*THEN) is present in a group that is called as a subroutine, its action
is limited to that group, even if the group does not contain any | characters.
.P
12. There are some differences that are concerned with the settings of captured
strings when part of a pattern is repeated. For example, matching "aba" against
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
.P
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 (?|(?<a>A)|(?<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
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 whitespace between ( and ? but PCRE never does, even if the
PCRE_EXTENDED option is set.
.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:
.sp
(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.
.sp
(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.
.sp
(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.)
.sp
(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.
.sp
(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.
.sp
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
.sp
(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
by the PCRE_BSR_ANYCRLF option.
.sp
(h) The callout facility is PCRE-specific.
.sp
(i) The partial matching facility is PCRE-specific.
.sp
(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.
.sp
(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
different way and is not Perl-compatible.
.sp
(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.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 04 October 2011
Copyright (c) 1997-2011 University of Cambridge.
.fi
1	nigel	79	.TH PCRECOMPAT 3
2	nigel	63	.SH NAME
3			PCRE - Perl-compatible regular expressions
4	nigel	75	.SH "DIFFERENCES BETWEEN PCRE AND PERL"
5	nigel	63	.rs
6			.sp
7			This document describes the differences in the ways that PCRE and Perl handle
8	ph10	456	regular expressions. The differences described here are with respect to Perl
9	ph10	562	versions 5.10 and above.
10	nigel	75	.P
11	nigel	87	1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
12			it does have are given in the
13	nigel	73	.\" HREF
14	ph10	678	\fBpcreunicode\fP
15	nigel	73	.\"
16			page.
17	nigel	75	.P
18	ph10	637	2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
19			not mean what you might think. For example, (?!a){3} does not assert that the
20			next three characters are not "a". It just asserts that the next character is
21	ph10	654	not "a" three times (in principle: PCRE optimizes this to run the assertion
22			just once). Perl allows repeat quantifiers on other assertions such as \eb, but
23	ph10	637	these do not seem to have any use.
24	nigel	75	.P
25	nigel	73	3. Capturing subpatterns that occur inside negative lookahead assertions are
26	nigel	63	counted, but their entries in the offsets vector are never set. Perl sets its
27			numerical variables from any such patterns that are matched before the
28			assertion fails to match something (thereby succeeding), but only if the
29			negative lookahead assertion contains just one branch.
30	nigel	75	.P
31	nigel	73	4. Though binary zero characters are supported in the subject string, they are
32	nigel	63	not allowed in a pattern string because it is passed as a normal C string,
33	nigel	75	terminated by zero. The escape sequence \e0 can be used in the pattern to
34	nigel	63	represent a binary zero.
35	nigel	75	.P
36			5. The following Perl escape sequences are not supported: \el, \eu, \eL,
37	ph10	654	\eU, and \eN when followed by a character name or Unicode value. (\eN on its
38	ph10	634	own, matching a non-newline character, is supported.) In fact these are
39			implemented by Perl's general string-handling and are not part of its pattern
40			matching engine. If any of these are encountered by PCRE, an error is
41			generated.
42	nigel	75	.P
43			6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
44			built with Unicode character property support. The properties that can be
45			tested with \ep and \eP are limited to the general category properties such as
46	nigel	87	Lu and Nd, script names such as Greek or Han, and the derived properties Any
47	ph10	450	and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
48			Perl documentation says "Because Perl hides the need for the user to understand
49			the internal representation of Unicode characters, there is no need to
50			implement the somewhat messy concept of surrogates."
51	nigel	75	.P
52	ph10	628	7. PCRE implements a simpler version of \eX than Perl, which changed to make
53	ph10	654	\eX match what Unicode calls an "extended grapheme cluster". This is more
54	ph10	628	complicated than an extended Unicode sequence, which is what PCRE matches.
55			.P
56			8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
57	nigel	63	between are treated as literals. This is slightly different from Perl in that $
58			and @ are also handled as literals inside the quotes. In Perl, they cause
59			variable interpolation (but of course PCRE does not have variables). Note the
60			following examples:
61	nigel	75	.sp
62	nigel	63	Pattern PCRE matches Perl matches
63	nigel	75	.sp
64			.\" JOIN
65			\eQabc$xyz\eE abc$xyz abc followed by the
66	nigel	63	contents of $xyz
67	nigel	75	\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
68			\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
69			.sp
70			The \eQ...\eE sequence is recognized both inside and outside character classes.
71			.P
72	ph10	628	9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
73	nigel	93	constructions. However, there is support for recursive patterns. This is not
74	ph10	453	available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
75	nigel	93	feature allows an external function to be called during pattern matching. See
76			the
77	nigel	75	.\" HREF
78			\fBpcrecallout\fP
79			.\"
80			documentation for details.
81			.P
82	ph10	716	10. Subpatterns that are called as subroutines (whether or not recursively) are
83			always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
84			There is a discussion of an example that explains this in more detail in the
85	ph10	453	.\" HTML <a href="pcrepattern.html#recursiondifference">
86			.\" </a>
87			section on recursion differences from Perl
88			.\"
89			in the
90			.\" HREF
91	ph10	456	\fBpcrepattern\fP
92	ph10	453	.\"
93			page.
94	nigel	93	.P
95	ph10	716	11. If (*THEN) is present in a group that is called as a subroutine, its action
96			is limited to that group, even if the group does not contain any \| characters.
97			.P
98			12. There are some differences that are concerned with the settings of captured
99	nigel	63	strings when part of a pattern is repeated. For example, matching "aba" against
100			the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
101	nigel	75	.P
102	ph10	716	13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
103	ph10	459	names is not as general as Perl's. This is a consequence of the fact the PCRE
104			works internally just with numbers, using an external table to translate
105			between numbers and names. In particular, a pattern such as (?\|(?<a>A)\|(?<b)B),
106			where the two capturing parentheses have the same number but different names,
107			is not supported, and causes an error at compile time. If it were allowed, it
108			would not be possible to distinguish which parentheses matched, because both
109			names map to capturing subpattern number 1. To avoid this confusing situation,
110			an error is given at compile time.
111	ph10	456	.P
112	ph10	716	14. Perl recognizes comments in some places that PCRE does not, for example,
113	ph10	654	between the ( and ? at the start of a subpattern. If the /x modifier is set,
114			Perl allows whitespace between ( and ? but PCRE never does, even if the
115	ph10	635	PCRE_EXTENDED option is set.
116	ph10	562	.P
117	ph10	716	15. PCRE provides some extensions to the Perl regular expression facilities.
118	ph10	456	Perl 5.10 includes new features that are not in earlier versions of Perl, some
119			of which (such as named parentheses) have been in PCRE for some time. This list
120			is with respect to Perl 5.10:
121			.sp
122			(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			.sp
126	nigel	63	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
127			meta-character matches only at the very end of the string.
128	nigel	75	.sp
129	nigel	63	(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
130	ph10	181	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
131			(Perl can be made to issue a warning.)
132	nigel	75	.sp
133	nigel	63	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
134			inverted, that is, by default they are not greedy, but if followed by a
135			question mark they are.
136	nigel	75	.sp
137			(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
138			only at the first matching position in the subject string.
139			.sp
140	ph10	442	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
141			PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
142	nigel	75	.sp
143	ph10	231	(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
144			by the PCRE_BSR_ANYCRLF option.
145	nigel	75	.sp
146	ph10	231	(h) The callout facility is PCRE-specific.
147	nigel	75	.sp
148	ph10	231	(i) The partial matching facility is PCRE-specific.
149			.sp
150			(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
151	ph10	716	different hosts that have the other endianness. However, this does not apply to
152			optimized data created by the just-in-time compiler.
153	nigel	77	.sp
154	ph10	231	(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
155	nigel	77	different way and is not Perl-compatible.
156	ph10	231	.sp
157			(l) PCRE recognizes some special sequences such as (*CR) at the start of
158			a pattern that set overall options that cannot be changed within the pattern.
159	ph10	99	.
160			.
161			.SH AUTHOR
162			.rs
163			.sp
164			.nf
165			Philip Hazel
166			University Computing Service
167			Cambridge CB2 3QH, England.
168			.fi
169			.
170			.
171			.SH REVISION
172			.rs
173			.sp
174			.nf
175	ph10	716	Last updated: 04 October 2011
176	ph10	596	Copyright (c) 1997-2011 University of Cambridge.
177	ph10	99	.fi
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svn:keywords	"Author Date Id Revision Url"