trunk/doc/pcrecompat.3

.TH PCRECOMPAT 3 "10 November 2013" "PCRE 8.34"
.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 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 sometimes
(but not always) sets its numerical variables from inside negative assertions.
.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 by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
\eU and \eu are interpreted as JavaScript interprets them.
.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 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
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
.\" HREF
\fBpcrecallout\fP
.\"
documentation for details.
.P
9. 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
.\" HTML <a href="pcrepattern.html#recursiondifference">
.\" </a>
section on recursion differences from Perl
.\"
in the
.\" HREF
\fBpcrepattern\fP
.\"
page.
.P
10. If any of the backtracking control verbs are used 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. Note that such subpatterns are
processed as anchored at the point where they are tested.
.P
11. If a pattern contains more than one backtracking control verb, the first
one that is backtracked onto acts. For example, in the pattern
A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
same as PCRE, but there are examples where it differs.
.P
12. Most backtracking verbs in assertions have their normal actions. They are
not confined to the assertion.
.P
13. 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
14. 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
15. 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 ? (though current Perls warn that this is
deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set.
.P
16. Perl, when in warning mode, gives warnings for character classes such as
[A-\ed] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE has no
warning features, so it gives an error in these cases because they are almost
certainly user mistakes.
.P
17. In PCRE, the upper/lower case character properties Lu and Ll are not
affected when case-independent matching is specified. For example, \ep{Lu}
always matches an upper case letter. I think Perl has changed in this respect;
in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all
letters, regardless of case, when case independence is specified.
.P
18. 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 functions (\fBpcre_dfa_exec()\fP,
\fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP,) match in a different way
and are 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: 10 November 2013
Copyright (c) 1997-2013 University of Cambridge.
.fi
1	.TH PCRECOMPAT 3 "10 November 2013" "PCRE 8.34"
2	.SH NAME
3	PCRE - Perl-compatible regular expressions
4	.SH "DIFFERENCES BETWEEN PCRE AND PERL"
5	.rs
6	.sp
7	This document describes the differences in the ways that PCRE and Perl handle
8	regular expressions. The differences described here are with respect to Perl
9	versions 5.10 and above.
10	.P
11	1. PCRE has only a subset of Perl's Unicode support. Details of what it does
12	have are given in the
13	.\" HREF
14	\fBpcreunicode\fP
15	.\"
16	page.
17	.P
18	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	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	these do not seem to have any use.
24	.P
25	3. Capturing subpatterns that occur inside negative lookahead assertions are
26	counted, but their entries in the offsets vector are never set. Perl sometimes
27	(but not always) sets its numerical variables from inside negative assertions.
28	.P
29	4. Though binary zero characters are supported in the subject string, they are
30	not allowed in a pattern string because it is passed as a normal C string,
31	terminated by zero. The escape sequence \e0 can be used in the pattern to
32	represent a binary zero.
33	.P
34	5. The following Perl escape sequences are not supported: \el, \eu, \eL,
35	\eU, and \eN when followed by a character name or Unicode value. (\eN on its
36	own, matching a non-newline character, is supported.) In fact these are
37	implemented by Perl's general string-handling and are not part of its pattern
38	matching engine. If any of these are encountered by PCRE, an error is
39	generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
40	\eU and \eu are interpreted as JavaScript interprets them.
41	.P
42	6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
43	built with Unicode character property support. The properties that can be
44	tested with \ep and \eP are limited to the general category properties such as
45	Lu and Nd, script names such as Greek or Han, and the derived properties Any
46	and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
47	Perl documentation says "Because Perl hides the need for the user to understand
48	the internal representation of Unicode characters, there is no need to
49	implement the somewhat messy concept of surrogates."
50	.P
51	7. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
52	between are treated as literals. This is slightly different from Perl in that $
53	and @ are also handled as literals inside the quotes. In Perl, they cause
54	variable interpolation (but of course PCRE does not have variables). Note the
55	following examples:
56	.sp
57	Pattern PCRE matches Perl matches
58	.sp
59	.\" JOIN
60	\eQabc$xyz\eE abc$xyz abc followed by the
61	contents of $xyz
62	\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
63	\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
64	.sp
65	The \eQ...\eE sequence is recognized both inside and outside character classes.
66	.P
67	8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
68	constructions. However, there is support for recursive patterns. This is not
69	available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
70	feature allows an external function to be called during pattern matching. See
71	the
72	.\" HREF
73	\fBpcrecallout\fP
74	.\"
75	documentation for details.
76	.P
77	9. Subpatterns that are called as subroutines (whether or not recursively) are
78	always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
79	Captured values that are set outside a subroutine call can be reference from
80	inside in PCRE, but not in Perl. There is a discussion that explains these
81	differences in more detail in the
82	.\" HTML <a href="pcrepattern.html#recursiondifference">
83	.\" </a>
84	section on recursion differences from Perl
85	.\"
86	in the
87	.\" HREF
88	\fBpcrepattern\fP
89	.\"
90	page.
91	.P
92	10. If any of the backtracking control verbs are used in a subpattern that is
93	called as a subroutine (whether or not recursively), their effect is confined
94	to that subpattern; it does not extend to the surrounding pattern. This is not
95	always the case in Perl. In particular, if (*THEN) is present in a group that
96	is called as a subroutine, its action is limited to that group, even if the
97	group does not contain any \| characters. Note that such subpatterns are
98	processed as anchored at the point where they are tested.
99	.P
100	11. If a pattern contains more than one backtracking control verb, the first
101	one that is backtracked onto acts. For example, in the pattern
102	A(COMMIT)B(PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
103	triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
104	same as PCRE, but there are examples where it differs.
105	.P
106	12. Most backtracking verbs in assertions have their normal actions. They are
107	not confined to the assertion.
108	.P
109	13. There are some differences that are concerned with the settings of captured
110	strings when part of a pattern is repeated. For example, matching "aba" against
111	the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
112	.P
113	14. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
114	names is not as general as Perl's. This is a consequence of the fact the PCRE
115	works internally just with numbers, using an external table to translate
116	between numbers and names. In particular, a pattern such as (?\|(?<a>A)\|(?<b)B),
117	where the two capturing parentheses have the same number but different names,
118	is not supported, and causes an error at compile time. If it were allowed, it
119	would not be possible to distinguish which parentheses matched, because both
120	names map to capturing subpattern number 1. To avoid this confusing situation,
121	an error is given at compile time.
122	.P
123	15. Perl recognizes comments in some places that PCRE does not, for example,
124	between the ( and ? at the start of a subpattern. If the /x modifier is set,
125	Perl allows white space between ( and ? (though current Perls warn that this is
126	deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set.
127	.P
128	16. Perl, when in warning mode, gives warnings for character classes such as
129	[A-\ed] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE has no
130	warning features, so it gives an error in these cases because they are almost
131	certainly user mistakes.
132	.P
133	17. In PCRE, the upper/lower case character properties Lu and Ll are not
134	affected when case-independent matching is specified. For example, \ep{Lu}
135	always matches an upper case letter. I think Perl has changed in this respect;
136	in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all
137	letters, regardless of case, when case independence is specified.
138	.P
139	18. PCRE provides some extensions to the Perl regular expression facilities.
140	Perl 5.10 includes new features that are not in earlier versions of Perl, some
141	of which (such as named parentheses) have been in PCRE for some time. This list
142	is with respect to Perl 5.10:
143	.sp
144	(a) Although lookbehind assertions in PCRE must match fixed length strings,
145	each alternative branch of a lookbehind assertion can match a different length
146	of string. Perl requires them all to have the same length.
147	.sp
148	(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
149	meta-character matches only at the very end of the string.
150	.sp
151	(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
152	meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
153	(Perl can be made to issue a warning.)
154	.sp
155	(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
156	inverted, that is, by default they are not greedy, but if followed by a
157	question mark they are.
158	.sp
159	(e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
160	only at the first matching position in the subject string.
161	.sp
162	(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
163	PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
164	.sp
165	(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
166	by the PCRE_BSR_ANYCRLF option.
167	.sp
168	(h) The callout facility is PCRE-specific.
169	.sp
170	(i) The partial matching facility is PCRE-specific.
171	.sp
172	(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
173	different hosts that have the other endianness. However, this does not apply to
174	optimized data created by the just-in-time compiler.
175	.sp
176	(k) The alternative matching functions (\fBpcre_dfa_exec()\fP,
177	\fBpcre16_dfa_exec()\fP and \fBpcre32_dfa_exec()\fP,) match in a different way
178	and are not Perl-compatible.
179	.sp
180	(l) PCRE recognizes some special sequences such as (*CR) at the start of
181	a pattern that set overall options that cannot be changed within the pattern.
182	.
183	.
184	.SH AUTHOR
185	.rs
186	.sp
187	.nf
188	Philip Hazel
189	University Computing Service
190	Cambridge CB2 3QH, England.
191	.fi
192	.
193	.
194	.SH REVISION
195	.rs
196	.sp
197	.nf
198	Last updated: 10 November 2013
199	Copyright (c) 1997-2013 University of Cambridge.
200	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"