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Revision 654 - (show annotations)
Tue Aug 2 11:00:40 2011 UTC (8 years ago) by ph10
File size: 7533 byte(s)
Documentation and general text tidies in preparation for test release.
1 .TH PCRECOMPAT 3
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 UTF-8 and Unicode support. Details of what
12 it does have are given in the
13 .\" HTML <a href="pcre.html#utf8support">
14 .\" </a>
15 section on UTF-8 support
16 .\"
17 in the main
18 .\" HREF
19 \fBpcre\fP
20 .\"
21 page.
22 .P
23 2. PCRE allows repeat quantifiers only on parenthesized assertions, but they do
24 not mean what you might think. For example, (?!a){3} does not assert that the
25 next three characters are not "a". It just asserts that the next character is
26 not "a" three times (in principle: PCRE optimizes this to run the assertion
27 just once). Perl allows repeat quantifiers on other assertions such as \eb, but
28 these do not seem to have any use.
29 .P
30 3. Capturing subpatterns that occur inside negative lookahead assertions are
31 counted, but their entries in the offsets vector are never set. Perl sets its
32 numerical variables from any such patterns that are matched before the
33 assertion fails to match something (thereby succeeding), but only if the
34 negative lookahead assertion contains just one branch.
35 .P
36 4. Though binary zero characters are supported in the subject string, they are
37 not allowed in a pattern string because it is passed as a normal C string,
38 terminated by zero. The escape sequence \e0 can be used in the pattern to
39 represent a binary zero.
40 .P
41 5. The following Perl escape sequences are not supported: \el, \eu, \eL,
42 \eU, and \eN when followed by a character name or Unicode value. (\eN on its
43 own, matching a non-newline character, is supported.) In fact these are
44 implemented by Perl's general string-handling and are not part of its pattern
45 matching engine. If any of these are encountered by PCRE, an error is
46 generated.
47 .P
48 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
49 built with Unicode character property support. The properties that can be
50 tested with \ep and \eP are limited to the general category properties such as
51 Lu and Nd, script names such as Greek or Han, and the derived properties Any
52 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
53 Perl documentation says "Because Perl hides the need for the user to understand
54 the internal representation of Unicode characters, there is no need to
55 implement the somewhat messy concept of surrogates."
56 .P
57 7. PCRE implements a simpler version of \eX than Perl, which changed to make
58 \eX match what Unicode calls an "extended grapheme cluster". This is more
59 complicated than an extended Unicode sequence, which is what PCRE matches.
60 .P
61 8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
62 between are treated as literals. This is slightly different from Perl in that $
63 and @ are also handled as literals inside the quotes. In Perl, they cause
64 variable interpolation (but of course PCRE does not have variables). Note the
65 following examples:
66 .sp
67 Pattern PCRE matches Perl matches
68 .sp
69 .\" JOIN
70 \eQabc$xyz\eE abc$xyz abc followed by the
71 contents of $xyz
72 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
73 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
74 .sp
75 The \eQ...\eE sequence is recognized both inside and outside character classes.
76 .P
77 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
78 constructions. However, there is support for recursive patterns. This is not
79 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
80 feature allows an external function to be called during pattern matching. See
81 the
82 .\" HREF
83 \fBpcrecallout\fP
84 .\"
85 documentation for details.
86 .P
87 10. Subpatterns that are called recursively or as "subroutines" are always
88 treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
89 is a discussion of an example that explains this in more detail in the
90 .\" HTML <a href="pcrepattern.html#recursiondifference">
91 .\" </a>
92 section on recursion differences from Perl
93 .\"
94 in the
95 .\" HREF
96 \fBpcrepattern\fP
97 .\"
98 page.
99 .P
100 11. There are some differences that are concerned with the settings of captured
101 strings when part of a pattern is repeated. For example, matching "aba" against
102 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
103 .P
104 12. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
105 names is not as general as Perl's. This is a consequence of the fact the PCRE
106 works internally just with numbers, using an external table to translate
107 between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
108 where the two capturing parentheses have the same number but different names,
109 is not supported, and causes an error at compile time. If it were allowed, it
110 would not be possible to distinguish which parentheses matched, because both
111 names map to capturing subpattern number 1. To avoid this confusing situation,
112 an error is given at compile time.
113 .P
114 13. Perl recognizes comments in some places that PCRE does not, for example,
115 between the ( and ? at the start of a subpattern. If the /x modifier is set,
116 Perl allows whitespace between ( and ? but PCRE never does, even if the
117 PCRE_EXTENDED option is set.
118 .P
119 14. PCRE provides some extensions to the Perl regular expression facilities.
120 Perl 5.10 includes new features that are not in earlier versions of Perl, some
121 of which (such as named parentheses) have been in PCRE for some time. This list
122 is with respect to Perl 5.10:
123 .sp
124 (a) Although lookbehind assertions in PCRE must match fixed length strings,
125 each alternative branch of a lookbehind assertion can match a different length
126 of string. Perl requires them all to have the same length.
127 .sp
128 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
129 meta-character matches only at the very end of the string.
130 .sp
131 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
132 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
133 (Perl can be made to issue a warning.)
134 .sp
135 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
136 inverted, that is, by default they are not greedy, but if followed by a
137 question mark they are.
138 .sp
139 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
140 only at the first matching position in the subject string.
141 .sp
142 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
143 PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
144 .sp
145 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
146 by the PCRE_BSR_ANYCRLF option.
147 .sp
148 (h) The callout facility is PCRE-specific.
149 .sp
150 (i) The partial matching facility is PCRE-specific.
151 .sp
152 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
153 different hosts that have the other endianness.
154 .sp
155 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
156 different way and is not Perl-compatible.
157 .sp
158 (l) PCRE recognizes some special sequences such as (*CR) at the start of
159 a pattern that set overall options that cannot be changed within the pattern.
160 .
161 .
162 .SH AUTHOR
163 .rs
164 .sp
165 .nf
166 Philip Hazel
167 University Computing Service
168 Cambridge CB2 3QH, England.
169 .fi
170 .
171 .
172 .SH REVISION
173 .rs
174 .sp
175 .nf
176 Last updated: 24 July 2011
177 Copyright (c) 1997-2011 University of Cambridge.
178 .fi

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