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Contents of /code/branches/pcre16/doc/pcrecompat.3

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Created a new branch for the development of 16-bit support.
3 PCRE - Perl-compatible regular expressions
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 .\" 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 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 .P
31 4. Though binary zero characters are supported in the subject string, they are
32 not allowed in a pattern string because it is passed as a normal C string,
33 terminated by zero. The escape sequence \e0 can be used in the pattern to
34 represent a binary zero.
35 .P
36 5. The following Perl escape sequences are not supported: \el, \eu, \eL,
37 \eU, and \eN when followed by a character name or Unicode value. (\eN on its
38 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 by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
42 \eU and \eu are interpreted as JavaScript interprets them.
43 .P
44 6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
45 built with Unicode character property support. The properties that can be
46 tested with \ep and \eP are limited to the general category properties such as
47 Lu and Nd, script names such as Greek or Han, and the derived properties Any
48 and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
49 Perl documentation says "Because Perl hides the need for the user to understand
50 the internal representation of Unicode characters, there is no need to
51 implement the somewhat messy concept of surrogates."
52 .P
53 7. PCRE implements a simpler version of \eX than Perl, which changed to make
54 \eX match what Unicode calls an "extended grapheme cluster". This is more
55 complicated than an extended Unicode sequence, which is what PCRE matches.
56 .P
57 8. PCRE does support the \eQ...\eE escape for quoting substrings. Characters in
58 between are treated as literals. This is slightly different from Perl in that $
59 and @ are also handled as literals inside the quotes. In Perl, they cause
60 variable interpolation (but of course PCRE does not have variables). Note the
61 following examples:
62 .sp
63 Pattern PCRE matches Perl matches
64 .sp
65 .\" JOIN
66 \eQabc$xyz\eE abc$xyz abc followed by the
67 contents of $xyz
68 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
69 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
70 .sp
71 The \eQ...\eE sequence is recognized both inside and outside character classes.
72 .P
73 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
74 constructions. However, there is support for recursive patterns. This is not
75 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
76 feature allows an external function to be called during pattern matching. See
77 the
78 .\" HREF
79 \fBpcrecallout\fP
80 .\"
81 documentation for details.
82 .P
83 10. Subpatterns that are called as subroutines (whether or not recursively) are
84 always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
85 Captured values that are set outside a subroutine call can be reference from
86 inside in PCRE, but not in Perl. There is a discussion that explains these
87 differences in more detail in the
88 .\" HTML <a href="pcrepattern.html#recursiondifference">
89 .\" </a>
90 section on recursion differences from Perl
91 .\"
92 in the
93 .\" HREF
94 \fBpcrepattern\fP
95 .\"
96 page.
97 .P
98 11. If (*THEN) is present in a group that is called as a subroutine, its action
99 is limited to that group, even if the group does not contain any | characters.
100 .P
101 12. There are some differences that are concerned with the settings of captured
102 strings when part of a pattern is repeated. For example, matching "aba" against
103 the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
104 .P
105 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
106 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 14. Perl recognizes comments in some places that PCRE does not, for example,
116 between the ( and ? at the start of a subpattern. If the /x modifier is set,
117 Perl allows whitespace between ( and ? but PCRE never does, even if the
118 PCRE_EXTENDED option is set.
119 .P
120 15. PCRE provides some extensions to the Perl regular expression facilities.
121 Perl 5.10 includes new features that are not in earlier versions of Perl, some
122 of which (such as named parentheses) have been in PCRE for some time. This list
123 is with respect to Perl 5.10:
124 .sp
125 (a) Although lookbehind assertions in PCRE must match fixed length strings,
126 each alternative branch of a lookbehind assertion can match a different length
127 of string. Perl requires them all to have the same length.
128 .sp
129 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
130 meta-character matches only at the very end of the string.
131 .sp
132 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
133 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
134 (Perl can be made to issue a warning.)
135 .sp
136 (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 .sp
140 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
141 only at the first matching position in the subject string.
142 .sp
144 PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
145 .sp
146 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
147 by the PCRE_BSR_ANYCRLF option.
148 .sp
149 (h) The callout facility is PCRE-specific.
150 .sp
151 (i) The partial matching facility is PCRE-specific.
152 .sp
153 (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
154 different hosts that have the other endianness. However, this does not apply to
155 optimized data created by the just-in-time compiler.
156 .sp
157 (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
158 different way and is not Perl-compatible.
159 .sp
160 (l) PCRE recognizes some special sequences such as (*CR) at the start of
161 a pattern that set overall options that cannot be changed within the pattern.
162 .
163 .
165 .rs
166 .sp
167 .nf
168 Philip Hazel
169 University Computing Service
170 Cambridge CB2 3QH, England.
171 .fi
172 .
173 .
175 .rs
176 .sp
177 .nf
178 Last updated: 14 November 2011
179 Copyright (c) 1997-2011 University of Cambridge.
180 .fi


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