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1 <html>
2 <head>
3 <title>pcrecompat specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrecompat man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 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 <br>
15 <br><b>
17 </b><br>
18 <P>
19 This document describes the differences in the ways that PCRE and Perl handle
20 regular expressions. The differences described here are with respect to Perl
21 versions 5.10 and above.
22 </P>
23 <P>
24 1. PCRE has only a subset of Perl's Unicode support. Details of what it does
25 have are given in the
26 <a href="pcreunicode.html"><b>pcreunicode</b></a>
27 page.
28 </P>
29 <P>
30 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 </P>
37 <P>
38 3. Capturing subpatterns that occur inside negative lookahead assertions are
39 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 4. Though binary zero characters are supported in the subject string, they are
46 not allowed in a pattern string because it is passed as a normal C string,
47 terminated by zero. The escape sequence \0 can be used in the pattern to
48 represent a binary zero.
49 </P>
50 <P>
51 5. The following Perl escape sequences are not supported: \l, \u, \L,
52 \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 generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
57 \U and \u are interpreted as JavaScript interprets them.
58 </P>
59 <P>
60 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 Lu and Nd, script names such as Greek or Han, and the derived properties Any
64 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 </P>
69 <P>
70 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 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
83 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
84 \Qabc\$xyz\E abc\$xyz abc\$xyz
85 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
86 </pre>
87 The \Q...\E sequence is recognized both inside and outside character classes.
88 </P>
89 <P>
90 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
91 constructions. However, there is support for recursive patterns. This is not
92 available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
93 feature allows an external function to be called during pattern matching. See
94 the
95 <a href="pcrecallout.html"><b>pcrecallout</b></a>
96 documentation for details.
97 </P>
98 <P>
99 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 <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
105 in the
106 <a href="pcrepattern.html"><b>pcrepattern</b></a>
107 page.
108 </P>
109 <P>
110 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 </P>
121 <P>
122 12. There are some differences that are concerned with the settings of captured
123 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 13. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
128 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 (?|(?&#60;a&#62;A)|(?&#60;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 14. Perl recognizes comments in some places that PCRE does not, for example,
139 between the ( and ? at the start of a subpattern. If the /x modifier is set,
140 Perl allows white space between ( and ? but PCRE never does, even if the
141 PCRE_EXTENDED option is set.
142 </P>
143 <P>
144 15. PCRE provides some extensions to the Perl regular expression facilities.
145 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 <br>
149 <br>
150 (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 <br>
154 <br>
155 (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 <br>
158 <br>
159 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
160 meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
161 (Perl can be made to issue a warning.)
162 <br>
163 <br>
164 (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 <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>
174 PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
175 <br>
176 <br>
177 (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
178 by the PCRE_BSR_ANYCRLF option.
179 <br>
180 <br>
181 (h) The callout facility is PCRE-specific.
182 <br>
183 <br>
184 (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 different hosts that have the other endianness. However, this does not apply to
189 optimized data created by the just-in-time compiler.
190 <br>
191 <br>
192 (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 <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 </P>
199 <br><b>
201 </b><br>
202 <P>
203 Philip Hazel
204 <br>
205 University Computing Service
206 <br>
207 Cambridge CB2 3QH, England.
208 <br>
209 </P>
210 <br><b>
212 </b><br>
213 <P>
214 Last updated: 01 June 2012
215 <br>
216 Copyright &copy; 1997-2012 University of Cambridge.
217 <br>
218 <p>
219 Return to the <a href="index.html">PCRE index page</a>.
220 </p>


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