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revision 63 by nigel, Sat Feb 24 21:40:03 2007 UTC revision 654 by ph10, Tue Aug 2 11:00:40 2011 UTC
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1  .TH PCRE 3  .TH PCRECOMPAT 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH DIFFERENCES FROM PERL  .SH "DIFFERENCES BETWEEN PCRE AND PERL"
5  .rs  .rs
6  .sp  .sp
7  This document describes the differences in the ways that PCRE and Perl handle  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  regular expressions. The differences described here are with respect to Perl
9  5.8.  versions 5.10 and above.
10    .P
11  1. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits  1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
12  them, but they do not mean what you might think. For example, (?!a){3} does  it does have are given in the
13  not assert that the next three characters are not "a". It just asserts that the  .\" HTML <a href="pcre.html#utf8support">
14  next character is not "a" three times.  .\" </a>
15    section on UTF-8 support
16  2. Capturing subpatterns that occur inside negative lookahead assertions are  .\"
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  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  numerical variables from any such patterns that are matched before the
33  assertion fails to match something (thereby succeeding), but only if the  assertion fails to match something (thereby succeeding), but only if the
34  negative lookahead assertion contains just one branch.  negative lookahead assertion contains just one branch.
35    .P
36  3. Though binary zero characters are supported in the subject string, they are  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,  not allowed in a pattern string because it is passed as a normal C string,
38  terminated by zero. The escape sequence "\\0" can be used in the pattern to  terminated by zero. The escape sequence \e0 can be used in the pattern to
39  represent a binary zero.  represent a binary zero.
40    .P
41  4. The following Perl escape sequences are not supported: \\l, \\u, \\L,  5. The following Perl escape sequences are not supported: \el, \eu, \eL,
42  \\U, \\P, \\p, \N, and \\X. In fact these are implemented by Perl's general  \eU, and \eN when followed by a character name or Unicode value. (\eN on its
43  string-handling and are not part of its pattern matching engine. If any of  own, matching a non-newline character, is supported.) In fact these are
44  these are encountered by PCRE, an error is generated.  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  5. PCRE does support the \\Q...\\E escape for quoting substrings. Characters in  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 $  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  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  variable interpolation (but of course PCRE does not have variables). Note the
65  following examples:  following examples:
66    .sp
67      Pattern            PCRE matches      Perl matches      Pattern            PCRE matches      Perl matches
68    .sp
69      \\Qabc$xyz\\E        abc$xyz           abc followed by the  .\" JOIN
70        \eQabc$xyz\eE        abc$xyz           abc followed by the
71                                             contents of $xyz                                             contents of $xyz
72      \\Qabc\\$xyz\\E       abc\\$xyz          abc\\$xyz      \eQabc\e$xyz\eE       abc\e$xyz          abc\e$xyz
73      \\Qabc\\E\\$\\Qxyz\\E   abc$xyz           abc$xyz      \eQabc\eE\e$\eQxyz\eE   abc$xyz           abc$xyz
74    .sp
75  In PCRE, the \\Q...\\E mechanism is not recognized inside a character class.  The \eQ...\eE sequence is recognized both inside and outside character classes.
76    .P
77  8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})  9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
78  constructions. However, there is some experimental support for recursive  constructions. However, there is support for recursive patterns. This is not
79  patterns using the non-Perl items (?R), (?number) and (?P>name). Also, the PCRE  available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
80  "callout" feature allows an external function to be called during pattern  feature allows an external function to be called during pattern matching. See
81  matching.  the
82    .\" HREF
83  9. There are some differences that are concerned with the settings of captured  \fBpcrecallout\fP
84    .\"
85    documentation for details.
86    .P
87    10. Subpatterns that are called recursively or as "subroutines" are always
88    treated as atomic groups in PCRE. This is like Python, but unlike Perl. There
89    is a discussion of an example that explains this in more detail in the
90    .\" HTML <a href="pcrepattern.html#recursiondifference">
91    .\" </a>
92    section on recursion differences from Perl
93    .\"
94    in the
95    .\" HREF
96    \fBpcrepattern\fP
97    .\"
98    page.
99    .P
100    11. There are some differences that are concerned with the settings of captured
101  strings when part of a pattern is repeated. For example, matching "aba" against  strings when part of a pattern is repeated. For example, matching "aba" against
102  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".  the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
103    .P
104  10. PCRE provides some extensions to the Perl regular expression facilities:  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  (a) Although lookbehind assertions must match fixed length strings, each  works internally just with numbers, using an external table to translate
107  alternative branch of a lookbehind assertion can match a different length of  between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
108  string. Perl requires them all to have the same length.  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 $  (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.  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  (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
132  meaning is faulted.  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  (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  inverted, that is, by default they are not greedy, but if followed by a
137  question mark they are.  question mark they are.
138    .sp
139  (e) PCRE_ANCHORED can be used to force a pattern to be tried only at the first  (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
140  matching position in the subject string.  only at the first matching position in the subject string.
141    .sp
142  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE  (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
143  options for \fBpcre_exec()\fR have no Perl equivalents.  PCRE_NO_AUTO_CAPTURE options for \fBpcre_exec()\fP have no Perl equivalents.
144    .sp
145  (g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern  (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
146  matching (Perl can do this using the (?p{code}) construct, which PCRE cannot  by the PCRE_BSR_ANYCRLF option.
147  support.)  .sp
148    (h) The callout facility is PCRE-specific.
149  (h) PCRE supports named capturing substrings, using the Python syntax.  .sp
150    (i) The partial matching facility is PCRE-specific.
151  (i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java  .sp
152  package.  (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  (j) The (R) condition, for testing recursion, is a PCRE extension.  .sp
155    (k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
156  (k) The callout facility is PCRE-specific.  different way and is not Perl-compatible.
157    .sp
158  .in 0  (l) PCRE recognizes some special sequences such as (*CR) at the start of
159  Last updated: 03 February 2003  a pattern that set overall options that cannot be changed within the pattern.
160  .br  .
161  Copyright (c) 1997-2003 University of Cambridge.  .
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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