Annotation of embedaddon/pcre/doc/pcrecompat.3, revision 1.1.1.1
1.1 misho 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: .\" 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
143: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
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: .
164: .SH AUTHOR
165: .rs
166: .sp
167: .nf
168: Philip Hazel
169: University Computing Service
170: Cambridge CB2 3QH, England.
171: .fi
172: .
173: .
174: .SH REVISION
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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