Annotation of embedaddon/pcre/doc/html/pcrecompat.html, revision 1.1.1.5
1.1 misho 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>
16: DIFFERENCES BETWEEN PCRE AND PERL
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>
1.1.1.2 misho 24: 1. PCRE has only a subset of Perl's Unicode support. Details of what it does
25: have are given in the
1.1 misho 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
1.1.1.4 misho 39: counted, but their entries in the offsets vector are never set. Perl sometimes
40: (but not always) sets its numerical variables from inside negative assertions.
1.1 misho 41: </P>
42: <P>
43: 4. Though binary zero characters are supported in the subject string, they are
44: not allowed in a pattern string because it is passed as a normal C string,
45: terminated by zero. The escape sequence \0 can be used in the pattern to
46: represent a binary zero.
47: </P>
48: <P>
49: 5. The following Perl escape sequences are not supported: \l, \u, \L,
50: \U, and \N when followed by a character name or Unicode value. (\N on its
51: own, matching a non-newline character, is supported.) In fact these are
52: implemented by Perl's general string-handling and are not part of its pattern
53: matching engine. If any of these are encountered by PCRE, an error is
54: generated by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
55: \U and \u are interpreted as JavaScript interprets them.
56: </P>
57: <P>
58: 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE is
59: built with Unicode character property support. The properties that can be
60: tested with \p and \P are limited to the general category properties such as
61: Lu and Nd, script names such as Greek or Han, and the derived properties Any
62: and L&. PCRE does support the Cs (surrogate) property, which Perl does not; the
63: Perl documentation says "Because Perl hides the need for the user to understand
64: the internal representation of Unicode characters, there is no need to
65: implement the somewhat messy concept of surrogates."
66: </P>
67: <P>
1.1.1.4 misho 68: 7. PCRE does support the \Q...\E escape for quoting substrings. Characters in
1.1 misho 69: between are treated as literals. This is slightly different from Perl in that $
70: and @ are also handled as literals inside the quotes. In Perl, they cause
71: variable interpolation (but of course PCRE does not have variables). Note the
72: following examples:
73: <pre>
74: Pattern PCRE matches Perl matches
75:
76: \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
77: \Qabc\$xyz\E abc\$xyz abc\$xyz
78: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
79: </pre>
80: The \Q...\E sequence is recognized both inside and outside character classes.
81: </P>
82: <P>
1.1.1.4 misho 83: 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
1.1 misho 84: constructions. However, there is support for recursive patterns. This is not
85: available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE "callout"
86: feature allows an external function to be called during pattern matching. See
87: the
88: <a href="pcrecallout.html"><b>pcrecallout</b></a>
89: documentation for details.
90: </P>
91: <P>
1.1.1.4 misho 92: 9. Subpatterns that are called as subroutines (whether or not recursively) are
1.1 misho 93: always treated as atomic groups in PCRE. This is like Python, but unlike Perl.
94: Captured values that are set outside a subroutine call can be reference from
95: inside in PCRE, but not in Perl. There is a discussion that explains these
96: differences in more detail in the
97: <a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
98: in the
99: <a href="pcrepattern.html"><b>pcrepattern</b></a>
100: page.
101: </P>
102: <P>
1.1.1.4 misho 103: 10. If any of the backtracking control verbs are used in a subpattern that is
104: called as a subroutine (whether or not recursively), their effect is confined
105: to that subpattern; it does not extend to the surrounding pattern. This is not
106: always the case in Perl. In particular, if (*THEN) is present in a group that
107: is called as a subroutine, its action is limited to that group, even if the
108: group does not contain any | characters. Note that such subpatterns are
109: processed as anchored at the point where they are tested.
110: </P>
111: <P>
112: 11. If a pattern contains more than one backtracking control verb, the first
113: one that is backtracked onto acts. For example, in the pattern
114: A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure in C
115: triggers (*PRUNE). Perl's behaviour is more complex; in many cases it is the
116: same as PCRE, but there are examples where it differs.
1.1 misho 117: </P>
118: <P>
1.1.1.4 misho 119: 12. Most backtracking verbs in assertions have their normal actions. They are
120: not confined to the assertion.
121: </P>
122: <P>
123: 13. There are some differences that are concerned with the settings of captured
1.1 misho 124: strings when part of a pattern is repeated. For example, matching "aba" against
125: the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
126: </P>
127: <P>
1.1.1.4 misho 128: 14. PCRE's handling of duplicate subpattern numbers and duplicate subpattern
1.1 misho 129: names is not as general as Perl's. This is a consequence of the fact the PCRE
130: works internally just with numbers, using an external table to translate
131: between numbers and names. In particular, a pattern such as (?|(?<a>A)|(?<b)B),
132: where the two capturing parentheses have the same number but different names,
133: is not supported, and causes an error at compile time. If it were allowed, it
134: would not be possible to distinguish which parentheses matched, because both
135: names map to capturing subpattern number 1. To avoid this confusing situation,
136: an error is given at compile time.
137: </P>
138: <P>
1.1.1.4 misho 139: 15. Perl recognizes comments in some places that PCRE does not, for example,
1.1 misho 140: between the ( and ? at the start of a subpattern. If the /x modifier is set,
1.1.1.5 ! misho 141: Perl allows white space between ( and ? (though current Perls warn that this is
! 142: deprecated) but PCRE never does, even if the PCRE_EXTENDED option is set.
1.1 misho 143: </P>
144: <P>
1.1.1.5 ! misho 145: 16. Perl, when in warning mode, gives warnings for character classes such as
! 146: [A-\d] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE has no
! 147: warning features, so it gives an error in these cases because they are almost
! 148: certainly user mistakes.
! 149: </P>
! 150: <P>
! 151: 17. In PCRE, the upper/lower case character properties Lu and Ll are not
1.1.1.4 misho 152: affected when case-independent matching is specified. For example, \p{Lu}
153: always matches an upper case letter. I think Perl has changed in this respect;
154: in the release at the time of writing (5.16), \p{Lu} and \p{Ll} match all
155: letters, regardless of case, when case independence is specified.
156: </P>
157: <P>
1.1.1.5 ! misho 158: 18. PCRE provides some extensions to the Perl regular expression facilities.
1.1 misho 159: Perl 5.10 includes new features that are not in earlier versions of Perl, some
160: of which (such as named parentheses) have been in PCRE for some time. This list
161: is with respect to Perl 5.10:
162: <br>
163: <br>
164: (a) Although lookbehind assertions in PCRE must match fixed length strings,
165: each alternative branch of a lookbehind assertion can match a different length
166: of string. Perl requires them all to have the same length.
167: <br>
168: <br>
169: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
170: meta-character matches only at the very end of the string.
171: <br>
172: <br>
173: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
174: meaning is faulted. Otherwise, like Perl, the backslash is quietly ignored.
175: (Perl can be made to issue a warning.)
176: <br>
177: <br>
178: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
179: inverted, that is, by default they are not greedy, but if followed by a
180: question mark they are.
181: <br>
182: <br>
183: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be tried
184: only at the first matching position in the subject string.
185: <br>
186: <br>
187: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
188: PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
189: <br>
190: <br>
191: (g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
192: by the PCRE_BSR_ANYCRLF option.
193: <br>
194: <br>
195: (h) The callout facility is PCRE-specific.
196: <br>
197: <br>
198: (i) The partial matching facility is PCRE-specific.
199: <br>
200: <br>
201: (j) Patterns compiled by PCRE can be saved and re-used at a later time, even on
202: different hosts that have the other endianness. However, this does not apply to
203: optimized data created by the just-in-time compiler.
204: <br>
205: <br>
1.1.1.4 misho 206: (k) The alternative matching functions (<b>pcre_dfa_exec()</b>,
207: <b>pcre16_dfa_exec()</b> and <b>pcre32_dfa_exec()</b>,) match in a different way
208: and are not Perl-compatible.
1.1 misho 209: <br>
210: <br>
211: (l) PCRE recognizes some special sequences such as (*CR) at the start of
212: a pattern that set overall options that cannot be changed within the pattern.
213: </P>
214: <br><b>
215: AUTHOR
216: </b><br>
217: <P>
218: Philip Hazel
219: <br>
220: University Computing Service
221: <br>
222: Cambridge CB2 3QH, England.
223: <br>
224: </P>
225: <br><b>
226: REVISION
227: </b><br>
228: <P>
1.1.1.5 ! misho 229: Last updated: 10 November 2013
1.1 misho 230: <br>
1.1.1.4 misho 231: Copyright © 1997-2013 University of Cambridge.
1.1 misho 232: <br>
233: <p>
234: Return to the <a href="index.html">PCRE index page</a>.
235: </p>
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