pcre-5.0/doc/pcreperform.3

.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH "PCRE PERFORMANCE"
.rs
.sp
Certain items that may appear in regular expression patterns are more efficient
than others. It is more efficient to use a character class like [aeiou] than a
set of alternatives such as (a|e|i|o|u). In general, the simplest construction
that provides the required behaviour is usually the most efficient. Jeffrey
Friedl's book contains a lot of useful general discussion about optimizing
regular expressions for efficient performance. This document contains a few
observations about PCRE.
.P
Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,
because PCRE has to scan a structure that contains data for over fifteen
thousand characters whenever it needs a character's property. If you can find
an alternative pattern that does not use character properties, it will probably
be faster.
.P
When a pattern begins with .* not in parentheses, or in parentheses that are
not the subject of a backreference, and the PCRE_DOTALL option is set, the
pattern is implicitly anchored by PCRE, since it can match only at the start of
a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
optimization, because the . metacharacter does not then match a newline, and if
the subject string contains newlines, the pattern may match from the character
immediately following one of them instead of from the very start. For example,
the pattern
.sp
  .*second
.sp
matches the subject "first\enand second" (where \en stands for a newline
character), with the match starting at the seventh character. In order to do
this, PCRE has to retry the match starting after every newline in the subject.
.P
If you are using such a pattern with subject strings that do not contain
newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
having to scan along the subject looking for a newline to restart at.
.P
Beware of patterns that contain nested indefinite repeats. These can take a
long time to run when applied to a string that does not match. Consider the
pattern fragment
.sp
  (a+)*
.sp
This can match "aaaa" in 33 different ways, and this number increases very
rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
times, and for each of those cases other than 0, the + repeats can match
different numbers of times.) When the remainder of the pattern is such that the
entire match is going to fail, PCRE has in principle to try every possible
variation, and this can take an extremely long time.
.P
An optimization catches some of the more simple cases such as
.sp
  (a+)*b
.sp
where a literal character follows. Before embarking on the standard matching
procedure, PCRE checks that there is a "b" later in the subject string, and if
there is not, it fails the match immediately. However, when there is no
following literal this optimization cannot be used. You can see the difference
by comparing the behaviour of
.sp
  (a+)*\ed
.sp
with the pattern above. The former gives a failure almost instantly when
applied to a whole line of "a" characters, whereas the latter takes an
appreciable time with strings longer than about 20 characters.
.P
In many cases, the solution to this kind of performance issue is to use an
atomic group or a possessive quantifier.
.P
.in 0
Last updated: 09 September 2004
.br
Copyright (c) 1997-2004 University of Cambridge.
1	.TH PCRE 3
2	.SH NAME
3	PCRE - Perl-compatible regular expressions
4	.SH "PCRE PERFORMANCE"
5	.rs
6	.sp
7	Certain items that may appear in regular expression patterns are more efficient
8	than others. It is more efficient to use a character class like [aeiou] than a
9	set of alternatives such as (a\|e\|i\|o\|u). In general, the simplest construction
10	that provides the required behaviour is usually the most efficient. Jeffrey
11	Friedl's book contains a lot of useful general discussion about optimizing
12	regular expressions for efficient performance. This document contains a few
13	observations about PCRE.
14	.P
15	Using Unicode character properties (the \ep, \eP, and \eX escapes) is slow,
16	because PCRE has to scan a structure that contains data for over fifteen
17	thousand characters whenever it needs a character's property. If you can find
18	an alternative pattern that does not use character properties, it will probably
19	be faster.
20	.P
21	When a pattern begins with .* not in parentheses, or in parentheses that are
22	not the subject of a backreference, and the PCRE_DOTALL option is set, the
23	pattern is implicitly anchored by PCRE, since it can match only at the start of
24	a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
25	optimization, because the . metacharacter does not then match a newline, and if
26	the subject string contains newlines, the pattern may match from the character
27	immediately following one of them instead of from the very start. For example,
28	the pattern
29	.sp
30	.*second
31	.sp
32	matches the subject "first\enand second" (where \en stands for a newline
33	character), with the match starting at the seventh character. In order to do
34	this, PCRE has to retry the match starting after every newline in the subject.
35	.P
36	If you are using such a pattern with subject strings that do not contain
37	newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
38	the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
39	having to scan along the subject looking for a newline to restart at.
40	.P
41	Beware of patterns that contain nested indefinite repeats. These can take a
42	long time to run when applied to a string that does not match. Consider the
43	pattern fragment
44	.sp
45	(a+)*
46	.sp
47	This can match "aaaa" in 33 different ways, and this number increases very
48	rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
49	times, and for each of those cases other than 0, the + repeats can match
50	different numbers of times.) When the remainder of the pattern is such that the
51	entire match is going to fail, PCRE has in principle to try every possible
52	variation, and this can take an extremely long time.
53	.P
54	An optimization catches some of the more simple cases such as
55	.sp
56	(a+)*b
57	.sp
58	where a literal character follows. Before embarking on the standard matching
59	procedure, PCRE checks that there is a "b" later in the subject string, and if
60	there is not, it fails the match immediately. However, when there is no
61	following literal this optimization cannot be used. You can see the difference
62	by comparing the behaviour of
63	.sp
64	(a+)*\ed
65	.sp
66	with the pattern above. The former gives a failure almost instantly when
67	applied to a whole line of "a" characters, whereas the latter takes an
68	appreciable time with strings longer than about 20 characters.
69	.P
70	In many cases, the solution to this kind of performance issue is to use an
71	atomic group or a possessive quantifier.
72	.P
73	.in 0
74	Last updated: 09 September 2004
75	.br
76	Copyright (c) 1997-2004 University of Cambridge.