trunk/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 discussion about optimizing regular expressions
for efficient performance.

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

  .*second

matches the subject "first\\nand second" (where \\n 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.

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.

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

  (a+)*

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.

An optimization catches some of the more simple cases such as

  (a+)*b

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

  (a+)*\\d

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.

.in 0
Last updated: 03 February 2003
.br
Copyright (c) 1997-2003 University of Cambridge.
1	nigel	63	.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 discussion about optimizing regular expressions
12			for efficient performance.
13
14			When a pattern begins with .* not in parentheses, or in parentheses that are
15			not the subject of a backreference, and the PCRE_DOTALL option is set, the
16			pattern is implicitly anchored by PCRE, since it can match only at the start of
17			a subject string. However, if PCRE_DOTALL is not set, PCRE cannot make this
18			optimization, because the . metacharacter does not then match a newline, and if
19			the subject string contains newlines, the pattern may match from the character
20			immediately following one of them instead of from the very start. For example,
21			the pattern
22
23			.*second
24
25			matches the subject "first\\nand second" (where \\n stands for a newline
26			character), with the match starting at the seventh character. In order to do
27			this, PCRE has to retry the match starting after every newline in the subject.
28
29			If you are using such a pattern with subject strings that do not contain
30			newlines, the best performance is obtained by setting PCRE_DOTALL, or starting
31			the pattern with ^.* to indicate explicit anchoring. That saves PCRE from
32			having to scan along the subject looking for a newline to restart at.
33
34			Beware of patterns that contain nested indefinite repeats. These can take a
35			long time to run when applied to a string that does not match. Consider the
36			pattern fragment
37
38			(a+)*
39
40			This can match "aaaa" in 33 different ways, and this number increases very
41			rapidly as the string gets longer. (The * repeat can match 0, 1, 2, 3, or 4
42			times, and for each of those cases other than 0, the + repeats can match
43			different numbers of times.) When the remainder of the pattern is such that the
44			entire match is going to fail, PCRE has in principle to try every possible
45			variation, and this can take an extremely long time.
46
47			An optimization catches some of the more simple cases such as
48
49			(a+)*b
50
51			where a literal character follows. Before embarking on the standard matching
52			procedure, PCRE checks that there is a "b" later in the subject string, and if
53			there is not, it fails the match immediately. However, when there is no
54			following literal this optimization cannot be used. You can see the difference
55			by comparing the behaviour of
56
57			(a+)*\\d
58
59			with the pattern above. The former gives a failure almost instantly when
60			applied to a whole line of "a" characters, whereas the latter takes an
61			appreciable time with strings longer than about 20 characters.
62
63			.in 0
64			Last updated: 03 February 2003
65			.br
66			Copyright (c) 1997-2003 University of Cambridge.