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<a href='#toc'>Table of Contents</a><p>
<h2><a name='sect0' href='#toc0'>Name</a></h2>
PCRE - Perl-compatible regular expressions
<h2><a name='sect1' href='#toc1'>Pcre Performance</a></h2>
<p>
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 \p,
\P, and \X 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
<p>
.*second<br>
<p>
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. <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 <p>
(a+)*<br>
<p>
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 <p>
(a+)*b<br>
<p>
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 <p>
(a+)*\d<br>
<p>
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>
Last updated: 09 September 2004 <br>
Copyright (c) 1997-2004 University of Cambridge. <p>
<hr><p>
<a name='toc'><b>Table of Contents</b></a><p>
<ul>
<li><a name='toc0' href='#sect0'>Name</a></li>
<li><a name='toc1' href='#sect1'>Pcre Performance</a></li>
</ul>
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