PCRE(3)PCRE(3)NAME
PCRE - Perl-compatible regular expressions
PCRE PERFORMANCE
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 gen‐
eral, 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 perfor‐
mance.
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 new‐
lines, 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 con‐
tain 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 sub‐
ject string, and if there is not, it fails the match immediately. How‐
ever, 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.
Last updated: 03 February 2003Copyright (c) 1997-2003 University of Cambridge.
PCRE(3)
