pcreperform
PCREPERFORM(3) PCREPERFORM(3)
NAME
PCRE - Perl-compatible regular expressions
PCRE PERFORMANCE
Two aspects of performance are discussed below: memory usage and pro-
cessing time. The way you express your pattern as a regular expression
can affect both of them.
MEMORY USAGE
Patterns are compiled by PCRE into a reasonably efficient byte code,
so that most simple patterns do not use much memory. However, there is
one case where memory usage can be unexpectedly large. When a paren-
thesized subpattern has a quantifier with a minimum greater than 1
and/or a limited maximum, the whole subpattern is repeated in the com-
piled code. For example, the pattern
(abc|def){2,4}
is compiled as if it were
(abc|def)(abc|def)((abc|def)(abc|def)?)?
(Technical aside: It is done this way so that backtrack points within
each of the repetitions can be independently maintained.)
For regular expressions whose quantifiers use only small numbers, this
is not usually a problem. However, if the numbers are large, and par-
ticularly if such repetitions are nested, the memory usage can become
an embarrassment. For example, the very simple pattern
((ab){1,1000}c){1,3}
uses 51K bytes when compiled. When PCRE is compiled with its default
internal pointer size of two bytes, the size limit on a compiled pat-
tern is 64K, and this is reached with the above pattern if the outer
repetition is increased from 3 to 4. PCRE can be compiled to use
larger internal pointers and thus handle larger compiled patterns, but
it is better to try to rewrite your pattern to use less memory if you
can.
One way of reducing the memory usage for such patterns is to make use
of PCRE’s "subroutine" facility. Re-writing the above pattern as
((ab)(?2){0,999}c)(?1){0,2}
reduces the memory requirements to 18K, and indeed it remains under
20K even with the outer repetition increased to 100. However, this
pattern is not exactly equivalent, because the "subroutine" calls are
treated as atomic groups into which there can be no backtracking if
there is a subsequent matching failure. Therefore, PCRE cannot do this
kind of rewriting automatically. Furthermore, there is a noticeable
loss of speed when executing the modified pattern. Nevertheless, if
the atomic grouping is not a problem and the loss of speed is accept-
able, this kind of rewriting will allow you to process patterns that
PCRE cannot otherwise handle.
PROCESSING TIME
Certain items in regular expression patterns are processed more effi-
ciently than others. It is more efficient to use a character class
like [aeiou] than a set of single-character 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 reg-
ular expressions for efficient performance. This document contains a
few observations about PCRE.
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.
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 con-
tain newlines, the best performance is obtained by setting
PCRE_DOTALL, or starting the pattern with ^.* or ^.*? to indicate
explicit anchoring. That saves PCRE from having to scan along the sub-
ject 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 16 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 or 4, 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, even for relatively short strings.
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 charac-
ters.
In many cases, the solution to this kind of performance issue is to
use an atomic group or a possessive quantifier.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
REVISION
Last updated: 06 March 2007
Copyright (c) 1997-2007 University of Cambridge.
PCREPERFORM(3)
