PCREPERFORM(3) Introduction to Library Functions PCREPERFORM(3)

NAME

PCRE - Perl-compatible regular expressions

PCRE PERFORMANCE

Two aspects of performance are discussed below: memory usage and
processing time. The way you express your pattern as a regular
expression can affect both of them.

COMPILED PATTERN MEMORY USAGE

Patterns are compiled by PCRE into a reasonably efficient
interpretive code, so that most simple patterns do not use much
memory. However, there is one case where the memory usage of a
compiled pattern can be unexpectedly large. If a parenthesized
subpattern has a quantifier with a minimum greater than 1 and/or a
limited maximum, the whole subpattern is repeated in the compiled
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
particularly 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 using the 8-bit library. When PCRE is
compiled with its default internal pointer size of two bytes, the
size limit on a compiled pattern is 64K data units, 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 acceptable, this kind of rewriting will allow you to process
patterns that PCRE cannot otherwise handle.

STACK USAGE AT RUN TIME

When pcre_exec() or pcre[16|32]_exec() is used for matching, certain
kinds of pattern can cause it to use large amounts of the process
stack. In some environments the default process stack is quite small,
and if it runs out the result is often SIGSEGV. This issue is
probably the most frequently raised problem with PCRE. Rewriting your
pattern can often help. The pcrestack documentation discusses this
issue in detail.

PROCESSING TIME

Certain items in regular expression patterns are processed more
efficiently 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 regular 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 use a multi-stage table lookup 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.

By default, the escape sequences \b, \d, \s, and \w, and the POSIX
character classes such as [:alpha:] do not use Unicode properties,
partly for backwards compatibility, and partly for performance
reasons. However, you can set PCRE_UCP if you want Unicode character
properties to be used. This can double the matching time for items
such as \d, when matched with a traditional matching function; the
performance loss is less with a DFA matching function, and in both
cases there is not much difference for \b.

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 ^.* or ^.*? 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 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
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 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: 25 August 2012
Copyright (c) 1997-2012 University of Cambridge.

PCRE 8.30 09 January 2012 PCREPERFORM(3)