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PerformanceCertain items that may appear in 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 .* 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
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
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
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