Optimizing

Why PHP’s RegEx Is Slow, And What You Can Do About It (if you happen to be a committer on the PHP project)

Regular Expression Matching Can Be Simple And Fast, by Russ Cox:

Perl [and PHP and others] could not now remove backreference support, of course, but they could employ much faster algorithms when presented with regular expressions that don’t have backreferences.

How much faster? About a million times (no, I do not exaggerate).

I use a lot of regular expressions, and relatively few of them use backreferences. It’d be worth optimizing.