The regex engine uses trie data structures when dealing with alternations (foo|biz|bar). So if you have massive regular expressions that make heavy use of alternation (like I do), you should see a performance boost.

A trie is a very cool data structure, quoting wikipedia "The worst-case lookup speed in an imperfect hash table is O(N) time, but far more typically is O(1), with O(m) time spent evaluating the hash." I can't do the explanation justice so go read it for yourself, it is a worthwhile learning experience.