Prefer over PCRE over EREG (much more powerful and faster!)

Bad written regex can consume tons of time (backtracking)

C implemented functions are always faster than regex

Is there really need for a regex or does a PHP function do the trick?

Replace all your EREGs with PCREs

Try to avoid backtracking as much as possible (U modifier, if necessary)

Reading recommendation: Mastering Regular Expressions (http://www.oreilly.com/catalog/regex/)