The performance of regular expressions on huge strings (assuming the way they are usually evaluated) really depends on the expression. It can be catastrophic, but it can also be linear (in regards to the string length). Sometimes executing multiple expressions is a lot faster then a single overly complex one (e.g. split/replace first, then search).
Also this may be useful (even if it is a bit annoying to read through). The basic idea is to force-enable Googles safe search and let it do its job: