If the set of HTML you're looking to parse with a regexp is known to conform to some sort of pattern. e.g. if you know there's no commented-out HTML, or complex scenarios etc.
e.g. I often preach that you shouldn't use regexps for HTML, but if I have a set of HTML that I'm familiar with, is straightforward and that I can check easily post-manipulation, then I have no qualms about using a regexp for that.
