Oh ... you're right, of course. In my limited regexp experience (not that I don't use regexps a lot, but when I do, they're usually very simple things), I used the (?!pattern) as a way to tell if the src attribute values in IMG tags started with http:// or not, so I could do some relative path stuff if necessary. In that experience, I labelled anything that started with (?...) as a lookahead.