I tend to try to use //x whenever I write code that I know someone else is going to have to maintain. At the same time, I also put the entire regex on one line in a comment just before the "real" version, so that someone reading the code can see it in both contexts.
I wouldn't exactly say this makes regexes easier to read, but for me it helps cement in my mind what each part of the regex does. If I have a comment that says "Here we're looking to extract X,Y, and Z", it's not as helpful as having the comment of "This extracts X" directly after the chunk of the RE that does that operation.
GuildensternNegaterd character class uber alles!