In general I don't know of a good way to do negations within regular expressions themselves.
Certainly we could add something like grep's -v, i.e. find all lines not matching a given expression. That would be easy enough if that's all you're looking for. I'm not sure how useful that would be in practice, though.
Doing anything more complicated, like (expr A matches) and !(expr B matches gets to be messy from a UI perspective.
FYI, the options that are currently present are closely modeled after what I've seen in modern programming IDE's.