Rather than just being treated as empty regex, broken filter regex should signal via UI.

[pobocks]

As a user who's using raw regex input, I should get different behavior when I have nothing in the filter and when I have some broken or incomplete regex, like:

wang(chung

My initial thought is to implement an "error" visual state for the filter, and enter/exit it based on filter state.

I think this is only relevant for the raw regex search mode - I don't think token mode can be broken in this way, because tokens are all regex-quoted.