I am trying to use a regex filter to do a match all strings except a string.
My regexp is [^(hello)].
I don't get any responses back. I do see there are documents which match this regex.
I think your regex might not be correct. As far as I can see it matches any character not in the list (helo), so it will return documents with terms like "a" or "x". Having said that, using regex query to get documents not matching anything might not be the right approach. Have you tried using bool query with a must_not clause?