Filter would be significantly more useful it it supported Regex or wildcards. Ideally under a different parameter. Wildcard could be with the * or the %. This would allow for "starts-with" or "ends-with" type queries. For example: wildcard=java% or wildcard=*java
I personally think that this would be more trouble than it's worth - and I'm speaking about the tremendous amount of work that the API maintainers would have to go through to filter malicious RegEx queries.
...and not only malicious, but also queries that would take a long time to parse/execute.
I think that if a web giant like Google doesn't support wildcard/RegEx, there's no need for StackOverflow to support it.