If not - why not?
The docs here Configuration reference - CircleCI (and “Ask AI”) both omit from saying that regex is supported for expression-based filters, leading to the conclusion that regex match is only possible with non-expression branches and tags filters
I want to filter branches based on a regex match of the branch name AND against a parameter value
There’s a similar (but I think different?) concept here, “Logic statements”:
these do support regex matches but the syntax is different from the ‘expression’ syntax for filters so I assume are not applicable 
@nathanwfish do you know the answer? I think this was one of your recent releases?
We support some regex but for security reasons its not a complete mapping. @rosie I think has more details and was planning on responding with those.