proxy based on number of numeric char.

I'd like to have a radius proxy setup where it can proxy users based on number of numeric characters in the userid. so for example, if the userid is abc123 (with 3 numeric char. at the end) it should proxy to an instance of radius running on the same box and if the user id is abcxxx it should proxy the request to a different instance of radius running on the same box, is this doable or did I miss something in the documentation?