  Regular expressions are often ambiguous. We present a novel method based on
Brzozowski's derivatives to aid the user in diagnosing ambiguous regular
expressions. We introduce a derivative-based finite state transducer to
generate parse trees and minimal counter-examples. The transducer can be easily
customized to either follow the POSIX or Greedy disambiguation policy and based
on a finite set of examples it is possible to examine if there are any
differences among POSIX and Greedy.
