Rex is a simple command-line tool that takes a .NET regex or several regexes and generates strings that match all the regexes. Rex is fast, using a novel approach to solve this problem. Rex enables penetration testing on any code using regexes to validate inputs.
http://research.microsoft.com/en-us/downloads/7f1d87be-f6d9-495d-a699-f12599cea030/default.aspx
Massimiliano