There is Xeger, a library for finding valid input to a regular expression. That means, it does the opposite of what you usually do with Regex. I'm using this for fuzz-testing at the moment.
I'm looking for a similar library in C#, preferably .NET 4.0. It should work on Windows Vista SP2 and later.
It needs to work with the MS toolchain, not necessarily with Mono.
It may be commercial.