One of the most important things to get right with my regexModeler application was the test suite, because I made several breaking changes many times along the way and would have been dead in the water without one. Because of all the random aspects of the program, it was an early challenge to get something for the unit tests to reliably measure. Now, if this is something you need to do too, bear in mind that object expressions can do much the same thing, but the limitation is that you need to define all of the methods for your solution to build. By taking the approach I outline below, you have more flexibility to define just the behavior you need to, while safely ignoring the rest. Also, if you want to take it a step further to add features similar to mocking libraries, you can do that.

If you happen to be enthusiastic about both F# and regex, I have the project for you.