I'm also a big fan of ragel, and have used it for various protocol implementations. One of the really nice features is that you can have it output graphviz dot

I'm also a big fan of ragel, and have used it for various protocol implementations. One of the really nice features is that you can have it output graphviz dot files, to get an actual visualisation of your state machine, so you know where you've missed a transition, or how you've accidentally hit a state explosion with a bad rule.