I've done a little toy language implementation with Nearley and really enjoyed it. If you have some JS experience under your belt, Nearley should be pretty easy

I've done a little toy language implementation with Nearley and really enjoyed it. If you have some JS experience under your belt, Nearley should be pretty easy to pick up and I felt it was easy to build on top of the basic building blocks. It's almost too easy to drop into custom JS with Nearley's inline JS syntax, so you can do all sorts of stuff with JS regexes or what not if you can't find a built-in feature that would do what you want. Unfortunately my project was not big enough that I'd be able to say anything about performace in scale, though. Being JS based has the advantage that you can (depending on your application needs) sometimes move some of that workload to the client :)