LingPipe is very nice and well documented. You can also take a look at:
The last one specifically might be of interest to you, although I don’t know whether there are any readily available Java implementations (and maybe that’s too big of a gun for your problem anyway 🙂
Paul’s idea of using a DSL is probably easier and faster to implement, and more reliable to use for your customers. I, too, would recommend looking into that first.