I have found relatively few implementations of adaptive grammars, though there is an adaptive parser generator called Dypgen. I also wrote an adaptive parser in Prolog that can "learn" new grammars from its input.
There is also a "self-modifying grammar interpreter" called OpenZZ, though it hasn't been updated in a few years.
It's also possible to write "self-modifying" parsers using Nearley, though this feature isn't documented yet.
