Proper code review takes as long as writing the damn thing in the first place and is infinitely more boring. And you still miss things that would have been obvious while writing.
In this special case, you'd have to reverse engineer the grammar from the parser, calculate first/follow sets and then see if the grammar even is what you intended it to be.
Author did review the (also generated) tests, which as long as they're comprehensive enough for his purposes, all pass and coverage is very high, means things work well enough. Attempting to manually edit that code is a whole other thing though.
