We consider from practical perspective the (generally undecidable) problem of checking equivalence of context-free grammars. We present both techniques for proving equivalence, as well as techniques for finding counter-examples that establish non-equivalence. Among the key building blocks of our approach is a novel algorithm for efficiently enumerating and sampling words and parse trees from arbitrary context-free grammars; the algorithm supports polynomial time random access to words belonging to the grammar. Furthermore, we propose an algorithm for proving equivalence of context-free grammars that is complete for LL grammars, yet can be invoked on any context-free grammar, including ambiguous grammars.
Our techniques successfully find discrepancies between different syntax specifications of several real-world languages, and is capable of detecting fine-grained incremental modifications performed on grammars. Our evaluation shows that our tool improves significantly on the existing available state of the art tools. In addition, we used these algorithms to develop an online tutoring system for grammars that we then used in an undergraduate course on computer language processing. On questions involving grammar constructions, our system was able to automatically evaluate the correctness of 95% equivalence questions: it disproved 74% of cases and proved 21% of them. This opens up the possibility of using our tool in massive open online courses to introduce grammars to large populations of students.