1. A method of refining a context-free grammar, the method comprising: generating backoff grammar rules from rules of an initial context-free grammar, each backoff grammar rule being generated based on at least one keyword identified from the initial context-free grammar; and recursively modifying the initial context-free grammar using a computer processing unit, wherein recursively modifying comprises: obtaining a relined context-free grammar by modifying the initial context-free grammar,during a first iteration, based at least in part on recognition of utterances using the backoff grammar rules; determining a recognition efficiency of the refined context-free grammar and the hackoff grammar rules to the utterances; and during each of at least one additional iteration, further modifying the refined context-free grammar based at least in part on recognition of utterances using the refined context free grammar and backoff grammar rules until the refined context-free grammar and the backoff grammar rules meet a threshold level of recognition efficiency.