Optimizing BNF grammars through source transformations

In this paper we explore the efficiency of various ways of expressing the form and meaning of natural language utterances as context-free grammars. We concentrate on the top-down parsing strategy employed in SpeechWorks 6.5, a strategy common to many systems. As with other search-based parsers, the key to efficiency is to limit the uncertainty of the parser at any given stage by reducing non-determinism in the grammar. Here we study the effects of different expressions of the same grammar in terms of efficiency. We also describe a methodology for transforming a source grammar into a more efficient expression of the same forms and meanings.