A method and apparatus are described for specifying arbitrary words in a rule-based grammar, such as a context-free grammar. A wildcard identifier is used in a context-free grammar file in place of a predefined category of words. Artificial phoneme combinations that represent generic words in a speech engine's vocabulary database, and that represent pronunciations of the predefined category of words are defined by rules. A speech engine uses the artificial phoneme combinations to determine acceptable words to listen to, and generates a results object comprising generic words corresponding to the artificial phoneme combinations, as well as non-generic words that do not exist in a given CFG, where the generic and non-generic words exist in the speech engine's vocabulary database. The word having the highest confidence level that is not a generic word is selected as the word most likely to have been spoken by the user.