High-performance spoken dialogue interfaces typically use a spoken command grammar, which defines what the user can say when talking to the system. For complex applications, implementation and maintenance of this grammar is a major task requiring specialised expertise in computational linguistics and software engineering. The resulting grammars are often hard to maintain, and do not port easily to new domains.

Regulus is an Open Source toolkit for construction of spoken command grammars, which has been developed since 2001 by a consortium whose main partners have been NASA Ames, the University of Geneva, and Fluency Voice Technology. Grammar development is carried out mainly using example-based methods, which greatly simplifies the process. Regulus has been used to build command grammars for Clarissa and several other major projects, including a medical speech translator, MedSLT and a speech enabled home control system.