English is an experiment at providing a natural language interface to the domain of email search. The hope is that it could also be applied to more interesting domains.

A formal grammar defines all meaningful queries and an attempt was made to make this grammar cover many cases is as generic a way as possible. Each rule in the grammar is accomponied by a function written in Lua. If the rule is a terminal symbol such as "three", the function simply returns the Lua number 3. If the rule is contains parameters as in "%number% days" then the value returned by the function which matched the %number% part of the subquery is passed into the Lua function bound to the variable number. In this case, the return value would be some Lua Date object. See the list of rules used in this grammar for more examples.