Definition:Backus-Naur Form/Rules of Formation

 Let,  ,  , ... ,   be metasymbols in an object language.

Backus-Naur form has the following rules of formation:



This means that  on the LHS may be replaced by the specified sequence of $n$ words on the RHS.



This means that  on the LHS may be replaced by one of the $n$ words on the RHS.