I have the following lexer rules in my grammar file: LINE : 'F' | 'G'; RULE : (('->' ('F' | 'G')) => 'F' | 'G' ) | LINE LINE + | LINE * (ROTATE + LINE+)+ ; fragment ROTATE : ('/' | '\\'); I'm basically trying to match productions that look like F - F/F\F\F/F. It successfully matches stuff like the above, but I'm guessing there's a problem with my syntactic predicate, since G - G produces a MismatchedTokenException. The predicate serves to disambiguate between single letters on the lhs of '-', which I want to be recognized as the LINE token, and those on the rhs, which should be RULEs. Any idea what I'm doing wrong?