The incoming Open Power Template 2.1 will have a new, more powerful expression parser. Basically speaking, expressions are those pieces of language that evaluate something, producing a value, i.e. variables and operators. Both OPT 1.1 and 2.0 use a nice, manually-written parser that compiles them to a PHP code, validating their syntax and performing some extra transformations. While it works well, I think I reached the end of possible ways to extend it. It would be simply too complicated to implement new features to it, so I decided I must rewrite it into a formal grammar. Today, the main works on it are finished.