A postgres parser related question

Hi everyone, I just want to know something about postgresql parser, because
I want to add a new feature for pgAdmin graphical query builder (GQB) that
allow an user to create a query graphical model from a sql statment, and I
just want to reuse postgres parser code (reuse this) to do the task of
create an abstract tree like structure for the sql statment.

I just remember that read at some place that I don't remember that the
syntax and semantic checks are done in two different stages, and as I can
understand, then is possible to just reuse only the parser stage in an "easy
way", but before even trying to do this task, I want some advices from
people that know a lot more than me from postgres internals.

Thanks in advance.
Regards,
Luis Ochoa.