Modifying SQL parser with extensions?

Hi everyone,

I am searching for the easiest way to let PostgreSQL parse a special
dialect of SQL. The dialect I want to parse augments common SQL in a way
that enables expressions within a WHERE clause to be annotated and is
thus not compatible with the standard SQL syntax anymore.

I am totally new to PostgreSQL extension development and skimming
through the Server Programming chapter I didn't notice an obvious way to
get down to the parser internals.

Is there any way of using existing extension techniques or do I have to
compile my own special version of PostgreSQL in order to modify the parser?

Any suggestions are appreciated.

Regards,
Matthias