PostgreSQL has a nice interface to the parser.

List* pg_parse_query(const char* query_string);

That is, you provide it with the query string and then you get a list structure that provides information about that query.

However, the PostgreSQL code uses setjmp/longjmp for error handling that will not work without modifications in a multi-threaded context. There are also other context variables that clearly will not work without modifications in an MT context.