https://github.com/postgres/postgres/blob/master/src/backend/parser/parser.c#L35

parser.c

/*
 * raw_parser
 * Given a query in string form, do lexical
 * and grammatical analysis.
 *
 * Returns a list of raw (un-analyzed) parse trees.
 */
List *
raw_parser(const char *str)
{
	...
}