parsing SQLERRM ?

(version == 9.1)

In my PL/pgSQL stored functions,
I want to be able to distinguish which FK-constraint caused the [foreign_key_violation] exception.
. . .
BEGIN
delete from MY_COOL_TABLE where id = 123 ;
EXCEPTION
WHEN foreign_key_violation THEN
CASE
WHEN (SQLERRM tells me it blew up because of FK X) THEN . . . ;
WHEN (SQLERRM tells me it blew up because of FK Y) THEN . . . ;
WHEN (SQLERRM tells me it blew up because of FK Z) THEN . . . ;
END;
WHEN others THEN
raise;
END;
. . .

Is a "robust enough" parsing of SQLERRM actually the best/only way to determine this ?

-dvs-