Store regexp_matches result in variables - regex

Question

In a psql function I use regexp_matches. The result should go into 2 var db_datum_von_string and db_datum_von_string. However the second is always null even if the pattern matches 2 strings.
The first value is correct. I guess the issue comes from the res[2] but I can't figure out what it is.
Notice that SELECT regexp_matches('1234ffdsafdsa 4554 ', '[0-9]+', 'g') works perfectly and return 2 rows.
CREATE OR REPLACE FUNCTION ajl_TEST_datum_check(
arg_datum_id integer,
req_datum integer,
flag integer
) RETURNS text AS $$
DECLARE
db_datum text;
db_datum_von_string text;
db_datum_bis_string text;
temp integer;
BEGIN
SELECT datum.datum INTO db_datum
FROM datum
WHERE datum_id = arg_datum_id;
--- the issue starts here
SELECT res[1], res[2] INTO db_datum_von_string, db_datum_bis_string
FROM (SELECT regexp_matches(db_datum, '[0-9]+', 'g') res) y;
--- end of trouble
IF db_datum_bis_string IS NULL THEN
RETURN db_datum_von_string;
ELSE
RETURN TRUE;
END IF;
END;
$$ LANGUAGE plpgsql;

Answer 1

Your query returns two rows of one-element array values:
SELECT res[1], res[2]
FROM (
SELECT regexp_matches('1234ffdsafdsa 4554 ', '[0-9]+', 'g') res
) y;
res | res
------+-----
1234 |
4554 |
(2 rows)
You should convert elements of the rows to an array:
SELECT res[1], res[2]
FROM (
SELECT array_agg(res) res
FROM (
SELECT unnest(regexp_matches('1234ffdsafdsa 4554 ', '[0-9]+', 'g')) res
) y
) x;
res | res
------+------
1234 | 4554
(1 row)