Postgres SE SSN constraint

ssn.sql

CREATE OR REPLACE FUNCTION se_ssn_valid (VARCHAR(12))
RETURNS boolean
AS $$
SELECT
char_length($1) = 12
AND date_part('year', age(now(), to_timestamp(substring($1, 1, 8), 'YYYYMMDD'))) > 0
AND (
    (select sum(t) from unnest(regexp_split_to_array((substring($1, 3, 1)::int * 2)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 4, 1)::int * 1)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 5, 1)::int * 2)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 6, 1)::int * 1)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 7, 1)::int * 2)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 8, 1)::int * 1)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 9, 1)::int * 2)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 10, 1)::int * 1)::text, '')::int[]) as t)
    + (select sum(t) from unnest(regexp_split_to_array((substring($1, 11, 1)::int * 2)::text, '')::int[]) as t)
    + substring($1, 12, 1)::int
) % 10 = 0;
$$
LANGUAGE SQL
IMMUTABLE;

create table person (
    id int GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
    ssn VARCHAR(12) null,
    CONSTRAINT valid_date CHECK (se_ssn_valid(ssn))
);