SELECT
   code,
   postcode_validate(code) AS valid,
   postcode_validate(code) = expect AS pass
FROM (
   VALUES
   -- allow
   ('L1 1AA', true),
   ('L0 0AA', true),
   ('L1 0AA', true),
   ('L0 1AA', true),
   -- forbid
   ('L00 0AA', false),
   ('L00 1AA', false),
   ('L01 0AA', false),
   ('L01 1AA', false),
   -- unsupported
   ('GIR 0AA', false)
) AS sample (code, expect);
  code   | valid | pass 
---------+-------+------
 L1 1AA  | t     | t
 L0 0AA  | t     | t
 L1 0AA  | t     | t
 L0 1AA  | t     | t
 L00 0AA | f     | t
 L00 1AA | f     | t
 L01 0AA | f     | t
 L01 1AA | f     | t
 GIR 0AA | f     | t
(9 rows)