SQL> with test as
  2    (select 'BWHPK2334M' col from dual union
  3     select 'abc123'         from dual union
  4     select 'BWHPK23344'     from dual union
  5     select '12345abcd6'     from dual union
  6     select 'AABWHPK2334MXX' from dual
  7    )
  8  select col
  9  from test
 10  where regexp_like(col, '^[[:alpha:]]{5}[[:digit:]]{4}[[:alpha:]]{1}$');

COL
--------------
BWHPK2334M

SQL>

if not regexp_like(:block.item, '^[[:alpha:]]{5}[[:digit:]]{4}[[:alpha:]]{1}$') then
   message('Invalid format');
   raise form_trigger_failure;
end if;