create or replace function middle3digits_helper(s) as  (
  select if (invalid_character, 'invalid character found',
             if (invalid_n, 'invalid number of digits',
                 s[p+1 : p+3])) as result
  from (select
         regexp_matches(s, '[^0-9]') as invalid_character,
         (length(s) < 3 or (length(s) % 2 = 0)) as invalid_n,
         (length(s) - 3) // 2 as p
  )
);

create or replace function middle3digits(number) as (
  middle3digits_helper(replace(trim(number::VARCHAR),'-',''))
);

## Examples
select middle3digits('xyzzy');

select n, middle3digits(n)
from unnest( [
  123, 12345, 1234567, 987654321, 10001, -10001, -123, -100, 100, -12345,
  1, 2, -1, -10, 2002, -2002, 0 ]) _(n) ;