Count number of digits in one field

I'm trying to come up with a code that would count the number of digits in one field. in this case it's passports I want to make sure they have valid digits and characters. select p.passport_nbr, CASE WHEN Number = 0 THEN 1 ELSE FLOOR(LOG10(ABS(p.passport_nbr))) + 1 AS NDigits , is there a different code I can use. I use oracle sql