code: def get_valid_byte(num, default):
          
          num_valid = re.match(r"\A([0-9]+)(M|G|m|g|)(B|b|\b)\Z", num, re.I)
          if num_valid:
              return num
          return default
docs: check if the format of number is (num)(G|m|B) i.e 500GB, 200mb. 400
          etc..