code: def _keynat(string):
      
      r = []
      for c in string:
          if c.isdigit():
              if r and isinstance(r[-1], int):
                  r[-1] = r[-1] * 10 + int(c)
              else:
                  r.append(int(c))
          else:
              r.append(9 + ord(c))
      return r
docs: A natural sort helper function for sort() and sorted()
      without using regular expression.