code: def tokens(self, format_string):
          
          if format_string not in self.format_string_cache:
              if python2 and isinstance(format_string, str):
                  format_string = format_string.decode("utf-8")
              tokens = list(re.finditer(self.reg_ex, format_string))
              self.format_string_cache[format_string] = tokens
          return self.format_string_cache[format_string]
docs: Get the tokenized format_string.
          Tokenizing is resource intensive so we only do it once and cache it