code: def __normalize(self,text):
          
          return self._normalizer_regex.sub(lambda mo:
                  self._normalizer[mo.string[mo.start():mo.end()]],
                      text.lower())
docs: Substitute words in the string, according to the specified Normal,
          e.g. "I'm" -> "I am"
  
          :type str: str
          :param str: The string to be mapped
          :rtype: str