<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def validate_one(cls, keystr):
 """ validates one key string """

        regex = r'%s$' % cls.ALLOWED_KEY
        if re.match(regex, keystr) is None:
            raise cls.Bad("Bad key syntax for: %s. Should be: key1.key2..." % (keystr))

        return True