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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def opponent_rank(self):
 """ Returns a ``string`` of the opponent's rank when the game was played and None if the team was unranked. """

        rank = re.findall(r'\d+', self._opponent_name)
        if len(rank) > 0:
            return int(rank[0])
        return None