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You forgot to use raw strings:
>>> text = "The cat ate the rat."
>>> print("before: %s" % text)
before: The cat ate the rat.
>>> text = re.sub("(\w+) ate the (\w+)", r"2円 ate the 1円", text)
>>> print("after : %s" % text)
after : The rat ate the cat.
>>> 
(Maybe you should reconsider writing yet another tutorial about regular expressions, and possibly submit patches to improve the official regex howto if you think it's not good enough.)