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The first example below works; the second one produces output containing garbage characters. (This came up while I was creating a set of examples for a tutorial on regular expressions).
import re
text= "The cat ate the rat."
print("before: %s" % text)
m= re.search("The (\w+) ate the (\w+)", text)
text= "The %s ate the %s." % (m.group(2), m.group(1))
print("after : %s" % text)
text= "The cat ate the rat."
print("before: %s" % text)
text= re.sub("(\w+) ate the (\w+)", "2円 ate the 1円", text)
print("after : %s" % text)

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.)