import re
x = "The sky is red"
r = re.compile ("red")
y = r.sub(x, "blue")
print x  # Prints "The sky is red"
print y  # Prints "blue"

>>> y = re.sub(r, 'blue', x)
>>> y
'The sky is blue'

>>> z = r.sub('blue', x)
>>> z
'The sky is blue'

r.sub(x, "blue")
# should be
r.sub("blue", x)

import re
x = "The sky is red"
r = re.compile ("red")
y = r.sub("blue", x)
print x  # Prints "The sky is red"
print y  # Prints "The sky is blue"

x= "The sky is red"
y= x.replace("red", "blue")
print y

x = r.sub("blue", x)