Add a space before the \d+.
>>> s = "This must not b3 delet3d, but the number at the end yes 134411"
>>> s = re.sub(" \d+", " ", s)
>>> s
'This must not b3 delet3d, but the number at the end yes '
Edit: After looking at the comments, I decided to form a more complete answer. I think this accounts for all the cases.
s = re.sub("^\d+\s|\s\d+\s|\s\d+$", " ", s)