rreplace - How to replace the last occurrence of an expression in a string?

>>> def rreplace(s, old, new, occurrence):
...  li = s.rsplit(old, occurrence)
...  return new.join(li)
... 
>>> s
'1232425'
>>> rreplace(s, '2', ' ', 2)
'123 4 5'
>>> rreplace(s, '2', ' ', 3)
'1 3 4 5'
>>> rreplace(s, '2', ' ', 4)
'1 3 4 5'
>>> rreplace(s, '2', ' ', 0)
'1232425'

I'm not going to pretend that this is the most efficient way of doing it, but it's a simple way. It reverses all the strings in question, performs an ordinary replacement using str.replace on the reversed strings, then reverses the result back the right way round:

>>> def rreplace(s, old, new, count):
...     return (s[::-1].replace(old[::-1], new[::-1], count))[::-1]
...
>>> rreplace('<div><div>Hello</div></div>', '</div>', '</bad>', 1)
'<div><div>Hello</div></bad>'

Just reverse the string, replace first occurrence and reverse it again:

mystr = "Remove last occurrence of a BAD word. This is a last BAD word."

removal = "BAD"
reverse_removal = removal[::-1]

replacement = "GOOD"
reverse_replacement = replacement[::-1]

newstr = mystr[::-1].replace(reverse_removal, reverse_replacement, 1)[::-1]
print ("mystr:", mystr)
print ("newstr:", newstr)

Output:

mystr: Remove last occurence of a BAD word. This is a last BAD word.
newstr: Remove last occurence of a BAD word. This is a last GOOD word.

If you know that the 'old' string does not contain any special characters you can do it with a regex:

In [44]: s = '<div><div>Hello</div></div>'

In [45]: import re

In [46]: re.sub(r'(.*)</div>', r'\1</bad>', s)
Out[46]: '<div><div>Hello</div></bad>'