How to replace the some characters from the end of a string?

Question

I want to replace characters at the end of a python string. I have this string:

 s = \"123123\"

I want to replace the last 2 with

Answer 1

Using regular expression function re.sub to replace words at end of string

import re
s = "123123"
s = re.sub('23$', 'penguins', s)
print s

Prints:

1231penguins

or

import re
s = "123123"
s = re.sub('^12', 'penguins', s)
print s

Prints:

penguins3123

Answer 2

Here is a solution based on a simplistic interpretation of your question. A better answer will require more information.

>>> s = "aaa bbb aaa bbb"
>>> separator = " "
>>> parts = s.split(separator)
>>> separator.join(parts[:-1] + ["xxx"])
'aaa bbb aaa xxx'

Update

(After seeing edited question) another very specific answer.

>>> s = "123123"
>>> separator = "2"
>>> parts = s.split(separator)
>>> separator.join(parts[:-1]) + "x" + parts[-1]
'1231x3'

Update 2

There is far better way to do this. Courtesy @mizipzor.

Answer 3

For the second example, I would recommend rsplit, as it is very simple to use and and directly achieves the goal. Here is a little function with it:

def replace_ending(sentence, old, new):
    if sentence.endswith(old):
        i = sentence.rsplit(old,1)
        new_sentence =new.join(i)
        return new_sentence
    return sentence

print(replace_ending("aaa bbb aaa bbb", "bbb", "xxx")) 

Output:

aaa bbb aaa xxx

Answer 4

I got a tricky answer, but it is not efficient enough

    >>> fname = '12345.png.pngasdfg.png'
    >>> suffix = '.png'
    >>> fname_rev = fname[::-1]
    >>> suffix_rev = suffix[::-1]
    >>> fullname_rev = fname_rev.replace(suffix_rev, '', 1)
    >>> fullname = fullname_rev[::-1]
    >>> fullname '12345.png.pngasdfg'

Built-in function replace() takes three arguments str.replace(old, new, max_time) So you can delete the last mached string from the origional string

Answer 5

This is exactly what the rpartition function is used for:

rpartition(...) S.rpartition(sep) -> (head, sep, tail)

Search for the separator sep in S, starting at the end of S, and return
the part before it, the separator itself, and the part after it.  If the
separator is not found, return two empty strings and S.

I wrote this function showing how to use rpartition in your use case:

def replace_last(source_string, replace_what, replace_with):
    head, _sep, tail = source_string.rpartition(replace_what)
    return head + replace_with + tail

s = "123123"
r = replace_last(s, '2', 'x')
print r

Output:

1231x3

Answer 6

>>> s = "aaa bbb aaa bbb"
>>> s[::-1].replace('bbb','xxx',1)[::-1]
'aaa bbb aaa xxx'

For your second example

>>> s = "123123"
>>> s[::-1].replace('2','x',1)[::-1]
'1231x3'