Right-to-left string replace in Python?

Question

I want to do a string replace in Python, but only do the first instance going from right to left. In an ideal world I’d have:

myStr = "mississippi"
print myStr.rreplace("iss","XXX",1)

> missXXXippi

What’s the best way of doing this, given that rreplace doesn’t exist?

Asked By: fredley

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Source

Answer 1

>>> myStr[::-1].replace("iss"[::-1], "XXX"[::-1], 1)[::-1]
'missXXXippi'

Answered By: eumiro

Answer 2

It’s kind of a dirty hack, but you could reverse the string and replace with also reversed strings.

"mississippi".reverse().replace('iss'.reverse(), 'XXX'.reverse(),1).reverse()

Answered By: sleeplessnerd

Answer 3

you may reverse a string like so:

myStr[::-1]

to replace just add the .replace:

print myStr[::-1].replace("iss","XXX",1)

however now your string is backwards, so re-reverse it:

myStr[::-1].replace("iss","XXX",1)[::-1]

and you’re done.
If your replace strings are static just reverse them in file to reduce overhead.
If not, the same trick will work.

myStr[::-1].replace("iss"[::-1],"XXX"[::-1],1)[::-1]

Answered By: Serdalis

Answer 4

>>> re.sub(r'(.*)iss',r'1XXX',myStr)
'missXXXippi'

The regex engine cosumes all the string and then starts backtracking untill iss is found. Then it replaces the found string with the needed pattern.

Some speed tests

The solution with [::-1] turns out to be faster.

The solution with re was only faster for long strings (longer than 1 million symbols).

Answered By: ovgolovin

Answer 5

def rreplace(s, old, new):
    try:
        place = s.rindex(old)
        return ''.join((s[:place],new,s[place+len(old):]))
    except ValueError:
        return s

Answered By: Anaphory

Answer 6

rsplit and join could be used to simulate the effects of an rreplace

>>> 'XXX'.join('mississippi'.rsplit('iss', 1))
'missXXXippi'

Answered By: Stephen Emslie

Answer 7

You could also use str.rpartition() which splits the string by the specified separator from right and returns a tuple:

myStr = "mississippi"

first, sep, last = myStr.rpartition('iss')
print(first + 'XXX' + last)
# missXXXippi

Answered By: Austin

Answer 8

Using the package fishhook (available through pip), you can add this functionality.

from fishhook import hook

@hook(str)
def rreplace(self, old, new, count=-1):
    return self[::-1].replace(old[::-1], new[::-1], count)[::-1]

print('abxycdxyef'.rreplace('xy', '--', count=1))

# 'abxycd--ef'

Answered By: Michal Punčochář

Right-to-left string replace in Python?

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