Case-insensitive string replacement in python

Question

I am able to replace a word in a string content using the following solution in a case insensitive method

http://code.activestate.com/recipes/552726/ 
import re

class str_cir(str):
        ''' A string with a built-in case-insensitive replacement method '''

        def ireplace(self,old,new,count=0):
        ''' Behaves like S.replace(), but does so in a case-insensitive
        fashion. '''
            pattern = re.compile(re.escape(old),re.I)
            return re.sub(pattern,new,self,count)

My problem is i need to replace exactly the word i provide like

para = "Train toy tram dog cat cow plane TOY  Joy   JoyTOY"

i need to replace the word "toy" with "ham" and i get

'Train HAM tram dog cat cow plane HAM  Joy   JoyHAM'

What i need is

'Train HAM tram dog cat cow plane HAM  Joy   JoyTOY'

Just to note - the indentation isn't correct - and I think the actual code is `para = str_cri("...")` — Jon Clements, Jun 26 '12 at 11:10
Possible duplicate of [Case insensitive replace](https://stackoverflow.com/questions/919056/case-insensitive-replace) — Martin Thoma, Jul 05 '17 at 11:39

nhahtdh · Accepted Answer · 2012-06-26T11:08:51.023

4

Add \b to the start and end of the keyword:

pattern = re.compile("\\b" + re.escape(old) + "\\b",re.I)

\b means word boundary, and it matches the empty string at the start and end of a word (defined by sequence of alphanumeric or underscore character). (Reference)

As @Tim Pietzcker pointed out, it won't work as you might think if there are non-word (not alphanumeric and not underscore) characters in the keyword.

edited Jun 26 '12 at 11:08

answered Jun 26 '12 at 11:01

nhahtdh

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This. (Let's just hope that his search "words" never start or end with a non-alphanumeric character) – Tim Pietzcker Jun 26 '12 at 11:04
Hi Guys this method works for me as the terms i search are always alphanumeric characters. Thanks – Rakesh Jun 26 '12 at 12:55

score 2 · Answer 2 · answered Jun 26 '12 at 11:00

2

Put \b at the beginning and ending of the regex.

answered Jun 26 '12 at 11:00

Ignacio Vazquez-Abrams

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score 2 · Answer 3 · answered Jun 26 '12 at 11:00

2

Wrap the word you're using in the regular expression with word boundaries (\b).

answered Jun 26 '12 at 11:00

alex

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putting `\b` at both ends would be same as putting `^` at the beginning and `$` at the end? – theharshest Jun 26 '12 at 11:01
@theharshest Nope, `\b` are word boundaries while `^` and `$` position at the start and end of string. – alex Jun 26 '12 at 11:02
but if I put `^` and `$` at start and end, then also it will match the exact re enclosed, solve the same purpose as `\b` does. Right? – theharshest Jun 26 '12 at 11:05

Case-insensitive string replacement in python

3 Answers3