Python – Case insensitive string replacement

Given a string of words. The task is to write a Python program to replace the given word irrespective of the case with the given string.

Examples

Input : String = "gfg is BeSt", replace = "good", substring = "best"
Output : gfg is good
Explanation : BeSt is replaced by "good" ignoring cases.

Case insensitive string replacement using re.IGNORECASE + re.escape() + re.sub()

In this, span>sub()span> of the regex is used to perform the task of replacement, and IGNORECASE ignores the cases and performs case-insensitive replacements.

Python3

import re

# initializing string
test_str = "gfg is BeSt"

# printing original string
print("The original string is : " + str(test_str))

# initializing replace string
repl = "good"

# initializing substring to be replaced
subs = "best"

# re.IGNORECASE ignoring cases
# compilation step to escape the word for all cases
compiled = re.compile(re.escape(subs), re.IGNORECASE)
res = compiled.sub(repl, test_str)

# printing result
print("Replaced String : " + str(res))

Output

The original string is : gfg is BeSt
Replaced String : gfg is good

Case insensitive string replacement using re.sub() + lambda + re.escape() 

Using particular ignore case regex also this problem can be solved. Rest, a lambda function is used to handle escape characters if present in the string. 

Python3

import re

# initializing string
test_str = "gfg is BeSt"

# printing original string
print("The original string is : " + str(test_str))

# initializing replace string
repl = "good"

# initializing substring to be replaced
subs = "best"

# regex used for ignoring cases
res = re.sub('(?i)'+re.escape(subs), lambda m: repl, test_str)

# printing result
print("Replaced String : " + str(res))

Output

The original string is : gfg is BeSt
Replaced String : gfg is good

Case insensitive string replacement using split(), lower() and string replace()

Here, you can also use upper() in place of lower().

Python3

# initializing string
test_str = "gfg is BeSt"

# printing original string
print("The original string is : " + str(test_str))

# initializing replace string
repl = "good"

# initializing substring to be replaced
subs = "best"

x = test_str.split()
for i in x:
    if(i.lower()==subs.lower()):
        test_str=test_str.replace(i,repl)
# printing result
print("Replaced String : " + test_str)

#contributed by Bhavya Koganti

Output

The original string is : gfg is BeSt
Replaced String : gfg is good

Time Complexity: O(n), where n is length of test_str.
Auxiliary Space: O(1)

Case insensitive string replacement using a List and Join()

Strings are not directly mutable so using lists can be an option. And then get back the string using join on the list.

Python3

# the given input string.
test_str = "gfg is BeSt"

# the replace string.
repl = "good"

# the string to be replaced.
subs = "best"

# create the list.
l = test_str.split(" ")

# loop and replace the target string.
for i, w in enumerate(l):
    if w.lower() == subs:
        l[i] = repl

# list to string using join.
output = " ".join([i for i in l])

print("The original string is :", test_str)
print("Replaced String :", output)

Output

The original string is : gfg is BeSt
Replaced String : gfg is good

Time Complexity: O(n), where n is 
Auxiliary Space: O(1)

Case insensitive string replacement using Regular Expression

In Python, we can use regular expressions for case-sensitive string replacement.

Python3

import re

# the given input string.
test_str = "gfg is BeSt"

# the replace string.
repl = "good"

# the string to be replaced.
subs = "best"

def replace_substring(test_str, repl, subs):
    # Replacing all occurrences of substring s1 with s2
    test_str = re.sub(r'(?i)'+subs, repl, test_str)
    return test_str


print("The original string is :", test_str)
print("Replaced String :", replace_substring(test_str, subs, repl))

Output

The original string is : gfg is BeSt
Replaced String : gfg is good

Time Complexity: O(n), where n is the length of the input string.
Auxiliary Space: O(1)