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Python - Replace Different Characters in String at Once

Replace a String character at given index in Python

Last Updated : 16 Apr, 2025

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In Python, strings are immutable, meaning they cannot be directly modified. We need to create a new string using various methods to replace a character at a specific index.

Using slicing

Slicing is one of the most efficient ways to replace a character at a specific index.

Python

s = "hello" idx = 1 replacement = "a" res = s[:idx] + replacement + s[idx+1:] print(res)

Output

hallo

Explanation:

The string is split into two parts: everything before the target index (text[:index]) and everything after (text[index+1:]).
The replacement character is inserted between these slices.
This method is simple, efficient, and works well for small or large strings.

Using list conversion

Converting the string to a list allows us to modify its characters directly before converting it back.

Python

s = "hello" idx = 1 replacement = "a" a = list(s) a[idx] = replacement res = ''.join(b) print(res)

Output

hallo

Explanation:

The string is converted to a list because lists are mutable.
We modify the character at the target index and then rejoin the list into a string.
This method is slightly less efficient but useful for multiple modifications.

Using regular expressions

Regular expressions can be used to replace a character at a specific position by matching patterns.

Python

import re  s = "hello"  idx = 1 replacement = "a"  # Create a pattern to match the character at the specific index pattern = f"^(.{{{idx}}})."  res = re.sub(pattern, rf"\1{replacement}", s) print(res)

Output

hallo

Explanation:

The pattern matches the character at the desired index using lookbehind.
The re.sub() method replaces the matched character with the specified replacement.
This method is powerful but less efficient and more complex for simple replacements.

Using a manual loop

We can iterate through the string and build a new one, replacing the character at the given index.

Python

s = "hello" idx = 1 replacement = "a" res = ""  for i in range(len(s)):     if i == idx:         res += replacement     else:         res += s[i]  print(res)

Output

hallo

Explanation:

The loop iterates through the string and appends characters to a new string.
At the target index, the replacement character is added instead.

Related Articles:

Python String
String Slicing in Python
Regex Tutorial
Loops in Python

Python - Replace Different Characters in String at Once

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