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Python - Replace occurrences by K except first character

Remove All Characters Except Letters and Numbers – Python

Last Updated : 19 Apr, 2025

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In this article, we’ll learn how to clean a string by removing everything except letters (A–Z, a–z) and numbers (0–9). This means getting rid of:

Special characters (@, #, !, etc.)
Punctuation marks
Whitespace

For example, consider a string s = “[email protected]”, after removing everything except the numbers and alphabets the string will become “Geeksno1”.

Let’s explore different ways to achieve this in Python:

Using Regular Expressions (re.sub())

re.sub() function replaces all characters that match a given pattern with a replacement. It’s perfect for pattern-based filtering.

Python

import re s1 = "Hello, World! 123 @Python$" s2 = re.sub(r'[^a-zA-Z0-9]', '', s1) print(s2)

Output

HelloWorld123Python

Explanation:

[^a-zA-Z0-9]: Matches any character that is not a letter or number.
re.sub() replaces these characters with an empty string.
The result is a clean string with only alphanumeric characters..

Using List Comprehension with str.isalnum()

List comprehension lets you filter characters efficiently in a single line.

Python

s1 = "Hello, World! 123 @Python$" s2 = ''.join([char for char in s1 if char.isalnum()]) print(s2)

Output

HelloWorld123Python

Explanation:

char.isalnum() returns True for letters and numbers.
Only those characters are included and joined into a new string.

Using filter() with str.isalnum()

The filter() function applies a condition (in this case, isalnum) to each character and filters out the rest.

Python

s1 = "Hello, World! 123 @Python$" s2 = ''.join(filter(str.isalnum, s1)) print(s2)

Output

HelloWorld123Python

Explanation:

filter(str.isalnum, s1) keeps only characters where isalnum() is True.
”.join() combines the valid characters into a new string.

Using a for Loop

Using a for loop, we can iterate through each character in a string and check if it is alphanumeric. If it is, we add it to a new string to create a cleaned version of the original.

Python

s1 = "Hello, World! 123 @Python$" s2 = '' for char in s1: if char.isalnum(): s2 += char print(s2)

Output

HelloWorld123Python

Explanation:

The loop checks each character.
If it’s alphanumeric, it’s added to the result string s2.

Also read: Strings, List comprehension, re.sub(), regular expressions, filter, str.isalnum(), ”.join() method.

Python - Replace occurrences by K except first character

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Article Tags :

Python
Python Programs
Python Regex-programs
Python string-programs
python-regex

Practice Tags :

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