Remove All Characters Except Letters and Numbers - Python

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.

import re

s1 = "Hello, World! 123 @Python$"

s2 = re.sub(r'[^a-zA-Z0-9]', '', s1)

print(s2)

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.

s1 = "Hello, World! 123 @Python$"

s2 = ''.join([char for char in s1 if char.isalnum()])

print(s2)

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.

s1 = "Hello, World! 123 @Python$"

s2 = ''.join(filter(str.isalnum, s1))

print(s2)

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.

s1 = "Hello, World! 123 @Python$"
s2 = ''

for char in s1:
    if char.isalnum():
        s2 += char

print(s2)

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.