How to Replace Using Regex in Python: Simple Guide

In Python, you can replace text using regular expressions with the re.sub() function. It takes a pattern, a replacement string, and the original text, then returns the modified text with matches replaced.

Syntax

The basic syntax of re.sub() is:

• pattern: The regex pattern to find in the text.
• replacement: The string to replace each match with.
• string: The original text where replacements happen.
• Optional count: Number of replacements to make (default is 0, meaning replace all).

python

import re

result = re.sub(pattern, replacement, string, count=0)

Example

This example replaces all digits in a string with the '#' character using regex:

python

import re

text = "My phone number is 123-456-7890."
new_text = re.sub(r"\d", "#", text)
print(new_text)

Output

My phone number is ###-###-####.

Common Pitfalls

Common mistakes include:

• Not using raw strings (prefix r) for regex patterns, which can cause errors with backslashes.
• Forgetting that re.sub() returns a new string and does not change the original.
• Using incorrect regex patterns that don't match what you expect.

python

import re

# Wrong: pattern without raw string
text = "abc123"
# This will cause an error or unexpected behavior
# new_text = re.sub("\d", "#", text)  # Incorrect

# Right: use raw string for pattern
new_text = re.sub(r"\d", "#", text)  # Correct
print(new_text)

Output

abc###

Quick Reference

Tips for using re.sub():

• Always use raw strings for regex patterns: r"pattern".
• Remember re.sub() returns a new string; assign it to a variable.
• Use count to limit replacements if needed.
• Use parentheses in patterns to capture groups and refer to them in replacements.

Key Takeaways

Use re.sub() with a regex pattern, replacement string, and target text to replace text in Python.

Always write regex patterns as raw strings (prefix with r) to avoid errors with backslashes.

re.sub() returns a new string; it does not modify the original text in place.

Use the optional count argument to limit how many replacements happen.

Test your regex patterns to ensure they match what you want to replace.