How to Insert Commas into Numbers Using Python Regex – Quick Guide

1. Introduction

In a recent Python community chat, a member asked how to format a positive integer string with thousand separators (e.g., 12345678 → 12,345,678) using regular expressions.

2. Implementation

The solution uses a regex that matches a digit followed by groups of three digits up to the end of the string: (\d)(?=(\d{3})+$) Applying re.sub inserts a comma after each matched digit:

import re

num_str = "12345678"
formatted_num_str = re.sub(r'(\d)(?=(\d{3})+$)', r'\1,', num_str)
print(formatted_num_str)  # Output: 12,345,678

For more details, refer to Python’s official re documentation.

3. Conclusion

The article demonstrates a concise regex‑based method to add thousand separators to numeric strings in Python, helping readers solve similar formatting problems.