X-Forwarded-For Header Value Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching x-forwarded-for header value, ported and verified for Python. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# X-Forwarded-For Header Value
# ReDoS-safe | RegexVault — Web & Network > HTTP

import re

xforwardedfor_header_value_pattern = re.compile(r'^(?:(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])(?:\.(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])){3}|[0-9a-fA-F]{0,4}(?::[0-9a-fA-F]{0,4}){2,7})(?:\s*,\s*(?:(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])(?:\.(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])){3}|[0-9a-fA-F]{0,4}(?::[0-9a-fA-F]{0,4}){2,7}))*$')

def validate_xforwardedfor_header_value(value: str) -> bool:
    return bool(xforwardedfor_header_value_pattern.fullmatch(value))

# Example
print(validate_xforwardedfor_header_value("192.168.1.1"))  # True

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > HTTP category and carries a ReDoS-safe certification. That matters for Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

The leftmost IP is typically the original client, subsequent IPs are proxies. Never trust X-Forwarded-For blindly — it can be spoofed by clients. Only trust it when set by a known, controlled proxy.