X-Forwarded-For Header Value Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching x-forwarded-for header value, ported and verified for JavaScript. A rigorously tested regex reduces debugging time and protects your application from edge-case failures. The snippet below is ready to drop into your JavaScript project — whether you're validating in an Express middleware, a Next.js API route, or a client-side form.

Javascript Implementation

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

const xforwardedforHeaderValueRegex = /^(?:(?: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}))*$/i;

function validateXforwardedforHeaderValue(input: string): boolean {
  return xforwardedforHeaderValueRegex.test(input);
}

// Example
console.log(validateXforwardedforHeaderValue("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 JavaScript developers because especially critical in long-running Node.js event loops where a ReDoS vulnerability can block the entire process. 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.