IPv4 Loopback Address Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching ipv4 loopback address, 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

// IPv4 Loopback Address
// ReDoS-safe | RegexVault — Web & Network > IPv4

const ipv4LoopbackAddressRegex = /^127\.(?:(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])\.){2}(?:25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9][0-9]|[0-9])$/;

function validateIpv4LoopbackAddress(input: string): boolean {
  return ipv4LoopbackAddressRegex.test(input);
}

// Example
console.log(validateIpv4LoopbackAddress("127.0.0.1")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > IPv4 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 entire 127.0.0.0/8 block is reserved for loopback. 127.0.0.1 is the default but any address in the block routes to the local machine on Linux.