Localhost Variants Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching localhost variants, 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

// Localhost Variants
// ReDoS-safe | RegexVault — Web & Network > Domain

const localhostVariantsRegex = /^(?:localhost|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])|::1)$/i;

function validateLocalhostVariants(input: string): boolean {
  return localhostVariantsRegex.test(input);
}

// Example
console.log(validateLocalhostVariants("localhost")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > Domain 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

Used in security middleware to detect and allow or block local connections. The i flag allows LOCALHOST or Localhost.