IPv6 with CIDR Prefix Length Regex for JavaScript

What this pattern does

This page provides a well-structured, multi-part regular expression for matching ipv6 with cidr prefix length, 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

// IPv6 with CIDR Prefix Length
// ReDoS-safe | RegexVault — Web & Network > IPv6

const ipv6WithCidrPrefixLengthRegex = /^[0-9a-fA-F:]+\\/(?:12[0-8]|1[01][0-9]|[1-9][0-9]|[0-9])$/i;

function validateIpv6WithCidrPrefixLength(input: string): boolean {
  return ipv6WithCidrPrefixLengthRegex.test(input);
}

// Example
console.log(validateIpv6WithCidrPrefixLength("2001:db8::/32")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Web & Network > IPv6 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 IPv6 portion uses a simplified [0-9a-fA-F:]+ to avoid duplicating the full IPv6 validation. Pair with net-ipv6-02 for full address validation before applying this pattern.