CIDR Notation (IPv4) Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching cidr notation (ipv4), ported and verified for JavaScript. In security-sensitive code, using an unverified regex can open the door to both false positives and denial-of-service attacks. 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

// CIDR Notation (IPv4)
// ReDoS-safe | RegexVault — Security > Network Security

const cidrNotationIpv4Regex = /^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\/(?:[0-9]|[12][0-9]|3[0-2])$/;

function validateCidrNotationIpv4(input: string): boolean {
  return cidrNotationIpv4Regex.test(input);
}

// Example
console.log(validateCidrNotationIpv4("192.168.1.0/24")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Security > Network Security 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

/0 = all IPs (default route), /32 = single host. Subnets should have a host part of all zeros for canonical CIDR notation (192.168.1.0/24, not 192.168.1.5/24). Common in firewall rules, security groups, and IP allowlists.