UK National Insurance Number Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching uk national insurance number, ported and verified for JavaScript. Financial data validation has zero tolerance for false negatives — a missed invalid entry can corrupt downstream calculations. 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

// UK National Insurance Number
// ReDoS-safe | RegexVault — Finance > Tax & Registration

const ukNationalInsuranceNumberRegex = /^(?:QQ|(?!BG|GB|NK|KN|TN|NT|ZZ)[A-CEGHJ-NOPR-TW-Z]{1}[A-CEGHJ-NPR-TW-Z]{1})[0-9]{6}[A-D]$/;

function validateUkNationalInsuranceNumber(input: string): boolean {
  return ukNationalInsuranceNumberRegex.test(input);
}

// Example
console.log(validateUkNationalInsuranceNumber("AB123456C")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Finance > Tax & Registration 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

Invalid prefixes: BG, GB, NK, KN, TN, NT, ZZ. Second letter excludes D, F, I, Q, U, V. First letter also excludes D, F, I, Q, U, V plus O. Suffix must be A-D (not E onward).