UK National Insurance Number (NI) Regex for JavaScript

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching uk national insurance number (ni), ported and verified for JavaScript. Identity and credential patterns need both correctness and safety, since they're frequent targets for adversarial input. 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 (NI)
// ReDoS-safe | RegexVault — Identity & PII > National Identity Numbers

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

function validateUkNationalInsuranceNumberNi(input: string): boolean {
  return ukNationalInsuranceNumberNiRegex.test(input);
}

// Example
console.log(validateUkNationalInsuranceNumberNi("AB123456A")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Identity & PII > National Identity Numbers 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 include BG, GB, KN, NK, NT, TN, ZZ. Letters D, F, I, O, Q, U, V are excluded from the second character. Letters C, I, K, M, O, V are excluded from the first. Suffixes A-D indicate the benefit payment week within a quarterly cycle.