UK National Insurance Number (NI) Regex for PHP

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching uk national insurance number (ni), ported and verified for PHP. 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 PHP project — whether you're validating in a Laravel validator, a WordPress plugin, or a standalone PHP script.

Php Implementation

<?php
// UK National Insurance Number (NI)
// ReDoS-safe | RegexVault — Identity & PII > National Identity Numbers

define('UK_NATIONAL_INSURANCE_NUMBER_NI_PATTERN', '/^(?!(?:BG|GB|KN|NK|NT|TN|ZZ))[A-CEGHJ-NPR-TW-Z][A-CEGHJ-NPR-TW-Z][0-9]{6}[A-D]$/');

function validate_uk_national_insurance_number_ni(string $input): bool {
    return (bool) preg_match(UK_NATIONAL_INSURANCE_NUMBER_NI_PATTERN, $input);
}

// Example
var_dump(validate_uk_national_insurance_number_ni("AB123456A")); // bool(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 PHP developers because especially relevant in PHP where PCRE backtracking limits can trigger silent failures on malicious input. 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.