UK National Insurance Number (NI) Regex for Python

What this pattern does

This page provides a comprehensive, battle-tested regular expression for matching uk national insurance number (ni), ported and verified for Python. 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 Python project — whether you're validating in a Django view, a FastAPI endpoint, or a standalone data processing script.

Python Implementation

# UK National Insurance Number (NI)
# ReDoS-safe | RegexVault — Identity & PII > National Identity Numbers

import re

uk_national_insurance_number_ni_pattern = re.compile(r'^(?!(?:BG|GB|KN|NK|NT|TN|ZZ))[A-CEGHJ-NPR-TW-Z][A-CEGHJ-NPR-TW-Z][0-9]{6}[A-D]$')

def validate_uk_national_insurance_number_ni(value: str) -> bool:
    return bool(uk_national_insurance_number_ni_pattern.fullmatch(value))

# Example
print(validate_uk_national_insurance_number_ni("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 Python developers because particularly important in Python web servers where CPU-bound regex operations can stall concurrent request handling. 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.