Developer Guide

UK Postcode Validation

How to validate UK postcodes in your application — regex patterns, edge cases, normalisation, and API-based validation.

The Standard Validation Regex

This regex handles all standard UK postcode formats:

/^[A-Z]{1,2}[0-9][0-9A-Z]?\s?[0-9][A-Z]{2}$/i

It accepts postcodes with or without a space, in upper or lower case, and covers all UK postcode area formats.

Validation in JavaScript

function isValidUKPostcode(postcode) {
  const regex = /^[A-Z]{1,2}[0-9][0-9A-Z]?\s?[0-9][A-Z]{2}$/i;
  return regex.test(postcode.trim());
}

isValidUKPostcode('NR21 8AB');   // true
isValidUKPostcode('SW1A 1AA');   // true
isValidUKPostcode('invalid');    // false
isValidUKPostcode('NR218AB');    // true (no space)

Validation in Python

import re

def is_valid_uk_postcode(postcode):
    pattern = re.compile(r'^[A-Z]{1,2}[0-9][0-9A-Z]?\s?[0-9][A-Z]{2}$', re.IGNORECASE)
    return bool(pattern.match(postcode.strip()))

is_valid_uk_postcode('NR21 8AB')  # True
is_valid_uk_postcode('ZZ99 9ZZ')  # False

Validation in PHP

function is_valid_uk_postcode(string $postcode): bool {
    $pattern = '/^[A-Z]{1,2}[0-9][0-9A-Z]?\s?[0-9][A-Z]{2}$/i';
    return (bool) preg_match($pattern, trim($postcode));
}

is_valid_uk_postcode('NR21 8AB');  // true
is_valid_uk_postcode('not valid'); // false

Normalising Before Validation

Always normalise a postcode before validating — strip whitespace, uppercase, and ensure correct spacing:

function normalisePostcode(raw) {
  const p = raw.toUpperCase().replace(/\s+/g, '').trim();
  return p.slice(0, -3) + ' ' + p.slice(-3);
}

// Then validate the normalised version
const postcode = normalisePostcode(userInput);
if (isValidUKPostcode(postcode)) {
  // safe to use
}

Edge Cases to Handle

Input	Issue	Normalised
`nr21 8ab`	Lowercase	`NR21 8AB`
`NR218AB`	No space	`NR21 8AB`
`NR21 8AB`	Leading/trailing spaces	`NR21 8AB`
`NR21 8AB`	Double space	`NR21 8AB`
`BT1 1AA`	Northern Ireland (BT prefix)	`BT1 1AA` ✓

API-Based Validation

For production applications, validate postcodes against a live dataset rather than just regex. The Job Bookers Routing API returns a 400 invalid_postcode error for malformed postcodes and a 404 not_found error for postcodes that don't exist in the dataset — giving you both format and existence validation in one call.

GET /v1/geocode?postcode=ZZ99+9ZZ

{
  "error": {
    "code": "invalid_postcode",
    "message": "Not a valid UK postcode."
  }
}

Validate postcodes and get routing data in one API

The Job Bookers Routing API validates UK postcodes and returns coordinates, drive times and street names. From £19/month.

View pricing