UK Postcode Regex Improved

uk_postcode_regex.php

<?php
/**
 * Notes:
 *
 * - Adapted from Bulk_Data_Transfer_-_additional_validation_valid_from_12_November_2015.pdf provided by gov.uk
 * - We don't care about GIR 0AA seen in many other regexes
 * - We accept (like the original) there will be a few false positives
 * - Allows a single, optional space between left and right hand side
 * - Case insensitive
 * - Example below shows how to return uppercase with 1 space standard format
 *
 * To allow for nicer formatting when taking postcodes:
 * - Capturing group $1 is the left hand side
 * - Capturing group $2 is the right hand side
 *
 * Play around with it: https://3v4l.org/ju94t
 * Feel free to make suggestions!
 */
$regex = '/^((?:[A-Z][0-9]{1,2})|(?:(?:[A-Z][A-HJ-Y][0-9]{1,2})|(?:(?:[A-Z][0-9][A-Z])|(?:[A-Z][A-HJ-Y][0-9]?[A-Z])))) ?([0-9][A-Z]{2})$/i';
$testPostCodes = [
    'HU1 1EN',
    'HU11EN',
    'hu1 1en',
    'hu11en',
];

foreach ($testPostCodes as $testPostCode) {
    $matches = [];
    if (!preg_match($regex, $testPostCode, $matches)) {
        throw new InvalidArgumentException("Invalid post code format");
    }
    $postcode = strtoupper($matches[1] . ' ' . $matches[2]);
    
    echo "$testPostCode matches as $postcode\n";
}