Check Greek postcodes at checkout

<?php

add_action(
	'woocommerce_after_checkout_validation',
	static function ( $data, $errors ) {
		foreach ( array( 'billing', 'shipping' ) as $type ) {
			if ( 'shipping' === $type && empty( $data['ship_to_different_address'] ) ) {
				continue;
			}
			if ( 'GR' !== ( $data[ $type . '_country' ] ?? '' ) ) {
				continue;
			}
			$postcode = preg_replace( '/\s+/', '', (string) ( $data[ $type . '_postcode' ] ?? '' ) );
			if ( '' !== $postcode && ! preg_match( '/^[1-8]\d{4}$/', $postcode ) ) {
				$errors->add( 'validation', 'Ο ταχυδρομικός κώδικας πρέπει να έχει 5 ψηφία, π.χ. 57001.' );
			}
		}
	},
	10,
	2
);