mypmpro-validate-uk-postal-code.php

<?php
/**
 * This recipe will validate a UK postal code when submitting on checkout. 
 * Validation will only occur if the billing country has been set to the UK.
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function mypmprouk_scripts(){
	$ajaxurl = admin_url( 'admin-ajax.php' );
	$nonce = wp_create_nonce( 'pmprouk_validate_postcode' );
	?>
	<script>
		var $ = jQuery;

		var form = $('#pmpro_form, .pmpro_form');

		$( '.pmpro_form' ).submit( function( event ) {
			
			event.preventDefault();

			var postal_code = $( '#bzipcode' ).val();
			var country = $( '#bcountry' ).val();

			var data = {
				action: 'pmprouk_validate_postcode',
				security: '<?php echo $nonce; ?>',
				postal_code: postal_code,
				country: country
			}

			jQuery.post( '<?php echo $ajaxurl; ?>', data, function( response ){

				response = JSON.parse( response );
				
				if( response.status == 200 ){
					//Valid postal code
					this.submit();
					return true;
				} else if( response.status == 404 ) {

					$( '#pmpro_message' ).text( 'Invalid Postal Code Entered' ).addClass( 'pmpro_error' ).removeClass( 'pmpro_alert' ).removeClass( 'pmpro_success' ).show();

					return false;

				} else {					
					this.submit();
					return true;
				}
			});
		});
	</script>
	<?php
}
add_action('wp_footer', 'mypmprouk_scripts');

function mypmprouk_validate_postcode(){

	check_ajax_referer( 'pmprouk_validate_postcode', 'security' );
	
	if( !empty( $_REQUEST['postal_code'] ) && !empty( $_REQUEST['country'] ) ){

		if( $_REQUEST['country'] == 'GB' ){
			$postal_code = sanitize_text_field( $_REQUEST['postal_code'] );

			$request = wp_remote_get( 'https://api.postcodes.io/postcodes/'.$postal_code );

			$response = wp_remote_retrieve_body( $request );

			if( !is_wp_error( $response ) ){
	 			echo $response;
			} else {
				echo '{"status":404,"error":"Invalid postcode"}';
			}
		} else {
			echo '{"status":999,"error":"Invalid Country"}';
		}

	}
	wp_die();
}
add_action( 'wp_ajax_pmprouk_validate_postcode', 'mypmprouk_validate_postcode' );
add_action( 'wp_ajax_nopriv_pmprouk_validate_postcode', 'mypmprouk_validate_postcode' );

This recipe is included in the blog post on "Adding UK Postal Code Validation to Your Website’s Checkout Form" at Paid Memberships Pro here: https://www.paidmembershipspro.com/uk-postal-code-validation-at-checkout/