Add `F-` in front of French postcodes in WooCommerce

functions.php

/* This will "normalize" the postcode entered by the customer, if the country selected is France.
 * If the customer entered their postcode as a 5-digit number, it will add `F-` in the front.
 * If they already entered `F-` the post code will stay as it is.
 */

add_filter( 'woocommerce_format_postcode', 'custom_add_post_code_format_for_france', 10, 2 );

function custom_add_post_code_format_for_france( $postcode, $country ){
	$postcode = wc_normalize_postcode( $postcode );
	switch ( $country ) {
		case 'FR' :
			if ( (bool) preg_match( '/^F([0-9]{5})$/i', $postcode ) ) {
				$postcode = trim( substr_replace( $postcode, '-', 1, 0 ) );			  
			} else if ( (bool) preg_match( '/^([0-9]{5})$/i', $postcode ) ) {
				$postcode = trim( substr_replace( $postcode, 'F-', 0, 0 ) );					  
			}

			break;
	}
	return $postcode;
}
/* This will validate the postcode entered by the customer and returned by the function above if the
 * country selected is France. This function checks to see if the post code is formatted as 
 * `F-` followed by exactly 5 digits 0-9.
 */

add_filter( 'woocommerce_validate_postcode', 'sd_validate_post_code_for_france', 10, 3 );

function sd_validate_post_code_for_france( $valid, $postcode, $country ){
	switch ( $country ) {
		case 'FR':
			$valid = (bool) preg_match( '/^F-([0-9]{5})$/', $postcode );
			break;
	}
	return $valid;
}