Checkout credit card expiration date formatter

add_action( 'wp_enqueue_scripts', function() {

	if( ! is_checkout() ) { return; }

	wp_enqueue_script( 'jquery' );

	wp_add_inline_script( 'jquery', "

		jQuery( document ).ready( function( $ ) {

			$( 'body.wfacp_main_wrapper' )
				.on( 'click paste keyup change', 'input#cc_expiry', function() {

				$( this ).val( formatExpiry( $( this ).val() ) );

			} );

		} );

		function formatExpiry( expiry ) {

			var mon, parts, sep, year;

			parts = expiry.match( /^\D*(\d{1,2})(\D+)?(\d{1,4})?/ );
			if( ! parts ) {
				return '';
			}

			mon = parts[1] || '';
			sep = parts[2] || '';
			year = parts[3] || '';
			if( year.length > 0 ) {
				sep = ' / ';
			} else if( sep === ' /' ) {
				mon = mon.substring( 0, 1 );
				sep = '';
			} else if( mon.length === 2 || sep.length > 0 ) {
				sep = ' / ';
			} else if(
				mon.length === 1
				&& ( mon !== '0' && mon !== '1' )
			) {
				mon = '0' + mon;
				sep = ' / ';
			}

			return mon + sep + year;

		}

	" );

} );