MC-XOBAHCK,
var url = prompt('Введите URL', 'https://javascript.ru/forum/misc/75836-pomogite-s-regulyarnym-vyrazheniem.html');
if( !url ) { alert(url); throw new Error(); }
domain = url.match(/(^https?):\/\/([^/]+)/);
if( !domain ) { alert(domain); throw new Error(); }
console.log(domain);
alert( domain );