removing http:// or http:// and www

/^(?:https?:\/\/)?(?:www\.)?/i where both https:// and www. should be optional (?) and non-capturing groups ((?:...)).

var url = prompt("url: ");

url = url.replace(/^(?:https?:\/\/)?(?:www\.)?/i, "").split('/')[0];

alert("url: " + url);

This will take care of http, https and www

url.replace(/^(?:https?:\/\/)?(?:www\.)?/i, "").split('/')[0]