Remove the string on the beginning of an URL

Depends on what you need, you have a couple of choices, you can do:

// this will replace the first occurrence of "www." and return "testwww.com"
"www.testwww.com".replace("www.", "");

// this will slice the first four characters and return "testwww.com"
"www.testwww.com".slice(4);

// this will replace the www. only if it is at the beginning
"www.testwww.com".replace(/^(www\.)/,"");

If the string has always the same format, a simple substr() should suffice.

var newString = originalString.substr(4)

Yes, there is a RegExp but you don't need to use it or any "smart" function:

var url = "www.testwww.com";
var PREFIX = "www.";
if (url.startsWith(PREFIX)) {
  // PREFIX is exactly at the beginning
  url = url.slice(PREFIX.length);
}