various url validation using regular expression

How can I validate the following urls:

1. http:www.google.com/test
2. http:www.google.com
3. www.google.com
4. google.com

In all the above cases, my regular expression will return true and also I want to extract the url from the first case.How we cam impl开发者_如何学Pythonement this feature?

My stab at it:

var match = url.match(/(http:\/\/www\.google\.com\/test)|http:\/\/(www\.)?google\.com/);
if(match){
  alert("found it");
  if(match.length>1 && match[1]){
    alert("captured it "+match[1]);
  }
}

Apparently, a capturing group in only one side of the | makes the capturing group to be there but undefined

PS: I have added slashes to the URLs, shouldn't they be there?