Fastest way to detect external URLs

Question

What\'s the fastest method to detect if foo=\'http://john.doe\' is an external url (in comparsion to window.location.href)?

Answer 1

I had to build on pseudosavant's and Jon's answers because, I needed to also catch cases of URLs beginning with "//" and URLs that do not include a sub-domain. Here's what worked for me:

var getDomainName = function(domain) {
    var parts = domain.split('.').reverse();
    var cnt = parts.length;
    if (cnt >= 3) {
        // see if the second level domain is a common SLD.
        if (parts[1].match(/^(com|edu|gov|net|mil|org|nom|co|name|info|biz)$/i)) {
            return parts[2] + '.' + parts[1] + '.' + parts[0];
        }
    }
    return parts[1]+'.'+parts[0];
};
var isExternalUrl = function(url) {
	var curLocationUrl = getDomainName(location.href.replace("http://", "").replace("https://", "").replace("//", "").split("/")[0].toLowerCase());
	var destinationUrl = getDomainName(url.replace("http://", "").replace("https://", "").replace("//", "").split("/")[0].toLowerCase());
	return !(curLocationUrl === destinationUrl)
};

$(document).delegate('a', 'click', function() {
	var aHrefTarget = $(this).attr('target');
	if(typeof aHrefTarget === 'undefined')
		return;
	if(aHrefTarget !== '_blank')
		return;  // not an external link
	var aHrefUrl = $(this).attr('href');
	if(aHrefUrl.substr(0,2) !== '//' && (aHrefUrl.substr(0,1) == '/' || aHrefUrl.substr(0,1) == '#'))
		return;  // this is a relative link or anchor link
	if(isExternalUrl(aHrefUrl))
		alert('clicked external link');
});

Internal URLs:

  
stackoverflow.com/questions/6238351/fastest-way-to-detect-external-urls
  
www.stackoverflow.com/questions/6238351/fastest-way-to-detect-external-urls
  
//stackoverflow.com/questions/6238351/fastest-way-to-detect-external-urls
  
//www.stackoverflow.com/questions/6238351/fastest-way-to-detect-external-urls

External URLs:

  
http://www.yahoo.com
  
yahoo.com
  
www.yahoo.com
  
//www.yahoo.com