// checks whether or not the url is external to the local site.
// generously provided by StackOverflow: http://stackoverflow.com/questions/6238351/fastest-way-to-detect-external-urls
function isExternal(url) {
    // parse the url into protocol, host, path, query, and fragment. More information can be found here: http://tools.ietf.org/html/rfc3986#appendix-B
    var match = url.match(/^([^:\/?#]+:)?(?:\/\/([^\/?#]*))?([^?#]+)?(\?[^#]*)?(#.*)?/);
    // match[1] matches a protocol if one exists in the url
    // if the protocol in the url does not match the protocol in the window's location, this url is considered external
    if (typeof match[1] === "string" && 
            match[1].length > 0 
            && match[1].toLowerCase() !== location.protocol) 
        return true;
    // match[2] matches the host if one exists in the url
    // if the host in the url does not match the host of the window location, this url is considered external
    if (typeof match[2] === "string" && 
            match[2].length > 0 && 
            // this regex removes the port number if it patches the current location's protocol
            match[2].replace(new RegExp(":("+{"http:":80,"https:":443}[location.protocol]+")?$"), "") !== location.host) 
        return true;
    return false;
}