AS3 problem isolating a substring

I hope someone here can explain to me how to isolate a file name from the end of a URL link. It’s driving me nuts. The URL’s will have varying lengths so I need to grab everything that comes after the equals Sign ( = )

    private function linkHandler(evt:MouseEvent):void {
        if(shareScreen.mouseY < 80) {
            stage.focus = shareField;
// The next line copies a URL to the clipboard so the user can copy and paste it elsewhere.
// a typical URL looks like this but the length will not always be the same.


// I need to get the file name, in this case " ccotest.mp4 ' into a text variable so the
// next 2 lines will work but I can't get it to work. Can someone advise me how.
//  I added these 2 lines to call a php routine that actually FORCE A
//  DOWNLOAD of the file instead.
            url = 'force_download.php?file='+encodeURI(config['link']);
            navigateToURL(new URLRequest(url));

Any help will sure be appreciated - Jim