var url;
var scripts = document.getElementsByTagName('script');
for (var i=0; i<scripts.length; i++) {
  if (scripts[i].innerHTML.search(/mp4/) != -1) {
    var ihtml = scripts[i].innerHTML;
    var regex = /sd_src[\\u0-9A-F]*(http.*?)thumbnail_src/;
    var m = regex.exec(ihtml);
    url = m[1];  // still escaped with unicode escapes
  }
}

var s = '{ "a": "'+url+'"}'; // create JSON object as string to get a succesful parse
url = JSON.parse(s).a; // parse as JSON and save url
url = unescape(url); // remove the URI encoding
url = url.replace(/\\/g, ''); // remove the escaped slashes
var r = /(.*?)"/;
url = r.exec(url)[1]; // trim the last ",

alert(url);