function parseExternalUrl(url) {
  //                                   single quotes [2]
  //                                   v           double quotes [3]
  //                                   v           v        no quotes [4]
  //                                   v           v        v
  var urlMatch = url.match(/url\(('([^']+)'|"([^"]+)"|([^'"\)]+))\)/) || [];
  return urlMatch[2] || urlMatch[3] || urlMatch[4];
}
Parse external URL.
@param url - CSS url string.