Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
function externISBN() {
  for (var i = 0; i < document.links.length; i++) 
    {       
         var ln = document.links[i].href.match(/isbn=(.*)/);
         if (ln) {
           document.links[i].href='http://www.amazon.com/exec/obidos/ASIN/'+RegExp.$1;
         }
    }

}

$(externISBN);
  NODES
Note 3