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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);