function regEvent(eventname, elem, handler) {
    if (window.addEventListener) {
        elem.addEventListener(eventname, handler, false);
    }
    else if (window.attachEvent) {
        elem.attachEvent("on" + eventname, handler);
    }
}

function externalLinks() {
 if (!document.getElementsByTagName) return;
 var anchors = document.getElementsByTagName("a");
 for (var i=0; i<anchors.length; i++) {
   var anchor = anchors[i];
   if (anchor.getAttribute("href") &&
       anchor.getAttribute("rel") == "external")
     anchor.target = "_blank";
 }
}
regEvent("load", window, externalLinks);