var aElems = document.getElementsByTagName("a");
for (var i=0, n=aElems.length; i<n; ++i) {
    if (/(?:^|\s+)_blank(?:\s+|$)/.test(aElems[i].className)) {
        aElems[i].onclick = function() {
            return !window.open(this.href, "_blank");
        }
    }
}