If this needs to work in IE 7 or lower you need to remember that getElementsByClassName does not exist in all browsers. Because of this you can create your own getElementsByClassName or you can try this.
var fooDiv = document.getElementById("foo");
for (var i = 0, childNode; i <= fooDiv.childNodes.length; i ++) {
childNode = fooDiv.childNodes[i];
if (/bar/.test(childNode.className)) {
childNode.innerHTML = "Goodbye world!";
}
}