getElementsByClassName.js

// Element.prototype.getElementsByClassName (using XPath or getElementsByTagName)
// Monkeypatching is evil
if(typeof Element.getElementsByClassName === "undefined") {
    if("evaluate" in document) {
        Element.prototype.getElementsByClassName = function(className) {
            var result = document.evaluate(".//*[contains(concat(' ', @class, ' '), '"+className+"')]", this, null, XPathResult.ANY_TYPE, null),
                node,
                elements = [];

            while(node = result.iterateNext()) {
                elements.push(node);
            }

            return elements;
        }
    } else {
        Element.prototype.getElementsByClassName = function(className) {
            var allChilds = this.getElementsByTagName("*"),
                re = new RegExp('\\b'+className+'\\b');
                elements = [];

            for(var i=0; i<allChilds.length; i++) {
                if(~allChilds[i].className.search(re)) {
                    elements.push(allChilds[i]);
                }
            }

            return elements;
        }
    }
}