getElementsByClassName-like function for all browsers (IE6+ / w3c)

index.js

var 
	/** @type {RegExp} @const */
	RE__getElementsByClassName = /\s*(\S+)\s*/g
	/** @type {string} @const */
	, STRING_FOR_RE__getElementsByClassName = '(?=(^|.*\\s)$1(\\s|$))'
	/** @type {boolean} @const */
	, IS_IELT9 = !/\[native\s+code\]/i.test(document.getElementsByClass + "")
	/** @type {RegExp} @const */
	, RE__querySelector__dottes = /\./g
;
/**
 * @param {(Document|Node|string)} root_or_klas if string -> klas. Class list with "." separator and start with "."
 * @param {string=} klas
 * @param {string=} tag
 */
function getElementsByClass(root_or_klas, klas, tag) {
	var result
		, preResult
		, i
		, child
	;

	if(typeof root_or_klas === "string") {
		tag = klas;
		klas = root_or_klas;
		root_or_klas = document;
	}

	tag = tag || "";

	if(!root_or_klas || klas == null) {
		return [];
	}

	if("querySelectorAll" in root_or_klas) {
		preResult = root_or_klas.querySelectorAll(tag + klas); // w3c and IE8

		if(IS_IELT9) {
			i = -1;
			result = [];
			while(child = preResult[++i]) {
				result.push(child);
			}
			return result;
		}
		else {
			return preResult;
		}
	}
	else { // IE < 8
		preResult = root_or_klas.getElementsByTagName(tag || "*");

		if(klas) {
			klas = klas.replace(RE__querySelector__dottes, " ");
	    		klas = new RegExp(klas.replace(RE__getElementsByClassName, STRING_FOR_RE__getElementsByClassName));
	    	}

		i = -1;
		result = [];

    		while(child = preResult[++i]) {
    			if(child.nodeType === 1 && (!klas || klas.test(child.className))) {
				result.push(child);
    			}
    		}

    		return result;
	}
}