[vk65535]

var ems = (function() {
	var re = /\<(\/?)em\>/g;
	return function(str) {
		var r = [], _, level = 0, last;
		for (re.lastIndex = 0; _ = re.exec(str); )
			if (!_[1]) {
				if (!level++) last = re.lastIndex;
			} else if (level && !--level)
				r.push(str.substr(last, _.index - last));
		if (level) r.push(str.substr(last));
		return r;
	};
})();

alert(ems('string <em>substring<em>substring</em>substring</em> xxx substring xxx <em>substring</em> xxx <em>substring</em>').join('\n'));