[рони]

Le7o,

var str = "tag.class1.class2.class3#id"
function fn(str) {
  var tag = "div", match, div = document.createElement("div"), elem;
  if (match = str.match(/^[^.#]+/)) {
    tag = match[0];
  }
  elem = document.createElement(tag);
  if (match = str.match(/#([^.#]+)/g)) {
    elem.id = match[0].slice(1);
  }
  if (match = str.match(/\.([^.#]+)/g)) {
    match.forEach(function(cls) {
      elem.classList.add(cls.slice(1));
    });
  }
  div.appendChild(elem);
  return div.innerHTML;
};
alert(fn(str))