SOURCE

  var oldEl = document.getElementById('old');
  var newEl = document.getElementById('new');
  //模拟ajax数据
  var data = oldEl.innerHTML;
  //处理实体字符
  data = data.replace(/&lt;/g, '<');
  data = data.replace(/&gt;/g, '>');
  var newData = data.match(/<xmls><\/xmls>([\s\S]+?)<xmle><\/xmle>/);
  //取第一个子表达式值
  newEl.innerHTML = newData[1];

old:<textarea rows="10" cols="50" id="old">
  <xmls></xmls>
  <div>
    <ul>
      <li>111</li>
      <li>222</li>
    </ul>
  </div>
  <xmle></xmle>
  <script>
    var demo = 1;
  </script>
</textarea>
<br>
new:<textarea rows="10" cols="50" id="new"></textarea>