html

<textarea   placeholder="内容框"  id="kw"></textarea>
  
  
  
<a id="ll">仅保留字母数字</a>

js

 document.getElementById('ll').onclick = function () {   
    var input = document.getElementById("kw");  
 var value = input.value;
 var newValue = value.replace(/[^a-zA-Z0-9]/g, ''); 
 input.value = newValue; };