<input type="text" id="txt">
  <input type="button" id="btn" value="校验">
  <script>
    var txt = document.getElementById("txt");
    var btn = document.getElementById("btn")
    btn.onclick = function(){
      var re = /^\w+@[a-z0-9]+\.[a-z]{2,4}$/;
      if(re.test(txt.value)){
        alert("你写对了")
      }else {
        alert("你写错了")
      }
    }
  </script>