<input type="text" placeholder="请输入邮箱" id="email">

<script>
    email.onchange = function(){
        var email = this.value;
        var reg = /^([a-zA-Z]|[0-9])(\w|\-)+@[a-zA-Z0-9]+\.([a-zA-Z]{2,4})$/;
        if(reg.test(email)){
            alert("邮箱格式正确");
        }else{
            alert("邮箱格式不正确");
        }
    }
</script>
</html>