表单验证

<input type="text"/ id="txt">
    <script type="text/javascript">
        var txt = document.getElementById("txt");
        txt.onblur = function()
        {
            if(txt.value.trim()=="")//为空且去除空格
            {
                alert("请输入用户名");
            }
        }
    </script>

<input type="password" id="psw1"/>
    <input type="password" id="psw2"/>
    <script type="text/javascript">
        var psw1 = document.getElementById("psw1");
        var psw2 = document.getElementById("psw2");
        psw2.onblur = function()
        {
            if(psw1.value==psw2.value)
            {
                alert("正确");
            }
            else
            {
                alert("两次输入的密码不相同");
            }
        }   
    </script>

<input type="text"/ id="age">
    <script type="text/javascript">
        var age = document.getElementById("age");
        age.onblur = function()
        {
            if(age.value>=18)
            {
                alert("通过");
            }
            else
            {
                alert("没有达到要求的年龄");
            }
        }
    </script>

<input type="text"/ id="email">
    <script type="text/javascript">
        var email = document.getElementById("email");
        email.onblur = function()
        {
            var zhz = /^(\w)+(\.\w+)*@(\w)+((\.\w{2,3}){1,3})$/;
            if(email.value.match(zhz)==null)//match方法内写正则表达式，符合会返回符合的内容，不符合会返回null
            {
                alert("请输入有效的邮箱")
            }
        }
    </script>