<input type=text maxlength='20' onblur="check(this)">
<script>
function check(id)
{
var reg = /^[a-zA-Z0-9]+$/;
if (!reg.test(id))
alert("只能是字符!");
id.focus();
}
</script>
<script>
function check(id)
{
var reg = /^[a-zA-Z0-9]+$/;
if (!reg.test(id))
alert("只能是字符!");
id.focus();
}
</script>
var max=9;
function check(str){
if(str.length>=max)
alert("输入的字符不能超过9个！");
if(str.search(/\W/)!=-1)
{alert("所输入的必须为字符！");}}
</script>
</head><body>
<input type="text" onblur="check(this.value)"></body></html>