<script language=javascript>
function check() {
if (isNaN(document.myform.username.value)) {
alert("你输入的不是数字");
document.myform.username.focus();
return false;
}
}
</script>
<form method=post name=myform action="" onsubmit="return check()" >
<input type=text name=username size=20>
<input type=submit value="ok">
</form>