这么简单的代码都不对，我也是醉了= =为什么这个就是不行呢？求指教
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>key_up事件</title>
<script>
function tck(){
if(isNaN(txt1.value))
{
alert("字符是非法的!");
txt1.value="";
}
}
</script>
</head>
<body>
<input type="text" name="txt1" onkeyup="tck();"/>
</body>
</html>
