扣扣位数限制(练习)
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
账号:<input type="text" id="a">
<br>
<input type="submit" id="c" value="登录">
<script>
var a=document.getElementById("a");
var c=document.getElementById("c");
c.onclick=function(){
if(a.value=="")
alert("null");
else if(!/^[0-9]\d{4,8}$/.test(a.value))
alert("请输入5-9位");
else
alert("ok");
}
</script>
</body>
</html>