<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<form id="login" action="" method="post">
<p>
<input type="text" name="username" placeholder="请输入用户名">
<span style="color: red; font-size: 12px"></span>
</p>
<p>
<input type="password" name="password" placeholder="请输入口令">
</p>
<p>
<input type="submit" value="登录">
</p>
</form>
<script src="js/jquery.min.js"></script>
<script>
$(function() {
$('input:first').val(localStorage.uid);
$('#login').on('submit', function(evt) {
evt.preventDefault();
var username = $('input:first').val();
if (/^[a-zA-Z][a-zA-Z0-9_]{5,19}$/.test(username)) {
localStorage.uid = username;
evt.target.submit();
} else {
$('input:first').next().text('请输入有效的用户名');
}
});
});
</script>
</body>
</html>