親フォルダ タブ
<!DOCTYPE html>
<html>
<head>
<meta content="width=device-width initial-scale=1.0 minimum-scale=1.0 maximum-scale=1.0 user-scalable=no" name="viewport">
<meta charset="UTF-8">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.1/css/bootstrap.css">

<style>
</style>

<script>
</script>

</head>
<body>
<div class="alert alert-primary">
正規表現で入力チェック( required は必須入力 )
</div>

<form method="post">

ユーザーID:
<input required type="text" name="userid" pattern="[0-9]+">
<input type="submit" name="send" value="ログイン">

</form>


</body>
</html>