<html>
<body>
<form action="demo-form.php">
Country code: <input type="text" name="country_code" pattern="[A-Za-z]{3}" title="Three letter country code">
<input type="submit">
</form>
<p><strong>注意:</strong> Internet Explorer 9及更早IE版本，或Safari不支持input标签的 pattern 属性。</p>
</body>
</html>