1.
<input type="text" name="formField" pattern="[a-zA-Z0-9]{6}" required>
2.
<form>
<input type="text" pattern="[a-zA-Z0-9]{6}" name="formName" />
<input type="submit" value="OK" />
</form>
3.
<html>
<body>
<form action="demo_form.asp">
Country code: <input type="text" name="country_code" pattern="[A-Za-z]{3}" title="Three letter country code">
<input type="submit">
</form>
</body>
</html>
4.