HTML Form Verification checks

I'm currently trying to do some verification checks for my form, here's a sample bit of code. I want it to allow any letters A-Z but have no characters such as 0-9 ? , > < @ : ; etc. etc.

<form action="AddClient.html" method="post">
First name: <input type="text" name="first_name" required><br>
<input type="submit" value="Submit">
</form>