Forcing form text to be lower-case

How could I force the text in the "username" text input to be lower-case regardless of what user types?

<div class="register">
   <label for="username">Select username:</label>
</div> 
<div class="registerform">
    <input name="username" class="registerfont" type="text"
           id="username" maxlength="15" style="height:35px; width:300px">
</div>