Komma ikke tillatt

Er der nogen der kan hjelpe med en "Alert" hvis der bruges "," (komma) i inputfelterne i nedenstående php-script.

<b>**** Registration Form ****</b><br><br>
<center><font color="#FF0000"><B>Please dont use , (comma) in the inputfields</B></font></center>
<form action="<?=$_SERVER["PHP_SELF"]?>" method="post" onsubmit="return validate_add();">
    <table border="0" cellpadding="5" width="600">
      <tr>
        <td>Firstname Lastname:</td>
        <td><input type="text" name="name" size="25"></td>
        <td>Email:</td>
        <td><input type="text" name="email" size="25"></td>
      </tr>
      <tr>
        <td>Webpage:</td>
        <td>http://<input type="text" name="url" size="18"></td>
        <td>Keyboard:</td>
        <td><input type="text" name="board" size="25"></td>
      </tr>
      <tr>
       
      <td>Town/City:</td>
        <td><input type="text" name="location" size="25"></td>
        <td>Country:</td>
        <td><input type="text" name="country" size="25"></td>
      </tr>
      <tr>
        <td></td>
        <td></td>
        <td><input type="hidden" name="action" value="add"></td>
        <td><input type="submit" value=" Add "></td>
      </tr>
    </table>
</form>