How to restrict user registrations based on postcode/zipcode

[j45p41]

I would like to restrict users with certain letters in the postcode from not registering for my site.

Does anybody know which php code I would edit for this? It seems like the postcode fields are validated in javascript using the following code:

function myValidator(f, t)
{
    f.task.value=t;
    if (document.formvalidator.isValid(f)) {
        f.submit();
        return true;
    } else {
        var msg = '<?php echo addslashes( JText::_('COM_VIRTUEMART_USER_FORM_MISSING_REQUIRED_JS') ); ?>';
        alert (msg);
    }
    return false;
}
in file: templates\\html\com_virtuemart\user\edit.php

the fields seems to be captured in edit_address.php.

I would just like to know where I can check for user entry of certain characters in the postcode string and send out a message to the user.