ZIP code validation

Hi there,

I've been having some troubles with GravityForms... Or users, to be more precise. They seem to be mistaking the ZIP code field for street number. So I decided I need a validation on the ZIP code. Problem is... I use the advanced address field you guys supplied, and I can't get the validation to work on that. Our zip code (postcode) is made up out of 4 numbers, an optional space and 2 letters. The problem is probably in the field id. I tried it with
$field_value = rgpost("input_{$field['id']}");
and
$field_value = rgpost("input_{$field['id']}.3");
since the zipcode is in input_x.3

Both fail. If I use it on a normal text line, the validation does work, but this is not an option.

Full code:

add_filter('gform_validation', 'validate_postcode');

function validate_postcode($validation_result) {

    $form = $validation_result["form"];
    $current_page = rgpost('gform_source_page_number_' . $form['id']) ? rgpost('gform_source_page_number_' . $form['id']) : 1;
    foreach($form['fields'] as &$field){
        if(strpos($field['cssClass'], 'postcode') === false)
            continue;

        $field_page = $field['pageNumber'];
        $is_hidden = RGFormsModel::is_field_hidden($form, $field, array());

        if($field_page != $current_page || $is_hidden)
            continue;

        $field_value = rgpost("input_{$field['id']}.3");
        $is_valid = is_postcode($field_value);
        if($is_valid)
            continue;

        $validation_result['is_valid'] = false;
        $field['failed_validation'] = true;
        $field['validation_message'] = 'De postcode is niet correct ingevuld.';

    }

    $validation_result['form'] = $form;
    return $validation_result;
}

function is_postcode($value) {
	if (preg_match('/^[1-9][0-9]{3} ?[a-zA-Z]{2}$/', $value)) {
		return true;
	} else {
		return false;
	}
}