// Method to test if two values are equal. To use this rule, the form
public function test(SimpleXMLElement $element, $value, $group = null, JRegistry $input = null, JForm $form = null)
{
    $field = (string) $element['field'];

    // Check that a validation field is set.
    if (!$field)
    {
        throw new UnexpectedValueException(sprintf('$field empty in %s::test', get_class($this)));
    }

    if (is_null($form))
    {
        throw new InvalidArgumentException(sprintf('The value for $form must not be null in %s', get_class($this)));
    }

    if (is_null($input))
    {
        throw new InvalidArgumentException(sprintf('The value for $input must not be null in %s', get_class($this)));
    }

    // HERE i added conditionnal to take account of group
    if(isset($group))
    {
        $test = $input->get($group.'.'.$field);
    }
    else
    {
        $test = $input->get($field);
    }

    // Test the two values against each other.
    if ($value == $test)
    {
        return true;
    }

    return false;
}