public function setExpectedRights($expectedRights)
    {
        
        if (!empty($expectedRights) && preg_match('/^r*w*x*$/', $expectedRights) !== 0) {
            $this->expectedRights = $expectedRights;    
        }
        else{
            throw new common_configuration_MalformedRightsException("Malformed rights. Expected format is r|rw|rwx.");
        }
        
    }