public function evaluate($values)
    {
        //evaluate identifier, starts with letter, contains letters, numbers and _, ends with letter, number
        if(preg_match("/^[a-z]+[a-z_0-9]*$/", $values) === 1){
            return true;
        } else {
            if (!$this->hasOption('message')) {
                $message = empty($values)
                    ? __('The index identifier should not be empty')
                    : __('"%s" is not a valid index identifier. It must start with a letter and contain letters, numbers or underscores only', $values)
                ; 
                $this->setMessage($message);
            }
        }
        return false;
    }