protected function validate_contains($field, $input, $param = null)
    {
        if (!isset($input[$field])) {
            return;
        }

        $param = trim(strtolower($param));

        $value = trim(strtolower($input[$field]));

        if (preg_match_all('#\'(.+?)\'#', $param, $matches, PREG_PATTERN_ORDER)) {
            $param = $matches[1];
        } else {
            $param = explode(chr(32), $param);
        }

        if (in_array($value, $param)) { // valid, return nothing
            return;
        }

        return array(
            'field' => $field,
            'value' => $value,
            'rule' => __FUNCTION__,
            'param' => $param,
        );
    }