ip- 這段Regex的含義？

ipphpRegex

整個函數是擷取真實IP的，

 function real_ip(){    static $realip = NULL;    if ($realip !== NULL)    {        return $realip;    }    if (isset($_SERVER))    {        if (isset($_SERVER['HTTP_X_FORWARDED_FOR']))        {            $arr = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);            /* 取X-Forwarded-For中第一個非unknown的有效IP字串 */            foreach ($arr AS $ip)            {                $ip = trim($ip);                if ($ip != 'unknown')                {                    $realip = $ip;                    break;                }            }        }        elseif (isset($_SERVER['HTTP_CLIENT_IP']))        {            $realip = $_SERVER['HTTP_CLIENT_IP'];        }        else        {            if (isset($_SERVER['REMOTE_ADDR']))            {                $realip = $_SERVER['REMOTE_ADDR'];            }            else            {                $realip = '0.0.0.0';            }        }    }    else    {        if (getenv('HTTP_X_FORWARDED_FOR'))        {            $realip = getenv('HTTP_X_FORWARDED_FOR');        }        elseif (getenv('HTTP_CLIENT_IP'))        {            $realip = getenv('HTTP_CLIENT_IP');        }        else        {            $realip = getenv('REMOTE_ADDR');        }    }    preg_match("/[\d\.]{7,15}/", $realip, $onlineip);    $realip = !empty($onlineip[0]) ? $onlineip[0] : '0.0.0.0';    return $realip;}

preg_match("/[\d.]{7,15}/", $realip, $onlineip); 這裡的Regex有點不明白，
為什麼數字或點要重複七到十五次？

