problem using Validate::isUrl

[zerros]

Hello,

Please forgive me for my bad english, I'm a french people.
I'm trying to verify if my input text in my form is in URL format.

if(Tools::isSubmit('submiturl')) {
    if(Validate::isUrl(Tools::getValue('myurl'))) {
        $smarty->assign('ERR', "URL OK" . Tools::getValue('myurl'));
    } else {
        $smarty->assign('ERR', "URL NOK" . Tools::getValue('myurl'));
    }    
}

This code returns "URL NOK" if myurl is empty, but always returns "URL OK" if anything is post.
Could you help me to verify if myurl is in URL format ? the Validate::isUrl seemd to not function.

Thanks in advance for your help.