[php] preg_match_all non trova niente

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$text= "blablabla http: //www.pippo.it blabla";
$url_pattern = "http:\/\/[[:alpha:],0-9._\/?!:&=()~°+_'-%]+";

preg_match_all($url_pattern, $text, $valore);
print_r($valore);

non cambia niente così...