'^d{1,2}/d{1,2}/d{4}^'  //  dd/mm/yyyy

'^d{1,2}/d{1,2}/d{2}^'  //  dd/mm/yy

$date_to_check = "22/06/2017";
$true = preg_match("/([0-9]{2})/([0-9]{2})/([0-9]{4})/", $date_to_check);
var_dump($true);

$date='99/99/2017';
$yearlen=strlen(substr($date,strrpos($date,'/')+1));
if($yearlen==2){
    echo 'two digit year';
}elseif($yearlen==4){
    echo 'four digit year';
}else{
    echo "unexpected year length ($yearlen)";
}

$date='99/99/2017';
$yearlen=strlen((preg_match('/d+$/',$date,$out)?$out[0]:''));
if($yearlen==2){
    echo 'two digit year';
}elseif($yearlen==4){
    echo 'four digit year';
}else{
    echo "unexpected year length ($yearlen)";
}

function validDate($date){
    $d=explode('/',$date,3);  // limit $d array to 3 elements
    if(sizeof($d)==3 && checkdate($d[1],$d[0],$d[2])){
        if(strlen($d[2])==4){
            return 'a valid date with four-digit year';
        }elseif(strlen($d[2])==2){
            return 'a valid date with two-digit year';
        }
    }
    return 'an invalid date';
}

$dates=['1/4/17','01/04/17','9/9/1999','32/12/17','99/99/9999'];
foreach($dates as $date){
    echo "$date is ",validDate($date),"n";
}

1/4/17 is a valid date with two-digit year
01/04/17 is a valid date with two-digit year
9/9/1999 is a valid date with four-digit year
32/12/17 is an invalid date
99/99/9999 is an invalid date