Description:
------------
(Sorry for my english)
Subpattern is not captured for backreference if it is followed by '?' and does not appear in subject string.

Reproduce code:
---------------
<?
$str=<<<EOD
<a href="aaa">
<a href='aaa'>
<a href=aaa>
EOD;

// Following RE works properly
$re='/<a href=(["\']?)(\w+)\1>/iS';
preg_match_all($re,$str,$regs,PREG_SET_ORDER);
print_r($regs);

// Following RE works wrong
$re='/<a href=(["\'])?(\w+)\1>/iS';
preg_match_all($re,$str,$regs,PREG_SET_ORDER);
print_r($regs);
?>


Expected result:
----------------
// Produced by 1st RE:
Array
(
    [0] => Array
        (
            [0] => <a href="aaa">
            [1] => "
            [2] => aaa
        )

    [1] => Array
        (
            [0] => <a href='aaa'>
            [1] => '
            [2] => aaa
        )

    [2] => Array
        (
            [0] => <a href=aaa>
            [1] => 
            [2] => aaa
        )

)


Actual result:
--------------
// Produced by 2nd RE:
Array
(
    [0] => Array
        (
            [0] => <a href="aaa">
            [1] => "
            [2] => aaa
        )

    [1] => Array
        (
            [0] => <a href='aaa'>
            [1] => '
            [2] => aaa
        )

)