Description:
------------
When using the /u modifier, preg_match, preg_match_all, and preg_replace all give incorrect results using /\S{2}/u but work fine with /\S\S/u which should be equivalent.

Reproduce code:
---------------
$s = "A\xc2\xa3BC";
preg_match_all('/\S\S/u', $s, $m);
print_r($m[0]);
preg_match_all('/\S{2}/u', $s, $m);
print_r($m[0]);


Expected result:
----------------
Array
(
    [0] => A£
    [1] => BC
)
Array
(
    [0] => A£
    [1] => BC
)


Actual result:
--------------
Array
(
    [0] => A£
    [1] => BC
)
Array
(
    [0] => A
    [1] => 
)