Description:
------------
mb_split() cannot divide into char by char using the expression '(?<!\A)(?!\z)'.

Test script:
---------------
$str = "あいうえお";
mb_regex_encoding('UTF-8');
print_r([
    'PCRE' => preg_split('/(?<!\A)(?!\z)/u', $str),
    '鬼車' => mb_split('(?<!\A)(?!\z)', $str)
]);

Expected result:
----------------
Array
(
    [PCRE] => Array
        (
            [0] => あ
            [1] => い
            [2] => う
            [3] => え
            [4] => お
        )

    [鬼車] => Array
        (
            [0] => あ
            [1] => い
            [2] => う
            [3] => え
            [4] => お
        )

)
[Finished in 0.5s]

Actual result:
--------------
Array
(
    [PCRE] => Array
        (
            [0] => あ
            [1] => い
            [2] => う
            [3] => え
            [4] => お
        )

    [鬼車] => Array
        (
            [0] => あ
            [1] => いうえお
        )

)
[Finished in 0.5s]