正则表达式怎么写

Question

$s = 'abc ABC;x';echo "?=\n";
preg_match_all('/\w+(?=;)/', $s, $m);
print_r($m);echo "?<=\n";
preg_match_all('/(?<=b)\w/i', $s, $m);
print_r($m);echo "?!\n";
preg_match_all('/\w+(?!;)/', $s, $m);
print_r($m);echo "?<!\n";
preg_match_all('/\w+(?<!;)/', $s, $m);
print_r($m);
?=
Array
(
    [0] => Array
        (
            [0] => ABC
        ))
?<=
Array
(
    [0] => Array
        (
            [0] => c
            [1] => C
        ))
?!
Array
(
    [0] => Array
        (
            [0] => abc
            [1] => AB
            [2] => x
        ))
?<!
Array
(
    [0] => Array
        (
            [0] => abc
            [1] => ABC
            [2] => x
        ))