preg_match_all("/([0-9]{8})/", $string, $match)

preg_match_all('/(?<!d)(d{8})(?!d)/', $string, $match)

preg_match_all('/^(d{8})$/', $string, $match)

preg_match_all('/^|D(d{8})D|$/', $string, $match)

preg_match_all('/(?<!d)(d{8})(?!d)/', $string, $match)

/b[0-9]{8}b/

$string = '34523452345 2352345234 13452345 45357567567567 24573257 35672456';
preg_match_all("/b[0-9]{8}b/", $string, $match);
print_r($match);

Array
(
    [0] => Array
        (
            [0] => 13452345
            [1] => 24573257
            [2] => 35672456
        )

)

preg_match_all("/(?:^|D)(d{8})(?:D|$)/", $string, $match);

preg_match_all("/[^d]([d]{8})[^d]/", $string, $match)