Array (
     [0] => word
     [1] => word
     [2] => dr
     [3] => word
     [4] => a.sh
     [5] => word
     [6] => a.k
     [7] => word     )

$special_words = array("dr.", "a.sh.", "a.k");
array_walk($special_words, function(&$item, $key){ $item= preg_quote($item, '~');});

$regex = '~(?<!w)(' . implode('|', $special_words) . '|w+)(?!w)~';
$str = 'word word, dr. word: a.sh. word a.k word?!..';
preg_match_all($regex, $str, $matches);
var_dump($matches[0]);