Regular assertion matching problem

Question

<?php
$str='hello wehe fasdsf sdff coming sf being';
$reg='/\b\w (?!ing\b)/';//Unable to meet expectations Effect
//$reg='/\b\w (?=ing\b)/';//Can achieve the expected effect, the result: com be
preg_match_all($reg, $str, $res) ;
print_r($res);
Why