good question. \b is a bit harder to understand; at least for me it was. \b is the boundary between a word character and a non word character. For example
$test='####tttt';
$test=preg_replace('/\b/','correct',$test);
echo "$test";
// this produces: ####correcttttt
I wasn't sure how to do what you wanted without \b, because it would have failed under certain circumstances.
