$re = '/(?:(?<=\h)|^)(\w*foo\w*)(?=\h|$)/m';
$str = 'foo bar quux foofoo foobar fooquux barfoo barbar barquux quuxfoo quuxbar quuxquux .
[foo] (foo) {foo} foofoo barfoo quuxfoo foo2foo foo2bar foo2quux foo2foo bar2foo quux2foo';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php