$re = '/(?<!\[|F|\[F)\w+(?!W|\]|W\]\])/';
$str = 'Fixing this [HFW] takes alot of [HFW] time [HFW] two';
$subst = "[CW]";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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