$re = '/\b([A-F0-9]{2})([A-F0-9]{2})([A-F0-9]{2})([A-F0-9]{2})-([A-F0-9]{2})([A-F0-9]{2})-([A-F0-9]{2})([A-F0-9]{2})-([A-F0-9]{2})([A-F0-9]{2})-([A-F0-9]{12})\b/m';
$str = 'f46279ee-aa28-497c-982c-403ac0ea84a6';
$subst = "X'\4\3\2\1\6\5\8\7\9\10\11'";
$result = preg_replace($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