$re = '/,(?=[^(]*\))/';
$str = 'Lorem ipsum dolor sit amet, consetetur sadipscing (elitr, sed diam nonumy eirmod tempor invidunt, ut labore, et dolore magna aliquyam erat), (sed diam) voluptua. (At, vero) eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.';
$subst = "+";
$result = preg_replace($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