--TEST-- preg_replace_callback() --FILE-- <?php $input = "plain [indent] deep [indent] [abcd]deeper[/abcd] [/indent] deep [/indent] plain"; function parseTagsRecursive($input) { $regex = '#\[indent]((?:[^[]|\[(?!/?indent])|(?R))+)\[/indent]#'; if (is_array($input)) { $input = '<div style="margin-left: 10px">'.$input[1].'</div>'; } return preg_replace_callback($regex, 'parseTagsRecursive', $input); } $output = parseTagsRecursive($input); echo $output, "\n"; ?> --EXPECT-- plain <div style="margin-left: 10px"> deep <div style="margin-left: 10px"> [abcd]deeper[/abcd] </div> deep </div> plain