I am a newbie to regex. Can anyone please help? I am trying to replace $number with a number 2 but instead it looks for argument number 12. What is the format or escape sequence format_string variable needs to be?

regex e("(\\$number)");

const char* format_string = "(?12)";

regex_replace(out, file_stream.begin(), file_stream.end(), e, format_string, match_default | format_all);

Put some parenthesis around the 2: const char* format_string = "(?1(2))"; John.