[Boost-users] regex_replace to replace a number?

Hi, 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); Thanks, Mike