the easiest way is just s/^(.*)\+_/\s\+_$1\+_/s
of course, there are a few caveats with that:
i don't believe that . matches the new line character, (unless you have the s at the end of the s///) and you will only end up with one space for all the stuff before the + that you replaced. did you want just one white space character, or 1 for every character that you replaced? and i just realized that you want to delete after the +, too. the first answer is much better than my quick regex. would tr/// work for this?