implement \1 captures manually in gsub block

.rb

#I have no idea what the correct way to do this is
#couldn't use .gsub(regex, 'need \1') because sometimes I needed to format the capture
#so i implmented it in a block to gsub

tex.gsub!(regex) do
  res = "thing with \1 and \2 and \3"
  Regexp.last_match.captures.each_with_index do |match, i|
    if match =~ /^請求項[\p{N},～、－及びおよ]+$/
      res = res.gsub(/\\#{i+1}/, format_invention_according_to(match))
    else
      res = res.gsub(/\\#{i+1}/, match)
    end
  end
  res
end