t.rb

module M
  refine String do
    def gsub(*args)
      super(*args) {
        yield $~
      }
    end
  end
end

p "foo123".gsub(/([a-z]+)([0-9]+)/) { |s|
  p s
  p $~
  $~[2] + $~[1]
}
using M
p "foo123".gsub(/([a-z]+)([0-9]+)/) { |m|
  p m
  p $~
  m[2] + m[1]
}