#!/usr/bin/env ruby

if ARGV.size < 1
  STDERR.puts "Usage: #$0 <regexp> [<subst-patt>]"
  exit 1
end

re = Regexp.new(ARGV.shift)
subst = ARGV.shift ||'\1'
STDIN.each do |line|
  line.gsub! re do
    match = Regexp.last_match
    subst.gsub(/\\n/, "\n").gsub(/\\(\d+)/) { match[$1.to_i] }
  end
  puts line
end