echobot.rb

require 'socket'

nick     = 'foo'
channels = %w[#foo #bar]

s = TCPSocket.new 'irc.freenode.net', 6667
s.puts "NICK #{nick}"
s.puts "USER #{nick} 0 * :#{nick}"
s.puts "JOIN #{channels * ?,}"

loop do
  msg = s.gets
  next s.puts("PONG #$1") if msg[/PING (\S+)/]
  /:(?<nick>[^!]+)\S+ PRIVMSG (?<channel>\S+) :(?<message>.+)/ =~ msg
  s.puts "PRIVMSG #{channel} :#{nick}: #{message}"
end