#bulk.rb

def bulkRename(re_src, pat_dst, fmt_dst)
  re_dst = Regexp.new(pat_dst)
  selected = Dir.glob(re_src)
  for name in selected
    newnam = name.gsub(re_dst, fmt_dst)
    puts("mv #{name} #{newnam}")
    File.rename(name, newnam)
  end 
end  

case ARGV.size # Just br size is OK
  when 0 then warn("Usage: #{$0} [select] re_dst fmt_dst")
  when 1 then puts(Dir.glob($*[0]))
  when 2 then bulkRename('*', $*[0], $*[1])  
  when 3 then bulkRename($*[0], $*[1], $*[2])
end

#move_back.rb
for ln in $<.each_line do
  next unless ln.start_with?('mv')
  _mv, src, dst = ln.split
  puts("mv #{dst} #{src}")
end

mv a.py a.txt

ruby s.rb '(.*)?!\.rb' '\1.txt'