mdexec.rb

#!/usr/bin/env ruby

require 'open3'

def replace(md, strip=false)
  rex = /```([a-zA-Z]+)\n(([^`\n]{3,}\n)*)```\s*```mdexec\n([^`\n]{3,}\n)*```/

  replaced = md.gsub(rex) do |rep| 
    int, code = $1, $2
    if !strip
      stdin, stdout, stderr = Open3.popen3(int)
      stdin.write(code)
      stdin.close
      out = stdout.read.strip + "\n"
    else
      out = ""
    end
    "```#{int}\n#{code}```\n```mdexec\n#{out}```" 
  end
  replaced
end

strip = !ARGV.delete("-s").nil?

files = ARGV
if files.size > 0
  files.each do |f|
    txt = File.read(f)  
    txt = replace(txt, strip)
    File.open(f, "w") { |file| file.write(txt) }
  end
else
  puts replace(STDIN.read) 
end