Convert textile to Markdown Extra - shamelessly derived from https://gist.github.com/othree/8563b7f9018730666b95

convert_textile_to_markdown.rake

task :convert_textile_to_markdown_extra => :environment do
  require 'tempfile'

  Issue.all.each do |version|

if (String(version.tracker) == "Approval" && Integer(version.id) > 72400 && Integer(version.id)  < 72500)
    #if (Integer(version.id) == 73293) for testing on specific documents
      puts 'Converting Issue...'
      puts version.id
      puts version.subject
      puts version.tracker
      textile = version.description
      #only if your descriptions have tag-like strings that are not actually meant to be tags
      textile.gsub!(/\<([^\.])/,'STARTTAG\\1')
      textile.gsub!(/\>/,'ENDTAG')
      #replaces ++ tags, or underlines, since pandoc converts them to asterisks
      textile.gsub!(/([\s\t\r\n\f])\+(.*)\+([\s\t\r\n\f])/,"\\1UNDERLINESTART\\2UNDERLINEEND\\3")

      src = Tempfile.new('textile')
      src.write(textile)
      src.open
      src.close
      dst = Tempfile.new('markdown')
      dst.close


      command = [
        "pandoc",
        "--no-wrap",
        "--smart",
        "+RTS",
        "-K64m",
        "-RTS",
        # "--strict",
        "-f",
        "textile",
        "-t",
        "markdown_phpextra",
        src.path,
        "-o",
        dst.path,
      ]


      system(*command) or raise "pandoc failed"

      dst.open

      markdown = dst.read

      # remove the \ pandoc puts before * and > at begining of lines
             markdown.gsub!(/^((\\[*>  ])+)/) { $1.gsub("\\", "") }
      #
                # add a blank line before lists
                         markdown.gsub!(/^([^*].*)\n\*/, "\\1\n\n*")
                  # remove the strange \ character pandoc puts after |
                          markdown.gsub!(/\|\\/,"|")

                          #Create headers in tables - specifically for a three column, left-aligned table
                          markdown.gsub!(/\n\|\*\\\<\.(.*)\n/,"\n\|\*\\\<\.\\1\n\|:-\|:-\|:-\|\n")

                          # remove the header attributes that pandoc doesn't convert -  *<. and _<.
                          markdown.gsub!(/\*\\<\./,"")
                          markdown.gsub!(/_\\\<\./,"")

                        # removes any solitary || with no purpose
                          markdown.gsub!(/\n\|\|\n/,"\n\n")

                          # removes |   , which screws up tables
                          markdown.gsub!(/\|  \n/,"\|\n")

                          #adds a line after every first line of a table
                          markdown.gsub!(/\n\n\|(.*)\|\n/,"\n\|\\1\|\n")

                          #opens up every cell that should be there but empty, as || is just ignored by markdown_extra
                          markdown.gsub!(/\|\|/,"| |")
                          markdown.gsub!(/\|\|\|/,"| | |")
                          markdown.gsub!(/\|\|\|\|/,"| | | |")

                          #catches underline tags from before, replaces them with appropriate ones
                          markdown.gsub!(/UNDERLINESTART/,"<u>")
                          markdown.gsub!(/UNDERLINEEND/,"</u>")

                          #catches tags from before, replaces them with appropriate ones, only if your descriptions have tag-like strings that are not actually meant to be tags
                           markdown.gsub!(/STARTTAG/,"&lt;")
                           markdown.gsub!(/ENDTAG/,">")

                           #probably as a result of the operations above, indented lists don't work, this fixes them
                          # markdown.gsub!(/\n\*\*\\\*/,"  *")
                           markdown.gsub!(/\n\*\* (.*)\n/,"  * \\1\n")

                               version.update_columns(description: markdown)

                               puts 'Done'

                             else

                            end
                                   end
                                     end

Put this rake task into lib/tasks/convert_textile_to_markdown_extra.rake and run with

bundle exec rake convert_textile_to_markdown_extra RAILS_ENV=production