convert_redmine_textile_to_md.rb

def convert_textile_to_markdown(textile)
    require 'tempfile'

    # Redmine support @ inside inline code marked with @ (such as "@git@github.com@"), but not pandoc.
    # So we inject a placeholder that will be replaced later on with a real backtick.
    tag_code = 'pandoc-unescaped-single-backtick'
    textile.gsub!(/@([\S]+@[\S]+)@/, tag_code + '\\1' + tag_code)

    # Drop table colspan/rowspan notation ("|\2." or "|/2.") because pandoc does not support it
    # See https://github.com/jgm/pandoc/issues/22
    textile.gsub!(/\|[\/\\]\d\. /, '| ')

    # Drop table alignement notation ("|>." or "|<." or "|=.") because pandoc does not support it
    # See https://github.com/jgm/pandoc/issues/22
    textile.gsub!(/\|[<>=]\. /, '| ')

    # Move the class from <code> to <pre> so pandoc can generate a code block with correct language
    textile.gsub!(/(<pre)(><code)( class="[^"]*")(>)/, '\\1\\3\\2\\4')

    # Remove the <code> directly inside <pre>, because pandoc would incorrectly preserve it
    textile.gsub!(/(<pre[^>]*>)<code>/, '\\1')
    textile.gsub!(/<\/code>(<\/pre>)/, '\\1')

    # Inject a class in all <pre> that do not have a blank line before them
    # This is to force pandoc to use fenced code block (```) otherwise it would
    # use indented code block and would very likely need to insert an empty HTML
    # comment "<!-- -->" (see http://pandoc.org/README.html#ending-a-list)
    # which are unfortunately not supported by Redmine (see http://www.redmine.org/issues/20497)
    tag_fenced_code_block = 'force-pandoc-to-ouput-fenced-code-block'
    textile.gsub!(/([^\n]<pre)(>)/, "\\1 class=\"#{tag_fenced_code_block}\"\\2")

    # Force <pre> to have a blank line before them
    # Without this fix, a list of items containing <pre> would not be interpreted as a list at all.
    textile.gsub!(/([^\n])(<pre)/, "\\1\n\n\\2")

    # Some malformed textile content make pandoc run extremely slow,
    # so we convert it to proper textile before hitting pandoc
    # see https://github.com/jgm/pandoc/issues/3020
    textile.gsub!(/-          # (\d+)/, "* \\1")

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

    command = [
      'pandoc',
      '--wrap=preserve',
      '-f',
      'textile',
      '--atx-headers',
      '-t',
      'gfm+smart',
      src.path,
      '-o',
      dst.path,
    ]
    system(*command, :out => $stdout) or raise 'pandoc failed'

    dst.open
    markdown = dst.read

    # Remove the \ pandoc puts before * and > at begining of lines
    markdown.gsub!(/^((\\[*>])+)/) { $1.gsub("\\", "") }

    # Remove the \ pandoc puts before #
    markdown.gsub!(/(\\[#]+)/) { $1.gsub("\\", "") }

    # Add a blank line before lists
    markdown.gsub!(/^([^*].*)\n\*/, "\\1\n\n*")

    # Remove the injected tag
    markdown.gsub!(' ' + tag_fenced_code_block, '')

    # Replace placeholder with real backtick
    markdown.gsub!(tag_code, '`')

    # Un-escape Redmine link syntax to wiki pages
    markdown.gsub!('\[\[', '[[')
    markdown.gsub!('\]\]', ']]')

    # Un-escape Redmine quotation mark "> " that pandoc is not aware of
    markdown.gsub!(/(^|\n)&gt; /, "\n> ")

    # Remove <!-- end list --> injected by pandoc because Redmine incorrectly
    # does not supported HTML comments: http://www.redmine.org/issues/20497
    markdown.gsub!(/\n\n<!-- end list -->\n/, "\n")

    # Unescape URL that could easily get mangled
    markdown.gsub!(/(https?:\/\/\S+)/) { |link| link.gsub(/\\([_#])/, "\\1") }

    return markdown
  end

Encoding.default_external = Encoding::UTF_8
Encoding.default_internal = Encoding::UTF_8

puts convert_textile_to_markdown(STDIN.read)