Fixed redcarpet view helper in Rails using :with_toc_data. Works around vmg/redcarpet#385

application_helper.rb

module ApplicationHelper
  def markdown_toc(&block)
    output = []

    content = capture(&block)

    unsafe_chars_regex = /[ $&+,\/:;=?@\"<>#%{}|\^~\[\]`]+/o

    # Render the Table of Contents
    renderer = Redcarpet::Render::HTML_TOC
    toc_text = Redcarpet::Markdown.new(renderer).render(content)

    # Fix redcarpet anchor links; see bug
    # https://github.com/vmg/redcarpet/issues/385
    toc_text.gsub!(/<a href="(.*)#(.*)">/) do |match|
      link_url = $1
      fixed_anchor = CGI.unescapeHTML($2).gsub(unsafe_chars_regex, '')

      "<a href=\"#{link_url}##{fixed_anchor}\">"
    end
    output << toc_text

    # Then render the content
    renderer = Redcarpet::Render::HTML.new(with_toc_data: true)
    body_text = Redcarpet::Markdown.new(renderer).render(content)

    # Fix redcarpet header IDs user for anchors; see bug
    # https://github.com/vmg/redcarpet/issues/385
    body_text.gsub!(/<h([1-6]) id="(.*)"/) do |match|
      header_level = $1
      fixed_anchor = CGI.unescapeHTML($2).gsub!(unsafe_chars_regex, '')

      "<h#{header_level} id=\"#{fixed_anchor}\""
    end
    output << body_text

    output.join('').html_safe
  end
end