extract_anchors

# File railties/guides/rails_guides/generator.rb, line 281
    def extract_anchors(html)
      # Textile generates headers with IDs computed from titles.
      anchors = Set.new
      html.scan(/<h\d\s+id="([^"]+)/).flatten.each do |anchor|
        if anchors.member?(anchor)
          puts "*** DUPLICATE ID: #{anchor}, please put and explicit ID, e.g. h4(#explicit-id), or consider rewording"
        else
          anchors << anchor
        end
      end

      # Footnotes.
      anchors += Set.new(html.scan(/<p\s+class="footnote"\s+id="([^"]+)/).flatten)
      anchors += Set.new(html.scan(/<sup\s+class="footnote"\s+id="([^"]+)/).flatten)
      return anchors
    end