extension.rb

require 'asciidoctor/extensions' unless RUBY_ENGINE == 'opal'

# TIP Asciidoctor has built-in support for skipping YAML-style front matter. This is just an example!
class FrontMatterPreprocessor < Asciidoctor::Extensions::Preprocessor
  def process document, reader
    # get unprocessed lines
    lines = reader.lines
    return reader if lines.empty?
    front_matter = []
    if lines.first.chomp == '---'
      original_lines = lines.dup
      lines.shift
      while !lines.empty? && lines.first.chomp != '---'
        front_matter << lines.shift
      end

      if (first = lines.first).nil? || first.chomp != '---'
        lines = original_lines
      else
        lines.shift
        document.attributes['front-matter'] = front_matter.join.chomp
        # advance the reader by the number of lines taken
        (front_matter.length + 2).times { reader.advance }
      end
    end
    reader
  end
end