An experiment to try parsing markdown using parslet gem.

markdown_parsing.rb

require 'parslet'

class Store
  class << self
    def headings
      @headings ||= []
    end

    def links
      @links ||= []
    end

    def add_heading(text)
      headings << text.to_s
    end

    def add_link(label, link)
      links << { label: label.to_s, link: link.to_s }
    end
  end
end

class MarkdownParser < Parslet::Parser
  rule(:text) { match('.').repeat(1) }

  rule(:space)  { match('\s').repeat(1) }
  rule(:space?) { space.maybe }

  rule(:start_label) { str('(') }
  rule(:end_label) { str(')') }
  rule(:label_content) { match('[\s\w]').repeat(1) }
  rule(:label) { start_label >> label_content.as(:label_text) >> end_label }

  rule(:start_link) { str('[') }
  rule(:end_link) { str(']') }
  rule(:http_path) { match('[\s\w\/\.]').repeat(1) }
  rule(:http_start) { str('http:') }
  rule(:https_start) { str('https:') }
  rule(:url) { http_start >> http_path }
  rule(:secure_url) { https_start >> http_path }
  rule(:link_content) { secure_url | url | http_path}
  rule(:link) { start_link >> link_content.as(:link_text) >> end_link }

  rule(:anchor) { label >> space? >> link }

  rule(:header_flag) { str('#').repeat(1) }
  rule(:header) { header_flag >> space? >> text.as(:header_text) }

  rule(:content) { header | anchor }

  root(:content)

  def self.parse(text)
    new.parse(text)
  end
end

class MarkdownTransformer < Parslet::Transform
  rule(header_text: simple(:header_text))  { Store.add_heading(header_text) }
  rule(label_text: simple(:label_text), link_text: simple(:link_text)) { Store.add_link(label_text, link_text) }

  def self.process(text)
    parsed = MarkdownParser.parse(text)
    new.apply(parsed)
  end
end


MarkdownTransformer.process('###bbbb')
MarkdownTransformer.process('### Goodbye and thanks for all the fish')
MarkdownTransformer.process('(this link)[http://example.com]')
MarkdownTransformer.process('(that link)[https://example.com/foo/bar]')
MarkdownTransformer.process('(another link)[/example/ho/hum]')

p Store.headings
p Store.links

The script outputs:

["bbbb", "Goodbye and thanks for all the fish"]
[{:label=>"this link", :link=>"http://example.com"}, {:label=>"that link", :link=>"https://example.com/foo/bar"}, {:label=>"another link", :link=>"/example/ho/hum"}]