application_helper.rb

module ApplicationHelper
  
  require 'asciidoctor'
  def asciidoctor(text)
    Asciidoctor.convert(text)
  end

  require 'coderay'
  def coderay(text)
    #I tried      data-lang="( ... etc)     since the browser output does that.
    text.gsub(/\<code( lang="(.+?)")?\>(.+?)\<\/code\>/m) do
      CodeRay.scan($3, $2).div(line_numbers: :table)#, :css => :class)
    end
  end
  
end

html browser output

<div class="listingblock">

    <div class="content">
        <pre class="highlight">
            <code class="language-ruby" data-lang="ruby">

               ORDERED_LIST_KEYWORDS = {
               'loweralpha' => 'a',
               'lowerroman' => 'i',
               'upperalpha' => 'A',
               'upperroman' => 'I'
               #'lowergreek' => 'a'
               #'arabic'     => '1'
               #'decimal'    => '1'
            }
            </code>
        </pre>
    </div>

</div>

note: the browser should have applied various classes for syntax highlighting, but it is absent.
If it's coderay's job to apply them, coderay is failing to do so.

markdown

/data displayed in show.html.slim

:coderay-linenums-mode: inline
:source-highlighter: coderay
:coderay-css: class

[source,ruby,linenums]
----
ORDERED_LIST_KEYWORDS = {
  'loweralpha' => 'a',
  'lowerroman' => 'i',
  'upperalpha' => 'A',
  'upperroman' => 'I'
   #'lowergreek' => 'a'
   #'arabic'     => '1'
   #'decimal'    => '1'
}
----

show.html.slim

/articles - show.html.slim

== coderay(asciidoctor(@article.content))