Allow language-xxx class detection on pre tags in ReverseMarkdown (#5299)

* Allow language-xxx class detection on pre tags in ReverseMarkdown

Solves thepracticaldev/dev.to#5298

* Remove comment

app/lib/reverse_markdown/converters/custom_pre.rb

@@ -25,13 +25,19 @@ module ReverseMarkdown
 
       def language(node)
         lang = language_from_highlight_class(node)
-        lang || language_from_confluence_class(node)
+        lang ||= language_from_language_class(node)
+        lang ||= language_from_confluence_class(node)
+        lang
       end
 
       def language_from_highlight_class(node)
         node.parent["class"].to_s[/highlight-([a-zA-Z0-9]+)/, 1]
       end
 
+      def language_from_language_class(node)
+        node["class"].to_s[/language-([a-zA-Z0-9]+)/, 1]
+      end
+
       def language_from_confluence_class(node)
         node["class"].to_s[/brush:\s?(:?.*);/, 1]
       end

spec/lib/reverse_markdown/converters/custom_pre_spec.rb

@@ -0,0 +1,28 @@
+require "rails_helper"
+
+RSpec.describe ReverseMarkdown::Converters::CustomPre, type: :lib do
+  def create_custom_pre
+    ReverseMarkdown.config.github_flavored = true
+    described_class.new
+  end
+
+  describe "#convert" do
+    it "detects languages from language-prefixed classnames" do
+      node = Nokogiri::HTML('<pre class="language-js"></pre>').search("pre")[0]
+      result = create_custom_pre.convert(node)
+      expect(result.split[0]).to eq("```js")
+    end
+
+    it "detects languages from highlight-prefixed classnames on the parent element" do
+      node = Nokogiri::HTML('<div class="highlight-ruby"><pre></pre></div>').search("pre")[0]
+      result = create_custom_pre.convert(node)
+      expect(result.split[0]).to eq("```ruby")
+    end
+
+    it "detects languages from confluence-style classnames" do
+      node = Nokogiri::HTML('<pre class="brush:html;"></pre>').search("pre")[0]
+      result = create_custom_pre.convert(node)
+      expect(result.split[0]).to eq("```html")
+    end
+  end
+end