Autolink emails and URLs at rendering time

app/helpers/html_helper.rb

@@ -0,0 +1,46 @@
+module HtmlHelper
+  def format_html(html)
+    fragment = Nokogiri::HTML.fragment(html)
+
+    auto_link(fragment)
+
+    fragment.to_html.html_safe
+  end
+
+  private
+    EXCLUDED_ELEMENTS = %w[ a figcaption pre code ]
+    EMAIL_REGEXP = /\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}\b/
+    URL_REGEXP = URI::DEFAULT_PARSER.make_regexp(%w[http https])
+
+    def auto_link(fragment)
+      fragment.traverse do |node|
+        next unless auto_linkable_node?(node)
+
+        content = node.text
+        linked_content = content.dup
+
+        auto_link_urls(linked_content)
+        auto_link_emails(linked_content)
+
+        if linked_content != content
+          node.replace(Nokogiri::HTML.fragment(linked_content))
+        end
+      end
+    end
+
+    def auto_linkable_node?(node)
+      node.text? && node.ancestors.none? { |ancestor| EXCLUDED_ELEMENTS.include?(ancestor.name) }
+    end
+
+    def auto_link_urls(linked_content)
+      linked_content.gsub!(URL_REGEXP) do |match|
+        %(<a href="#{match}" rel="noreferrer">#{match}</a>)
+      end
+    end
+
+    def auto_link_emails(text)
+      text.gsub!(EMAIL_REGEXP) do |match|
+        %(<a href="mailto:#{match}">#{match}</a>)
+      end
+    end
+end