Fix mentions, hashtags and custom emojis being formatted with Markdown

app/lib/advanced_text_formatter.rb

@@ -98,6 +98,8 @@
   private
 
   def format_markdown(html)
+    # Force escape underscores in mentions, custom emojis and hashtags before formatting
+    html = html.gsub(/#{Account::MENTION_RE}|#{CustomEmoji::SCAN_RE}|#{Tag::HASHTAG_RE}/o) { |re| re.gsub('_', '\\_') }
     html = markdown_formatter.render(html)
     html.delete("\r").delete("\n")
   end

spec/lib/advanced_text_formatter_spec.rb

@@ -75,6 +75,15 @@
         it 'creates a mention link' do
           expect(subject).to include '<a href="https://cb6e6126.ngrok.io/@alice" class="u-url mention">@<span>alice</span></a></span>'
         end
+
+        context 'when username contains underscores' do
+          let(:preloaded_accounts) { [Fabricate(:account, username: '_bob_')] }
+          let(:text) { '@_bob_' }
+
+          it 'creates a mention link' do
+            expect(subject).to include '<a href="https://cb6e6126.ngrok.io/@_bob_" class="u-url mention">@<span>_bob_</span></a></span>'
+          end
+        end
       end
 
       context 'with text containing unlinkable mentions' do
@@ -272,6 +281,14 @@
         end
       end
 
+      context 'with text containing a hashtag with underscores' do
+        let(:text) { '#_hashtag_' }
+
+        it 'creates a hashtag link' do
+          expect(subject).to include '/tags/_hashtag_" class="mention hashtag" rel="tag">#<span>_hashtag_</span></a>'
+        end
+      end
+
       context 'with text with a stand-alone xmpp: URI' do
         let(:text) { 'xmpp:user@instance.com' }