Fix mentions regex to exclude usernames in Medium URLs

spec/models/account_spec.rb

@@ -111,4 +111,28 @@ RSpec.describe Account, type: :model do
   describe '.find_local' do
     pending
   end
+
+  describe 'MENTION_RE' do
+    subject { Account::MENTION_RE }
+
+    it 'matches usernames in the middle of a sentence' do
+      expect(subject.match('Hello to @alice from me')[1]).to eq 'alice'
+    end
+
+    it 'matches usernames in the beginning of status' do
+      expect(subject.match('@alice Hey how are you?')[1]).to eq 'alice'
+    end
+
+    it 'matches dot-prepended usernames' do
+      expect(subject.match('.@alice I want everybody to see this')[1]).to eq 'alice'
+    end
+
+    it 'does not match e-mails' do
+      expect(subject.match('Drop me an e-mail at alice@example.com')).to be_nil
+    end
+
+    it 'does not match URLs' do
+      expect(subject.match('Check this out https://medium.com/@alice/some-article#.abcdef123')).to be_nil
+    end
+  end
 end