Port of ccr/regexp_anchor to EE

changelogs/unreleased/ccr-incoming-email-regex-anchor.yml

+title: Add anchor for incoming email regex
+merge_request: !18917
+type: added

lib/gitlab/incoming_email.rb

@@ -59,7 +59,7 @@ module Gitlab
 
         regex = Regexp.escape(wildcard_address)
         regex = regex.sub(Regexp.escape(WILDCARD_PLACEHOLDER), '(.+)')
-        Regexp.new(regex).freeze
+        Regexp.new(/\A#{regex}\z/).freeze
       end
     end
   end

spec/lib/gitlab/incoming_email_spec.rb

@@ -84,6 +84,10 @@ describe Gitlab::IncomingEmail do
       expect(described_class.key_from_address("replies+key@example.com")).to eq("key")
     end
 
+    it 'does not match emails with extra bits' do
+      expect(described_class.key_from_address('somereplies+somekey@example.com.someotherdomain.com')).to be nil
+    end
+
     context "on GitLab.com" do
       before do
         allow(Gitlab).to receive(:com?).and_return(true)