[ruby/irb] Avoid slow symbol completion when completion target is an

empty symbol
(https://github.com/ruby/irb/pull/534)

https://github.com/ruby/irb/commit/35697f3ef3

lib/irb/completion.rb

@@ -215,7 +215,7 @@ module IRB
           select_message(receiver, message, proc_candidates | hash_candidates)
         end
 
-      when /^(:[^:.]*)$/
+      when /^(:[^:.]+)$/
         # Symbol
         if doc_namespace
           nil

test/irb/test_completion.rb

@@ -292,6 +292,8 @@ module TestIRB
       _ = :aiueo
       assert_include(IRB::InputCompletor.retrieve_completion_data(":a", bind: binding), ":aiueo")
       assert_empty(IRB::InputCompletor.retrieve_completion_data(":irb_unknown_symbol_abcdefg", bind: binding))
+      # Do not complete empty symbol for performance reason
+      assert_empty(IRB::InputCompletor.retrieve_completion_data(":", bind: binding))
     end
 
     def test_complete_invalid_three_colons