diff --git a/lib/irb/completion.rb b/lib/irb/completion.rb
index 97f97dfaf..45c696622 100644
--- a/lib/irb/completion.rb
+++ b/lib/irb/completion.rb
@@ -166,10 +166,12 @@ def self.retrieve_files_to_require_relative_from_current_dir
 
     def self.retrieve_completion_data(input, bind: IRB.conf[:MAIN_CONTEXT].workspace.binding, doc_namespace: false)
       case input
-      when /^((["'`]).*\2)\.([^.]*)$/
+      # this regexp only matches the closing character because of irb's Reline.completer_quote_characters setting
+      # details are described in: https://github.com/ruby/irb/pull/523
+      when /^(.*["'`])\.([^.]*)$/
         # String
         receiver = $1
-        message = $3
+        message = $2
 
         if doc_namespace
           "String.#{message}"
@@ -178,7 +180,9 @@ def self.retrieve_completion_data(input, bind: IRB.conf[:MAIN_CONTEXT].workspace
           select_message(receiver, message, candidates)
         end
 
-      when /^(\/[^\/]*\/)\.([^.]*)$/
+      # this regexp only matches the closing character because of irb's Reline.completer_quote_characters setting
+      # details are described in: https://github.com/ruby/irb/pull/523
+      when /^(.*\/)\.([^.]*)$/
         # Regexp
         receiver = $1
         message = $2
diff --git a/test/irb/test_completion.rb b/test/irb/test_completion.rb
index b93b5e804..4ae1d63bf 100644
--- a/test/irb/test_completion.rb
+++ b/test/irb/test_completion.rb
@@ -14,11 +14,15 @@ def setup
     class TestMethodCompletion < TestCompletion
       def test_complete_string
         assert_include(IRB::InputCompletor.retrieve_completion_data("'foo'.up", bind: binding), "'foo'.upcase")
+        # completing 'foo bar'.up
+        assert_include(IRB::InputCompletor.retrieve_completion_data("bar'.up", bind: binding), "bar'.upcase")
         assert_equal("String.upcase", IRB::InputCompletor.retrieve_completion_data("'foo'.upcase", bind: binding, doc_namespace: true))
       end
 
       def test_complete_regexp
         assert_include(IRB::InputCompletor.retrieve_completion_data("/foo/.ma", bind: binding), "/foo/.match")
+        # completing /foo bar/.ma
+        assert_include(IRB::InputCompletor.retrieve_completion_data("bar/.ma", bind: binding), "bar/.match")
         assert_equal("Regexp.match", IRB::InputCompletor.retrieve_completion_data("/foo/.match", bind: binding, doc_namespace: true))
       end