diff -r 97ceab0bd6f8 Doc/library/imaplib.rst
--- a/Doc/library/imaplib.rst	Fri Nov 21 21:55:39 2014 +0200
+++ b/Doc/library/imaplib.rst	Tue Dec 22 23:06:16 2015 -0800
@@ -470,6 +470,13 @@
          M.store(num, '+FLAGS', '\\Deleted')
       M.expunge()
 
+   .. note::
+
+      Creating flags containing ']' violates the :rfc:`3501` and the IMAP
+      protocol. However, popular IMAP servers, such as Gmail, accepts flags
+      with brackets like "[test]" despite the violation. Thus, :mod:`imaplib`
+      allows you create and access flags with brackets for backwards
+      compatibility reasons.
 
 .. method:: IMAP4.subscribe(mailbox)
 
diff -r 97ceab0bd6f8 Lib/imaplib.py
--- a/Lib/imaplib.py	Fri Nov 21 21:55:39 2014 +0200
+++ b/Lib/imaplib.py	Tue Dec 22 23:06:16 2015 -0800
@@ -108,7 +108,10 @@
         br'"')
 Literal = re.compile(br'.*{(?P<size>\d+)}$', re.ASCII)
 MapCRLF = re.compile(br'\r\n|\r|\n')
-Response_code = re.compile(br'\[(?P<type>[A-Z-]+)( (?P<data>[^\]]*))?\]')
+# We don't exclude the ']' character, even though it violates the RFC. This
+# is because popular IMAP servers, such as Gmail, allow flags with ']'. Thus,
+# we are purposefully violating the RFC for backward compatibility reasons.
+Response_code = re.compile(br'\[(?P<type>[A-Z-]+)( (?P<data>.*))?\]')
 Untagged_response = re.compile(br'\* (?P<type>[A-Z-]+)( (?P<data>.*))?')
 Untagged_status = re.compile(
     br'\* (?P<data>\d+) (?P<type>[A-Z-]+)( (?P<data2>.*))?', re.ASCII)
diff -r 97ceab0bd6f8 Lib/test/test_imaplib.py
--- a/Lib/test/test_imaplib.py	Fri Nov 21 21:55:39 2014 +0200
+++ b/Lib/test/test_imaplib.py	Tue Dec 22 23:06:16 2015 -0800
@@ -228,6 +228,49 @@
             client.shutdown()
 
     @reap_threads
+    def test_bracket_flags(self):
+
+        class GmailHandler(SimpleIMAPHandler):
+
+            def handle(self):
+                self.flags = ['Answered', 'Flagged', 'Deleted', 'Seen', 'Draft']
+                super(GmailHandler, self).handle()
+
+            def cmd_AUTHENTICATE(self, tag, args):
+                self._send_textline('+')
+                self.server.response = yield
+                self._send_tagged(tag, 'OK', 'FAKEAUTH successful')
+
+            def cmd_SELECT(self, tag, args):
+                flag_msg = ' \\'.join(self.flags)
+                self._send_line(('* FLAGS (%s)' % flag_msg).encode('ascii'))
+                self._send_line(b'* 2 EXISTS')
+                self._send_line(b'* 0 RECENT')
+                msg = ('* OK [PERMANENTFLAGS %s \\*)] Flags permitted.'
+                        % flag_msg)
+                self._send_line(msg.encode('ascii'))
+                self._send_tagged(tag, 'OK', '[READ-WRITE] SELECT completed.')
+
+            def cmd_STORE(self, tag, args):
+                new_flags = args[2].strip('(').strip(')').split()
+                self.flags.extend(new_flags)
+                flags_msg = '(FLAGS (%s))' % ' \\'.join(self.flags)
+                msg = '* %s FETCH %s' % (args[0], flags_msg)
+                self._send_line(msg.encode('ascii'))
+                self._send_tagged(tag, 'OK', 'STORE completed.')
+
+        with self.reaped_pair(GmailHandler) as (server, client):
+            code, data = client.authenticate('MYAUTH', lambda x: b'fake')
+            self.assertEqual(code, 'OK')
+            self.assertEqual(server.response, b'ZmFrZQ==\r\n')
+            client.select('test')
+            typ, [data] = client.store(b'1', "+FLAGS", "[test]")
+            self.assertIn(b'[test]', data)
+            client.select('test')
+            typ, [data] = client.response('PERMANENTFLAGS')
+            self.assertIn(b'[test]', data)
+
+    @reap_threads
     def test_issue5949(self):
 
         class EOFHandler(socketserver.StreamRequestHandler):