diff --git a/django/utils/regex_helper.py b/django/utils/regex_helper.py
index 622d8227597..d046b71d96e 100644
--- a/django/utils/regex_helper.py
+++ b/django/utils/regex_helper.py
@@ -7,7 +7,10 @@ should be good enough for a large class of URLS, however.
 """
 from __future__ import unicode_literals
 
+import warnings
+
 from django.utils import six
+from django.utils.deprecation import RemovedInDjango21Warning
 from django.utils.six.moves import zip
 
 # Mapping of an escape character to a representative of that class. So, e.g.,
@@ -59,9 +62,7 @@ def normalize(pattern):
     (3) Select the first (essentially an arbitrary) element from any character
         class. Select an arbitrary character for any unordered class (e.g. '.'
         or '\w') in the pattern.
-    (4) Ignore comments, look-ahead and look-behind assertions, and any of the
-        reg-exp flags that won't change what we construct ("iLmsu"). "(?x)" is
-        an error, however.
+    (4) Ignore look-ahead and look-behind assertions.
     (5) Raise an error on any disjunctive ('|') constructs.
 
     Django's URLs for forward resolving are either all positional arguments or
@@ -128,10 +129,16 @@ def normalize(pattern):
                     walk_to_end(ch, pattern_iter)
                 else:
                     ch, escaped = next(pattern_iter)
-                    if ch in "iLmsu#!=<":
+                    if ch in '!=<':
                         # All of these are ignorable. Walk to the end of the
                         # group.
                         walk_to_end(ch, pattern_iter)
+                    elif ch in 'iLmsu#':
+                        warnings.warn(
+                            'Using (?%s) in url() patterns is deprecated.' % ch,
+                            RemovedInDjango21Warning
+                        )
+                        walk_to_end(ch, pattern_iter)
                     elif ch == ':':
                         # Non-capturing group
                         non_capturing_groups.append(len(result))
diff --git a/docs/internals/deprecation.txt b/docs/internals/deprecation.txt
index 29f4479c5a2..c1a1c41cd6a 100644
--- a/docs/internals/deprecation.txt
+++ b/docs/internals/deprecation.txt
@@ -48,6 +48,9 @@ details on these changes.
 
 * The ``Model._meta.has_auto_field`` attribute will be removed.
 
+* Support for regular expression groups with ``iLmsu#`` in ``url()`` will be
+  removed.
+
 .. _deprecation-removed-in-2.0:
 
 2.0
diff --git a/docs/releases/1.11.txt b/docs/releases/1.11.txt
index 24247f7d75c..48849f3b4d9 100644
--- a/docs/releases/1.11.txt
+++ b/docs/releases/1.11.txt
@@ -747,3 +747,10 @@ Miscellaneous
 
 * ``Model._meta.has_auto_field`` is deprecated in favor of checking if
   ``Model._meta.auto_field is not None``.
+
+* Using regular expression groups with ``iLmsu#`` in ``url()`` is deprecated.
+  The only group that's useful is ``(?i)`` for case-insensitive URLs, however,
+  case-insensitive URLs aren't a good practice because they create multiple
+  entries for search engines, for example. An alternative solution could be to
+  create a :data:`~django.conf.urls.handler404` that looks for uppercase
+  characters in the URL and redirects to a lowercase equivalent.
diff --git a/tests/urlpatterns_reverse/test_deprecated.py b/tests/urlpatterns_reverse/test_deprecated.py
new file mode 100644
index 00000000000..c918dd5a179
--- /dev/null
+++ b/tests/urlpatterns_reverse/test_deprecated.py
@@ -0,0 +1,33 @@
+import warnings
+
+from django.conf.urls import url
+from django.test import SimpleTestCase, override_settings
+from django.urls import reverse
+
+from .views import empty_view
+
+urlpatterns = [
+    url(r'^(?i)CaseInsensitive/(\w+)', empty_view, name="insensitive"),
+    url(r'^(?i)test/2/?$', empty_view, name="test2"),
+]
+
+
+@override_settings(ROOT_URLCONF='urlpatterns_reverse.test_deprecated')
+class URLPatternReverse(SimpleTestCase):
+
+    def test_urlpattern_reverse(self):
+        test_data = (
+            ('insensitive', '/CaseInsensitive/fred', ['fred'], {}),
+            ('test2', '/test/2', [], {}),
+        )
+        with warnings.catch_warnings(record=True) as warns:
+            warnings.simplefilter('always')
+            warnings.filterwarnings(
+                'ignore', 'Flags not at the start',
+                DeprecationWarning, module='django.urls.resolvers'
+            )
+            for i, (name, expected, args, kwargs) in enumerate(test_data):
+                got = reverse(name, args=args, kwargs=kwargs)
+                self.assertEqual(got, expected)
+                msg = str(warns[i].message)
+                self.assertEqual(msg, 'Using (?i) in url() patterns is deprecated.')
diff --git a/tests/urlpatterns_reverse/tests.py b/tests/urlpatterns_reverse/tests.py
index 4b31e382e1a..b02898e3458 100644
--- a/tests/urlpatterns_reverse/tests.py
+++ b/tests/urlpatterns_reverse/tests.py
@@ -198,9 +198,7 @@ test_data = (
     ('repeats', '/repeats/a/', [], {}),
     ('repeats2', '/repeats/aa/', [], {}),
     ('repeats3', '/repeats/aa/', [], {}),
-    ('insensitive', '/CaseInsensitive/fred', ['fred'], {}),
     ('test', '/test/1', [], {}),
-    ('test2', '/test/2', [], {}),
     ('inner-nothing', '/outer/42/', [], {'outer': '42'}),
     ('inner-nothing', '/outer/42/', ['42'], {}),
     ('inner-nothing', NoReverseMatch, ['foo'], {}),
diff --git a/tests/urlpatterns_reverse/urls.py b/tests/urlpatterns_reverse/urls.py
index 35e2ff707c8..731c97146b3 100644
--- a/tests/urlpatterns_reverse/urls.py
+++ b/tests/urlpatterns_reverse/urls.py
@@ -49,9 +49,7 @@ urlpatterns = [
     url(r'^repeats/a{1,2}/$', empty_view, name="repeats"),
     url(r'^repeats/a{2,4}/$', empty_view, name="repeats2"),
     url(r'^repeats/a{2}/$', empty_view, name="repeats3"),
-    url(r'^(?i)CaseInsensitive/(\w+)', empty_view, name="insensitive"),
     url(r'^test/1/?', empty_view, name="test"),
-    url(r'^(?i)test/2/?$', empty_view, name="test2"),
     url(r'^outer/(?P<outer>[0-9]+)/', include('urlpatterns_reverse.included_urls')),
     url(r'^outer-no-kwargs/([0-9]+)/', include('urlpatterns_reverse.included_no_kwargs_urls')),
     url('', include('urlpatterns_reverse.extra_urls')),
diff --git a/tests/utils_tests/test_regex_helper.py b/tests/utils_tests/test_regex_helper.py
index 055da89ee09..f91fd1cec7e 100644
--- a/tests/utils_tests/test_regex_helper.py
+++ b/tests/utils_tests/test_regex_helper.py
@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 import unittest
+import warnings
 
 from django.utils import regex_helper
 
@@ -27,8 +28,12 @@ class NormalizeTests(unittest.TestCase):
     def test_group_ignored(self):
         pattern = r"(?i)(?L)(?m)(?s)(?u)(?#)"
         expected = [('', [])]
-        result = regex_helper.normalize(pattern)
+        with warnings.catch_warnings(record=True) as warns:
+            warnings.simplefilter('always')
+            result = regex_helper.normalize(pattern)
         self.assertEqual(result, expected)
+        for i, char in enumerate('iLmsu#'):
+            self.assertEqual(str(warns[i].message), 'Using (?%s) in url() patterns is deprecated.' % char)
 
     def test_group_noncapturing(self):
         pattern = r"(?:non-capturing)"