diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index 4a77f0c..3d38673 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -735,8 +735,13 @@ def _parse(source, state, verbose):
                     if flags is None:  # global flags
                         if pos != 3:  # "(?x"
                             import warnings
-                            warnings.warn('Flags not at the start of the expression',
-                                          DeprecationWarning, stacklevel=7)
+                            warnings.warn(
+                                'Flags not at the start of the expression %s%s' % (
+                                    source.string[:20],  # truncate long regexes
+                                    ' (truncated)' if len(source.string) > 20 else '',
+                                ),
+                                DeprecationWarning, stacklevel=7
+                            )
                         continue
                     add_flags, del_flags = flags
                     group = None
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index eb1aba3..9ffbb4d 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -1323,8 +1323,19 @@ class ReTests(unittest.TestCase):
         self.assertTrue(re.match('(?ixu) ' + upper_char, lower_char))
         self.assertTrue(re.match('(?ixu) ' + lower_char, upper_char))
 
-        with self.assertWarns(DeprecationWarning):
+        with self.assertWarns(DeprecationWarning) as warns:
             self.assertTrue(re.match(upper_char + '(?i)', lower_char))
+        self.assertEqual(
+            str(warns.warnings[0].message),
+            'Flags not at the start of the expression Ạ(?i)'
+        )
+
+        with self.assertWarns(DeprecationWarning) as warns:
+            self.assertTrue(re.match(upper_char + '(?i)%s' % ('.?' * 100), lower_char))
+        self.assertEqual(
+            str(warns.warnings[0].message),
+            'Flags not at the start of the expression Ạ(?i).?.?.?.?.?.?.?. (truncated)'
+        )
 
     def test_dollar_matches_twice(self):
         "$ matches the end of string, and just before the terminating \n"