bpo-30605: Fix compiling binary regexs with BytesWarnings enabled. (#2016)

Running our unit tests with `-bb` enabled triggered this failure.

Lib/sre_parse.py

@@ -765,7 +765,7 @@ def _parse(source, state, verbose, nested, first=False):
                         if not first or subpattern:
                             import warnings
                             warnings.warn(
-                                'Flags not at the start of the expression %s%s' % (
+                                'Flags not at the start of the expression %r%s' % (
                                     source.string[:20],  # truncate long regexes
                                     ' (truncated)' if len(source.string) > 20 else '',
                                 ),

Lib/test/test_re.py

@@ -1368,7 +1368,7 @@ class ReTests(unittest.TestCase):
             self.assertTrue(re.match(p, lower_char))
         self.assertEqual(
             str(warns.warnings[0].message),
-            'Flags not at the start of the expression %s' % p
+            'Flags not at the start of the expression %r' % p
         )
         self.assertEqual(warns.warnings[0].filename, __file__)
 
@@ -1377,10 +1377,22 @@ class ReTests(unittest.TestCase):
             self.assertTrue(re.match(p, lower_char))
         self.assertEqual(
             str(warns.warnings[0].message),
-            'Flags not at the start of the expression %s (truncated)' % p[:20]
+            'Flags not at the start of the expression %r (truncated)' % p[:20]
         )
         self.assertEqual(warns.warnings[0].filename, __file__)
 
+        # bpo-30605: Compiling a bytes instance regex was throwing a BytesWarning
+        with warnings.catch_warnings():
+            warnings.simplefilter('error', BytesWarning)
+            p = b'A(?i)'
+            with self.assertWarns(DeprecationWarning) as warns:
+                self.assertTrue(re.match(p, b'a'))
+            self.assertEqual(
+                str(warns.warnings[0].message),
+                'Flags not at the start of the expression %r' % p
+            )
+            self.assertEqual(warns.warnings[0].filename, __file__)
+
         with self.assertWarns(DeprecationWarning):
             self.assertTrue(re.match('(?s).(?i)' + upper_char, '\n' + lower_char))
         with self.assertWarns(DeprecationWarning):

Misc/ACKS

@@ -1685,6 +1685,7 @@ Jakub Wilk
 Gerald S. Williams
 Jason Williams
 John Williams
+Roy Williams
 Sue Williams
 Carol Willing
 Steven Willis

Misc/NEWS

@@ -350,6 +350,9 @@ Extension Modules
 Library
 -------
 
+- bpo-30605: re.compile() no longer raises a BytesWarning when compiling a
+  bytes instance with misplaced inline modifier.  Patch by Roy Williams.
+
 - bpo-29870: Fix ssl sockets leaks when connection is aborted in asyncio/ssl 
   implementation. Patch by Michaël Sghaïer.