diff -r b6ebc726d5fe Lib/re.py
--- a/Lib/re.py	Sat Jul 06 13:25:44 2013 +0200
+++ b/Lib/re.py	Sat Jul 06 16:37:09 2013 +0400
@@ -120,6 +120,7 @@
 
 import sys
 import sre_compile
+import sre_constants
 import sre_parse
 import functools
 
@@ -167,6 +168,8 @@
     if a string, backslash escapes in it are processed.  If it is
     a callable, it's passed the match object and must return
     a replacement string to be used."""
+    if isinstance(count, sre_constants.Flags):
+        raise TypeError("count argument does not accept flags, use flags argument")
     return _compile(pattern, flags).sub(repl, string, count)
 
 def subn(pattern, repl, string, count=0, flags=0):
@@ -178,6 +181,8 @@
     callable; if a string, backslash escapes in it are processed.
     If it is a callable, it's passed the match object and must
     return a replacement string to be used."""
+    if isinstance(count, sre_constants.Flags):
+        raise TypeError("count argument does not accept flags, use flags argument")
     return _compile(pattern, flags).subn(repl, string, count)
 
 def split(pattern, string, maxsplit=0, flags=0):
@@ -188,6 +193,8 @@
     list.  If maxsplit is nonzero, at most maxsplit splits occur,
     and the remainder of the string is returned as the final element
     of the list."""
+    if isinstance(maxsplit, sre_constants.Flags):
+        raise TypeError("maxsplit argument does not accept flags, use flags argument")
     return _compile(pattern, flags).split(string, maxsplit)
 
 def findall(pattern, string, flags=0):
diff -r b6ebc726d5fe Lib/sre_constants.py
--- a/Lib/sre_constants.py	Sat Jul 06 13:25:44 2013 +0200
+++ b/Lib/sre_constants.py	Sat Jul 06 16:37:09 2013 +0400
@@ -16,6 +16,7 @@
 MAGIC = 20031017
 
 from _sre import MAXREPEAT
+from enum import IntEnum
 
 # SRE standard exception (access as sre.error)
 # should this really be here?
@@ -200,15 +201,26 @@
 }
 
 # flags
-SRE_FLAG_TEMPLATE = 1 # template mode (disable backtracking)
-SRE_FLAG_IGNORECASE = 2 # case insensitive
-SRE_FLAG_LOCALE = 4 # honour system locale
-SRE_FLAG_MULTILINE = 8 # treat target as multiline string
-SRE_FLAG_DOTALL = 16 # treat target as a single string
-SRE_FLAG_UNICODE = 32 # use unicode "locale"
-SRE_FLAG_VERBOSE = 64 # ignore whitespace and comments
-SRE_FLAG_DEBUG = 128 # debugging
-SRE_FLAG_ASCII = 256 # use ascii "locale"
+class Flags(IntEnum):
+    SRE_FLAG_TEMPLATE = 1 # template mode (disable backtracking)
+    SRE_FLAG_IGNORECASE = 2 # case insensitive
+    SRE_FLAG_LOCALE = 4 # honour system locale
+    SRE_FLAG_MULTILINE = 8 # treat target as multiline string
+    SRE_FLAG_DOTALL = 16 # treat target as a single string
+    SRE_FLAG_UNICODE = 32 # use unicode "locale"
+    SRE_FLAG_VERBOSE = 64 # ignore whitespace and comments
+    SRE_FLAG_DEBUG = 128 # debugging
+    SRE_FLAG_ASCII = 256 # use ascii "locale"
+
+SRE_FLAG_TEMPLATE = Flags.SRE_FLAG_TEMPLATE
+SRE_FLAG_IGNORECASE = Flags.SRE_FLAG_IGNORECASE
+SRE_FLAG_LOCALE = Flags.SRE_FLAG_LOCALE
+SRE_FLAG_MULTILINE = Flags.SRE_FLAG_MULTILINE
+SRE_FLAG_DOTALL = Flags.SRE_FLAG_DOTALL
+SRE_FLAG_UNICODE = Flags.SRE_FLAG_UNICODE
+SRE_FLAG_VERBOSE = Flags.SRE_FLAG_VERBOSE
+SRE_FLAG_DEBUG = Flags.SRE_FLAG_DEBUG
+SRE_FLAG_ASCII = Flags.SRE_FLAG_ASCII
 
 # flags for INFO primitive
 SRE_INFO_PREFIX = 1 # has prefix
diff -r b6ebc726d5fe Lib/test/test_re.py
--- a/Lib/test/test_re.py	Sat Jul 06 13:25:44 2013 +0200
+++ b/Lib/test/test_re.py	Sat Jul 06 16:37:09 2013 +0400
@@ -1040,6 +1040,19 @@
         with self.assertRaisesRegex(sre_constants.error, '\?foo'):
             re.compile('(?P<?foo>)')
 
+    def test_argument_not_flags(self):
+        # Issue 11957: Common mistake to pass flags argument in a wrong position to sub, subn and split
+        self.assertRaises(TypeError, re.sub, r'', '', '', re.U)
+        self.assertRaises(TypeError, re.subn, r'', '', '', re.U)
+        self.assertRaises(TypeError, re.split, r'', '', re.U)
+
+    def test_ints_can_be_passed_as_flags(self):
+        # Issue 11957: Common mistake to pass flags argument in a wrong position to sub, subn and split
+        # Checking for backward compatibility
+        self.assertEqual(re.sub(r'', '', '', 0, int(re.U)), '')
+        self.assertEqual(re.subn(r'', '', '', 0, int(re.U)), ('', 1))
+        self.assertEqual(re.split(r'', '', 0, int(re.U)), [''])
+
 
 def run_re_tests():
     from test.re_tests import tests, SUCCEED, FAIL, SYNTAX_ERROR