diff --git a/Lib/_strptime.py b/Lib/_strptime.py
index feac05a..8eb2718 100644
--- a/Lib/_strptime.py
+++ b/Lib/_strptime.py
@@ -254,8 +254,8 @@
         # format directives (%m, etc.).
         regex_chars = re_compile(r"([\\.^$*+?\(\){}\[\]|])")
         format = regex_chars.sub(r"\\\1", format)
-        whitespace_replacement = re_compile('\s+')
-        format = whitespace_replacement.sub('\s+', format)
+        whitespace_replacement = re_compile(r'\s+')
+        format = whitespace_replacement.sub(r'\\s+', format)
         while '%' in format:
             directive_index = format.index('%')+1
             processed_format = "%s%s%s" % (processed_format,
diff --git a/Lib/sre_compile.py b/Lib/sre_compile.py
index c5a7e89..b6689fa 100644
--- a/Lib/sre_compile.py
+++ b/Lib/sre_compile.py
@@ -435,7 +435,7 @@
     # this contains min/max pattern width, and an optional literal
     # prefix or a character map
     lo, hi = pattern.getwidth()
-    if lo == 0:
+    if not lo and hi:
         return # not worth it
     # look for a literal prefix
     prefix = []
diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index 75f488b..e0d003e 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -23,6 +23,7 @@
 
 OCTDIGITS = set("01234567")
 HEXDIGITS = set("0123456789abcdefABCDEF")
+ASCIILETTERS = set("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
 
 WHITESPACE = set(" \t\n\r\v\f")
 
@@ -260,6 +261,15 @@
         elif c in DIGITS:
             raise error, "bogus escape: %s" % repr(escape)
         if len(escape) == 2:
+            if sys.py3kwarning and c in ASCIILETTERS:
+                import warnings
+                if c in 'Uu':
+                    warnings.warn('bad escape %s; Unicode escapes are '
+                                  'supported only since Python 3.3' % escape,
+                                  FutureWarning, stacklevel=8)
+                else:
+                    warnings.warnpy3k('bad escape %s' % escape,
+                                      DeprecationWarning, stacklevel=8)
             return LITERAL, ord(escape[1])
     except ValueError:
         pass
@@ -309,6 +319,15 @@
                 return GROUPREF, group
             raise ValueError
         if len(escape) == 2:
+            if sys.py3kwarning and c in ASCIILETTERS:
+                import warnings
+                if c in 'Uu':
+                    warnings.warn('bad escape %s; Unicode escapes are '
+                                  'supported only since Python 3.3' % escape,
+                                  FutureWarning, stacklevel=8)
+                else:
+                    warnings.warnpy3k('bad escape %s' % escape,
+                                      DeprecationWarning, stacklevel=8)
             return LITERAL, ord(escape[1])
     except ValueError:
         pass
@@ -714,6 +733,12 @@
     pattern.str = str
 
     p = _parse_sub(source, pattern, 0)
+    if (sys.py3kwarning and
+        (p.pattern.flags & SRE_FLAG_LOCALE) and
+        (p.pattern.flags & SRE_FLAG_UNICODE)):
+        import warnings
+        warnings.warnpy3k("LOCALE and UNICODE flags are incompatible",
+                          DeprecationWarning, stacklevel=5)
 
     tail = source.get()
     if tail == ")":
@@ -801,7 +826,10 @@
                 try:
                     this = makechar(ESCAPES[this][1])
                 except KeyError:
-                    pass
+                    if sys.py3kwarning and c in ASCIILETTERS:
+                        import warnings
+                        warnings.warnpy3k('bad escape %s' % this,
+                                          DeprecationWarning, stacklevel=4)
                 literal(this)
         else:
             literal(this)
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 5725a99..174c5ca 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -3,7 +3,7 @@
     verbose, run_unittest, import_module,
     precisionbigmemtest, _2G, cpython_only,
     captured_stdout, have_unicode, requires_unicode, u,
-    check_warnings)
+    check_warnings, check_py3k_warnings)
 import locale
 import re
 from re import Scanner
@@ -66,11 +66,13 @@
         self.assertEqual(re.sub('(?P<unk>x)', '\g<unk>\g<unk>', 'xx'), 'xxxx')
         self.assertEqual(re.sub('(?P<unk>x)', '\g<1>\g<1>', 'xx'), 'xxxx')
 
-        self.assertEqual(re.sub('a',r'\t\n\v\r\f\a\b\B\Z\a\A\w\W\s\S\d\D','a'),
-                         '\t\n\v\r\f\a\b\\B\\Z\a\\A\\w\\W\\s\\S\\d\\D')
-        self.assertEqual(re.sub('a', '\t\n\v\r\f\a', 'a'), '\t\n\v\r\f\a')
-        self.assertEqual(re.sub('a', '\t\n\v\r\f\a', 'a'),
-                         (chr(9)+chr(10)+chr(11)+chr(13)+chr(12)+chr(7)))
+        self.assertEqual(re.sub('a', r'\t\n\v\r\f\a\b', 'a'), '\t\n\v\r\f\a\b')
+        self.assertEqual(re.sub('a', '\t\n\v\r\f\a\b', 'a'), '\t\n\v\r\f\a\b')
+        self.assertEqual(re.sub('a', '\t\n\v\r\f\a\b', 'a'),
+                         (chr(9)+chr(10)+chr(11)+chr(13)+chr(12)+chr(7)+chr(8)))
+        for c in 'cdehijklmopqsuwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ':
+            with check_py3k_warnings():
+                self.assertEqual(re.sub('a', '\\' + c, 'a'), '\\' + c)
 
         self.assertEqual(re.sub('^\s*', 'X', 'test'), 'Xtest')
 
@@ -223,11 +225,11 @@
 
     def test_re_split(self):
         self.assertEqual(re.split(":", ":a:b::c"), ['', 'a', 'b', '', 'c'])
-        self.assertEqual(re.split(":*", ":a:b::c"), ['', 'a', 'b', 'c'])
-        self.assertEqual(re.split("(:*)", ":a:b::c"),
+        self.assertEqual(re.split(":+", ":a:b::c"), ['', 'a', 'b', 'c'])
+        self.assertEqual(re.split("(:+)", ":a:b::c"),
                          ['', ':', 'a', ':', 'b', '::', 'c'])
-        self.assertEqual(re.split("(?::*)", ":a:b::c"), ['', 'a', 'b', 'c'])
-        self.assertEqual(re.split("(:)*", ":a:b::c"),
+        self.assertEqual(re.split("(?::+)", ":a:b::c"), ['', 'a', 'b', 'c'])
+        self.assertEqual(re.split("(:)+", ":a:b::c"),
                          ['', ':', 'a', ':', 'b', ':', 'c'])
         self.assertEqual(re.split("([b:]+)", ":a:b::c"),
                          ['', ':', 'a', ':b::', 'c'])
@@ -237,13 +239,34 @@
         self.assertEqual(re.split("(?:b)|(?::+)", ":a:b::c"),
                          ['', 'a', '', '', 'c'])
 
+        for sep, expected in [
+            (':*', ['', 'a', 'b', 'c']),
+            ('(?::*)', ['', 'a', 'b', 'c']),
+            ('(:*)', ['', ':', 'a', ':', 'b', '::', 'c']),
+            ('(:)*', ['', ':', 'a', ':', 'b', ':', 'c']),
+        ]:
+            with check_py3k_warnings(('', FutureWarning)):
+                self.assertEqual(re.split(sep, ':a:b::c'), expected)
+
+        for sep, expected in [
+            ('', [':a:b::c']),
+            (r'\b', [':a:b::c']),
+            (r'(?=:)', [':a:b::c']),
+            (r'(?<=:)', [':a:b::c']),
+        ]:
+            with check_py3k_warnings():
+                self.assertEqual(re.split(sep, ':a:b::c'), expected)
+
     def test_qualified_re_split(self):
         self.assertEqual(re.split(":", ":a:b::c", 2), ['', 'a', 'b::c'])
         self.assertEqual(re.split(':', 'a:b:c:d', 2), ['a', 'b', 'c:d'])
         self.assertEqual(re.split("(:)", ":a:b::c", 2),
                          ['', ':', 'a', ':', 'b::c'])
-        self.assertEqual(re.split("(:*)", ":a:b::c", 2),
+        self.assertEqual(re.split("(:+)", ":a:b::c", 2),
                          ['', ':', 'a', ':', 'b::c'])
+        with check_py3k_warnings(('', FutureWarning)):
+            self.assertEqual(re.split("(:*)", ":a:b::c", maxsplit=2),
+                             ['', ':', 'a', ':', 'b::c'])
 
     def test_re_findall(self):
         self.assertEqual(re.findall(":+", "abc"), [])
@@ -404,6 +427,29 @@
             self.assertEqual(re.search(r"\d\D\w\W\s\S",
                                        "1aa! a", re.UNICODE).group(0), "1aa! a")
 
+    def test_other_escapes(self):
+        self.assertRaises(re.error, re.compile, "\\")
+        self.assertEqual(re.match(r"\(", '(').group(), '(')
+        self.assertIsNone(re.match(r"\(", ')'))
+        self.assertEqual(re.match(r"\\", '\\').group(), '\\')
+        self.assertEqual(re.match(r"[\]]", ']').group(), ']')
+        self.assertIsNone(re.match(r"[\]]", '['))
+        self.assertEqual(re.match(r"[a\-c]", '-').group(), '-')
+        self.assertIsNone(re.match(r"[a\-c]", 'b'))
+        self.assertEqual(re.match(r"[\^a]+", 'a^').group(), 'a^')
+        self.assertIsNone(re.match(r"[\^a]+", 'b'))
+        re.purge()  # for warnings
+        for c in 'ceghijklmopquyzCEFGHIJKLMNOPQRTUVXY':
+            warn = FutureWarning if c in 'Uu' else DeprecationWarning
+            with check_py3k_warnings(('', warn)):
+                self.assertEqual(re.match('\\%c$' % c, c).group(), c)
+                self.assertIsNone(re.match('\\%c' % c, 'a'))
+        for c in 'ceghijklmopquyzABCEFGHIJKLMNOPQRTUVXYZ':
+            warn = FutureWarning if c in 'Uu' else DeprecationWarning
+            with check_py3k_warnings(('', warn)):
+                self.assertEqual(re.match('[\\%c]$' % c, c).group(), c)
+                self.assertIsNone(re.match('[\\%c]' % c, 'a'))
+
     def test_string_boundaries(self):
         # See http://bugs.python.org/issue10713
         self.assertEqual(re.search(r"\b(abc)\b", "abc").group(1),
@@ -931,6 +977,19 @@
         self.assertTrue(re.match('(?ixu) ' + upper_char, lower_char))
         self.assertTrue(re.match('(?ixu) ' + lower_char, upper_char))
 
+        # Incompatibilities
+        re.purge()
+        with check_py3k_warnings():
+            re.compile('', re.LOCALE|re.UNICODE)
+        with check_py3k_warnings():
+            re.compile('(?L)', re.UNICODE)
+        with check_py3k_warnings():
+            re.compile('(?u)', re.LOCALE)
+        with check_py3k_warnings():
+            re.compile('(?Lu)')
+        with check_py3k_warnings():
+            re.compile('(?uL)')
+
     def test_dollar_matches_twice(self):
         "$ matches the end of string, and just before the terminating \n"
         pattern = re.compile('$')
@@ -967,8 +1026,9 @@
     def test_bug_13899(self):
         # Issue #13899: re pattern r"[\A]" should work like "A" but matches
         # nothing. Ditto B and Z.
-        self.assertEqual(re.findall(r'[\A\B\b\C\Z]', 'AB\bCZ'),
-                         ['A', 'B', '\b', 'C', 'Z'])
+        with check_py3k_warnings():
+            self.assertEqual(re.findall(r'[\A\B\b\C\Z]', 'AB\bCZ'),
+                             ['A', 'B', '\b', 'C', 'Z'])
 
     @precisionbigmemtest(size=_2G, memuse=1)
     def test_large_search(self, size):
@@ -1261,7 +1321,11 @@
 
 def test_main():
     run_unittest(ReTests)
-    run_re_tests()
+    deprecations = [
+        ('bad escape', DeprecationWarning),
+    ]
+    with check_py3k_warnings(*deprecations):
+        run_re_tests()
 
 if __name__ == "__main__":
     test_main()
diff --git a/Misc/NEWS b/Misc/NEWS
index dd6ec1b..3f34c6c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -42,6 +42,10 @@
 Library
 -------
 
+- bpo-30363: Running Python with the -3 option now warns about regular
+  expression syntax that is invalid or has different semantic in Python 3
+  or will change the behavior in future Python versions.
+
 - bpo-30365: Running Python with the -3 option now emits deprecation warnings
   for getchildren() and getiterator() methods of the Element class in the
   xml.etree.cElementTree module and when pass the html argument to
diff --git a/Modules/_sre.c b/Modules/_sre.c
index 8e16c1d..6fd3aff 100644
--- a/Modules/_sre.c
+++ b/Modules/_sre.c
@@ -2267,6 +2267,20 @@
     if (!string)
         return NULL;
 
+    if (Py_Py3kWarningFlag &&
+        (self->code[0] != SRE_OP_INFO || self->code[3] == 0))
+    {
+        if (self->code[0] == SRE_OP_INFO && self->code[4] == 0) {
+            if (PyErr_WarnPy3k("split() requires a non-empty pattern match.",
+                               1) < 0)
+                return NULL;
+        }
+        else if (PyErr_WarnEx(PyExc_FutureWarning,
+                              "split() requires a non-empty pattern match.",
+                              1) < 0)
+            return NULL;
+    }
+
     string = state_init(&state, self, string, 0, PY_SSIZE_T_MAX);
     if (!string)
         return NULL;