diff --git a/Doc/library/re.rst b/Doc/library/re.rst
index 8e20496..0d305d5 100644
--- a/Doc/library/re.rst
+++ b/Doc/library/re.rst
@@ -297,6 +297,9 @@
       >>> m.group(0)
       'egg'
 
+   .. versionchanged: 3.5
+      Added support for group references of fixed length.
+
 ``(?<!...)``
    Matches if the current position in the string is not preceded by a match for
    ``...``.  This is called a :dfn:`negative lookbehind assertion`.  Similar to
diff --git a/Lib/re.py b/Lib/re.py
index bc3056f..788fa6b 100644
--- a/Lib/re.py
+++ b/Lib/re.py
@@ -351,10 +351,11 @@
         s = sre_parse.Pattern()
         s.flags = flags
         for phrase, action in lexicon:
+            gid = s.opengroup()
             p.append(sre_parse.SubPattern(s, [
-                (SUBPATTERN, (len(p)+1, sre_parse.parse(phrase, flags))),
+                (SUBPATTERN, (gid, sre_parse.parse(phrase, flags))),
                 ]))
-        s.groups = len(p)+1
+            s.closegroup(gid, p[-1])
         p = sre_parse.SubPattern(s, [(BRANCH, (None, p))])
         self.scanner = sre_compile.compile(p)
     def scan(self, string):
diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index 1a7d316..98afd7c 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -68,12 +68,15 @@
     # master pattern object.  keeps track of global attributes
     def __init__(self):
         self.flags = 0
-        self.open = []
-        self.groups = 1
         self.groupdict = {}
+        self.subpatterns = [None]  # group 0
+        self.lookbehindgroups = None
+    @property
+    def groups(self):
+        return len(self.subpatterns)
     def opengroup(self, name=None):
         gid = self.groups
-        self.groups = gid + 1
+        self.subpatterns.append(None)
         if self.groups > MAXGROUPS:
             raise error("groups number is too large")
         if name is not None:
@@ -82,12 +85,19 @@
                 raise error("redefinition of group name %r as group %d; "
                             "was group %d" % (name, gid,  ogid))
             self.groupdict[name] = gid
-        self.open.append(gid)
         return gid
-    def closegroup(self, gid):
-        self.open.remove(gid)
+    def closegroup(self, gid, p):
+        self.subpatterns[gid] = p
     def checkgroup(self, gid):
-        return gid < self.groups and gid not in self.open
+        return gid < self.groups and self.subpatterns[gid] is not None
+
+    def checklookbehindgroup(self, gid, source):
+        if self.lookbehindgroups is not None:
+            if not self.checkgroup(gid):
+                raise source.error('cannot refer to an open group')
+            if gid >= self.lookbehindgroups:
+                raise source.error('cannot refer to group defined in the same '
+                                   'lookbehind subpattern')
 
 class SubPattern:
     # a subpattern, in intermediate form
@@ -183,7 +193,21 @@
             elif op in _UNITCODES:
                 lo = lo + 1
                 hi = hi + 1
-            elif op == SUCCESS:
+            elif op is GROUPREF:
+                i, j = self.pattern.subpatterns[av].getwidth()
+                lo = lo + i
+                hi = hi + j
+            elif op is GROUPREF_EXISTS:
+                i, j = av[1].getwidth()
+                if av[2] is not None:
+                    l, h = av[2].getwidth()
+                    i = min(i, l)
+                    j = max(j, h)
+                else:
+                    i = 0
+                lo = lo + i
+                hi = hi + j
+            elif op is SUCCESS:
                 break
         self.width = min(lo, MAXREPEAT - 1), min(hi, MAXREPEAT)
         return self.width
@@ -379,6 +403,7 @@
                 if not state.checkgroup(group):
                     raise source.error("cannot refer to open group",
                                        len(escape))
+                state.checklookbehindgroup(group, source)
                 return GROUPREF, group
             raise ValueError
         if len(escape) == 2:
@@ -641,6 +666,7 @@
                         if gid is None:
                             msg = "unknown group name: {0!r}".format(name)
                             raise source.error(msg, len(name) + 1)
+                        state.checklookbehindgroup(gid, source)
                         subpatternappend((GROUPREF, gid))
                         continue
                     else:
@@ -668,7 +694,13 @@
                         if char is None or char not in "=!":
                             raise source.error("syntax error")
                         dir = -1 # lookbehind
+                        lookbehindgroups = state.lookbehindgroups
+                        if lookbehindgroups is None:
+                            state.lookbehindgroups = state.groups
                     p = _parse_sub(source, state)
+                    if dir < 0:
+                        if lookbehindgroups is None:
+                            state.lookbehindgroups = None
                     if not sourcematch(")"):
                         raise source.error("unbalanced parenthesis")
                     if char == "=":
@@ -701,6 +733,7 @@
                         if condgroup >= MAXGROUPS:
                             raise source.error("the group number is too large",
                                                len(condname) + 1)
+                    state.checklookbehindgroup(condgroup, source)
                 elif char in FLAGS:
                     # flags
                     state.flags |= FLAGS[char]
@@ -726,7 +759,7 @@
                 if not sourcematch(")"):
                     raise source.error("unbalanced parenthesis")
                 if group is not None:
-                    state.closegroup(group)
+                    state.closegroup(group, p)
                 subpatternappend((SUBPATTERN, (group, p)))
             else:
                 while True:
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 2fb4764..0fbf8c5 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -604,7 +604,7 @@
         self.assertEqual(re.match("a.*b", "a\n\nb", re.DOTALL).group(0),
                          "a\n\nb")
 
-    def test_non_consuming(self):
+    def test_lookahead(self):
         self.assertEqual(re.match("(a(?=\s[^a]))", "a b").group(1), "a")
         self.assertEqual(re.match("(a(?=\s[^a]*))", "a b").group(1), "a")
         self.assertEqual(re.match("(a(?=\s[abc]))", "a b").group(1), "a")
@@ -618,6 +618,46 @@
         self.assertEqual(re.match(r"(a)(?!\s\1)", "a b").group(1), "a")
         self.assertEqual(re.match(r"(a)(?!\s(abc|a))", "a b").group(1), "a")
 
+        # Group reference.
+        self.assertTrue(re.match(r'(a)b(?=\1)a', 'aba'))
+        self.assertIsNone(re.match(r'(a)b(?=\1)c', 'abac'))
+        # Conditional group reference.
+        self.assertTrue(re.match(r'(?:(a)|(x))b(?=(?(2)x|c))c', 'abc'))
+        self.assertIsNone(re.match(r'(?:(a)|(x))b(?=(?(2)c|x))c', 'abc'))
+        self.assertTrue(re.match(r'(?:(a)|(x))b(?=(?(2)x|c))c', 'abc'))
+        self.assertIsNone(re.match(r'(?:(a)|(x))b(?=(?(1)b|x))c', 'abc'))
+        self.assertTrue(re.match(r'(?:(a)|(x))b(?=(?(1)c|x))c', 'abc'))
+        # Group used before defined.
+        self.assertTrue(re.match(r'(a)b(?=(?(2)x|c))(c)', 'abc'))
+        self.assertIsNone(re.match(r'(a)b(?=(?(2)b|x))(c)', 'abc'))
+        self.assertTrue(re.match(r'(a)b(?=(?(1)c|x))(c)', 'abc'))
+
+    def test_lookbehind(self):
+        self.assertTrue(re.match(r'ab(?<=b)c', 'abc'))
+        self.assertIsNone(re.match(r'ab(?<=c)c', 'abc'))
+        self.assertIsNone(re.match(r'ab(?<!b)c', 'abc'))
+        self.assertTrue(re.match(r'ab(?<!c)c', 'abc'))
+        # Group reference.
+        self.assertTrue(re.match(r'(a)a(?<=\1)c', 'aac'))
+        self.assertIsNone(re.match(r'(a)b(?<=\1)a', 'abaa'))
+        self.assertIsNone(re.match(r'(a)a(?<!\1)c', 'aac'))
+        self.assertTrue(re.match(r'(a)b(?<!\1)a', 'abaa'))
+        # Conditional group reference.
+        self.assertIsNone(re.match(r'(?:(a)|(x))b(?<=(?(2)x|c))c', 'abc'))
+        self.assertIsNone(re.match(r'(?:(a)|(x))b(?<=(?(2)b|x))c', 'abc'))
+        self.assertTrue(re.match(r'(?:(a)|(x))b(?<=(?(2)x|b))c', 'abc'))
+        self.assertIsNone(re.match(r'(?:(a)|(x))b(?<=(?(1)c|x))c', 'abc'))
+        self.assertTrue(re.match(r'(?:(a)|(x))b(?<=(?(1)b|x))c', 'abc'))
+        # Group used before defined.
+        self.assertRaises(re.error, re.compile, r'(a)b(?<=(?(2)b|x))(c)')
+        self.assertIsNone(re.match(r'(a)b(?<=(?(1)c|x))(c)', 'abc'))
+        self.assertTrue(re.match(r'(a)b(?<=(?(1)b|x))(c)', 'abc'))
+        # Group defined in the same lookbehind pattern
+        self.assertRaises(re.error, re.compile, r'(a)b(?<=(.)\2)(c)')
+        self.assertRaises(re.error, re.compile, r'(a)b(?<=(?P<a>.)(?P=a))(c)')
+        self.assertRaises(re.error, re.compile, r'(a)b(?<=(a)(?(2)b|x))(c)')
+        self.assertRaises(re.error, re.compile, r'(a)b(?<=(.)(?<=\2))(c)')
+
     def test_ignore_case(self):
         self.assertEqual(re.match("abc", "ABC", re.I).group(0), "ABC")
         self.assertEqual(re.match(b"abc", b"ABC", re.I).group(0), b"ABC")
diff --git a/Misc/NEWS b/Misc/NEWS
index c4ae6f2..ad03e0a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -13,6 +13,9 @@
 Library
 -------
 
+- Issues #814253, #9179: Group references and conditional group references now
+  work in lookbehind assertions in regular expressions.
+
 - Issue #23215: Multibyte codecs with custom error handlers that ignores errors
   consumed too much memory and raised SystemError or MemoryError.
   Original patch by Aleksi Torhamo.