diff --git a/Lib/json/decoder.py b/Lib/json/decoder.py
index dfcc628..1b43238 100644
--- a/Lib/json/decoder.py
+++ b/Lib/json/decoder.py
@@ -62,6 +62,16 @@
 
 DEFAULT_ENCODING = "utf-8"
 
+def _decode_uXXXX(s, pos):
+    esc = s[pos + 1:pos + 5]
+    if len(esc) == 4 and esc[1] not in 'xX':
+        try:
+            return int(esc, 16)
+        except ValueError:
+            pass
+    msg = "Invalid \\uXXXX escape"
+    raise ValueError(errmsg(msg, s, pos))
+
 def py_scanstring(s, end, encoding=None, strict=True,
         _b=BACKSLASH, _m=STRINGCHUNK.match):
     """Scan the string s for a JSON string. End is the index of the
@@ -116,25 +126,16 @@
             end += 1
         else:
             # Unicode escape sequence
-            esc = s[end + 1:end + 5]
-            next_end = end + 5
-            if len(esc) != 4:
-                msg = "Invalid \\uXXXX escape"
-                raise ValueError(errmsg(msg, s, end))
-            uni = int(esc, 16)
+            uni = _decode_uXXXX(s, end)
+            end += 5
             # Check for surrogate pair on UCS-4 systems
-            if 0xd800 <= uni <= 0xdbff and sys.maxunicode > 65535:
-                msg = "Invalid \\uXXXX\\uXXXX surrogate pair"
-                if not s[end + 5:end + 7] == '\\u':
-                    raise ValueError(errmsg(msg, s, end))
-                esc2 = s[end + 7:end + 11]
-                if len(esc2) != 4:
-                    raise ValueError(errmsg(msg, s, end))
-                uni2 = int(esc2, 16)
-                uni = 0x10000 + (((uni - 0xd800) << 10) | (uni2 - 0xdc00))
-                next_end += 6
+            if sys.maxunicode > 65535 and \
+               0xd800 <= uni <= 0xdbff and s[end:end + 2] == '\\u':
+                uni2 = _decode_uXXXX(s, end + 1)
+                if 0xdc00 <= uni2 <= 0xdfff:
+                    uni = 0x10000 + (((uni - 0xd800) << 10) | (uni2 - 0xdc00))
+                    end += 6
             char = unichr(uni)
-            end = next_end
         # Append the unescaped character
         _append(char)
     return u''.join(chunks), end
diff --git a/Lib/json/tests/test_scanstring.py b/Lib/json/tests/test_scanstring.py
index 4fef8cb..d17ebdd 100644
--- a/Lib/json/tests/test_scanstring.py
+++ b/Lib/json/tests/test_scanstring.py
@@ -5,10 +5,6 @@
 class TestScanstring(object):
     def test_scanstring(self):
         scanstring = self.json.decoder.scanstring
-        self.assertEqual(
-            scanstring('"z\\ud834\\udd20x"', 1, None, True),
-            (u'z\U0001d120x', 16))
-
         if sys.maxunicode == 65535:
             self.assertEqual(
                 scanstring(u'"z\U0001d120x"', 1, None, True),
@@ -94,6 +90,57 @@
             scanstring('["Bad value", truth]', 2, None, True),
             (u'Bad value', 12))
 
+    def test_surrogates(self):
+        scanstring = self.json.decoder.scanstring
+        def assertScan(given, expect):
+            self.assertEqual(scanstring(given, 1, None, True),
+                             (expect, len(given)))
+            if not isinstance(given, unicode):
+                given = unicode(given)
+                self.assertEqual(scanstring(given, 1, None, True),
+                                 (expect, len(given)))
+
+        assertScan('"z\\ud834\\u0079x"', u'z\ud834yx')
+        assertScan('"z\\ud834\\udd20x"', u'z\U0001d120x')
+        assertScan('"z\\ud834\\ud834\\udd20x"', u'z\ud834\U0001d120x')
+        assertScan('"z\\ud834x"', u'z\ud834x')
+        assertScan(u'"z\\ud834\udd20x12345"', u'z\ud834\udd20x12345')
+        assertScan('"z\\udd20x"', u'z\udd20x')
+        assertScan(u'"z\ud834\udd20x"', u'z\ud834\udd20x')
+        assertScan(u'"z\ud834\\udd20x"', u'z\ud834\udd20x')
+        assertScan(u'"z\ud834x"', u'z\ud834x')
+
+    def test_bad_escapes(self):
+        scanstring = self.json.decoder.scanstring
+        bad_escapes = [
+            '"\\"',
+            '"\\x"',
+            '"\\u"',
+            '"\\u0"',
+            '"\\u01"',
+            '"\\u012"',
+            '"\\uz012"',
+            '"\\u0z12"',
+            '"\\u01z2"',
+            '"\\u012z"',
+            '"\\u0x12"',
+            '"\\u0X12"',
+            '"\\ud834\\"',
+            '"\\ud834\\u"',
+            '"\\ud834\\ud"',
+            '"\\ud834\\udd"',
+            '"\\ud834\\udd2"',
+            '"\\ud834\\uzdd2"',
+            '"\\ud834\\udzd2"',
+            '"\\ud834\\uddz2"',
+            '"\\ud834\\udd2z"',
+            '"\\ud834\\u0x20"',
+            '"\\ud834\\u0X20"',
+        ]
+        for s in bad_escapes:
+            with self.assertRaises(ValueError):
+                scanstring(s, 1, None, True)
+
     def test_issue3623(self):
         self.assertRaises(ValueError, self.json.decoder.scanstring, b"xxx", 1,
                           "xxx")