PYTHON-3028 $regex as a field name does not allow for non-string values (#807)

(cherry picked from commit 70f7fe7542)

bson/json_util.py

@@ -505,7 +505,7 @@ def object_hook(dct, json_options=DEFAULT_JSON_OPTIONS):
 def _parse_legacy_regex(doc):
     pattern = doc["$regex"]
     # Check if this is the $regex query operator.
-    if isinstance(pattern, Regex):
+    if not isinstance(pattern, (text_type, bytes)):
         return doc
     flags = 0
     # PyMongo always adds $options but some other tools may not.

test/test_json_util.py

@@ -262,6 +262,15 @@ class TestJsonUtil(unittest.TestCase):
             '{"$regex": ".*", "$options": "mx"}',
             json_util.dumps(re.compile(b'.*', re.M | re.X)))
 
+    def test_regex_validation(self):
+        non_str_types = [10, {}, []]
+        docs = [{"$regex": i} for i in non_str_types]
+        for doc in docs:
+            self.assertEqual(doc, json_util.loads(json.dumps(doc)))
+
+        doc = {"$regex": ""}
+        self.assertIsInstance(json_util.loads(json.dumps(doc)), Regex)
+
     def test_minkey(self):
         self.round_trip({"m": MinKey()})