diff --git a/stix2/properties.py b/stix2/properties.py
index 33d3917..5e6c3c3 100644
--- a/stix2/properties.py
+++ b/stix2/properties.py
@@ -309,7 +309,7 @@ class HexProperty(Property):
 
 
 REF_REGEX = re.compile("^[a-z][a-z-]+[a-z]--[0-9a-fA-F]{8}-[0-9a-fA-F]{4}"
-                       "-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$")
+                       "-4[0-9a-fA-F]{3}-[89abAB][0-9a-fA-F]{3}-[0-9a-fA-F]{12}$")
 
 
 class ReferenceProperty(Property):
diff --git a/stix2/test/test_datastore_filters.py b/stix2/test/test_datastore_filters.py
index b12eaa8..76ae6de 100644
--- a/stix2/test/test_datastore_filters.py
+++ b/stix2/test/test_datastore_filters.py
@@ -97,7 +97,7 @@ filters = [
     Filter("granular_markings.marking_ref", "=", "marking-definition--5e57c739-391a-4eb3-b6be-7d15ca92d5ed"),
     Filter("external_references.external_id", "in", "CVE-2014-0160,CVE-2017-6608"),
     Filter("created_by_ref", "=", "identity--f1350682-3290-4e0d-be58-69e290537647"),
-    Filter("object_marking_refs", "=", "marking-definition--613f2e26-0000-0000-0000-b8e91df99dc9"),
+    Filter("object_marking_refs", "=", "marking-definition--613f2e26-0000-4000-8000-b8e91df99dc9"),
     Filter("granular_markings.selectors", "in", "description"),
     Filter("external_references.source_name", "=", "CVE"),
     Filter("objects", "=", {"0": {"type": "file", "name": "HAL 9000.exe"}})
@@ -262,7 +262,7 @@ def test_apply_common_filters9():
 
 
 def test_apply_common_filters10():
-    # "Return any object that matches marking-definition--613f2e26-0000-0000-0000-b8e91df99dc9 in object_marking_refs" (None)
+    # "Return any object that matches marking-definition--613f2e26-0000-4000-8000-b8e91df99dc9 in object_marking_refs" (None)
     resp = list(apply_common_filters(stix_objs, [filters[11]]))
     assert len(resp) == 0
 
diff --git a/stix2/test/test_memory.py b/stix2/test/test_memory.py
index 88078d7..44f90ba 100644
--- a/stix2/test/test_memory.py
+++ b/stix2/test/test_memory.py
@@ -155,7 +155,7 @@ def test_memory_source_get(mem_source):
 
 
 def test_memory_source_get_nonexistant_object(mem_source):
-    resp = mem_source.get("tool--d81f86b8-975b-bc0b-775e-810c5ad45a4f")
+    resp = mem_source.get("tool--8d0b222c-7a3b-44a0-b9c6-31b051efb32e")
     assert resp is None
 
 
diff --git a/stix2/test/test_properties.py b/stix2/test/test_properties.py
index 462735c..b2e49b5 100644
--- a/stix2/test/test_properties.py
+++ b/stix2/test/test_properties.py
@@ -245,10 +245,14 @@ def test_boolean_property_invalid(value):
 def test_reference_property():
     ref_prop = ReferenceProperty()
 
-    assert ref_prop.clean("my-type--3a331bfe-0566-55e1-a4a0-9a2cd355a300")
+    assert ref_prop.clean("my-type--00000000-0000-4000-8000-000000000000")
     with pytest.raises(ValueError):
         ref_prop.clean("foo")
 
+    # This is not a valid V4 UUID
+    with pytest.raises(ValueError):
+        ref_prop.clean("my-type--00000000-0000-0000-0000-000000000000")
+
 
 @pytest.mark.parametrize("value", [
     '2017-01-01T12:34:56Z',