Validate JSON Pointer array index spelling

referencing/_core.py

@@ -264,6 +264,16 @@ def pointer(self, pointer: str, resolver: Resolver[D]) -> Resolved[D]:
         segments: list[int | str] = []
         for segment in unquote(pointer[1:]).split("/"):
             if isinstance(contents, Sequence):
+                # 2026-10-11: Validate unsigned ASCII spelling before int().
+                if (
+                    not segment.isascii()
+                    or not segment.isdecimal()
+                    or (len(segment) > 1 and segment.startswith("0"))
+                ):
+                    raise exceptions.PointerToNowhere(
+                        ref=pointer,
+                        resource=self,
+                    )
                 segment = int(segment)
             else:
                 segment = segment.replace("~1", "/").replace("~0", "~")

referencing/tests/test_core.py

@@ -762,6 +762,69 @@ def test_lookup_non_existent_pointer_to_array_index(self):
             resource=resource,
         )
 
+    @pytest.mark.parametrize("index", [-1, -2, -4])
+    def test_lookup_negative_array_index(self, index):
+        # 2026-10-08: JSON arrays have no negative-index members.
+        resource = Resource.opaque([1, 2, 4, 8])
+        resolver = Registry({"http://example.com/1": resource}).resolver()
+        pointer = f"/{index}"
+        with pytest.raises(exceptions.PointerToNowhere) as exc:
+            resolver.lookup(f"http://example.com/1#{pointer}")
+        assert exc.value == exceptions.PointerToNowhere(
+            ref=pointer,
+            resource=resource,
+        )
+
+    def test_lookup_negative_integer_object_key(self):
+        # 2026-10-08: Object member names remain literal strings.
+        resource = Resource.opaque({"-1": "value"})
+        resolver = Registry().resolver_with_root(resource)
+        assert resolver.lookup("#/-1").contents == "value"
+
+    # 2026-10-11: Array tokens retain their spelling until validation.
+    @pytest.mark.parametrize(
+        "index",
+        [
+            "-0",
+            "+1",
+            "01",
+            "00",
+            " 1",
+            "1 ",
+            "\u0661",
+            "\uff11",
+            "1_0",
+            "",
+            "-",
+            "foo",
+            "1x",
+        ],
+    )
+    def test_lookup_invalid_array_index_spelling(self, index):
+        resource = Resource.opaque([10, 20, 30])
+        resolver = Registry().resolver_with_root(resource)
+        pointer = f"/{index}"
+        with pytest.raises(exceptions.PointerToNowhere) as exc:
+            resolver.lookup(f"#{pointer}")
+        assert exc.value == exceptions.PointerToNowhere(
+            ref=pointer,
+            resource=resource,
+        )
+
+    @pytest.mark.parametrize("index", ["0", "1", "2"])
+    def test_lookup_unsigned_array_index(self, index):
+        resolver = Registry().resolver_with_root(Resource.opaque([10, 20, 30]))
+        assert (
+            resolver.lookup(f"#/{index}").contents == [10, 20, 30][int(index)]
+        )
+
+    @pytest.mark.parametrize("key", ["-0", "+1", "01"])
+    def test_lookup_array_index_spelling_as_object_key(self, key):
+        resolver = Registry().resolver_with_root(
+            Resource.opaque({key: "value"}),
+        )
+        assert resolver.lookup(f"#/{key}").contents == "value"
+
     def test_lookup_pointer_to_empty_string(self):
         resolver = Registry().resolver_with_root(Resource.opaque({"": {}}))
         assert resolver.lookup("#/").contents == {}