Adds warning when invalid error codes are parsed for ignore or extend-ignore from config file

tests/unit/test_defaults.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+import pytest
+
+from flake8.defaults import VALID_CODE_PREFIX
+
+
+@pytest.mark.parametrize(
+    "s",
+    (
+        "E",
+        "E1",
+        "E123",
+        "ABC",
+        "ABC1",
+        "ABC123",
+    ),
+)
+def test_valid_plugin_prefixes(s):
+    assert VALID_CODE_PREFIX.match(s)
+
+
+@pytest.mark.parametrize(
+    "s",
+    (
+        "",
+        "A1234",
+        "ABCD",
+        "abc",
+        "a-b",
+        "☃",
+        "A𝟗",
+    ),
+)
+def test_invalid_plugin_prefixes(s):
+    assert VALID_CODE_PREFIX.match(s) is None