Match the email local part without Unicode case folding

The local-part pattern lists the characters it allows explicitly: ASCII
letters and digits, the RFC 5322 specials and U+00A0 to U+024F. Compiled
with re.IGNORECASE but not re.ASCII, a character also matches when its
case-folded form is in a class, which admitted 31 characters outside
those ranges: the Kelvin sign (U+212A) through "k", the Angstrom sign
(U+212B), Greek mu, U+1E9E and IPA letters. The Kelvin and Angstrom
signs render as "K" and "A with ring above", so "Kelvin@example.com"
could be registered next to a lookalike.

With re.ASCII, case-insensitive matching applies to ASCII letters only.
Both cases of the allowed Latin letters are already in the ranges, so
nothing inside them is lost.

Author: nandgator

src/validators/email.py

@@ -95,7 +95,9 @@ def email(
             + r"!#-\[\]-\177]|\\[\011.]"
             + r')*")$',
             username_part,
-            re.IGNORECASE,
+            # re.ASCII: Unicode case folding would also admit characters outside
+            # these ranges, such as the Kelvin sign as a "K"
+            re.IGNORECASE | re.ASCII,
         )
         else False
     )

tests/test_email.py

@@ -72,3 +72,29 @@ def test_rejects_newline_after_local_part(local_part: str):
 def test_returns_failed_validation_on_idna_mapped_lookalike_domain(domain_part: str):
     """Test returns failed validation on email domains that only IDNA 2003 mapping makes valid."""
     assert isinstance(email(f"user@{domain_part}"), ValidationError)
+
+
+@pytest.mark.parametrize(
+    "local_part",
+    [
+        "\u212aelvin",  # Kelvin sign, folds to k
+        "\u212belvin",  # Angstrom sign, folds to a with ring above
+        "\u1e9eelvin",  # capital sharp s, folds to sharp s
+        "\u039celvin",  # Greek capital mu, folds to micro sign
+        "\u2c65elvin",  # a with stroke, folds to capital a with stroke
+    ],
+)
+@pytest.mark.parametrize("quoted", [False, True])
+def test_returns_failed_validation_on_case_folded_local_part(local_part: str, quoted: bool):
+    """Test returns failed validation on characters allowed only through Unicode case folding."""
+    if quoted:
+        local_part = f'"{local_part}"'
+    assert isinstance(email(f"{local_part}@example.com"), ValidationError)
+
+
+@pytest.mark.parametrize(
+    "local_part", ["KELVIN", "Kelvin", "\u0141\u00d3KA\u015a\u0106", "\u00ff\u0178", '"Kelvin"']
+)
+def test_returns_true_on_mixed_case_local_part(local_part: str):
+    """Test returns true on upper and lower case letters inside the allowed ranges."""
+    assert email(f"{local_part}@example.com")