Add UK-postcode format validity to the Postcode value object 🟩

A pure, offline regex check (no network) so a frozen value object stays pure.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/postcode.py

@@ -1,7 +1,13 @@
 from __future__ import annotations
 
+import re
 from dataclasses import dataclass
 
+# UK postcode format, matched against the canonical (no-space, upper) value.
+# A pure structural check — it does not confirm the postcode actually exists
+# (that would need a network lookup, which a value object must not do).
+_UK_POSTCODE_RE = re.compile(r"[A-Z]{1,2}\d[A-Z\d]?\d[A-Z]{2}")
+
 
 @dataclass(frozen=True)
 class Postcode:
@@ -13,3 +19,8 @@ class Postcode:
 
     def __str__(self) -> str:
         return self.value
+
+    def is_valid(self) -> bool:
+        """Whether the canonical value is a well-formed UK postcode (format
+        only — no existence check, so it is pure and offline)."""
+        return _UK_POSTCODE_RE.fullmatch(self.value) is not None

tests/domain/test_postcode.py

@@ -57,3 +57,18 @@ def test_postcode_is_frozen() -> None:
     # act / assert
     with pytest.raises(dataclasses.FrozenInstanceError):
         postcode.value = "OTHER"  # type: ignore[misc]
+
+
+@pytest.mark.parametrize(
+    "raw",
+    ["sw1a 1aa", "M1 1AA", "b33 8th", "cr2 6xh", "dn55 1pt", "ec1a 1bb", "w1a 0ax"],
+)
+def test_valid_uk_postcode_formats_are_valid(raw: str) -> None:
+    # act / assert — a pure format check on the canonical value, no network.
+    assert Postcode(raw).is_valid() is True
+
+
+@pytest.mark.parametrize("raw", ["", "   ", "NONSENSE", "12345", "SW1A 1A", "ZZ"])
+def test_malformed_or_empty_postcodes_are_invalid(raw: str) -> None:
+    # act / assert
+    assert Postcode(raw).is_valid() is False