[patch] Require the 978/979 Bookland prefix on an ISBN-13

Semantics.Strings.Identifiers/IsIsbnAttribute.cs

@@ -8,8 +8,8 @@ namespace ktsu.Semantics.Strings.Identifiers;
 
 /// <summary>
 /// Validates that the string is an ISBN-10 (weighted mod-11, final digit may be X) or an ISBN-13
-/// (mod-10), evaluated on the separator-stripped value. Registration-group and publisher ranges are
-/// not validated.
+/// (mod-10, with the <c>978</c> or <c>979</c> Bookland prefix), evaluated on the separator-stripped
+/// value. Registration-group and publisher ranges are not validated.
 /// </summary>
 [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
 public sealed class IsIsbnAttribute : NativeSemanticStringValidationAttribute
@@ -63,6 +63,12 @@ private static bool IsValidIsbn10(string s)
 
 		private static bool IsValidIsbn13(string s)
 		{
+			// An ISBN-13 is an EAN-13 in the Bookland range; any other prefix is a non-book barcode.
+			if (!s.StartsWith("978", StringComparison.Ordinal) && !s.StartsWith("979", StringComparison.Ordinal))
+			{
+				return false;
+			}
+
 			int sum = 0;
 			for (int i = 0; i < 13; i++)
 			{

Semantics.Strings.Identifiers/Isbn.cs

@@ -7,8 +7,9 @@ namespace ktsu.Semantics.Strings.Identifiers;
 /// <summary>
 /// An International Standard Book Number in ISBN-10 or ISBN-13 form, e.g. <c>0306406152</c> or
 /// <c>9780306406157</c>. Hyphens and spaces are stripped and the value is uppercased (for the ISBN-10
-/// <c>X</c> check digit) on creation. The check digit is validated; registration-group and publisher
-/// ranges are not.
+/// <c>X</c> check digit) on creation. The check digit is validated, and an ISBN-13 must start with
+/// the <c>978</c> or <c>979</c> Bookland prefix; registration-group and publisher ranges are not
+/// validated.
 /// </summary>
 [IsIsbn]
 public sealed record Isbn : SemanticString<Isbn>

Semantics.Strings.Identifiers/README.md

@@ -87,7 +87,7 @@ Each type is a `sealed record` deriving from `SemanticString<T>`. None declare t
 | `Uuid` | Trim, strip wrapping `{}` / `()`, lowercase | Canonical 8-4-4-4-12 hex (RFC 4122 layout) | Any variant/version accepted, including the nil UUID. Not version-checked. |
 | `Ulid` | Trim, uppercase | 26 Crockford base32 chars, first char `0`-`7` | Timestamp is not otherwise decoded. |
 | `Iban` | Strip spaces, uppercase | Length 15-34, country/check prefix, ISO 7064 mod-97-10 checksum | Country-specific BBAN structure is not enforced. |
-| `Isbn` | Strip `-` and spaces, uppercase | ISBN-10 (weighted mod-11, `X` allowed) or ISBN-13 (mod-10) | Registration-group/publisher ranges are not validated. |
+| `Isbn` | Strip `-` and spaces, uppercase | ISBN-10 (weighted mod-11, `X` allowed) or ISBN-13 (`978`/`979` prefix, mod-10) | Registration-group/publisher ranges are not validated. |
 | `CreditCardNumber` | Strip spaces and hyphens | 13-19 digits, Luhn (mod-10) checksum | Luhn only. No issuer/network detection, no PCI guarantee. The value is sensitive, do not log it. |
 | `JwtToken` | None (stored verbatim, case-sensitive) | Three `.`-separated segments, non-empty header and payload that base64url-decode to UTF-8 text delimited as `{ ... }` | Structural only: the header and payload bodies are not parsed, so malformed JSON between the braces is accepted. Signature is not decoded or verified. `alg`, claims, and expiry are not inspected. |
 

Semantics.Test/Identifiers/IsbnTests.cs

@@ -40,6 +40,30 @@ public void Create_BadIsbn13CheckDigit_Throws()
 		Assert.ThrowsExactly<ArgumentException>(() => Isbn.Create("978-0-306-40615-8"));
 	}
 
+	[TestMethod]
+	public void Create_ValidIsbn13With979Prefix_Succeeds()
+	{
+		Isbn isbn = Isbn.Create("979-10-90636-07-1");
+		Assert.AreEqual("9791090636071", isbn.WeakString);
+	}
+
+	[TestMethod]
+	[DataRow("4006381333931")]
+	[DataRow("5901234123457")]
+	[DataRow("9770317847001")]
+	public void Create_Ean13OutsideBooklandRange_Throws(string ean13)
+	{
+		Assert.ThrowsExactly<ArgumentException>(() => Isbn.Create(ean13));
+	}
+
+	[TestMethod]
+	public void TryCreate_Ean13OutsideBooklandRange_ReturnsFalse()
+	{
+		bool created = Isbn.TryCreate("4006381333931", out Isbn? result);
+		Assert.IsFalse(created);
+		Assert.IsNull(result);
+	}
+
 	[TestMethod]
 	public void Create_WrongLength_Throws()
 	{