fix: reject invalid SemVer identifiers when parsing

Per SemVer 2.0.0 the core version numbers and numeric prerelease
identifiers must not include leading zeroes, and prerelease/build-metadata
identifiers must be composed only of ASCII alphanumerics and hyphens.

Add IsValidIdentifiers using a vectorized SearchValues lookup for the
allowed character set and ulong.TryParse to detect numeric identifiers
for the leading-zero check. Add failing-then-passing tests for both
invalid and still-valid inputs.

CHANGELOG.md

@@ -16,6 +16,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Changed
 
+### Fixed
+
+- Reject invalid SemVer identifiers when parsing versions.
+
 ### Removed
 
 [1.0.0]: https://code.geekeey.de/geekeey/semver/releases/tag/1.0.0

src/semver.tests/SemanticVersionTests.cs

@@ -186,6 +186,32 @@ internal sealed class SemanticVersionTests
 		await Assert.That(v.Metadata).IsEqualTo("789");
 	}
 
+	[Test]
+	[Arguments("1.2.3-@#")]
+	[Arguments("01.2.3")]
+	[Arguments("1.2.3-01")]
+	[Arguments("1.2.3-alpha_1")]
+	[Arguments("1.2.3+build.@")]
+	[Arguments("1.2.3-")]
+	[Arguments("1.2.3-..")]
+	public async Task I_can_not_parse_invalid_versions(string input)
+	{
+		await Assert.That(SemanticVersion.TryParse(input, out _)).IsFalse();
+	}
+
+	[Test]
+	[Arguments("1.2.3")]
+	[Arguments("1.2.3-alpha.1")]
+	[Arguments("1.0.0-0.3.7")]
+	[Arguments("1.0.0-x.7.z.92")]
+	[Arguments("10.20.30")]
+	[Arguments("1.2.3+build.01")]
+	[Arguments("1.0.0-alpha-b")]
+	public async Task I_can_parse_valid_versions(string input)
+	{
+		await Assert.That(SemanticVersion.TryParse(input, out _)).IsTrue();
+	}
+
 	[Test]
 	public async Task I_can_handle_invalid_json_token()
 	{

src/semver/SemanticVersion.Parsing.cs

@@ -85,6 +85,9 @@ public readonly partial record struct SemanticVersion : ISpanParsable<SemanticVe
 	// components = 1: wildcard at minor
 	// components = 2: wildcard at patch
 	// components = 3: no wildcards
+	private static readonly System.Buffers.SearchValues<char> ValidIdentifierChars =
+		System.Buffers.SearchValues.Create("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-");
+
 	internal static bool TryParsePartially(ReadOnlySpan<char> s, out SemanticVersion version, out int components)
 	{
 		version = default;
@@ -146,10 +149,76 @@ public readonly partial record struct SemanticVersion : ISpanParsable<SemanticVe
 				return false;
 			}
 
+			// Leading zeroes are forbidden for the numeric core components (SemVer 2.0.0 §9).
+			if (segment.Length > 1 && segment[0] is '0')
+			{
+				return false;
+			}
+
 			component[components++] = value;
 		}
 
+		// Per SemVer 2.0.0 the core version numbers and numeric prerelease identifiers
+		// MUST NOT include leading zeroes, and prerelease/metadata identifiers MUST be
+		// composed only of ASCII alphanumerics and hyphens.
+		if (metadata is not null && !IsValidIdentifiers(metadata, forbidLeadingZeroInNumerics: false))
+		{
+			return false;
+		}
+
+		if (prerelease is not null && !IsValidIdentifiers(prerelease, forbidLeadingZeroInNumerics: true))
+		{
+			return false;
+		}
+
 		version = new SemanticVersion(component[0], component[1], component[2], prerelease, metadata);
 		return true;
 	}
+
+	private static bool IsValidIdentifiers(ReadOnlySpan<char> value, bool forbidLeadingZeroInNumerics)
+	{
+		if (value.IsEmpty)
+		{
+			return false;
+		}
+
+		// Validation is per-identifier (dot-separated), not per-string: an empty
+		// segment ("a..b", "a.") is invalid, and the leading-zero ban applies only
+		// to numeric identifiers (so "0abc" stays valid while "02" does not).
+		var start = 0;
+		for (var i = 0; i <= value.Length; i++)
+		{
+			if (i != value.Length && value[i] is not '.')
+			{
+				continue;
+			}
+
+			var identifier = value[start..i];
+
+			// Reject empty segments produced by adjacent or trailing dots.
+			if (identifier.IsEmpty)
+			{
+				return false;
+			}
+
+			// Every character must be an allowed identifier character.
+			if (identifier.IndexOfAnyExcept(ValidIdentifierChars) >= 0)
+			{
+				return false;
+			}
+
+			// Numeric identifiers must not include leading zeroes (SemVer 2.0.0 §9/§10).
+			// The TryParse gate ensures the rule fires only for all-digit identifiers,
+			// leaving an alphanumeric "0abc" (which is allowed) untouched.
+			if (forbidLeadingZeroInNumerics && identifier.Length > 1 && identifier[0] is '0' &&
+				ulong.TryParse(identifier, NumberStyles.None, CultureInfo.InvariantCulture, out _))
+			{
+				return false;
+			}
+
+			start = i + 1;
+		}
+
+		return true;
+	}
 }