feat: introduce IdentifierList for prerelease and build-metadata segments

Add a value type wrapping the dot-separated prerelease and build-metadata
identifiers, replacing the previous nullable string properties on
SemanticVersion. Implements IReadOnlyList<string>, zero-alloc
ReadOnlySpan<char> enumeration, Ordinal equality with implicit string
conversions, a JSON converter, and ISpanParsable/IParsable parsing
validated via IdentifierListFormatInfo.

Closes #1

CHANGELOG.md

@@ -17,13 +17,19 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - `SemanticVersion` and `SemanticVersionRange` can now be used as `System.Text.Json`
   dictionary keys. The custom converters override `ReadAsPropertyName`/`WriteAsPropertyName`
   so `Dictionary<TKey, TValue>` round-trips correctly.
+- New `IdentifierList` value type for prerelease/build-metadata identifiers.
 
 ### Changed
 
+- **Breaking:** `SemanticVersion.Prerelease` and `SemanticVersion.Metadata` are now `IdentifierList`
+  instead of `string?`. Implicit `string` conversions keep most call sites compiling, but the property
+  type change is breaking and requires a recompile. Constructors additionally accept `IdentifierList`.
 - `SemanticVersion` `==`/`Equals`/`GetHashCode` now ignore build metadata, matching precedence comparison rules.
 - `SemanticVersionRange` equality (`==`, `Equals`, `GetHashCode`) is now structural and based
   on the constraint sets, so ranges that parse to the same comparators compare as equal
   (previously it was reference-sensitive on the backing array).
+- `SemanticVersionComparer` now enumerates the `IdentifierList` segments directly instead of
+  re-splitting the joined identifier strings.
 
 ### Fixed
 

src/semver.tests/IdentifierListTests.cs

@@ -0,0 +1,219 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+using System.Text.Json;
+
+namespace Geekeey.SemVer.Tests;
+
+internal sealed class IdentifierListTests
+{
+	[Test]
+	public async Task I_can_represent_an_empty_list()
+	{
+		var empty = IdentifierList.Empty;
+
+		await Assert.That(empty.IsEmpty).IsTrue();
+		await Assert.That(empty.Count).IsEqualTo(0);
+		await Assert.That(empty.ToString()).IsEqualTo(string.Empty);
+
+		var collected = new List<string>();
+		foreach (var segment in empty)
+		{
+			collected.Add(segment.ToString());
+		}
+
+		await Assert.That(collected).IsEmpty();
+	}
+
+	[Test]
+	public async Task I_can_construct_from_an_empty_string()
+	{
+		var list = new IdentifierList(string.Empty);
+
+		await Assert.That(list.IsEmpty).IsTrue();
+		await Assert.That(list.Count).IsEqualTo(0);
+	}
+
+	[Test]
+	[Arguments("alpha")]
+	[Arguments("alpha.beta")]
+	[Arguments("alpha.beta.1")]
+	[Arguments("1.2.3.4.5")]
+	public async Task I_can_count_dot_separated_segments(string joined)
+	{
+		var list = new IdentifierList(joined);
+		var expected = joined.Split('.').Length;
+
+		await Assert.That(list.Count).IsEqualTo(expected);
+		await Assert.That(list.IsEmpty).IsFalse();
+	}
+
+	[Test]
+	public async Task I_can_get_each_segment_by_index()
+	{
+		var list = new IdentifierList("alpha.beta.1");
+
+		await Assert.That(list[0]).IsEqualTo("alpha");
+		await Assert.That(list[1]).IsEqualTo("beta");
+		await Assert.That(list[2]).IsEqualTo("1");
+	}
+
+	[Test]
+	public async Task I_can_throw_out_of_range_for_an_invalid_index()
+	{
+		var list = new IdentifierList("alpha");
+
+		await Assert.That(() => list[1]).Throws<ArgumentOutOfRangeException>();
+		await Assert.That(() => list[-1]).Throws<ArgumentOutOfRangeException>();
+	}
+
+	[Test]
+	public async Task I_can_enumerate_segments_as_spans()
+	{
+		var list = new IdentifierList("alpha.beta.1");
+		var collected = new List<string>();
+
+		foreach (var segment in list)
+		{
+			collected.Add(segment.ToString());
+		}
+
+		await Assert.That(collected).IsEquivalentTo(["alpha", "beta", "1"]);
+	}
+
+	[Test]
+	public async Task I_can_enumerate_segments_as_strings()
+	{
+		var list = new IdentifierList("alpha.beta.1");
+		var collected = new List<string>();
+
+		foreach (var segment in (IEnumerable<string>)list)
+		{
+			collected.Add(segment);
+		}
+
+		await Assert.That(collected).IsEquivalentTo(["alpha", "beta", "1"]);
+	}
+
+	[Test]
+	public async Task I_can_compare_ordinally_with_other_lists_and_strings()
+	{
+		var list = new IdentifierList("alpha.beta");
+
+		await Assert.That(list.Equals(new IdentifierList("alpha.beta"))).IsTrue();
+		await Assert.That(list == "alpha.beta").IsTrue();
+		await Assert.That("alpha.beta" == list).IsTrue();
+		await Assert.That(list.Equals("alpha.beta")).IsTrue();
+		await Assert.That(list == new IdentifierList("alpha.BETA")).IsFalse();
+		await Assert.That(list == "alpha.BETA").IsFalse();
+	}
+
+	[Test]
+	public async Task I_can_report_inequality_with_a_null_string()
+	{
+		var list = new IdentifierList("alpha");
+
+		await Assert.That(list == (string?)null).IsFalse();
+		await Assert.That(list.Equals((string?)null)).IsFalse();
+		await Assert.That(IdentifierList.Empty == (string?)null).IsFalse();
+	}
+
+	[Test]
+	public async Task I_can_report_equal_empty_lists()
+	{
+		await Assert.That(IdentifierList.Empty == new IdentifierList(string.Empty)).IsTrue();
+	}
+
+	[Test]
+	public async Task I_can_convert_implicitly_to_and_from_string()
+	{
+		IdentifierList list = "alpha.beta";
+		string text = list;
+
+		await Assert.That(text).IsEqualTo("alpha.beta");
+		await Assert.That(list == new IdentifierList("alpha.beta")).IsTrue();
+	}
+
+	[Test]
+	public async Task I_can_produce_consistent_hash_codes_for_equal_values()
+	{
+		await Assert.That(new IdentifierList("alpha").GetHashCode())
+			.IsEqualTo(new IdentifierList("alpha").GetHashCode());
+	}
+
+	[Test]
+	public async Task I_can_round_trip_via_json_as_a_string()
+	{
+		var list = new IdentifierList("alpha.beta.1");
+
+		var json = JsonSerializer.Serialize(list);
+		await Assert.That(json).IsEqualTo("\"alpha.beta.1\"");
+
+		var roundTripped = JsonSerializer.Deserialize<IdentifierList>(json);
+		await Assert.That(roundTripped == list).IsTrue();
+	}
+
+	[Test]
+	public async Task I_can_deserialize_null_json_to_an_empty_list()
+	{
+		var roundTripped = JsonSerializer.Deserialize<IdentifierList>("null");
+
+		await Assert.That(roundTripped.IsEmpty).IsTrue();
+	}
+
+	[Test]
+	[Arguments("alpha")]
+	[Arguments("alpha.beta")]
+	[Arguments("alpha.beta.1")]
+	[Arguments("0abc")]
+	public async Task I_can_parse_valid_dot_separated_identifiers(string joined)
+	{
+		await Assert.That(IdentifierList.TryParse(joined, out var list)).IsTrue();
+		await Assert.That(list.ToString()).IsEqualTo(joined);
+	}
+
+	[Test]
+	[Arguments("a..b")]
+	[Arguments("a.")]
+	[Arguments(".a")]
+	[Arguments("a.b!")]
+	[Arguments("a b")]
+	public async Task I_can_reject_invalid_identifiers(string invalid)
+	{
+		await Assert.That(IdentifierList.TryParse(invalid, out _)).IsFalse();
+	}
+
+	[Test]
+	public async Task I_can_forbid_leading_zero_numerics_via_format_info()
+	{
+		var prereleaseRule = new IdentifierListFormatInfo(forbidLeadingZeroInNumerics: true);
+
+		await Assert.That(IdentifierList.TryParse("01", prereleaseRule, out _)).IsFalse();
+		await Assert.That(IdentifierList.TryParse("1", prereleaseRule, out _)).IsTrue();
+		await Assert.That(IdentifierList.TryParse("0", prereleaseRule, out _)).IsTrue();
+		await Assert.That(IdentifierList.TryParse("0abc", prereleaseRule, out _)).IsTrue();
+	}
+
+	[Test]
+	public async Task I_can_allow_leading_zero_numerics_for_metadata_rule()
+	{
+		var metadataRule = new IdentifierListFormatInfo(forbidLeadingZeroInNumerics: false);
+
+		await Assert.That(IdentifierList.TryParse("01", metadataRule, out _)).IsTrue();
+		await Assert.That(IdentifierList.TryParse("0.1", metadataRule, out var list)).IsTrue();
+		await Assert.That(list.ToString()).IsEqualTo("0.1");
+	}
+
+	[Test]
+	public async Task I_can_throw_format_exception_for_invalid_parse()
+	{
+		await Assert.That(() => IdentifierList.Parse("a..b")).Throws<FormatException>();
+		await Assert.That(() => IdentifierList.Parse("a.b!".AsSpan())).Throws<FormatException>();
+	}
+
+	[Test]
+	public async Task I_can_expose_default_format_info_allowing_leading_zeroes()
+	{
+		await Assert.That(IdentifierListFormatInfo.Default.ForbidLeadingZeroInNumerics).IsFalse();
+	}
+}

src/semver.tests/SemanticVersionTests.cs

@@ -26,8 +26,8 @@ internal sealed class SemanticVersionTests
 		await Assert.That(v.Major).IsEqualTo((ulong)major);
 		await Assert.That(v.Minor).IsEqualTo((ulong)minor);
 		await Assert.That(v.Patch).IsEqualTo((ulong)build);
-		await Assert.That(v.Prerelease).IsNull();
-		await Assert.That(v.Metadata).IsNull();
+		await Assert.That(v.Prerelease.IsEmpty).IsTrue();
+		await Assert.That(v.Metadata.IsEmpty).IsTrue();
 	}
 
 	[Test]
@@ -40,8 +40,8 @@ internal sealed class SemanticVersionTests
 		await Assert.That(v.Major).IsEqualTo(major);
 		await Assert.That(v.Minor).IsEqualTo(0UL);
 		await Assert.That(v.Patch).IsEqualTo(0UL);
-		await Assert.That(v.Prerelease).IsNull();
-		await Assert.That(v.Metadata).IsNull();
+		await Assert.That(v.Prerelease.IsEmpty).IsTrue();
+		await Assert.That(v.Metadata.IsEmpty).IsTrue();
 	}
 
 	[Test]
@@ -53,8 +53,8 @@ internal sealed class SemanticVersionTests
 		await Assert.That(v.Major).IsEqualTo(major);
 		await Assert.That(v.Minor).IsEqualTo(minor);
 		await Assert.That(v.Patch).IsEqualTo(0UL);
-		await Assert.That(v.Prerelease).IsNull();
-		await Assert.That(v.Metadata).IsNull();
+		await Assert.That(v.Prerelease.IsEmpty).IsTrue();
+		await Assert.That(v.Metadata.IsEmpty).IsTrue();
 	}
 
 	[Test]
@@ -66,8 +66,8 @@ internal sealed class SemanticVersionTests
 		await Assert.That(v.Major).IsEqualTo(major);
 		await Assert.That(v.Minor).IsEqualTo(minor);
 		await Assert.That(v.Patch).IsEqualTo(patch);
-		await Assert.That(v.Prerelease).IsNull();
-		await Assert.That(v.Metadata).IsNull();
+		await Assert.That(v.Prerelease.IsEmpty).IsTrue();
+		await Assert.That(v.Metadata.IsEmpty).IsTrue();
 	}
 
 	[Test]
@@ -80,8 +80,8 @@ internal sealed class SemanticVersionTests
 		await Assert.That(v.Major).IsEqualTo(major);
 		await Assert.That(v.Minor).IsEqualTo(minor);
 		await Assert.That(v.Patch).IsEqualTo(patch);
-		await Assert.That(v.Prerelease).IsEqualTo(prerelease);
-		await Assert.That(v.Metadata).IsEqualTo(metadata);
+		await Assert.That(v.Prerelease.ToString()).IsEqualTo(prerelease ?? string.Empty);
+		await Assert.That(v.Metadata.ToString()).IsEqualTo(metadata ?? string.Empty);
 	}
 
 	[Test]
@@ -133,8 +133,8 @@ internal sealed class SemanticVersionTests
 		await Assert.That(result.Major).IsEqualTo(1UL);
 		await Assert.That(result.Minor).IsEqualTo(2UL);
 		await Assert.That(result.Patch).IsEqualTo(3UL);
-		await Assert.That(result.Prerelease).IsEqualTo("alpha");
-		await Assert.That(result.Metadata).IsEqualTo("build.1");
+		await Assert.That(result.Prerelease.ToString()).IsEqualTo("alpha");
+		await Assert.That(result.Metadata.ToString()).IsEqualTo("build.1");
 	}
 
 	[Test]
@@ -182,8 +182,8 @@ internal sealed class SemanticVersionTests
 		await Assert.That(v.Major).IsEqualTo(1UL);
 		await Assert.That(v.Minor).IsEqualTo(2UL);
 		await Assert.That(v.Patch).IsEqualTo(3UL);
-		await Assert.That(v.Prerelease).IsEqualTo("beta");
-		await Assert.That(v.Metadata).IsEqualTo("789");
+		await Assert.That(v.Prerelease.ToString()).IsEqualTo("beta");
+		await Assert.That(v.Metadata.ToString()).IsEqualTo("789");
 	}
 
 	[Test]

src/semver/IdentifierList.cs

@@ -0,0 +1,516 @@
+// Copyright (c) The Geekeey Authors
+// SPDX-License-Identifier: EUPL-1.2
+
+using System.Buffers;
+using System.Collections;
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Geekeey.SemVer;
+
+/// <summary>
+/// A read-only, dot-separated list of semantic version identifiers (prerelease or build metadata).
+/// </summary>
+/// <remarks>
+/// <para>
+/// The value behaves as an implicit <see cref="string"/> holding the original, joined identifier text (for example
+/// <c>"alpha.1"</c>). It additionally exposes the individual segments without allocating a string per segment: it
+/// implements <see cref="IReadOnlyList{T}"/> of <see cref="string"/> and yields <see cref="ReadOnlySpan{T}"/> of
+/// <see cref="char"/> when enumerated with <see langword="foreach"/>.
+/// </para>
+/// <para>
+/// Equality is always performed ordinally on the joined text. Semantic-version precedence (numeric versus alphanumeric
+/// prerelease ordering) is intentionally not part of this type and lives in <see cref="SemanticVersionComparer"/>.
+/// </para>
+/// </remarks>
+[JsonConverter(typeof(IdentifierListJsonConverter))]
+public readonly struct IdentifierList : IReadOnlyList<string>, IEquatable<IdentifierList>, IEquatable<string?>, ISpanParsable<IdentifierList>
+{
+	private readonly string? _value;
+
+	/// <summary>
+	/// Gets an <see cref="IdentifierList"/> that contains no identifiers.
+	/// </summary>
+	public static readonly IdentifierList Empty = default;
+
+	/// <summary>
+	/// The set of characters that are legal within a single SemVer identifier.
+	/// </summary>
+	private static readonly SearchValues<char> ValidIdentifierChars =
+		SearchValues.Create("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-");
+
+	/// <summary>
+	/// Initializes a new instance of the <see cref="IdentifierList"/> struct from a joined identifier string.
+	/// </summary>
+	/// <param name="value">
+	/// The dot-separated identifier text, or <see langword="null"/>/empty to create <see cref="Empty"/>.
+	/// </param>
+	/// <remarks>
+	/// The value is stored verbatim; splitting into segments happens lazily during enumeration or indexing. No
+	/// identifier validation is performed here — that is the responsibility of <see cref="SemanticVersion"/> parsing.
+	/// </remarks>
+	public IdentifierList(string? value)
+	{
+		_value = value;
+	}
+
+	/// <summary>
+	/// Gets a value indicating whether this instance contains no identifiers.
+	/// </summary>
+	public bool IsEmpty => string.IsNullOrEmpty(_value);
+
+	/// <inheritdoc />
+	public int Count
+	{
+		get
+		{
+			if (IsEmpty)
+			{
+				return 0;
+			}
+
+			var count = 1;
+			for (var i = 0; i < _value!.Length; i++)
+			{
+				if (_value[i] is '.')
+				{
+					count++;
+				}
+			}
+
+			return count;
+		}
+	}
+
+	/// <inheritdoc />
+	public string this[int index]
+	{
+		get
+		{
+			if (index < 0 || index >= Count)
+			{
+				throw new ArgumentOutOfRangeException(nameof(index));
+			}
+
+			var start = 0;
+			for (var i = 0; i < index; i++)
+			{
+				start = _value!.IndexOf('.', start) + 1;
+			}
+
+			var end = _value!.IndexOf('.', start);
+			if (end < 0)
+			{
+				end = _value.Length;
+			}
+
+			return _value[start..end];
+		}
+	}
+
+	/// <summary>
+	/// Returns an enumerator that iterates over the identifiers as <see cref="ReadOnlySpan{T}"/> of <see cref="char"/>
+	/// without allocating a string per segment.
+	/// </summary>
+	/// <returns>An <see cref="Enumerator"/> positioned before the first identifier.</returns>
+	public Enumerator GetEnumerator()
+	{
+		return new Enumerator(_value);
+	}
+
+	/// <inheritdoc />
+	public override string ToString()
+	{
+		return _value ?? string.Empty;
+	}
+
+	/// <inheritdoc />
+	public bool Equals(IdentifierList other)
+	{
+		return string.Equals(_value ?? string.Empty, other._value ?? string.Empty, StringComparison.Ordinal);
+	}
+
+	/// <inheritdoc />
+	public bool Equals(string? other)
+	{
+		return string.Equals(_value ?? string.Empty, other, StringComparison.Ordinal);
+	}
+
+	/// <inheritdoc />
+	public override bool Equals(object? obj)
+	{
+		return obj is IdentifierList list ? Equals(list) : obj is string s && Equals(s);
+	}
+
+	/// <inheritdoc />
+	public override int GetHashCode()
+	{
+		return (_value ?? string.Empty).GetHashCode();
+	}
+
+	/// <summary>Determines whether two <see cref="IdentifierList"/> values are ordinally equal.</summary>
+	public static bool operator ==(IdentifierList left, IdentifierList right)
+	{
+		return left.Equals(right);
+	}
+
+	/// <summary>Determines whether two <see cref="IdentifierList"/> values differ.</summary>
+	public static bool operator !=(IdentifierList left, IdentifierList right)
+	{
+		return !left.Equals(right);
+	}
+
+	/// <summary>Determines whether an <see cref="IdentifierList"/> and a <see cref="string"/> are ordinally equal.</summary>
+	public static bool operator ==(IdentifierList left, string? right)
+	{
+		return left.Equals(right);
+	}
+
+	/// <summary>Determines whether an <see cref="IdentifierList"/> and a <see cref="string"/> differ.</summary>
+	public static bool operator !=(IdentifierList left, string? right)
+	{
+		return !left.Equals(right);
+	}
+
+	/// <summary>Determines whether a <see cref="string"/> and an <see cref="IdentifierList"/> are ordinally equal.</summary>
+	public static bool operator ==(string? left, IdentifierList right)
+	{
+		return right.Equals(left);
+	}
+
+	/// <summary>Determines whether a <see cref="string"/> and an <see cref="IdentifierList"/> differ.</summary>
+	public static bool operator !=(string? left, IdentifierList right)
+	{
+		return !right.Equals(left);
+	}
+
+	/// <summary>Defines an implicit conversion from <see cref="IdentifierList"/> to its joined <see cref="string"/> form.</summary>
+	public static implicit operator string(IdentifierList value)
+	{
+		return value._value ?? string.Empty;
+	}
+
+	/// <summary>Defines an implicit conversion from a joined <see cref="string"/> to an <see cref="IdentifierList"/>.</summary>
+	public static implicit operator IdentifierList(string? value)
+	{
+		return new IdentifierList(value);
+	}
+
+	#region IParsable
+
+	/// <summary>Parses a string into an <see cref="IdentifierList"/> using the default identifier rules.</summary>
+	/// <param name="s">The string to parse.</param>
+	/// <returns>The parsed <see cref="IdentifierList"/>.</returns>
+	/// <exception cref="FormatException"><paramref name="s"/> is not a valid dot-separated identifier list.</exception>
+	public static IdentifierList Parse(string? s)
+	{
+		return Parse(s.AsSpan(), null);
+	}
+
+	/// <summary>
+	/// Parses a string into an <see cref="IdentifierList"/>, validating it against the SemVer identifier rules.
+	/// </summary>
+	/// <param name="s">The string to parse.</param>
+	/// <param name="provider">See <see cref="Parse(ReadOnlySpan{char}, IFormatProvider?)"/>.</param>
+	/// <returns>The parsed <see cref="IdentifierList"/>.</returns>
+	/// <exception cref="FormatException"><paramref name="s"/> is not a valid dot-separated identifier list.</exception>
+	public static IdentifierList Parse(string? s, IFormatProvider? provider)
+	{
+		return Parse(s.AsSpan(), provider);
+	}
+
+	/// <summary>Tries to parse a string into an <see cref="IdentifierList"/> using the default identifier rules.</summary>
+	public static bool TryParse(string? s, out IdentifierList result)
+	{
+		return TryParse(s.AsSpan(), null, out result);
+	}
+
+	/// <summary>
+	/// Tries to parse a string into an <see cref="IdentifierList"/>.
+	/// </summary>
+	/// <param name="s">The string to parse.</param>
+	/// <param name="provider">See <see cref="Parse(ReadOnlySpan{char}, IFormatProvider?)"/>.</param>
+	/// <param name="result">The parsed list, or <see cref="Empty"/> on failure.</param>
+	/// <returns><see langword="true"/> if <paramref name="s"/> was parsed successfully; otherwise <see langword="false"/>.</returns>
+	public static bool TryParse(string? s, IFormatProvider? provider, out IdentifierList result)
+	{
+		return TryParse(s.AsSpan(), provider, out result);
+	}
+
+	#endregion
+
+	/// <summary>Parses a span of characters into an <see cref="IdentifierList"/> using the default identifier rules.</summary>
+	/// <param name="s">The span of characters to parse.</param>
+	/// <returns>The parsed <see cref="IdentifierList"/>.</returns>
+	/// <exception cref="FormatException"><paramref name="s"/> is not a valid dot-separated identifier list.</exception>
+	public static IdentifierList Parse(ReadOnlySpan<char> s)
+	{
+		return Parse(s, null);
+	}
+
+	/// <summary>
+	/// Parses a span of characters into an <see cref="IdentifierList"/>, validating it against the SemVer identifier rules.
+	/// </summary>
+	/// <param name="s">The span of characters to parse.</param>
+	/// <param name="provider">
+	/// An <see cref="IFormatProvider"/> that supplies an <see cref="IdentifierListFormatInfo"/>; its
+	/// <see cref="IdentifierListFormatInfo.ForbidLeadingZeroInNumerics"/> controls whether all-numeric identifiers may carry
+	/// leading zeroes. When omitted, leading zeroes are allowed (build-metadata rules).
+	/// </param>
+	/// <returns>The parsed <see cref="IdentifierList"/>.</returns>
+	/// <exception cref="FormatException"><paramref name="s"/> is not a valid dot-separated identifier list.</exception>
+	public static IdentifierList Parse(ReadOnlySpan<char> s, IFormatProvider? provider)
+	{
+		if (!TryParse(s, provider, out var result))
+		{
+			throw new FormatException($"The identifier list '{s}' was not in a correct format.");
+		}
+
+		return result;
+	}
+
+	/// <summary>Tries to parse a span of characters into an <see cref="IdentifierList"/> using the default identifier rules.</summary>
+	public static bool TryParse(ReadOnlySpan<char> s, out IdentifierList result)
+	{
+		return TryParse(s, null, out result);
+	}
+
+	/// <summary>
+	/// Tries to parse a span of characters into an <see cref="IdentifierList"/>.
+	/// </summary>
+	/// <param name="s">The span of characters to parse.</param>
+	/// <param name="provider">See <see cref="Parse(ReadOnlySpan{char}, IFormatProvider?)"/>.</param>
+	/// <param name="result">The parsed list, or <see cref="Empty"/> on failure.</param>
+	/// <returns><see langword="true"/> if <paramref name="s"/> was parsed successfully; otherwise <see langword="false"/>.</returns>
+	public static bool TryParse(ReadOnlySpan<char> s, IFormatProvider? provider, out IdentifierList result)
+	{
+		var formatInfo = IdentifierListFormatInfo.GetInstance(provider);
+
+		if (!IsValidIdentifiers(s, formatInfo.ForbidLeadingZeroInNumerics))
+		{
+			result = Empty;
+			return false;
+		}
+
+		result = new IdentifierList(new string(s));
+		return true;
+	}
+
+	/// <inheritdoc />
+	IEnumerator<string> IEnumerable<string>.GetEnumerator()
+	{
+		if (string.IsNullOrEmpty(_value))
+		{
+			return ((IEnumerable<string>)[]).GetEnumerator();
+		}
+
+		var enumerator = GetEnumerator();
+		var segments = new List<string>(Count);
+		while (enumerator.MoveNext())
+		{
+			segments.Add(new string(enumerator.Current));
+		}
+
+		return segments.GetEnumerator();
+	}
+
+	/// <inheritdoc />
+	IEnumerator IEnumerable.GetEnumerator()
+	{
+		return ((IEnumerable<string>)this).GetEnumerator();
+	}
+
+	/// <summary>
+	/// Determines whether the given dot-separated identifier text conforms to the SemVer 2.0.0 identifier rules.
+	/// </summary>
+	/// <param name="value">The joined identifier text to validate.</param>
+	/// <param name="forbidLeadingZeroInNumerics">
+	/// <see langword="true"/> to reject numeric identifiers with leading zeroes (used for prerelease identifiers);
+	/// <see langword="false"/> to allow them (used for build metadata identifiers).
+	/// </param>
+	/// <returns><see langword="true"/> if every dot-separated segment is a valid SemVer identifier; otherwise <see langword="false"/>.</returns>
+	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 caller 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;
+	}
+
+	/// <summary>
+	/// Enumerates the identifiers of an <see cref="IdentifierList"/> as <see cref="ReadOnlySpan{T}"/> of
+	/// <see cref="char"/>, avoiding per-segment string allocations.
+	/// </summary>
+	public ref struct Enumerator
+	{
+		private readonly string? _value;
+		private int _next;
+
+		internal Enumerator(string? value)
+		{
+			_value = value;
+			_next = 0;
+			Current = default;
+		}
+
+		/// <summary>
+		/// Gets the identifier at the current position as a <see cref="ReadOnlySpan{T}"/> of <see cref="char"/>.
+		/// </summary>
+		public ReadOnlySpan<char> Current { get; private set; }
+
+		/// <summary>
+		/// Advances the enumerator to the next identifier.
+		/// </summary>
+		/// <returns><see langword="true"/> if an identifier is available; otherwise <see langword="false"/>.</returns>
+		public bool MoveNext()
+		{
+			if (string.IsNullOrEmpty(_value))
+			{
+				return false;
+			}
+
+			if (_next > _value.Length)
+			{
+				return false;
+			}
+
+			var dot = _value.IndexOf('.', _next);
+			if (dot < 0)
+			{
+				Current = _value.AsSpan(_next);
+				_next = _value.Length + 1;
+				return true;
+			}
+
+			Current = _value.AsSpan(_next, dot - _next);
+			_next = dot + 1;
+			return true;
+		}
+	}
+}
+
+/// <summary>
+/// Supplies the SemVer identifier rules used when parsing an <see cref="IdentifierList"/>.
+/// </summary>
+public sealed class IdentifierListFormatInfo : IFormatProvider
+{
+	/// <summary>
+	/// Gets an <see cref="IdentifierListFormatInfo"/> that allows leading zeroes in all-numeric identifiers
+	/// (the build-metadata rule).
+	/// </summary>
+	public static readonly IdentifierListFormatInfo Default = new();
+
+	/// <summary>
+	/// Initializes a new instance of the <see cref="IdentifierListFormatInfo"/> class.
+	/// </summary>
+	/// <param name="forbidLeadingZeroInNumerics">
+	/// <see langword="true"/> to reject all-numeric identifiers with leading zeroes (the prerelease rule);
+	/// <see langword="false"/> to allow them (the build-metadata rule).
+	/// </param>
+	public IdentifierListFormatInfo(bool forbidLeadingZeroInNumerics = false)
+	{
+		ForbidLeadingZeroInNumerics = forbidLeadingZeroInNumerics;
+	}
+
+	/// <summary>
+	/// Gets a value indicating whether all-numeric identifiers are forbidden from carrying leading zeroes.
+	/// </summary>
+	public bool ForbidLeadingZeroInNumerics { get; }
+
+	/// <inheritdoc />
+	public object? GetFormat(Type? formatType)
+	{
+		return formatType == typeof(IdentifierListFormatInfo) ? this : null;
+	}
+
+	/// <summary>
+	/// Gets the <see cref="IdentifierListFormatInfo"/> associated with the specified <see cref="IFormatProvider"/>.
+	/// </summary>
+	/// <param name="provider">The format provider, or <see langword="null"/> for <see cref="Default"/>.</param>
+	/// <returns>
+	/// <paramref name="provider"/> when it is an <see cref="IdentifierListFormatInfo"/>, the instance it returns from
+	/// <see cref="GetFormat"/>, or <see cref="Default"/> otherwise.
+	/// </returns>
+	public static IdentifierListFormatInfo GetInstance(IFormatProvider? provider)
+	{
+		if (provider is IdentifierListFormatInfo info)
+		{
+			return info;
+		}
+
+		if (provider?.GetFormat(typeof(IdentifierListFormatInfo)) is IdentifierListFormatInfo fromProvider)
+		{
+			return fromProvider;
+		}
+
+		return Default;
+	}
+}
+
+/// <summary>
+/// Converts an <see cref="IdentifierList"/> to and from its joined <see cref="string"/> JSON representation.
+/// </summary>
+internal sealed class IdentifierListJsonConverter : JsonConverter<IdentifierList>
+{
+	/// <inheritdoc />
+	public override IdentifierList Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+	{
+		if (reader.TokenType is JsonTokenType.Null)
+		{
+			return IdentifierList.Empty;
+		}
+
+		if (reader.TokenType is not JsonTokenType.String || reader.GetString() is not { } value)
+		{
+			throw new JsonException("Expected string");
+		}
+
+		return new IdentifierList(value);
+	}
+
+	/// <inheritdoc />
+	public override void Write(Utf8JsonWriter writer, IdentifierList value, JsonSerializerOptions options)
+	{
+		writer.WriteStringValue(value.ToString());
+	}
+}

src/semver/SemanticVersion.Formatting.cs

@@ -79,7 +79,7 @@ public readonly partial struct SemanticVersion : ISpanFormattable
 		destination = destination[written..];
 		charsWritten += written;
 
-		if (Prerelease is { Length: > 0 } && format is "s" or "f")
+		if (!Prerelease.IsEmpty && format is "s" or "f")
 		{
 			if (!destination.TryWrite(provider, $"-{Prerelease}", out written))
 			{
@@ -91,7 +91,7 @@ public readonly partial struct SemanticVersion : ISpanFormattable
 			charsWritten += written;
 		}
 
-		if (Metadata is { Length: > 0 } && format is "f")
+		if (!Metadata.IsEmpty && format is "f")
 		{
 			if (!destination.TryWrite(provider, $"+{Metadata}", out written))
 			{

src/semver/SemanticVersion.Parsing.cs

@@ -31,7 +31,7 @@ public readonly partial struct SemanticVersion : ISpanParsable<SemanticVersion>
 	/// <see cref="TryParse(string, IFormatProvider, out SemanticVersion)"/>
 	public static bool TryParse([NotNullWhen(true)] string? s, [MaybeNullWhen(false)] out SemanticVersion result)
 	{
-		return TryParse(s, null, out result);
+		return TryParse(s.AsSpan(), null, out result);
 	}
 
 	/// <inheritdoc />
@@ -85,9 +85,6 @@ public readonly partial struct SemanticVersion : ISpanParsable<SemanticVersion>
 	// 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;
@@ -98,7 +95,7 @@ public readonly partial struct SemanticVersion : ISpanParsable<SemanticVersion>
 			return false;
 		}
 
-		var metadata = default(string);
+		var metadata = IdentifierList.Empty;
 		if (s.IndexOf('+') is >= 0 and var metadataIndex)
 		{
 			if (s[(metadataIndex + 1)..] is not { IsEmpty: false } value)
@@ -106,11 +103,15 @@ public readonly partial struct SemanticVersion : ISpanParsable<SemanticVersion>
 				return false;
 			}
 
-			metadata = new string(value);
+			if (!IdentifierList.TryParse(value, new IdentifierListFormatInfo(forbidLeadingZeroInNumerics: false), out metadata))
+			{
+				return false;
+			}
+
 			s = s[..metadataIndex];
 		}
 
-		var prerelease = default(string);
+		var prerelease = IdentifierList.Empty;
 		if (s.IndexOf('-') is >= 0 and var prereleaseIndex)
 		{
 			if (s[(prereleaseIndex + 1)..] is not { IsEmpty: false } value)
@@ -118,7 +119,11 @@ public readonly partial struct SemanticVersion : ISpanParsable<SemanticVersion>
 				return false;
 			}
 
-			prerelease = new string(value);
+			if (!IdentifierList.TryParse(value, new IdentifierListFormatInfo(forbidLeadingZeroInNumerics: true), out prerelease))
+			{
+				return false;
+			}
+
 			s = s[..prereleaseIndex];
 		}
 
@@ -158,67 +163,7 @@ public readonly partial struct SemanticVersion : ISpanParsable<SemanticVersion>
 			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;
-	}
 }

src/semver/SemanticVersion.cs

@@ -27,7 +27,7 @@ public readonly partial struct SemanticVersion : IEquatable<SemanticVersion>
 	/// </summary>
 	/// <param name="major">The major version number.</param>
 	public SemanticVersion(ulong major)
-		: this(major, 0, 0, default, default)
+		: this(major, 0, 0, IdentifierList.Empty, IdentifierList.Empty)
 	{
 	}
 
@@ -37,7 +37,7 @@ public readonly partial struct SemanticVersion : IEquatable<SemanticVersion>
 	/// <param name="major">The major version number.</param>
 	/// <param name="minor">The minor version number.</param>
 	public SemanticVersion(ulong major, ulong minor)
-		: this(major, minor, 0, default, default)
+		: this(major, minor, 0, IdentifierList.Empty, IdentifierList.Empty)
 	{
 	}
 
@@ -48,7 +48,7 @@ public readonly partial struct SemanticVersion : IEquatable<SemanticVersion>
 	/// <param name="minor">The minor version number.</param>
 	/// <param name="patch">The patch version number.</param>
 	public SemanticVersion(ulong major, ulong minor, ulong patch)
-		: this(major, minor, patch, default, default)
+		: this(major, minor, patch, IdentifierList.Empty, IdentifierList.Empty)
 	{
 	}
 
@@ -61,6 +61,19 @@ public readonly partial struct SemanticVersion : IEquatable<SemanticVersion>
 	/// <param name="prerelease">The prerelease identifiers.</param>
 	/// <param name="metadata">The build metadata identifiers.</param>
 	public SemanticVersion(ulong major, ulong minor, ulong patch, string? prerelease, string? metadata)
+		: this(major, minor, patch, new IdentifierList(prerelease), new IdentifierList(metadata))
+	{
+	}
+
+	/// <summary>
+	/// Constructs a new instance of the <see cref="SemanticVersion" /> class.
+	/// </summary>
+	/// <param name="major">The major version number.</param>
+	/// <param name="minor">The minor version number.</param>
+	/// <param name="patch">The patch version number.</param>
+	/// <param name="prerelease">The prerelease identifiers.</param>
+	/// <param name="metadata">The build metadata identifiers.</param>
+	public SemanticVersion(ulong major, ulong minor, ulong patch, IdentifierList prerelease, IdentifierList metadata)
 	{
 		Major = major;
 		Minor = minor;
@@ -91,20 +104,20 @@ public readonly partial struct SemanticVersion : IEquatable<SemanticVersion>
 	public ulong Patch { get; }
 
 	/// <summary>
-	/// Gets the prerelease label of the semantic version. This value is an optional string
-	/// indicating a version that is still in development or not yet considered stable.
-	/// Examples of prerelease labels include "alpha", "beta", or "rc".
-	/// When present, the prerelease label is used to differentiate between versions with
+	/// Gets the prerelease identifiers of the semantic version. This value is an
+	/// <see cref="IdentifierList"/> indicating a version that is still in development or not yet considered stable.
+	/// Examples of prerelease identifiers include "alpha", "beta", or "rc".
+	/// When present, the prerelease identifiers are used to differentiate between versions with
 	/// the same major, minor, and patch numbers.
 	/// </summary>
-	public string? Prerelease { get; }
+	public IdentifierList Prerelease { get; }
 
 	/// <summary>
 	/// Gets the build metadata associated with the semantic version. This optional value provides
 	/// additional information about the build, such as build number, commit hash, or other relevant
 	/// details. It does not affect the precedence of the version.
 	/// </summary>
-	public string? Metadata { get; }
+	public IdentifierList Metadata { get; }
 
 	/// <inheritdoc />
 	public bool Equals(SemanticVersion other)

src/semver/SemanticVersionComparer.cs

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: EUPL-1.2
 
 using System.Collections;
+using System.Globalization;
 
 namespace Geekeey.SemVer;
 
@@ -111,7 +112,7 @@ public abstract class SemanticVersionComparer : IEqualityComparer<SemanticVersio
 			return v1.Major == v2.Major &&
 				   v1.Minor == v2.Minor &&
 				   v1.Patch == v2.Patch &&
-				   string.Equals(v1.Prerelease, v2.Prerelease, StringComparison.Ordinal);
+				   v1.Prerelease.Equals(v2.Prerelease);
 		}
 
 		public override int GetHashCode(SemanticVersion? v)
@@ -192,8 +193,8 @@ public abstract class SemanticVersionComparer : IEqualityComparer<SemanticVersio
 			return v1.Major == v2.Major &&
 				   v1.Minor == v2.Minor &&
 				   v1.Patch == v2.Patch &&
-				   string.Equals(v1.Prerelease, v2.Prerelease, StringComparison.Ordinal) &&
-				   string.Equals(v1.Metadata, v2.Metadata, StringComparison.Ordinal);
+				   v1.Prerelease.Equals(v2.Prerelease) &&
+				   v1.Metadata.Equals(v2.Metadata);
 		}
 
 		public override int GetHashCode(SemanticVersion? v)
@@ -210,38 +211,55 @@ public abstract class SemanticVersionComparer : IEqualityComparer<SemanticVersio
 		public override int Compare(SemanticVersion? x, SemanticVersion? y)
 		{
 			var cmp = Priority.Compare(x, y);
-			return cmp is not 0 ? cmp : CompareMetadata(x?.Metadata, y?.Metadata);
+			return cmp is not 0 ? cmp : CompareMetadata(x?.Metadata ?? IdentifierList.Empty, y?.Metadata ?? IdentifierList.Empty);
 		}
 	}
 
-	private static int ComparePrerelease(string? x, string? y)
+	private static int ComparePrerelease(IdentifierList x, IdentifierList y)
 	{
-		if (string.Equals(x, y, StringComparison.Ordinal))
+		if (x.Equals(y))
 		{
 			return 0;
 		}
 
-		if (x is null)
+		if (x.IsEmpty)
 		{
 			return 1;
 		}
 
-		if (y is null)
+		if (y.IsEmpty)
 		{
 			return -1;
 		}
 
-		var xParts = x.Split('.');
-		var yParts = y.Split('.');
+		var xEnumerator = x.GetEnumerator();
+		var yEnumerator = y.GetEnumerator();
 
-		var length = Math.Min(xParts.Length, yParts.Length);
-		for (var i = 0; i < length; i++)
+		while (true)
 		{
-			var xPart = xParts[i];
-			var yPart = yParts[i];
+			var xHas = xEnumerator.MoveNext();
+			var yHas = yEnumerator.MoveNext();
 
-			var xIsNumeric = ulong.TryParse(xPart, out var xNum);
-			var yIsNumeric = ulong.TryParse(yPart, out var yNum);
+			if (!xHas && !yHas)
+			{
+				return 0;
+			}
+
+			if (!xHas)
+			{
+				return -1;
+			}
+
+			if (!yHas)
+			{
+				return 1;
+			}
+
+			var xPart = xEnumerator.Current;
+			var yPart = yEnumerator.Current;
+
+			var xIsNumeric = ulong.TryParse(xPart, NumberStyles.None, CultureInfo.InvariantCulture, out var xNum);
+			var yIsNumeric = ulong.TryParse(yPart, NumberStyles.None, CultureInfo.InvariantCulture, out var yNum);
 
 			if (xIsNumeric && yIsNumeric)
 			{
@@ -261,47 +279,60 @@ public abstract class SemanticVersionComparer : IEqualityComparer<SemanticVersio
 			}
 			else
 			{
-				var cmp = string.CompareOrdinal(xPart, yPart);
+				var cmp = xPart.SequenceCompareTo(yPart);
 				if (cmp is not 0)
 				{
 					return cmp;
 				}
 			}
 		}
-
-		return xParts.Length.CompareTo(yParts.Length);
 	}
 
-	private static int CompareMetadata(string? x, string? y)
+	private static int CompareMetadata(IdentifierList x, IdentifierList y)
 	{
-		if (string.Equals(x, y, StringComparison.Ordinal))
+		if (x.Equals(y))
 		{
 			return 0;
 		}
 
-		if (x is null)
+		if (x.IsEmpty)
 		{
 			return -1;
 		}
 
-		if (y is null)
+		if (y.IsEmpty)
 		{
 			return 1;
 		}
 
-		var xParts = x.Split('.');
-		var yParts = y.Split('.');
+		var xEnumerator = x.GetEnumerator();
+		var yEnumerator = y.GetEnumerator();
 
-		var length = Math.Min(xParts.Length, yParts.Length);
-		for (var i = 0; i < length; i++)
+		while (true)
 		{
-			var cmp = string.CompareOrdinal(xParts[i], yParts[i]);
+			var xHas = xEnumerator.MoveNext();
+			var yHas = yEnumerator.MoveNext();
+
+			if (!xHas && !yHas)
+			{
+				return 0;
+			}
+
+			if (!xHas)
+			{
+				return -1;
+			}
+
+			if (!yHas)
+			{
+				return 1;
+			}
+
+			var cmp = xEnumerator.Current.SequenceCompareTo(yEnumerator.Current);
 			if (cmp is not 0)
 			{
 				return cmp;
 			}
 		}
-
-		return xParts.Length.CompareTo(yParts.Length);
 	}
 }

src/semver/SemanticVersionRange.Formatting.cs

@@ -150,7 +150,7 @@ public readonly partial struct SemanticVersionRange : ISpanFormattable
 			return false;
 		}
 
-		if (upper.Operation is Comparison.Lt && hi is { Minor: 0, Patch: 0, Prerelease: not { Length: > 0 } })
+		if (upper.Operation is Comparison.Lt && hi is { Minor: 0, Patch: 0, Prerelease.IsEmpty: true })
 		{
 			if (lo is { Major: > 0 } && hi.Major == lo.Major + 1)
 			{
@@ -168,7 +168,7 @@ public readonly partial struct SemanticVersionRange : ISpanFormattable
 			}
 		}
 
-		if (upper.Operation is Comparison.Lt && hi is { Patch: 0, Prerelease: not { Length: > 0 } })
+		if (upper.Operation is Comparison.Lt && hi is { Patch: 0, Prerelease.IsEmpty: true })
 		{
 			if (hi.Major == lo.Major && hi.Minor == lo.Minor + 1)
 			{