Implemented the "subtract(_: )" function for the String+Helpers extension in the library target.

Sources/HummingbirdDocC/Internal/Extensions/String+Helpers.swift

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+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the Hummingbird DocC Middleware open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the Hummingbird DocC Middleware project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of Hummingbird DocC Middleware project authors
+//
+// ===----------------------------------------------------------------------===
+
+import RegexBuilder
+
+extension String {
+    
+    /// Subtracts a prefix from a string.
+    /// - Parameters:
+    ///   - prefix: A prefix to remove from a string.
+    /// - Returns: A new string with a prefix removed, if any.
+    func subtract(
+        _ prefix: String
+    ) -> String? {
+        let reference = Reference(String.self)
+        let pattern = Regex {
+            prefix
+            Optionally {
+                Capture(as: reference) {
+                    OneOrMore(.anyNonNewline)
+                } transform: { output in
+                    String(output)
+                }
+            }
+        }
+        
+        guard let matches = self.prefixMatch(of: pattern) else {
+            return nil
+        }
+        
+        return matches.output.1
+    }
+    
+}

Tests/HummingbirdDocC/Tests/Internal/Extensions/String_HelpersTests.swift

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+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the Hummingbird DocC Middleware open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the Hummingbird DocC Middleware project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of Hummingbird DocC Middleware project authors
+//
+// ===----------------------------------------------------------------------===
+
+import Testing
+
+@testable import HummingbirdDocC
+
+@Suite("String Helpers", .tags(.extension))
+struct String_HelpersTests {
+    
+    // MARK: Functions tests
+    
+#if swift(>=6.2)
+    @Test(arguments: zip(
+        Input.prefixesToSubtract,
+        Output.prefixesToSubtract
+    ))
+    func `subtract`(
+        prefix: String,
+        expects newString: String?
+    ) {
+        assertSubtract(
+            string: .sample,
+            prefix: prefix,
+            expects: newString
+        )
+    }
+#else
+    @Test("subtract", arguments: zip(
+        Input.prefixesToSubtract,
+        Output.prefixesToSubtract
+    ))
+    func subtract(
+        prefix: String,
+        expects newString: String?
+    ) {
+        assertSubtract(
+            string: .sample,
+            prefix: prefix,
+            expects: newString
+        )
+    }
+#endif
+    
+}
+
+// MARK: - Assertions
+
+private extension String_HelpersTests {
+    
+    // MARK: Functions
+    
+    /// Asserts a string subtraction.
+    /// - Parameters:
+    ///   - string: A string from where the subtraction will occur.
+    ///   - prefix: A prefix to subtract from a string.
+    ///   - newString: An expected new string created out of the subtraction, if any.
+    func assertSubtract(
+        string: String,
+        prefix: String,
+        expects newString: String?
+    ) {
+        // GIVEN
+        // WHEN
+        let result = string.subtract(prefix)
+        
+        // THEN
+        #expect(result == newString)
+    }
+    
+}
+
+// MARK: - Constants
+
+private extension Input {
+    /// A list of prefix strings.
+    static let prefixesToSubtract: [String] = ["Some", .sample, "some", "Else"]
+}
+
+private extension Output {
+    /// A list of outcomes that are expected from subtracting the prefix substrings out of the sample string.
+    static let prefixesToSubtract: [String?] = ["thing", .empty, nil, nil]
+}
+
+private extension String {
+    /// A sample string.
+    static let sample = "Something"
+}