Implemented the URLValidationRule type in the library target.

Sources/DiscogsService/Internal/Errors/InputValidationError.swift

@@ -24,8 +24,10 @@ enum InputValidationError: Error {
     case inputNotConsumerKey
     /// An input does not comply with the consumer secret requirements.
     case inputNotConsumerSecret
-    /// An input does not comply with the semantic versioning requirements.
+    /// An input is not a semantic version.
     case inputNotSemanticVersion
+    /// An input is not a URL.
+    case inputNotURL
     /// An input does not comply with the user token requirements.
     case inputNotUserToken
 }

Sources/DiscogsService/Internal/Validation Rules/URLValidationRule.swift

@@ -1,8 +1,83 @@
+// ===----------------------------------------------------------------------===
 //
-//  File.swift
-//  discogs-service
+// This source file is part of the DiscogsService open source project
 //
-//  Created by Javier Cicchelli on 13/10/2025.
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
 //
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
 
-import Foundation
+/// A validation rule type that checks whether an input is a URL or not.
+///
+/// This validation rule doesn't necessarily follow the [RFC 3986](https://www.rfc-editor.org/rfc/rfc3986) standard.
+/// Thus it doesn't implement a complex regular expression pattern such as [this one](https://rgxdb.com/r/5JXUI5A2).
+/// Instead this validation implements a regular expression sufficient enough to satisfy the requirements for a [user agent definition](https://www.discogs.com/developers/#page:home,header:home-general-information).
+struct URLValidationRule: InputValidationRule {
+    
+    // MARK: Functions
+    
+#if swift(>=6.0)
+    func validate(_ input: String?) throws(InputValidationError) -> Bool {
+        try validate(input: input)
+    }
+#else
+    func validate(_ input: String?) throws -> Bool {
+        try validate(input: input)
+    }
+#endif
+    
+}
+
+// MARK: - Definitions
+
+extension InputValidationRule where Self == URLValidationRule {
+    
+    // MARK: Constants
+    
+    /// A validation rule that checks whether an input is a URL or not.
+    static var url: Self { .init() }
+    
+}
+
+// MARK: - Helpers
+
+private extension URLValidationRule {
+    
+    // MARK: Functions
+    
+    /// Validates a given input.
+    ///
+    /// > note: This helper function would not be necessary when support for *Swift 5.10* is discontinued.
+    ///
+    /// - Parameter input: An input to be validated.
+    /// - Returns: A flag that indicates whether a given input has been validated or not.
+    /// - Throws: An error of type ``InputValidatorError`` in case the validation failed.
+    func validate(input: String?) throws -> Bool {
+        guard let input else {
+            return false
+        }
+        
+        guard input.fullyMatch(
+            pattern: .init(format: .Pattern.url)
+        ) else {
+            throw InputValidationError.inputNotURL
+        }
+        
+        return true
+    }
+    
+}
+
+// MARK: - Constants
+
+private extension String.Pattern {
+    /// A regular expression pattern that represents URL inputs.
+    ///
+    /// This regular expression is based on [this regular expression](https://regex101.com/r/3fYy3x/1) found while researching the topic.
+    static let url = "https?:\\/\\/(www\\.)?[-a-zA-Z0-9@:%._\\+~#=]{2,256}\\.[a-z]{2,4}\\b([-a-zA-Z0-9@:%_\\+.~#?&//=]*)"
+}