feat: add characters validation rule (#67)

* feat: add characters validation rule

* test: add unit tests for the characters validation rule

* docs: update `README.md`

Author: ns-vasilev

README.md

@@ -303,6 +303,7 @@ struct RegistrationView: View {
 | `URLValidationRule` | Validates URL format | `URLValidationRule(error: "Please enter a valid URL")` |
 | `CreditCardValidationRule` | Validates credit card numbers (Luhn algorithm) | `CreditCardValidationRule(error: "Invalid card number")` |
 | `EmailValidationRule` | Validates email format | `EmailValidationRule(error: "Please enter a valid email")` |
+| `CharactersValidationRule` | Validates that a string contains only characters from the allowed CharacterSet | `CharactersValidationRule(characterSet: .letters, error: "Invalid characters")` |
 
 ## Custom Validators
 

Sources/ValidatorCore/Classes/Rules/CharactersValidationRule.swift

@@ -0,0 +1,33 @@
+//
+// Validator
+// Copyright © 2025 Space Code. All rights reserved.
+//
+
+import Foundation
+
+/// A characters validation rule.
+public struct CharactersValidationRule: IValidationRule {
+    // MARK: Types
+
+    public typealias Input = String
+
+    // MARK: Properties
+
+    public let characterSet: CharacterSet
+
+    /// The validation error.
+    public let error: IValidationError
+
+    // MARK: Initialization
+
+    public init(characterSet: CharacterSet, error: IValidationError) {
+        self.characterSet = characterSet
+        self.error = error
+    }
+
+    // MARK: IValidationRule
+
+    public func validate(input: String) -> Bool {
+        input.rangeOfCharacter(from: characterSet.inverted) == .none
+    }
+}

Tests/ValidatorCoreTests/UnitTests/Rules/CharactersValidationRuleTests.swift

@@ -0,0 +1,100 @@
+//
+// Validator
+// Copyright © 2025 Space Code. All rights reserved.
+//
+
+@testable import ValidatorCore
+import XCTest
+
+// MARK: - CharactersValidationRuleTests
+
+final class CharactersValidationRuleTests: XCTestCase {
+    // MARK: - Properties
+
+    private var sut: CharactersValidationRule!
+
+    // MARK: - Setup / Teardown
+
+    override func setUp() {
+        super.setUp()
+        sut = CharactersValidationRule(characterSet: .letters, error: String.error)
+    }
+
+    override func tearDown() {
+        sut = nil
+        super.tearDown()
+    }
+
+    // MARK: - Valid Input Tests
+
+    func testValidate_WithOnlyAllowedCharacters_ReturnsTrue() {
+        // Given
+        let input = "HelloWorld"
+
+        // When
+        let result = sut.validate(input: input)
+
+        // Then
+        XCTAssertTrue(result)
+    }
+
+    // MARK: - Invalid Input Tests
+
+    func testValidate_WithDisallowedCharacters_ReturnsFalse() {
+        // Given
+        let input = "Hello123"
+
+        // When
+        let result = sut.validate(input: input)
+
+        // Then
+        XCTAssertFalse(result)
+    }
+
+    func testValidate_WithSpecialCharacters_ReturnsFalse() {
+        // Given
+        let input = "Hi!"
+
+        // When
+        let result = sut.validate(input: input)
+
+        // Then
+        XCTAssertFalse(result)
+    }
+
+    // MARK: - Edge Cases
+
+    func testValidate_EmptyString_ReturnsTrue() {
+        // Given
+        let input = ""
+
+        // When
+        let result = sut.validate(input: input)
+
+        // Then
+        XCTAssertTrue(result)
+    }
+
+    func testValidate_UnicodeCharacters_WhenNotAllowed_ReturnsTrue() {
+        // Given
+        let input = "Привет"
+
+        // When
+        let result = sut.validate(input: input)
+
+        // Then
+        XCTAssertTrue(result)
+    }
+
+    // MARK: - Error Property
+
+    func testErrorProperty_ReturnsProvidedError() {
+        XCTAssertEqual(sut.error as? String, String.error)
+    }
+}
+
+// MARK: Constants
+
+private extension String {
+    static let error = "Invalid characters"
+}