feat: add contains substring validation rule

README.md

@@ -311,6 +311,7 @@ struct RegistrationView: View {
 | `NilValidationRule` | Validates that value is nil | `NilValidationRule(error: "Value must be nil")`
 | `PositiveNumberValidationRule` | Validates that value is positive | `PositiveNumberValidationRule(error: "Value must be positive")`
 | `NoWhitespaceValidationRule` | Validates that a string does not contain any whitespace characters | `NoWhitespaceValidationRule(error: "Spaces are not allowed")`
+| `ContainsValidationRule` | Validates that a string contains a specific substring | `ContainsValidationRule(substring: "@", error: "Must contain @")`
 
 ## Custom Validators
 

Sources/ValidatorCore/Classes/Rules/ContainsValidationRule.swift

@@ -0,0 +1,43 @@
+//
+// Validator
+// Copyright © 2025 Space Code. All rights reserved.
+//
+
+/// Validates that a string contains a specific substring.
+///
+/// # Example:
+/// ```swift
+/// let rule = ContainsValidationRule(substring: "@", error: "Must contain @")
+/// rule.validate(input: "user@example.com") // true
+/// ```
+public struct ContainsValidationRule: IValidationRule {
+    // MARK: Types
+
+    public typealias Input = String
+
+    // MARK: Properties
+
+    /// The substring that the input must contain.
+    public let substring: String
+
+    /// The validation error.
+    public let error: IValidationError
+
+    // MARK: Initialization
+
+    /// Creates a validation rule that checks whether the input contains a required substring.
+    ///
+    /// - Parameters:
+    ///   - substring: The string the input must contain.
+    ///   - error: The validation error associated with failed validation.
+    public init(substring: String, error: IValidationError) {
+        self.substring = substring
+        self.error = error
+    }
+
+    // MARK: IValidationRule
+
+    public func validate(input: String) -> Bool {
+        input.contains(substring)
+    }
+}

Sources/ValidatorCore/Validator.docc/Overview.md

@@ -34,6 +34,7 @@ ValidatorCore contains all core validation rules, utilities, and mechanisms for
 - ``NilValidationRule``
 - ``PositiveNumberValidationRule``
 - ``NoWhitespaceValidationRuleTests``
+- ``ContainsValidationRule``
 
 ### Articles
 

Tests/ValidatorCoreTests/UnitTests/Rules/ContainsValidationRuleTests.swift

@@ -0,0 +1,73 @@
+//
+// Validator
+// Copyright © 2025 Space Code. All rights reserved.
+//
+
+@testable import ValidatorCore
+import XCTest
+
+// MARK: - ContainsValidationRuleTests
+
+final class ContainsValidationRuleTests: XCTestCase {
+    // MARK: Properties
+
+    private var sut: ContainsValidationRule!
+
+    // MARK: XCTestCase
+
+    override func setUp() {
+        super.setUp()
+        sut = ContainsValidationRule(substring: "@", error: String.error)
+    }
+
+    override func tearDown() {
+        sut = nil
+        super.tearDown()
+    }
+
+    // MARK: Tests
+
+    func test_thatContainsValidationRuleSetsProperties() {
+        // then
+        XCTAssertEqual(sut.substring, "@")
+        XCTAssertEqual(sut.error.message, .error)
+    }
+
+    func test_thatRuleReturnsTrue_whenInputContainsSubstring() {
+        // when
+        let result = sut.validate(input: "user@example.com")
+
+        // then
+        XCTAssertTrue(result)
+    }
+
+    func test_thatRuleReturnsFalse_whenInputDoesNotContainSubstring() {
+        // when
+        let result = sut.validate(input: "userexample.com")
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_thatRuleReturnsFalse_whenInputIsEmpty() {
+        // when
+        let result = sut.validate(input: "")
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_thatRuleReturnsTrue_whenSubstringAppearsMultipleTimes() {
+        // when
+        let result = sut.validate(input: "@user@domain.com")
+
+        // then
+        XCTAssertTrue(result)
+    }
+}
+
+// MARK: - Constants
+
+private extension String {
+    static let error = "Must contain @"
+}