feat: add JSON validation rule (#116)

Author: ns-vasilev

README.md

@@ -319,7 +319,8 @@ struct RegistrationView: View {
 | `PostalCodeValidationRule` | Validates postal/ZIP codes for different countries | `PostalCodeValidationRule(country: .uk, error: "Invalid post code")`
 | `Base64ValidationRule` | | `Base64ValidationRule(error: "The input is not valid Base64.")`
 | `UUIDValidationRule` | Validates UUID format | `UUIDValidationRule(error: "Please enter a valid UUID")` |
-
+| `JSONValidationRule` | Validates that a string represents valid JSON | `JSONValidationRule(error: "Invalid JSON")`
+ 
 ## Custom Validators
 
 Create custom validation rules by conforming to `IValidationRule`:

Sources/ValidatorCore/Classes/Rules/JSONValidationRule.swift

@@ -0,0 +1,71 @@
+//
+// Validator
+// Copyright © 2026 Space Code. All rights reserved.
+//
+
+import Foundation
+
+/// Validates that a string represents valid JSON.
+///
+/// # Example:
+/// ```swift
+/// let rule = JSONValidationRule(error: "Invalid JSON")
+/// rule.validate(input: "{\"key\": \"value\"}") // true
+/// rule.validate(input: "not json") // false
+/// ```
+public struct JSONValidationRule: IValidationRule {
+    // MARK: Types
+
+    public typealias Input = String
+
+    // MARK: Properties
+
+    /// The validation error returned if the input is not valid JSON.
+    public let error: IValidationError
+
+    // MARK: Initialization
+
+    /// Initializes a JSON validation rule.
+    ///
+    /// - Parameter error: The validation error returned if input fails validation.
+    public init(error: IValidationError) {
+        self.error = error
+    }
+
+    // MARK: IValidationRule
+
+    public func validate(input: String) -> Bool {
+        guard !input.isEmpty else { return false }
+        guard let data = input.data(using: .utf8) else { return false }
+
+        do {
+            _ = try JSONSerialization.jsonObject(with: data, options: [])
+        } catch {
+            return false
+        }
+
+        return isStrictJSON(input)
+    }
+
+    // MARK: Private Methods
+
+    /// Validates strict JSON syntax to catch issues like trailing commas.
+    private func isStrictJSON(_ input: String) -> Bool {
+        let trimmed = input.trimmingCharacters(in: .whitespacesAndNewlines)
+
+        let trailingCommaPatterns = [
+            ",\\s*}",
+            ",\\s*\\]",
+        ]
+
+        for pattern in trailingCommaPatterns where trimmed.range(of: pattern, options: .regularExpression) != nil {
+            return false
+        }
+
+        if trimmed.range(of: ",,", options: .literal) != nil {
+            return false
+        }
+
+        return true
+    }
+}

Sources/ValidatorCore/Classes/Rules/URLValidationRule.swift

@@ -5,14 +5,14 @@
 
 import Foundation
 
-/// Validates that a string represents a valid URL.
-///
-/// # Example:
-/// ```swift
-/// let rule = URLValidationRule(error: "Invalid URL")
-/// rule.validate(input: "https://example.com") // true
-/// rule.validate(input: "not_a_url") // false
-/// ```
+// Validates that a string represents a valid URL.
+//
+// # Example:
+// ```swift
+// let rule = URLValidationRule(error: "Invalid URL")
+// rule.validate(input: "https://example.com") // true
+// rule.validate(input: "not_a_url") // false
+// ```
 public struct URLValidationRule: IValidationRule {
     // MARK: Types
 

Sources/ValidatorCore/Validator.docc/Overview.md

@@ -42,6 +42,7 @@ ValidatorCore contains all core validation rules, utilities, and mechanisms for
 - ``PostalCodeValidationRule``
 - ``Base64ValidationRule``
 - ``UUIDValidationRule``
+- ``JSONValidationRule``
 
 ### Articles
 

Tests/ValidatorCoreTests/UnitTests/Rules/JSONValidationRuleTests.swift

@@ -0,0 +1,189 @@
+//
+// Validator
+// Copyright © 2026 Space Code. All rights reserved.
+//
+
+import ValidatorCore
+import XCTest
+
+// MARK: - JSONValidationRuleTests
+
+final class JSONValidationRuleTests: XCTestCase {
+    // MARK: - Properties
+
+    private var sut: JSONValidationRule!
+
+    // MARK: - Setup
+
+    override func setUp() {
+        super.setUp()
+        sut = JSONValidationRule(error: String.error)
+    }
+
+    override func tearDown() {
+        sut = nil
+        super.tearDown()
+    }
+
+    // MARK: - Tests
+
+    func test_validate_validJSONObject_shouldReturnTrue() {
+        // given
+        let json = "{\"key\": \"value\"}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertTrue(result)
+    }
+
+    func test_validate_validJSONArray_shouldReturnTrue() {
+        // given
+        let json = "[1, 2, 3]"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertTrue(result)
+    }
+
+    func test_validate_validNestedJSON_shouldReturnTrue() {
+        // given
+        let json = "{\"user\": {\"name\": \"John\", \"age\": 30}}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertTrue(result)
+    }
+
+    func test_validate_validJSONWithNumbers_shouldReturnTrue() {
+        // given
+        let json = "{\"count\": 42, \"price\": 19.99}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertTrue(result)
+    }
+
+    func test_validate_validJSONWithBooleans_shouldReturnTrue() {
+        // given
+        let json = "{\"active\": true, \"deleted\": false}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertTrue(result)
+    }
+
+    func test_validate_validJSONWithNull_shouldReturnTrue() {
+        // given
+        let json = "{\"value\": null}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertTrue(result)
+    }
+
+    func test_validate_invalidJSON_shouldReturnFalse() {
+        // given
+        let json = "{key: value}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_validate_incompleteBraces_shouldReturnFalse() {
+        // given
+        let json = "{\"key\": \"value\""
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_validate_trailingComma_shouldReturnFalse() {
+        // given
+        let json = "{\"key\": \"value\",}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_validate_plainText_shouldReturnFalse() {
+        // given
+        let json = "not json"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_validate_emptyString_shouldReturnFalse() {
+        // given
+        let json = ""
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_validate_whitespaceString_shouldReturnFalse() {
+        // given
+        let json = "    "
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_validate_singleQuotesJSON_shouldReturnFalse() {
+        // given
+        let json = "{'key': 'value'}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+
+    func test_validate_missingQuotesOnKey_shouldReturnFalse() {
+        // given
+        let json = "{key: \"value\"}"
+
+        // when
+        let result = sut.validate(input: json)
+
+        // then
+        XCTAssertFalse(result)
+    }
+}
+
+// MARK: Constants
+
+private extension String {
+    static let error = "JSON is invalid"
+}