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Q42/ValidationKit 1.0.1
A Swift library for validating user input, for example in forms.
⭐️ 5
🕓 20 weeks ago
.package(url: "https://github.com/Q42/ValidationKit.git", from: "1.0.1")


A lightweight Swift library for validating and normalizing user input, for example in forms.


Documentation for ValidationKit can be found on GitHub Pages. Documentation is written using DocC, with a documentation catalog included inside the Swift package.


Using the built in validators

import ValidationKit

// Validators can be combined using the operators `&&` and `||`
let tweetValidator: Validator = .notEmpty && .maxLength(280)

let tweet = "Hello, world!"
let result = tweetValidator.validate(input: tweet)

switch result {
case .valid(let value):
  print("Tweet '\(value)' is valid")
case .invalid(let error):
  print(String(format: "Tweet did not validate: %@", error.localizedDescription))

The built-in validators are:

  • isEmpty: String must be empty
  • notEmpty: String must not be empty
  • minLength(_ length: Int): String length must be greater than or equal...
  • maxLength(_ length: Int): String length must be less than or equal...
  • exactLength(_ length: Int): String length must be exactly...
  • isDate(formatter: DateFormatter): String must be parsable as a date using the given formatter
  • isInThePast: Date must be in the past
  • anyOf: Value must be one of certain given options
  • accepted: Value must be boolean true
  • hasPrefix: String must have a certain prefix

Implementing a custom validator

A custom validator can be implemented as follows. The example uses a regular expression, but of course you are free to use any kind of logic you want.

public extension Validator {
  static var isDutchPostalCode: Validator<String, String> {
    Validator<String, String> { input in
      if let range = input.range(of: #"^\d{4}\s?[a-zA-Z]{2}$"#, options: .regularExpression) {
        let output = String(input[range])
        return .valid(output)
      } else {
        return .invalid(.invalidDutchPostalCode)

public extension ValidationError {
  static let invalidDutchPostalCode = ValidationError(localizedDescription: NSLocalizedString("Invalid Dutch postal code", comment: "Validation error text"))

We can then use it like so:

let postalCodeValidator: Validator = .notEmpty && .isDutchPostalCode
let result = postalCodeValidator.validate(input: "2516 AH") // Equals ValidationResult.valid("2516 AH")

The input of a validator does not have to be of the same type as its output. For example, this validator converts a string into an array of digits (integers from 0-9).

extension Validator {
  /// Validates whether a string consists of a sequence of digits (0-9).
  static var numericCode: Validator<String, [Int]> {
    Validator<String, [Int]> { input in
      let numericCode: [Int] = input
        .map(String.init) // Convert the string to individual characters first

      if numericCode.count == input.count {
        return .valid(numericCode)
      } else {
        return .invalid(.notNumericCode)

extension ValidationError {
  static let notNumericCode = ValidationError(localizedDescription: NSLocalizedString("Not a numeric code", comment: "Validation error text"))

To validate an 8-digit code, we can use it like so:

let validator: Validator = .exactLength(8) && .numericCode
let result = validator.validate(input: "13374242)
result.isValid // True
result.value // [1, 3, 3, 7, 4, 2, 4, 2]


This library contains localized strings for English and Dutch. Pull requests that add support for additional languages are welcome.

If you desire a different error text for a certain validation, you must write your own validator and validation error. See the files Validators.swift and ValidationError.swift for examples how to do this.


  • 1.0.1 - 2023-10-12 - Added isEmpty/notEmpty validators for non-optional string types
  • 1.0.0 - 2023-05-25 - After more than two years since the public release, and having been used in multiple production projects, it is time for a 1.0 release!
  • 0.0.1 - 2021-03-28 - Initial public release
  • 0.0.0 - 2019-12-02 - Initial private version for a project at Q42

Licence & Credits

ValidationKit is written by Mathijs Bernson and Tim van Steenis.

It is available under the MIT license, so feel free to use it in commercial and non-commercial projects.


Stars: 5
Last commit: 20 weeks ago
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Release Notes

20 weeks ago
  • Add isEmpty/notEmpty validator for non-optional strings

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