Здесь есть несколько хороших ответов, но я принял совершенно другой подход и думал, что буду делиться, если это поможет.

Чтобы начать, я разбил шаги и компоненты форматирования в свои собственные обязанности.

Формат телефонного номера обычно можно разбить на локальный, внутренний или международный формат, который зависит от длины строки.

Я определил типы:

/// Defines the three different types of formatting phone numbers use
///
/// - local: Numbers used locally.
/// - domestic: Numbers used locally including area codes.
/// - international: Numbers used internationally with country codes.
public enum PhoneFormatType {
    case local
    case domestic
    case international
}

Затем определили разделители, доступные для форматирования строки телефонного номера:

// Defines separators that are available for use in formatting
// phone number strings.
public enum PhoneFormatSeparator {
    case hyphen
    case plus
    case space
    case parenthesisLH
    case parenthesisRH
    case slash
    case backslash
    case pipe
    case asterisk

    public var value: String {
        switch self {
        case .hyphen: return "-"
        case .plus: return "+"
        case .space: return " "
        case .parenthesisLH: return "("
        case .parenthesisRH: return ")"
        case .slash: return "/"
        case .backslash: return "\\"
        case .pipe: return "|"
        case .asterisk: return "*"
        }
    }
}

Далее я определил правила форматирования, которые укажите индекс (в строке номера телефона), где вставлены разделители, такие как +, - и т. д.

// defines the separators that should be inserted in a phone number string
// and the indexes where they should be applied
public protocol PhoneNumberFormatRule {

    // the index in a phone number where this separator should be applied
    var index: Int { get set }

    // the priority in which this rule should be applied. Sorted in inverse, 0 is highest priority, higher numbers are lower priority
    var priority: Int { get set }

    // the separator to use at this index
    var separator: PhoneFormatSeparator { get set }
}

/// Default implementation of PhoneNumberFormatRule
open class PNFormatRule: PhoneNumberFormatRule {
    public var index: Int
    public var priority: Int
    public var separator: PhoneFormatSeparator

    public init(_ index: Int, separator: PhoneFormatSeparator, priority: Int = 0) {
        self.index = index
        self.separator = separator
        self.priority = priority
    }
}

С этими определенными я создал набор правил, которые связывают правила с заданным типом формата.

/// Defines the rule sets associated with a given phone number type.
/// e.g. international/domestic/local
public protocol PhoneFormatRuleset {

    /// The type of phone number formatting to which these rules apply
    var type: PhoneFormatType { get set }

    /// A collection of rules to apply for this phone number type.
    var rules: [PhoneNumberFormatRule] { get set }

    /// The maximum length a number using this format ruleset should be. (Inclusive)
    var maxLength: Int { get set }
}

Со всем, что определено таким образом, вы можете быстро установить набор правил в соответствии с любым форматом, который вам нужен.

Вот пример набора правил, который определяет 3 правила для телефона с дефисным форматированием номер строки, обычно используемой в США:

    // Formats phone numbers:
    //  .local: 123-4567
    //  .domestic: 123-456-7890
    //  .international: +1 234-567-8901
    static func usHyphen() -> [PhoneFormatRuleset] {
        return [
            PNFormatRuleset(.local, rules: [
                PNFormatRule(3, separator: .hyphen)
                ], maxLength: 7),
            PNFormatRuleset(.domestic, rules: [
                PNFormatRule(3, separator: .hyphen),
                PNFormatRule(6, separator: .hyphen)
                ], maxLength: 10),
            PNFormatRuleset(.international, rules: [
                PNFormatRule(0, separator: .plus),
                PNFormatRule(1, separator: .space),
                PNFormatRule(4, separator: .hyphen),
                PNFormatRule(7, separator: .hyphen)
                ], maxLength: 11)
        ]
    }

. Тяжелый подъем логики форматирования происходит (не так):

// formats a string using the format rule provided at initialization
public func format(number: String) -> String {

    // strip non numeric characters
    let n = number.components(separatedBy: CharacterSet.decimalDigits.inverted).joined()

    // bail if we have an empty string, or if no ruleset is defined to handle formatting
    guard n.count > 0, let type = type(for: n.count), let ruleset = ruleset(for: type) else {
        return n
    }

    // this is the string we'll return
    var formatted = ""

    // enumerate the numeric string
    for (i,character) in n.enumerated() {

        // bail if user entered more numbers than allowed for our formatting ruleset
        guard i <= ruleset.maxLength else {
            break
        }

        // if there is a separator defined to be inserted at this index then add it to the formatted string
        if let separator = ruleset.separator(for: i) {
            formatted+=separator
        }

        // now append the character
        formatted+="\(character)"
    }

    return formatted
} 

Я создал структуру с пример проекта, вы можете посмотреть здесь: https://github.com/appteur/phoneformat

Вот как он работает по мере ввода:

Я также установил его, чтобы вы могли просто импортировать его с помощью cocoapods.

pod 'SwiftPhoneFormat', '1.0.0'

Затем используйте его:

import SwiftPhoneFormat

var formatter = PhoneFormatter(rulesets: PNFormatRuleset.usParethesis())
let formatted = formatter.format(number: numberString)

Вы можете использовать эту библиотеку https://github.com/luximetr/AnyFormatKit

Пример

let textInputController = TextInputController()

let textInput = TextInputField() // or TextInputView or any TextInput
textInputController.textInput = textInput // setting textInput

let formatter = TextInputFormatter(textPattern: "### (###) ###-##-##", prefix: "+12")
textInputController.formatter = formatter // setting formatter

Просто установите для вашего текстового поля этот textInputController и будет форматировать текст с шаблоном, который вы установили.

Swift 3 & amp; 4

Это решение удаляет любые нечисловые символы перед применением форматирования. Он возвращает nil, если исходный номер телефона не может быть отформатирован в соответствии с предположениями.

Swift 4

Решение Swift 4 учитывает устаревание CharacterView и Sting, становящееся совокупностью символов

import Foundation

func format(phoneNumber sourcePhoneNumber: String) -> String? {
    // Remove any character that is not a number
    let numbersOnly = sourcePhoneNumber.components(separatedBy: CharacterSet.decimalDigits.inverted).joined()
    let length = numbersOnly.count
    let hasLeadingOne = numbersOnly.hasPrefix("1")

    // Check for supported phone number length
    guard length == 7 || length == 10 || (length == 11 && hasLeadingOne) else {
        return nil
    }

    let hasAreaCode = (length >= 10)
    var sourceIndex = 0

    // Leading 1
    var leadingOne = ""
    if hasLeadingOne {
        leadingOne = "1 "
        sourceIndex += 1
    }

    // Area code
    var areaCode = ""
    if hasAreaCode {
        let areaCodeLength = 3
        guard let areaCodeSubstring = numbersOnly.substring(start: sourceIndex, offsetBy: areaCodeLength) else {
            return nil
        }
        areaCode = String(format: "(%@) ", areaCodeSubstring)
        sourceIndex += areaCodeLength
    }

    // Prefix, 3 characters
    let prefixLength = 3
    guard let prefix = numbersOnly.substring(start: sourceIndex, offsetBy: prefixLength) else {
        return nil
    }
    sourceIndex += prefixLength

    // Suffix, 4 characters
    let suffixLength = 4
    guard let suffix = numbersOnly.substring(start: sourceIndex, offsetBy: suffixLength) else {
        return nil
    }

    return leadingOne + areaCode + prefix + "-" + suffix
}

extension String {
    /// This method makes it easier extract a substring by character index where a character is viewed as a human-readable character (grapheme cluster).
    internal func substring(start: Int, offsetBy: Int) -> String? {
        guard let substringStartIndex = self.index(startIndex, offsetBy: start, limitedBy: endIndex) else {
            return nil
        }

        guard let substringEndIndex = self.index(startIndex, offsetBy: start + offsetBy, limitedBy: endIndex) else {
            return nil
        }

        return String(self[substringStartIndex ..< substringEndIndex])
    }
}

Swift 3

import Foundation

func format(phoneNumber sourcePhoneNumber: String) -> String? {

    // Remove any character that is not a number
    let numbersOnly = sourcePhoneNumber.components(separatedBy: CharacterSet.decimalDigits.inverted).joined()
    let length = numbersOnly.characters.count
    let hasLeadingOne = numbersOnly.hasPrefix("1")

    // Check for supported phone number length
    guard length == 7 || length == 10 || (length == 11 && hasLeadingOne) else {
        return nil
    }

    let hasAreaCode = (length >= 10)
    var sourceIndex = 0

    // Leading 1
    var leadingOne = ""
    if hasLeadingOne {
        leadingOne = "1 "
        sourceIndex += 1
    }

    // Area code
    var areaCode = ""
    if hasAreaCode {
        let areaCodeLength = 3
        guard let areaCodeSubstring = numbersOnly.characters.substring(start: sourceIndex, offsetBy: areaCodeLength) else {
            return nil
        }
        areaCode = String(format: "(%@) ", areaCodeSubstring)
        sourceIndex += areaCodeLength
    }

    // Prefix, 3 characters
    let prefixLength = 3
    guard let prefix = numbersOnly.characters.substring(start: sourceIndex, offsetBy: prefixLength) else {
        return nil
    }
    sourceIndex += prefixLength

    // Suffix, 4 characters
    let suffixLength = 4
    guard let suffix = numbersOnly.characters.substring(start: sourceIndex, offsetBy: suffixLength) else {
        return nil
    }

    return leadingOne + areaCode + prefix + "-" + suffix
}

extension String.CharacterView {
    /// This method makes it easier extract a substring by character index where a character is viewed as a human-readable character (grapheme cluster).
    internal func substring(start: Int, offsetBy: Int) -> String? {
        guard let substringStartIndex = self.index(startIndex, offsetBy: start, limitedBy: endIndex) else {
            return nil
        }

        guard let substringEndIndex = self.index(startIndex, offsetBy: start + offsetBy, limitedBy: endIndex) else {
            return nil
        }

        return String(self[substringStartIndex ..< substringEndIndex])
    }
}

Пример

func testFormat(sourcePhoneNumber: String) -> String {
    if let formattedPhoneNumber = format(phoneNumber: sourcePhoneNumber) {
        return "'\(sourcePhoneNumber)' => '\(formattedPhoneNumber)'"
    }
    else {
        return "'\(sourcePhoneNumber)' => nil"
    }
}

print(testFormat(sourcePhoneNumber: "1 800 222 3333"))
print(testFormat(sourcePhoneNumber: "18002223333"))
print(testFormat(sourcePhoneNumber: "8002223333"))
print(testFormat(sourcePhoneNumber: "2223333"))
print(testFormat(sourcePhoneNumber: "18002223333444"))
print(testFormat(sourcePhoneNumber: "Letters8002223333"))

Пример вывода

'1 800 222 3333' => '1 (800) 222-3333'

'18002223333' => '1 (800) 222-3333'

'8002223333' => '(800) 222-3333'

'2223333' => '222-3333'

'18002223333444' => nil

'Letters8002223333' => '(800) 222-3333'

Swift 4

let s = "05554446677"
let s2 = String(format: "%@ (%@) %@ %@ %@",
    s.substring(to: s.index(s.startIndex, offsetBy: 1)),
    s.substring(with: s.index(s.startIndex, offsetBy: 1) ..< s.index(s.startIndex, offsetBy: 4)),
    s.substring(with: s.index(s.startIndex, offsetBy: 4) ..< s.index(s.startIndex, offsetBy: 7)),
    s.substring(with: s.index(s.startIndex, offsetBy: 7) ..< s.index(s.startIndex, offsetBy: 9)),
    s.substring(with: s.index(s.startIndex, offsetBy: 9) ..< s.index(s.startIndex, offsetBy: 11))
    )

Но это не работает, если строковые символы считаются более или менее 11

Маскированный номер, набранный

private func formattedNumber(number: String) -> String {
    var cleanPhoneNumber = number!.components(separatedBy: CharacterSet.decimalDigits.inverted).joined()
    var mask = "+X (XXX) XXX XX-XX"

    var result = ""
    var index = cleanPhoneNumber.startIndex
    for ch in mask.characters {
        if index == cleanPhoneNumber.endIndex {
            break
        }
        if ch == "X" {
            result.append(cleanPhoneNumber[index])
            index = cleanPhoneNumber.index(after: index)
        } else {
            result.append(ch)
        }
    }
    return result
}

Итак, это лучше работает.

"" => ""
"0" => "+0"
"412" => "+4 (12"
"12345678901" => "+1 (234) 567 89-01"

Swift 3, но также следует перевести на Swift 4

1. ErrorHandling

   enum PhoneNumberFormattingError: Error {
       case wrongCharactersInPhoneNumber
       case phoneNumberLongerThanPatternAllowes
   }
   

2. Создать шаблоны

   enum PhoneNumberFormattingPatterns: String {
       case mobile = "+xx (yxx) xxxxxxxxxxx"
       case home = "+xx (yxxx) xxxx-xxx"
   }
   

3. Вставить функцию

   /**
        Formats a phone-number to correct format
        - Parameter pattern: The pattern to format the phone-number.
        - Example:
           - x: Says that this should be a digit.
           - y: Says that this digit cannot be a "0".
           - The length of the pattern restricts also the length of allowed phone-number digits.
               - phone-number: "+4306641234567"
               - pattern: "+xx (yxx) xxxxxxxxxxx"
               - result: "+43 (664) 1234567"
   
        - Throws:
           - PhoneNumberFormattingError
               - wrongCharactersInPhoneNumber: if phone-number contains other characters than digits.
               - phoneNumberLongerThanPatternAllowes: if phone-number is longer than pattern allows.
        - Returns:
           - The formatted phone-number due to the pattern.
        */
   extension String {
       func vpToFormattedPhoneNumber(withPattern pattern: PhoneNumberFormattingPatterns) throws -> String {
           let phoneNumber = self.replacingOccurrences(of: "+", with: "")
           var retVal: String = ""
           var index = 0
           for char in pattern.rawValue.lowercased().characters {
               guard index < phoneNumber.characters.count else {
                   return retVal
               }
   
               if char == "x" {
                   let charIndex = phoneNumber.index(phoneNumber.startIndex, offsetBy: index)
                   let phoneChar = phoneNumber[charIndex]
                   guard "0"..."9" ~= phoneChar else {
                       throw PhoneNumberFormattingError.wrongCharactersInPhoneNumber
                   }
                   retVal.append(phoneChar)
                   index += 1
               } else if char == "y" {
                   var charIndex = phoneNumber.index(phoneNumber.startIndex, offsetBy: index)
                   var indexTemp = 1
                   while phoneNumber[charIndex] == "0" {
                       charIndex = phoneNumber.index(phoneNumber.startIndex, offsetBy: index + indexTemp)
                       indexTemp += 1
                   }
   
                   let phoneChar = phoneNumber[charIndex]
                   guard "0"..."9" ~= phoneChar else {
                       throw PhoneNumberFormattingError.wrongCharactersInPhoneNumber
                   }
                   retVal.append(phoneChar)
                   index += indexTemp
               } else {
                   retVal.append(char)
               }
           }
   
           if phoneNumber.endIndex > phoneNumber.index(phoneNumber.startIndex, offsetBy: index) {
               throw PhoneNumberFormattingError.phoneNumberLongerThanPatternAllowes
           }
   
           return retVal
       }
   }
   

4. Использование

   let phoneNumber = "+4306641234567"
   let phoneNumber2 = "4343211234567"
   
   do {
       print(try phoneNumber.vpToFormattedPhoneNumber(withPattern: .mobile))
       print(try phoneNumber2.vpToFormattedPhoneNumber(withPattern: .home))
   } catch let error as PhoneNumberFormattingError {
       switch error {
       case .wrongCharactersInPhoneNumber:
           print("wrong characters in phone number")
       case .phoneNumberLongerThanPatternAllowes:
           print("too long phone number")
       default:
           print("unknown error")
       }
   } catch {
       print("something other went wrong")
   }
   
   // output: +43 (664) 1234567
   // output: +43 (4321) 1234-567
   

Действительно простое решение:

extension String {
    func applyPatternOnNumbers(pattern: String, replacmentCharacter: Character) -> String {
        var pureNumber = self.replacingOccurrences( of: "[^0-9]", with: "", options: .regularExpression)
        for index in 0 ..< pattern.count {
            guard index < pureNumber.count else { return pureNumber }
            let stringIndex = String.Index(encodedOffset: index)
            let patternCharacter = pattern[stringIndex]
            guard patternCharacter != replacmentCharacter else { continue }
            pureNumber.insert(patternCharacter, at: stringIndex)
        }
        return pureNumber
    }
}

Использование:

guard let text = textField.text else { return }
textField.text = text.applyPatternOnNumbers(pattern: "+# (###) ###-####", replacmentCharacter: "#")