Extension to add Phone Number mask on a textfield in iOS

UITextField+PhoneMask.swift

//
//  UITextField+PhoneMask.swift
//
//  Created by Rigoberto Sáenz Imbacuán (https://www.linkedin.com/in/rsaenzi/)
//  Copyright © 2019. All rights reserved.
//

import UIKit

extension UITextField {
    
    func applyPhoneMask(on phoneNumber: String, shouldRemoveLastDigit: Bool = false) {
        
        guard !phoneNumber.isEmpty else {
            self.text = ""
            return
        }
        
        guard let regex = try? NSRegularExpression(pattern: "[\\s-\\(\\)]", options: .caseInsensitive) else {
            self.text = ""
            return
        }
        
        let r = NSString(string: phoneNumber).range(of: phoneNumber)
        var number = regex.stringByReplacingMatches(in: phoneNumber, options: .init(rawValue: 0), range: r, withTemplate: "")
        
        if number.count > 10 {
            let tenthDigitIndex = number.index(number.startIndex, offsetBy: 10)
            number = String(number[number.startIndex..<tenthDigitIndex])
        }
        
        if shouldRemoveLastDigit {
            let end = number.index(number.startIndex, offsetBy: number.count-1)
            number = String(number[number.startIndex..<end])
        }
        
        if number.count < 7 {
            let end = number.index(number.startIndex, offsetBy: number.count)
            let range = number.startIndex..<end
            number = number.replacingOccurrences(of: "(\\d{3})(\\d+)", with: "$1-$2", options: .regularExpression, range: range)
            
        } else {
            let end = number.index(number.startIndex, offsetBy: number.count)
            let range = number.startIndex..<end
            number = number.replacingOccurrences(of: "(\\d{3})(\\d{3})(\\d+)", with: "$1-$2-$3", options: .regularExpression, range: range)
        }
        
        self.text = number
    }
}


// ---------------
// Usage example:
// ---------------

extension MyViewController: UITextFieldDelegate {
    
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
        // Deletion is enabled
        if string.count == 0 {
            return true
        }
        
        // Only numbers are permitted
        guard string.rangeOfCharacter(from: CharacterSet.decimalDigits.inverted) == nil else {
            return false
        }
        
        // Max digits allowed
        let fullLength: Int = textField.text!.count + string.count
        if fullLength > 12 { // 10 digits + 2 mask characters
            return false
        }
        
        // Phone mask
        let fullString: String = "\(textField.text!)\(string)"
        
        if range.length == 1 {
            textField.applyPhoneMask(on: fullString, shouldRemoveLastDigit: true)
        } else {
            textField.applyPhoneMask(on: fullString)
        }
        
        return false
    }
}