title: append :00 to whatever is entered in the textfield
body: <p>My code below restricts the user to only input 2 numbers into a textfield. All I want to do is append ':00' to whatever is entered into whatever is entered into the textfield. So if the user enters 30. The textfield should be 30:00. All code is listed below. </p>
  
  <pre class="lang-swift prettyprint-override"><code>import UIKit
  
  class ViewController: UIViewController, UITextFieldDelegate {
  
     @IBOutlet weak var txtMobileNumber: UITextField!
  
     let MAX_LENGTH_PHONENUMBER = 2
     let ACCEPTABLE_NUMBERS     = "0123456789"
  
     func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -&gt; Bool {                   
        let newLength: Int = textField.text!.count + string.count - range.length
        let numberOnly = NSCharacterSet.init(charactersIn: ACCEPTABLE_NUMBERS).inverted
        let strValid = string.rangeOfCharacter(from: numberOnly) == nil
        return (strValid &amp;&amp; (newLength &lt;= MAX_LENGTH_PHONENUMBER))
     }
  
     override func viewDidLoad() {
         super.viewDidLoad()
         // Do any additional setup after loading the view, typically from a nib.
         txtMobileNumber.delegate = self
     }
  }
  </code></pre>