title: User should not be able to delete given symbol in textfield which is a text
body: <p><a href="https://i.stack.imgur.com/ImoQv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ImoQv.png" alt="enter image description here"></a></p>
  
  <p>func textField(textField: UITextField,shouldChangeCharactersInRange range: NSRange,replacementString string: String) -> Bool
      {
  return true
  }</p>
  
  <p>I have to restrict user to delete this symbol from textfield in swift. User can delete any thing from textfield without deleting this symbol.</p>