Variable size fields about akmaskfield HOT 4 OPEN

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
       //Getting the textfield text
        var text: NSString = textField.text! as NSString
        text = text.replacingCharacters(in: range, with: string) as NSString
        
        //Adding the textfield text on a mutable string
        var mutableString: NSMutableString = text.mutableCopy() as! NSMutableString
        
        //Verifying if the user not tapped the delete key
        if(!(range.length == 1 && string.lengthOfBytes(using: String.Encoding.utf8) == 0)) {
            
            //Adding the formatter character at the correct position
            if (mutableString.length > 1) {
                mutableString = (mutableString.replacingOccurrences(of: "-", with: "") as NSString).mutableCopy() as! NSMutableString
                mutableString.insert("-", at: mutableString.length - 1)
            }
        }
        
        let char = string.cString(using: String.Encoding.utf8)!
        let isBackSpace = strcmp(char, "\\b")
        
        if (isBackSpace == -92 && mutableString.length > 1) {
            mutableString = (mutableString.replacingCharacters(in: NSMakeRange(mutableString.length - 1, 1), with: "") as NSString).mutableCopy() as! NSMutableString
            
            if (mutableString.length > 1) {
                mutableString = (mutableString.replacingOccurrences(of: "-", with: "") as NSString).mutableCopy() as! NSMutableString
                mutableString.insert("-", at: mutableString.length - 1)
            }
        }
        
        //Putting the string with the formatter character on the textfield
        textField.text = mutableString as String
        
        return false
    }