Call function from text field delegate method and get valid username.

let USERNAMERANGE = "ABCDEFGHIJKLMONPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_."

let KALERTVALIDUSERNAME = "Username can only use letters, numbers, underscore and periods."

public func setUsernameFormate(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String)-> Bool{
    var strUserName = textField.text! as String
    if string == " " {
        strUserName = strUserName + "_"
        return false
    }
    let currentCharacterCount = textField.text?.characters.count ?? 0
    if (range.length + range.location > currentCharacterCount){
        return false
    }
    let newLength = currentCharacterCount + string.characters.count - range.length
    if newLength <= 20 {
        
        let set = NSCharacterSet(charactersIn: USERNAMERANGE).inverted
        
        if string.rangeOfCharacter(from: set) != nil{
            topAlertActionError(msgAlert: KALERTVALIDUSERNAME)
        }
        return string.rangeOfCharacter(from: set) == nil
    }
    else{
        return false
    }
}