Valid currency input

ValidCurrencyInput.swift

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    if string.count > 0 {
        if string.count == 1, textField.text.isNilOrEmpty {
            if let single = string[0], (single == "." || single == "0") {// auto set . or 0 to 0.
                textField.text = "0."
                let newPosition = textField.endOfDocument
                textField.selectedTextRange = textField.textRange(from: newPosition, to: newPosition)
                textField.sendActions(for: .valueChanged)
                return false
            }
        }
        let toString = (textField.text as? NSString)?.replacingCharacters(in: range, with: string) ?? ""
        if toString.count > 0 {
            let regex = "(\\+)?(([0]|(0[.]\\d{0,18}))|([1-9]\\d{0,19}(([.]\\d{0,18})?)))?"
            let predicate = NSPredicate(format: "SELF MATCHES %@", regex)
            let matched = predicate.evaluate(with: toString)
            return matched
        }
    } else { // delete input
        if textField.text == "0" || textField.text == "0." || textField.text == "." {
            textField.text = ""
            textField.sendActions(for: .valueChanged)
        }
        return true
    }
    return true
}