class TestViewController: UIViewController,UITextFieldDelegate{

}

textField.delegate = self

func textFieldDidBeginEditing(_ textField: UITextField){

}

 func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

        var newString = (textField.text! as NSString).replacingCharacters(in: range, with: string)

        let expression = "^[0-9]*(?:\\.[0-9]{0,2})?$"

        do {

            let regex: NSRegularExpression = try NSRegularExpression(pattern: expression, options: NSRegularExpression.Options.caseInsensitive)

            let numberOfMatches = regex.matches(in: newString, options: NSRegularExpression.MatchingOptions.reportProgress, range: NSMakeRange(0, newString.characters.count))

            if numberOfMatches.count == 0{

                return false

            }else{

                if newString != "" {

                    if newString[newString.startIndex] == "." {

                        newString = "0" + newString

                    }

                    if newString[newString.index(before: newString.endIndex)] == "." {

                        newString = newString + "0"

                    }

                    if (Double(newString)! > 100.0){

                        account_textfield.text = "100"

                        return false

                    }

                }

                return true

            }

        }

        catch {

            return false

        }

    }