gistify388983.stringIsValidDecimalFormat.swift

func stringIsValidDecimalFormat(input: String) -> Bool {
    let validDecimalCharacters = "0123456789.-"
    for character in input.characters {
        let characterAsString = String(character)
        if validDecimalCharacters.rangeOfString(characterAsString) == nil {
            return false
        } else if characterAsString == "." && (input.componentsSeparatedByString(".").count) > 2 {
            return false
        }
    }
    return true
}

func textField(textField: UITextField,
    shouldChangeCharactersInRange range: NSRange,
    replacementString newString: String) -> Bool {
        if stringIsValidDecimalFormat(textField.text! + newString) == false {
            return false
        } else {
            return true
        }
}