How to check is a string or number?

Here is a small Swift version using String extension :

Swift 3/Swift 4 :

extension String  {
    var isNumber: Bool {
        return !isEmpty && rangeOfCharacter(from: CharacterSet.decimalDigits.inverted) == nil
    }
}

Swift 2 :

   extension String  {
        var isNumber : Bool {
            get{
                return !self.isEmpty && self.rangeOfCharacterFromSet(NSCharacterSet.decimalDigitCharacterSet().invertedSet) == nil
            }
        }
   }