String validation (Swift)

gistfile1.txt

extension String {

     func isValidString() -> Bool {

        var charSet = NSCharacterSet(charactersInString: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890_")
        charSet = charSet.invertedSet

        let range = (self as NSString).rangeOfCharacterFromSet(charSet)

        if range.location != NSNotFound {
            return false
        }

        return true
    }
}