Validate credit card number with Luhn algorithm

ValidateLuhnString.swift

/**
 * String extension to validate credit card number.
 * Automatically strips any whitespace from the string but otherwise the string
 * should not contain and non-numeric characters.
 */
extension String {
    func validateLuhn() -> Bool {
        let nsStringVersionNoSpaces = (self.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceCharacterSet()) as NSString)
        if (self.rangeOfString("^[0-9]+$", options: .RegularExpressionSearch) == nil) {
            return false
        }
        let lastOdd = self.characters.count - 2
        let lastEven = self.characters.count - 3
        var result = 0
        var i: Int
        for i = lastOdd; i >= 0; i-=2 {
            var oddNumber = Int(nsStringVersionNoSpaces.substringWithRange(NSRange(location: i, length: 1)))! * 2
            if oddNumber >= 10 {
                oddNumber -= 9
            }
            result += oddNumber
        }
        for i = lastEven; i >= 0; i-=2 {
            result += Int(nsStringVersionNoSpaces.substringWithRange(NSRange(location: i, length: 1)))!
        }
        result += Int(nsStringVersionNoSpaces.substringWithRange(NSRange(location: self.characters.count-1, length: 1)))!
        if result % 10 == 0 {
            return true
        }
        else {
            return false
        }
    }
}