Extension to string that adds the toDouble() function with some error checking.

String.toDouble()

extension String {
    func toDouble() -> Double? {
     
        var chars: [Character] = Array(self)
        
        if (chars.filter {$0 == "."}.count > 1) {
            return nil
        }
        
        var dotIndex: Int? = find(chars, ".")
 
        if dotIndex != nil {
            chars.removeAtIndex(dotIndex!)
        }
        
        if (String(seq: chars).toInt() == nil) {
            return nil
        }

        return NSNumberFormatter().numberFromString(self)!.doubleValue
    }
}