Date string conversion doesn't return decimal miliseconds Swift - swift

Question

I need to check if a date string is of this exact format 2017-01-01 00:00:00.000 with 3 decimal milliseconds, otherwise return.
I'm using yyyy-MM-dd hh:mm:ss.SSS but this doesn't return any decimal milliseconds. Why is that? When I convert Date() to string with DateFormatter, then it returns correct ss.SSS.
let str = "2017-01-01 00:00:00.000"
let df = DateFormatter()
df.dateFormat = "yyyy-MM-dd HH:mm:ss.SSS"
df.timeZone = NSTimeZone(abbreviation: "GMT") as TimeZone!
if let date = df.date(from: str) {
print("date: \(date)")
} else {
print("no")
}
output: date: 2017-01-01 00:00:00 +0000
More, there is no error if I add ss.S/ss.SS all return true. why?

Answer 1

The date formatter patterns says that 1...n digits for the milliseconds is valid. You can write as many capital S as you want - it makes no difference. You have to check for the second condition separately:
extension String {
var endsWithThreeDecimalDigits: Bool {
return self.range(of: "\\.\\d{3}$", options: .regularExpression) != nil
}
}
let df = DateFormatter()
df.dateFormat = "yyyy-MM-dd HH:mm:ss.SSS"
df.timeZone = TimeZone(abbreviation: "GMT")
if let date = df.date(from: str), str.endsWithThreeDecimalDigits {
print("date: \(date)")
} else {
print("no")
}