So far in this course we have only covered the Swift Standard Library, but when it comes to strings we must also include the Foundation framework, as it contains a lot of both basic and advanced text functionality that is missing from the Swift Standard Library.

Foundation is available on all Apple platforms and has been around for a long time (there is also a version for other platforms, re-implemented in Swift; see: https://github.com/apple/swift-corelibs-foundation). It is written in and for Objective-C, but a lot of its API has been updated to be easier to work with from Swift. Not all of it has been though, and as we'll see, you might run into some problems when converting Foundation types to Swift types.

Foundation's string type is NSString, and it works directly with UTF-16 encoded text. It does not know what the Character type is, and does not necessarily handle Unicode text correctly like Swift does. NSString can be used as Swift String and vice versa as they...