This is an S-expression parser written in pure Swift 2.0, where pure means without making any necessary usage of Foundation APIs at its core. This means: neither NSScanner, nor NSRegularExpression, nor NSCharacterSet, not even NSString.
S-expressions is a notation format for nested list data on which Lisp languages are based.
While S-expressions have a very simple base syntax and there are appealing approaches on using a functional-compositional style to implement a parser in general in Swift, this implementation goes a different way. It is imperative and makes intense usage of Swift specific features, in particular Optionals, the try/throw/do-catch error handling and mutating functions on value types.
The top-down Parser operates without prior Lexing directly on the String through Scanner, which is a minimalistic attempt to wrap access on a Character-oriented basis. The current state is given at any time through the call-stack, which also means that the implementation is heavily based on recursion.