String Parser in Swift

stringParser.swift

import Foundation

typealias ParseResult = (output: Any, rest: Substring)?

func stringParser(input: Substring) -> ParseResult {
    if input[input.startIndex] != "\"" {
        return nil
    }
    var isEscape = true
    let index = input.index() {
        if $0 == "\"" && !isEscape {
            return true
        }
        isEscape = $0 == "\\" ? true : false
        return false
    }
    if let index = index {
        return (input[input.index(after: input.startIndex)...input.index(before: index)],
                input[input.index(after: index)...])
    }
    return nil
}