String Parser in Swift 4

string-parser.swift

import Foundation

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

func stringParser(input: Substring) -> ParseResult {
    if input[input.startIndex] != "\"" {
        return nil
    }
    var isEscape = true
    func inspectChar(char: Character) -> Bool {
        if char == "\"" && !isEscape {
            return true
        }
        if char == "\\" {
            isEscape = true
        } else {
            isEscape = false
        }
        return false
    }
    if let index = input.index(where: inspectChar) {
        return (input[input.index(after: input.startIndex)...input.index(before: index)],
                input[input.index(after: index)...])
    }
    return nil
}