SrtParser.swift

struct SubtitleEntry {
    let index: Int
    let startTime, endTime: TimeInterval
    let text: String
}

extension String {
    subscript(nsrange: NSRange) -> String? {
        return Range(nsrange, in: self)
            .flatMap { range in String(self[range]) }
    }
}

extension TimeInterval {
    init?(timeline: String) {
        let components = timeline.components(separatedBy: ":")
        guard components.count == 3,
            let hours = Double(components[0]),
            let minutes = Double(components[1]),
            let seconds = Double(components[2].replacingOccurrences(of: ",", with: "."))
            else { return nil }
        self = hours * 60 * 60 + minutes * 60 + seconds
    }
}

enum SrtParser {
    private static var pattern: String {
        return ["(?<index>^\\d+$)\\n^",
                "(?<startTime>\\d\\d:[0-5]\\d:[0-5]\\d,\\d{1,3}) --> ",
                "(?<endTime>\\d\\d:[0-5]\\d:[0-5]\\d,\\d{1,3})$\\n",
                "(?<text>(?:^.+$\\n?)+)"].joined()
    }
    private static let expression = try? NSRegularExpression(pattern: pattern, options: .anchorsMatchLines)
    
    static func parse(string: String) -> [SubtitleEntry] {
        return expression?
            .matches(in: string, range: NSRange(..<string.endIndex, in: string))
            .compactMap { result -> SubtitleEntry? in
                guard result.numberOfRanges >= 4,
                    let indexString = string[result.range(withName: "index")],
                    let index = Int(indexString),
                    let startString = string[result.range(withName: "startTime")],
                    let startTime = TimeInterval(timeline: startString),
                    let endString = string[result.range(withName: "endTime")],
                    let endTime = TimeInterval(timeline: endString),
                    let text = string[result.range(withName: "text")]
                    else { return nil }
                return SubtitleEntry(index: index, startTime: startTime, endTime: endTime,
                                     text: text.trimmingCharacters(in: .whitespacesAndNewlines))
            } ?? []
    }
}