Parse an SRT File (single line subtitles) [Kotlin]

SrtFile.kt

import java.io.File
import java.nio.charset.Charset

class SrtFile(private val file: File, private val defaultCharset: Charset = Charset.forName("UTF-8")) {
    private val _subtitles by lazy { parse() }
    val subtitles: List<Subtitle> get() = _subtitles

    private fun parse(): MutableList<Subtitle> {
        val list = mutableListOf<Subtitle>()

        var id = ""
        var timeCode = ""
        var content = ""

        file.readLines(defaultCharset).let { lines ->
            lines.forEach { value ->
                when(value.type) {
                    Matches.Id -> id = value
                    Matches.TimeCode -> timeCode = value
                    Matches.Content -> {
                        content = value
                        list.add(Subtitle(id, timeCode, content))
                        id = ""
                        timeCode = ""
                        content = ""
                    }
                }
            }
        }

        return list
    }

    data class Subtitle(val id: String, val timeCode: String, val text: String)

    private sealed class Matches {
        object Content: Matches()
        object TimeCode: Matches()
        object Id: Matches()
        object None: Matches()
    }

    private val String.type: Matches get() {
        val id = "^\\d+\$".toRegex()
        val timeCode = "([\\d]{2}:[\\d]{2}:[\\d]{2},[\\d]{3}).*([\\d]{2}:[\\d]{2}:[\\d]{2},[\\d]{3})".toRegex()
        val content = "^[a-zA-Z].*".toRegex()

        return when {
            matches(id) -> Matches.Id
            matches(timeCode) -> Matches.TimeCode
            matches(content) -> Matches.Content
            else -> Matches.None
        }
    }
}