public static void keepLetters() throws IOException {
        BufferedReader sourceReader = new BufferedReader(new FileReader("input.txt"));

        String source = sourceReader.readLine();
        String updatedSource = source.replaceAll("[A-Za-z]", "");

        try (Writer writer = new BufferedWriter(new OutputStreamWriter(
                             new FileOutputStream("output.txt"), "UTF-8"))) {
                writer.write(updatedSource);
        }
    }
}