var rx = new Regex(@"(?<line>[^\r\n]*)(?<eol>\r\n|\r|\n|$)");

foreach (Match m in rx.Matches(str31))
{
    var line0 = m.Groups["line"].Value;
    var eol0 = m.Groups["eol"].Value;

    switch (eol0)
    {
        case "\r\n":
            // Proper CRLF – no issue
            break;

        case "\n":
            if (!b_err3)
            {
                sw.WriteLine($"Please check the lines having the issues below in the file - '{fileName}'.");
                sw.WriteLine();
            }
            sw.WriteLine("It is missing CR for the line below:");
            sw.WriteLine(line0);  // ✅ Only the actual line
            sw.WriteLine();
            b_err3 = true;
            break;

        case "\r":
            if (!b_err3)
            {
                sw.WriteLine($"Please check the lines having the issues below in the file - '{fileName}'.");
                sw.WriteLine();
            }
            sw.WriteLine("It is missing LF for the line below:");
            sw.WriteLine(line0);  // ✅
            sw.WriteLine();
            b_err3 = true;
            break;

        case "":
            if (!string.IsNullOrEmpty(line0))
            {
                if (!b_err3)
                {
                    sw.WriteLine($"Please check the lines having the issues below in the file - '{fileName}'.");
                    sw.WriteLine();
                }
                sw.WriteLine("The line below is missing both LF and CR:");
                sw.WriteLine(line0);  // ✅
                sw.WriteLine();
                b_err3 = true;
            }
            break;
    }
}