Final Corrected Code Block:


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


This ensures:
No false positives for empty lines.
Correct detection of missing CR, LF, or both.
Handles last line without newline properly.


Here’s a quick C# test snippet you can use to verify the corrected regex and logic:
using System;
using System.IO;
using System.Text.RegularExpressions;
class Program
{
    static void Main()
    {
        string str31 = 
@"H+CNQIZTQZ+ARRI+250729+0920+Z1.0++++TQZ
L+SEGU2150230+250723+1502+BACK+NIL+E++++CNQIZCLT++CNQIZTQZ+++++ZIM+ZIM++^^^^^^N^^^^^^^^^^^^^^^^^1^^^^+^^^^^^^^^^+^
X+TEST+LINE+WITH+CRLF\r\n
Y+TEST+LINE+WITH+LF\n
Z+TEST+LINE+WITH+CR\r";
        using (var sw = new StringWriter())
        {
            bool b_err3 = false;
            string fileName = "SampleFile.txt";
            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":
                        break; // Proper CRLF
                    case "\n":
                        if (!b_err3)
                        {
                            sw.WriteLine($"Please check the lines having the issues below in the file - '{fileName}'.");
                            sw.WriteLine();
                        }
                        sw.WriteLine("It is missing CR for the line below:");
                        sw.WriteLine(line0);
                        sw.WriteLine();
                        b_err3 = true;
                        break;
                    case "\r":
                        if (!b_err3)
                        {
                            sw.WriteLine($"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 (!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;
                }
            }
            Console.WriteLine(sw.ToString());
        }
    }
}

``Expected Output:
Please check the lines having the issues below in the file - 'SampleFile.txt'. The line below is missing both LF and CR: H+CNQIZTQZ+ARRI+250729+0920+Z1.0++++TQZ The line below is missing both LF and CR: L+SEGU2150230+250723+1502+BACK+NIL+E++++CNQIZCLT++CNQIZTQZ+++++ZIM+ZIM++^^^^^^N^^^^^^^^^^^^^^^^^1^^^^+^^^^^^^^^^+^ It is missing CR for the line below: Y+TEST+LINE+WITH+LF It is missing LF for the line below: Z+TEST+LINE+WITH+CR
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