standard UNIX sed ported to C#

// Original behavior/reference: sed (Lee E. McMahon)
// Ported to .NET by Timothy J. Bruce <[email protected]>

namespace Icod.CoreUtils.Sed;

using System;
using System.Collections.Generic;
using System.IO;
using System.Text.RegularExpressions;
using System.Text;

/// <summary>
/// sed: stream editor (simplified).
/// Supported features:
///   -n        suppress automatic printing
///   -e script add script to the commands to run
/// Script forms supported (simple subset):
///   s/old/new/g   substitute (supports regex) with optional g flag
/// Multiple -e accepted; first script applied if none specified.
/// This is a small, portable subset for common use cases.
/// </summary>
public static class Command
{
    public static int Run(string[] args, TextReader? stdin = null, TextWriter? stdout = null, TextWriter? stderr = null)
    {
        stdin ??= Console.In;
        stdout ??= Console.Out;
        stderr ??= Console.Error;

        var scripts = new List<string>();
        var suppress = false;
        var files = new List<string>();

        var i = 0;
        for (; i < args.Length; i++)
        {
            var a = args[i];
            if (!a.StartsWith("-"))
            {
                break;
            }

            if (a == "-n")
            {
                suppress = true;
            }
            else if (a == "-e")
            {
                if (i + 1 < args.Length)
                {
                    i++;
                    scripts.Add(args[i]);
                }
            }
            else
            {
                // ignore other options
            }
        }

        for (; i < args.Length; i++)
        {
            files.Add(args[i]);
        }

        if (scripts.Count == 0)
        {
            stderr.WriteLine("sed: no scripts provided");
            return 2;
        }

        if (files.Count == 0)
        {
            files.Add("-");
        }

        try
        {
            foreach (var path in files)
            {
                TextReader reader;
                if (path == "-")
                {
                    reader = stdin;
                }
                else
                {
                    reader = new StreamReader(path, Encoding.UTF8, detectEncodingFromByteOrderMarks: true);
                }

                using (reader)
                {
                    string? line;
                    while ((line = reader.ReadLine()) is not null)
                    {
                        var outLine = line;
                        foreach (var script in scripts)
                        {
                            if (script.StartsWith("s/"))
                            {
                                // parse s/old/new/flags
                                var m = Regex.Match(script, @"^s/(?<old>.*?)/(?<new>.*?)/(?<flags>.*)$");
                                if (!m.Success)
                                {
                                    m = Regex.Match(script, @"^s/(?<old>.*?)/(?<new>.*)$");
                                }

                                if (m.Success)
                                {
                                    var oldPat = m.Groups["old"].Value;
                                    var newText = m.Groups["new"].Value;
                                    var flags = m.Groups["flags"].Success ? m.Groups["flags"].Value : string.Empty;
                                    var regexOptions = RegexOptions.None;
                                    var replaceCount = flags.Contains("g") ? -1 : 1;
                                    outLine = RegexReplace(outLine, oldPat, newText, regexOptions, replaceCount);
                                }
                            }
                            else
                            {
                                // unsupported script: ignore
                            }
                        }

                        if (!suppress)
                        {
                            stdout.WriteLine(outLine);
                        }
                    }
                }
            }

            return 0;
        }
        catch (Exception ex)
        {
            stderr.WriteLine($"sed: {ex.Message}");
            return 1;
        }
    }

    private static string RegexReplace(string input, string pattern, string replacement, RegexOptions options, int maxReplacements)
    {
        try
        {
            if (maxReplacements == -1)
            {
                return Regex.Replace(input, pattern, replacement, options);
            }

            var regex = new Regex(pattern, options);
            var count = 0;
            return regex.Replace(input, m =>
            {
                count++;
                if (count <= maxReplacements)
                {
                    return replacement;
                }

                return m.Value;
            });
        }
        catch
        {
            return input;
        }
    }
}