using System;

using System.Text.RegularExpressions;

public class Program

{   

    private static string ToggleRichText(string text, string tag, int start, int end, string args = "")

    {

        string pretext = text.Substring(0, start);

        string selected = text.Substring(start, end - start);       

        string postext =  text.Substring(end);

        var regex = new Regex($@"<\/?{tag}([^>]*)>", RegexOptions.RightToLeft);

        var pretextmatch = regex.Match(pretext);        

        var startsTagged = pretextmatch.Success && pretextmatch.Value != $"</{tag}>";

        var pretextArgs = startsTagged ? pretextmatch.Groups[1].Captures[0].Value : "";

        var selectedmatch = regex.Match(selected);

        var containsTags = selectedmatch.Success;

        var endsUntagged = containsTags && selectedmatch.Value == $"</{tag}>";

        var posttextArgs = containsTags && !endsUntagged ? selectedmatch.Groups[1].Captures[0].Value : pretextArgs;

        selected = regex.Replace(selected, "");

        if (startsTagged && args != pretextArgs && !endsUntagged){

            selected = $"</{tag}><{tag}{args}>" + selected + $"</{tag}><{tag}{posttextArgs}>";

        }else if (startsTagged && args != pretextArgs && endsUntagged){

            selected = $"</{tag}><{tag}{args}>" + selected + $"</{tag}>";

        }else if (startsTagged && endsUntagged){

            selected += $"</{tag}>";

        }else if (!startsTagged && containsTags && !endsUntagged){

            selected = $"<{tag}{args}>" + selected;

        }else if (startsTagged){

            selected = $"</{tag}>" + selected + $"<{tag}{args}>";

        }else{          

            selected = $"<{tag}{args}>" + selected + $"</{tag}>";

        }

        return pretext + selected + postext;

    }

    public static void Main()

    {

        // characterIndicies:  8  11          23          35    41        51   56

        var content = "This is the string that has <b>bold typed text</b> in it"; //string with bold tags       

        Console.WriteLine(ToggleRichText(content, "b", 8, 11) + "\n"); //make normal text bold

        Console.WriteLine(ToggleRichText(content, "b", 23, 35) + "\n"); //extend bold backwards from 35 to 23

        Console.WriteLine(ToggleRichText(content, "b", 11, 56) + "\n"); //bold over the original, essentially though its make normal text bold

        Console.WriteLine(ToggleRichText(content, "b", 35, 56) + "\n"); //extend bold fwd from 46 to 56

        Console.WriteLine(ToggleRichText(content, "b", 35, 42) + "\n"); //unbold bold text

        //                             15  19

        var content2 = "<style=h1>This is a H1</style>"; //string with style tags       

        Console.WriteLine(ToggleRichText(content2, "style", 15, 19, "=h2") + "\n");// make the middle H2 style but keep the start and end H1

        //                                                     39                                               88

        var content3 = "<align=left>This is left aligned text, but I want to make this middle bit right aligned, and keep this end bit left aligned</align>"; //string with align tags      

        Console.WriteLine(ToggleRichText(content3, "align", 39, 88, "=right") + "\n"); //make the middle right aligned but keep the rest left

        //                                              31                                        84

        var content4 = "<color=00>This is colored text, but I want to make</color> <color=11>this middle bit different, and keep this end bit</color>"; //string with color tags        

        Console.WriteLine(ToggleRichText(content4, "color", 31, 84, "=22") + "\n"); //change the color of the middle section from mixed to 22

    }   

}