using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var str = "\"C:/Users/Max/AppData/Local/Temp/TelegramMirrorTemp/AttachmentsVideos/5436346166193181538_2_5226645564787015400 (8).mp4\"";

        //string pattern = @"([""']?)([a-zA-Z]:[\\/]|\/)[^""'\s]+\1";       

        //string pattern = @"([""']?)([a-zA-Z]?:?([\\/][^""'\s]*)+)\1";

        //string pattern = @"([""']?)([a-zA-Z]?:?([\\/][^""'\s]*)+(\s*\([^)]+\))?(\.[a-zA-Z0-9]+)?)\1";

        //string pattern = @"([""']?)([a-zA-Z]?:?[\\/](?:[^""'\\s]|\\.)+)\1";

        string pattern = @"([""']?)([a-zA-Z]?:?([\\/][^""'\\s]*)+)\1";

        var regex = new Regex(pattern, RegexOptions.Compiled);

​

        var matches = regex.Matches(str);

        Console.WriteLine(matches[0].Value);

    }

}