using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

            int length = 1234;

            string input = $"[lime on white]File size:[/] [bold]\t{length,-10}[/]";

​

            Console.WriteLine(input.StripCodes());

    }

}

    public static class StringExtensions

    {

        private static readonly Regex Whitespace = new(@"\s+");

​

        public static string Flatten(this string sender)

            => sender is null or "" ?

                sender :

                Whitespace.Replace(sender.Trim(), " ");

​

        public static string StripCodes(this string sender)

            => Regex.Replace(sender, @"\[[^]]*\]", "")

                .Flatten();

    }