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

public class Test
{
	public static void Main()
	{
		var markdown = "[url=https://stackoverflow.com/a b]SO[/url]";
		var p = @"(?i)\[url=([^]]+)](.*?)\[/url]";
		var result = Regex.Replace(markdown, p, x => 
			$"[{x.Groups[2].Value}]({x.Groups[1].Value.Replace(" ","%20")})");
        Console.WriteLine(result);
	}
}