using System;
using  System.Text.RegularExpressions;

public class Test
{
    public static void Main()
    {
        string s = "fsdfs [b]abc[/b] dddfs [b]abc[/b] fdsfdsfs [b]abcfsdfs";
        string result = Regex.Replace(s, @"\[b\](.*?)\[/b\]", @"<strong>$1</strong>");
        Console.WriteLine(result);
    }
}