using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        // \s : Matches a space

        // t : Exact match t

        // \d{2} : Any digit, 2 repetitions

        // t : Exact match b

        // \d{2} : Any digit, 2 repetitions

        var match = Regex.Match("I want to find t56b45 in a string", @".*(?=\st\d{2}b\d{2})");

        if(match.Success)

            Console.WriteLine("\"" + match.Value + "\"");

        else

            Console.WriteLine("Nothing found.");

    }

}

​

​