using System;

using System.Text.RegularExpressions;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var str = @"Tx1

        some data

        some data

        Tx2

        some data 

        some data

        Tx3

        some data 

        some data

        Tx99

        some data

        some data";

        string[] lines = Regex.Split(str, @"(Tx[0-9]+)");

        for(int i=1; i<lines.Count(); i++){

            Console.WriteLine("Filename: " + lines[i]);

            Console.WriteLine("Data: " + lines[++i]);

        }

    }

}