using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var input = @"

            <Project>

                <VersionPrefix>2.0.0-clinteastwood</VersionPrefix>

            </Project>

        ";

        var version = "3.0.0-foo";

        var output = Regex.Replace(input, "(<VersionPrefix>).+<", "$1" + version + "<");

        Console.WriteLine(output);

    }

}