gistfile1.txt

using System;
using System.Text.RegularExpressions;

class Test
{
    static void Main()
    {
        string text = @"A Foobar is cool stuff, as we can see in Figure 1.1:

  Figure 1.1  This is a Foobar";                       
        text = Regex.Replace(text, "^ +(Figure.*)$", "##### _$1_", RegexOptions.Multiline);
        Console.WriteLine($"'{text}'");
    }
}