using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		string myString = "<tr class=\"discussion r0\"><td class=\"topic starter\"><a href=\"SITE?d=6638\">Test di matematica</a></td>";
Regex rx = new Regex(@"<a.*?>(.*?)</a>");
MatchCollection matches = rx.Matches(myString);
if (matches.Count > 0)
{
    Match match = matches[0]; // only one match in this case
    GroupCollection groupCollection = match.Groups;
    Console.WriteLine( groupCollection[1].ToString());
}
	}
}