Could this be better? .NET 2.0 compatibility for SQL Server 2005:

public static SqlString RegexSubstring(SqlString regexpattern, 
                                       SqlString sourcetext, 
                                       SqlInt32 start_position)
{
   SqlString result = null;

   if (!regexpattern.IsNull && !sourcetext.IsNull && !start_position.IsNull)
   {
      int start_location = (int)start_position >= 0 ? (int)start_position : 0;

      Regex RegexInstance = new Regex(regexpattern.ToString());
      result = new SqlString(RegexInstance.Match(sourcetext.ToString(), 
                                                 (int)start_position).Value);
   }

   return result;
}