var result = (new Regex(@"<li>(.*?)<li>")).Matches(str).Cast<Match>()
.Select(m => m.Value)
.ToArray()
.Select(x => x.Replace("<li>", "").Replace("</li>", ""));
it returns the contents of <li><li> as a string collection.
if you want to keep the <li><li> just delete the last select line