无需正则表达式的解决方案：

string ExtractString(string s, string tag) {
     // You should check for errors in real-world code, omitted for brevity
     var startTag = "<" + tag + ">";
     int startIndex = s.IndexOf(startTag) + startTag.Length;
     int endIndex = s.IndexOf("</" + tag + ">", startIndex);
     return s.Substring(startIndex, endIndex - startIndex);
}