查找两个已知值之间的字符串

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 pre, 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);}