C#

I want to read the text file on a specific line and perform a task according to the information.

example: I have a string with the value ON, in case the program will read the text file online, if you have the information ON on line 4 performs a task, if OFF accomplishes another task can anyone help me?

<pre>  var webClient = new WebClient();
            var pageSC = webClient.DownloadString("http://en.wikipedia.org/w/api.php?format=xml&action=query&prop=extracts&titles=" + textBox1.Text + "&redirects=true");
            XmlDocument doc = new XmlDocument();
            doc.LoadXml(pageSC);

            var fnode = doc.GetElementsByTagName("extract")[0];
            try
            {
                string ss = fnode.InnerText;
                Regex regex = new Regex("\\<[^\\>]*\\>");
                string.Format("Before:{0}", ss);

                ss = regex.Replace(ss, string.Empty);
                string result = String.Format(ss);
                richTextBox2.Text = result;
            }
            catch
            {
                richTextBox2.Text = "error";
            }

I have a long string like  
     <abcd><xyz>testingmessage like this</xyz></abcd><abcd>
     <xyz>testingmessage like this</xyz></abcd><abcd>
     <xyz>testingmessage like this</xyz></abcd><abcd>
     <xyz>testingmessage like this</xyz></abcd>

I want to create a separate xml file based on <abcd><xyz> and </xyz></abcd> these xml tags eg : <abcd><xyz>testingmessage like this</xyz></abcd> consider as one xml file . I know it's a substring search in a string in C# but is there any fastest way to do this or any other way to achieve this . Any clue regarding this will be helpful . Please help . I have to do this in C# or can I do this using javascript .

EDIT I want to create separate xml file based on <abcd><xyz> and </xyz></abcd> these xml tags in a string .