saving text from txt in a list or variable and extracting words with "S" load it to another file and send in a e mail

Question

I'm making a windows form in C# that would allow me to load a text from a text file, extract only the words that have a "S", load these words in another text file, separate these words with ";", and finally send this new file through an email. So far, I have managed to read the file and print the text on screen, but how do I save the text in a variable (for instance "a") so I can then use regex to extract the words with "S" and then send them in a email?

Here is a sample of the text:

The Snowman It was nearly Christmas. Katie woke up and found that the world was white and magical.

• Snow, she shouted, snow for Christmas.

And here is the code for the form:

namespace Lectura_y_escritura
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void button2_Click(object sender, EventArgs e)
        {
            StreamWriter escritura = new StreamWriter(@"C:\Users\Joséguillermo\Desktop\C#\Texto.txt",true);
            try
            {
                escritura.WriteLine("\n");
                escritura.WriteLine(" " + textBox1.Text);
                escritura.WriteLine("\n");
            }
            catch {
                MessageBox.Show("Error de escritura");
            }
            escritura.Close();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            List<string> texto = new List<string>();
            StreamReader lectura = new StreamReader(@"C:\Users\Joséguillermo\Desktop\C#\Texto.txt", true) ;
            string linea;
            try
            {
                linea = lectura.ReadLine();
                while (linea != null)
                {
                    richTextBox1.AppendText(linea + "\n");
                    linea = lectura.ReadLine();
                    texto.Add(linea);
      
                }

                foreach (string j in texto)
                {
                    Console.WriteLine(j);

                }
            }
            catch {
                MessageBox.Show("Error de lectura");
            }
            lectura.Close();
        }

        private void button4_Click(object sender, EventArgs e)
        {

        }

        private void button3_Click(object sender, EventArgs e)
        {
            string pattern = @"\b[Ss]\b";
        }
    }
}

and here is the form (or a photo of it)

C# form

I really apreciate any help i could get since i have no experience in c#.

Thanks in advance