Highlight words in multiple files

Question

Hey everyone. What I want to do and don't know how is to find and highlight these 2 words "game" and "player" in .txt files. 

No richtext. 

so mostly like this code: 

  private void button1_Click(object sender, EventArgs e)

        {

            this.openFileDialog1.Filter =

                "TEXT (*.xml;*.txt|";

            this.openFileDialog1.Multiselect = true;

            this.openFileDialog1.Title = "My text editor";

            DialogResult dr = openFileDialog1.ShowDialog();

            if (dr == DialogResult.OK)

                foreach (String file in openFileDialog1.FileNames)

                    try

                    {

                        StreamReader reader = new StreamReader(file);

                        string content = reader.ReadToEnd();

                        reader.Close();

                        content = Regex.Replace(content, "game", "gaming"); 

content = Regex.Replace(content, "player", "playing"); 

so mostly like this code, but not actually replacing the word, just find it and highlight it. Help ???