Highlight all searched words

Question

In my RichtextBox, if I have written as below.

This is my pen,
his pen is beautiful.

Now I search word \"is\"

Answer 1

    private void button3_Click(object sender, EventArgs e)
    {
        if (textBox1.Text != "")
        {
            for (int i = 0; i < richTextBox1.TextLength; i++)
            {
                richTextBox1.Find(textBox1.Text, i, RichTextBoxFinds.None);
                richTextBox1.SelectionBackColor = Color.Red;
            }
        }
        else
        {
            for (int i = 0; i < richTextBox1.TextLength; i++)
            {
                richTextBox1.SelectAll();
                richTextBox1.SelectionBackColor = Color.White;
            }
        }
    }[lets make it!][1]