Dim index As Integer = 0

'TextBox1 = text to be searched in the rich text box
'RichTextBox1 = text being searched

While index < RichTextBox1.Text.LastIndexOf(TextBox1.Text)
    RichTextBox1.Find(TextBox1.Text, index, RichTextBox1.TextLength, RichTextBoxFinds.None)
    RichTextBox1.SelectionBackColor = Color.Turquoise
    index = RichTextBox1.Text.IndexOf(TextBox1.Text, index) + 1
End While