Поиск с выделение слов

    Private Sub CheckBox1_CheckedChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles CheckBox1.CheckedChanged
        If TextBox2.Text = "" Then Exit Sub
        If TextBox2.Text = " " Then Exit Sub
        Dim Cursor As Integer
        Cursor = RichTextBox1.SelectionStart
        Dim c, a As Integer
        Do
            a = InStr(c + 1, RichTextBox1.Text, TextBox2.Text)
            If a > 0 Then
                RichTextBox1.SelectionStart = a - 1
                RichTextBox1.SelectionLength = Len(TextBox2.Text)
                If CheckBox1.Checked Then
                    RichTextBox1.SelectionBackColor = Color.Yellow
                Else
                    RichTextBox1.SelectionBackColor = Color.White
                End If
                RichTextBox1.SelectionStart = Len(RichTextBox1.Text)
                c = a
            End If
        Loop While a > 0
        RichTextBox1.SelectionStart = Cursor
    End Sub

With RichTextBox1
    .SelectionStart = 0
    .SelectionLength = .TextLength
    .SelectionBackColor = Drawing.Color.Transparent
End With