RichTextBox - How To Replace Character / Word In Text

How can I replace character/word in text (loaded to richboxtext) but only for those which are not on the "block" list ?
Dim str As String = RichTextBox2.Text
RichTextBox3.Text = str.Replace("1"c, "A"c)

But I want add list of words which should be excluded. I thought that I can do something like :
Dim str As String = RichTextBox2.Text
If Regex.IsMatch(RichTextBox2.Text, "shows") Then
Else
RichTextBox3.Text = str.Replace("%"c, " "c)
End If
RichTextBox3.Text = str.Replace("1"c, "A"c)
Dim str2 As String = RichTextBox3.Text
RichTextBox3.Text = str2.Replace("2"c, "B"c)

But it's not working as it will just skip replace of % for whole text and I want just just exclude particular word from list from being replaced...