I've been using 'Find and Replace' to edit text from Memorizer.me website.
Here's a little sample:

H____ u_ h_____ e_ l_ i___ d_ H_____ a_ q__ l______ K____.

I want to replace some of the letters above with a red underline symbol. I'm trying to make it more challenging to memorize something.
When I use [A-f] in wildcard mode it selects all the existing underline symbols shown.

'[A-f]' is supposed to find only the letters 'a' through 'f', uppercase and lowercase.
However, it includes all the existing underline symbols in the find along with the letters a through f.
I know I've done this before a couple of months ago and it didn't do this.
What am I missing here?