[DiapDealer]

You're saying that \x{00E1} doesn't find the á character on line 22 of the code in your screenshot?

It finds it OK when I paste your code above into Sigil and search for \x{00E1}. The Sigil Unicode property setting shouldn't matter in this particular case. In fact none of the Regex settings should really affect that particular search. I tested on Windows and Linux.

Note: you won't be able to use \x{FFFF} syntax in Sigil's replace field (you'll have to use the character you want), but I got the impression it was finding á with \x{00E1} that you were having problems with.