That is a perfectly valid regex that ought to do what you want, so here is my guess: are you opening the advanced filtration area and not switching the match type drop-down from Containing to Matching RegExp?
As a side note, that regex will match what you want, but I suspect not in the way you intend.
FR_PLT* matches pages containing
FR_PL with or without
T on the end, and
FR_Country* matches pages containing
FR_Countr with or without
y on the end. In both cases the match is not case sensitive, so if you had a page under
fr_pl it would also match the first regex.
FR_PLT with anything after it, you'd use the dot wildcard character:
FR_PLT.*. However, since regex matches within the string by default, rather than requiring a match on the entire string, you don't match any more or less with
FR_PLT.* than with just