Doesn't work for me on linux using 5.1.8(1)-release (x86_64-pc-linux-gnu). Output with `set -vx` at top (I guessed "fooba"):

    ++ [[ false != true ]]
    ++ guess_count=1
    ++ [[ 1 -le 6 ]]
    ++ echo 'Enter your guess (1 / 6):'
    Enter your guess (1 / 6):
    ++ read guess
    fooba
    +++ echo fooba
    +++ tr '[a-z]' '[A-Z]'
    ++ guess=FOOBA

    [... ÉTUDE  =~  FOOBA  ]]
    ++ echo 'Please enter a valid word with 5 letters!'
    Please enter a valid word with 5 letters!
    ++ guess_count=0
    ++ [[ false != true ]]
    ++ guess_count=1
    ++ [[ 1 -le 6 ]]
    ++ echo 'Enter your guess (1 / 6):'
    Enter your guess (1 / 6):
    ++ read guess