sString$ = "[[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8 []]"

FOR siCount = 1 TO LEN(sString$)
    IF INSTR("[] ,", MID$(sString$, siCount, 1)) = 0 THEN
        sFlatter$ = sFlatter$ + sComma$ + MID$(sString$, siCount, 1)
        sComma$ = ", "
    END IF
NEXT siCount

PRINT "["; sFlatter$; "]"
END