n$ = "[[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8 []]"
for i = 1 to len(n$)
 if instr("[] ,",mid$(n$,i,1)) = 0 then
  flatten$ = flatten$ + c$ + mid$(n$,i,1)
  c$ = ","
 end if
next i
print "[";flatten$;"]"