> x = as.factor(c("3","$$$av","1","2","^ab", "^ba", "3","aba","4","-ab","ba",'3',"ba"))
> x
 [1] 3     $$$av 1     2     ^ab   ^ba   3     aba   4     -ab   ba    3     ba
Levels: 1 2 3 4 ^ab -ab aba $$$av ba ^ba

if (missing(levels)) {
    y <- unique(x, nmax = nmax)
    ind <- sort.list(y)
    y <- as.character(y)
    levels <- unique(y[ind])
}