How to split a string from right-to-left, like Python's rsplit()?

Question

Suppose a vector: ``` xx.1 <- c("zz_ZZ_uu_d", "II_OO_d") ``` I want to get a new vector splitted from right most and only split once. The expected results would be: ``` c("zz_ZZ_uu", "d", "II_OO", "d"). ``` It would be like python's `rsplit()` function. My current idea is to reverse the string, and split the with `str_split()` in `stringr`. Any better solutions? update Here is my solution returning n splits, depending on stringr and stringi. It would be nice that someone provides a version with base functions. ``` rsplit <- function (x, s, n) { cc1 <- unlist(stringr::str_split(stringi::stri_reverse(x), s, n)) cc2 <- rev(purrr::map_chr(cc1, stringi::stri_reverse)) return(cc2) } ```

Accepted Answer

Negative lookahead:

```
unlist(strsplit(xx.1, "_(?!.*_)", perl = TRUE))
# [1] "zz_ZZ_uu" "d"        "II_OO"    "d"     
```

Where `a(?!b)` says to find such an `a` which is not followed by a `b`. In this case `.*_` means that no matter how far (`.*`) there should not be any more `_`'s.

However, it seems to be not that easy to generalise this idea. First, note that it can be rewritten as positive lookahead with `_(?=[^_]*$)` (find `_` followed by anything but `_`, here `$` signifies the end of a string). Then a not very elegant generalisation would be

```
rsplit <- function(x, s, n) {
  p <- paste0("[^", s, "]*")
  rx <- paste0(s, "(?=", paste(rep(paste0(p, s), n - 1), collapse = ""), p, "$)")
  unlist(strsplit(x, rx, perl = TRUE))
}

rsplit(vec, "_", 1)
# [1] "a_b_c_d_e_f" "g"           "a"           "b"          
rsplit(vec, "_", 3)
# [1] "a_b_c_d" "e_f_g"   "a_b"    
```

where e.g. in case `n=3` this function uses `_(?=[^_]*_[^_]*_[^_]*$)`.

How to split a string from right-to-left, like Python's rsplit()?

Solution

Problem