Regex expression to match every nth occurence of a pattern

Question

Consider this string,

str = "abc-de-fghi-j-k-lm-n-o-p-qrst-u-vw-x-yz"

I'd like to separate the string at every nth occurrence of a pattern, here -:

f(str, n = 2)
[1] "abc-de" "fghi-j" "k-lm" "n-o"...

f(str, n = 3)
[1] "abc-de-fghi" "j-k-lm" "n-o-p" "qrst-u-vw"...

I know I could do it like this:

spl <- str_split(str, "-", )[[1]]
unname(sapply(split(spl, ceiling(seq(spl) / 2)), paste, collapse = "-"))
[1] "abc-de" "fghi-j" "k-lm"   "n-o"    "p-qrst" "u-vw"   "x-yz" 

But I'm looking for a shorter and cleaner solution

What are the possibilities?

Answer 1

What about the following (where 'n-1' is a placeholder for a number):

(?:[^-]*(?:-[^-]*){n-1})\K-

See an online demo

---

• (?: - Open 1st non-capture group;
  • [^-]* - Match 0+ characters other hyphen;
  • (?: - Open a nested 2nd non-capture group;
    • -[^-]* - Match an hyphen and 0+ characters other than hyphen;
    • ){n} - Close nested non-capture group and match n-times;
  • ) - Close 1st non-capture group;
• \K- - Forget what we just matched and match the trailing hyphen.

Note: The use of \K means we must use PCRE (perl=TRUE)

---

To create the 'n-1' we can use sprintf() functionality to use a variable:

str <- "abc-de-fghi-j-k-lm-n-o-p-qrst-u-vw-x-yz"
for (n in 1:10) {
  print(strsplit(str, sprintf("(?:[^-]*(?:-[^-]*){%s})\\K-", n-1), perl=TRUE)[[1]])
}

Prints: