Replace Nth occurrence of a character in a string with something else

Question

Consider a = paste(1:10,collapse=", ") which results in

a = "1, 2, 3, 4, 5, 6, 7, 8, 9, 10"

I would like to replace every n-th (say 4-th) occurrences of "," and replace it with something else (say "\n"). The desired output would be:

"1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"

I am looking for a code that uses gsub (or something equivalent) and some form of regular expression to achieve this goal.

Answer 1

You can replace ((?:\d+, ){3}\d), with \1\n

You basically capture everything till fourth comma in group1 and comma separately and replace it with \1\n which replaces matched text with group1 text and newline, giving you the intended results.

Regex Demo

R Code demo

gsub("((?:\\d+, ){3}\\d),", "\\1\n", "1, 2, 3, 4, 5, 6, 7, 8, 9, 10")

Prints,

[1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"

Edit:

To generalize above solution to any text, we can change \d to [^,]

New R code demo

gsub("((?:[^,]+, ){3}[^,]+),", "\\1\n", "1, 2, 3, 4, 5, 6, 7, 8, 9, 10")
gsub("((?:[^,]+, ){3}[^,]+),", "\\1\n", "a, bb, ccc, dddd, 500, 600, 700, 800, 900, 1000")

Output,

[1] "1, 2, 3, 4\n 5, 6, 7, 8\n 9, 10"
[1] "a, bb, ccc, dddd\n 500, 600, 700, 800\n 900, 1000"