Regular Expression: replace the n-th occurence - r

Question

does someone know how to find the n-th occurcence of a string within an expression and how to replace it by regular expression?
for example I have the following string
txt <- "aaa-aaa-aaa-aaa-aaa-aaa-aaa-aaa-aaa-aaa"
and I want to replace the 5th occurence of '-' by '|'
and the 7th occurence of '-' by "||" like
[1] aaa-aaa-aaa-aaa-aaa|aaa-aaa||aaa-aaa-aaa
How do I do this?
Thanks,
Florian

Answer 1

(1) sub It can be done in a single regular expression with sub:
> sub("(^(.*?-){4}.*?)-(.*?-.*?)-", "\\1|\\3||", txt, perl = TRUE)
[1] "aaa-aaa-aaa-aaa-aaa|aaa-aaa||aaa-aaa-aaa"
(2) sub twice or this variation which calls sub twice:
> txt2 <- sub("(^(.*?-){6}.*?)-", "\\1|", txt, perl = TRUE)
> sub("(^(.*?-){4}.*?)-", "\\1||", txt2, perl = TRUE)
[1] "aaa-aaa-aaa-aaa-aaa|aaa-aaa||aaa-aaa-aaa"
(3) sub.fun or this variation which creates a function sub.fun which does one substitute. it makes use of fn$ from the gsubfn package to substitute n-1, pat, and value into the sub arguments. First define the indicated function and then call it twice.
library(gsubfn)
sub.fun <- function(x, pat, n, value) {
fn$sub( "(^(.*?-){`n-1`}.*?)$pat", "\\1$value", x, perl = TRUE)
}
> sub.fun(sub.fun(txt, "-", 7, "||"), "-", 5, "|")
[1] "aaa-aaa-aaa-aaa-aaa|aaa-aaa||aaa-aaa-aaa"
(We could have modified the arguments to sub in the body of sub.fun using paste or sprintf to give a base R solution but at the expense of some additional verbosity.)
This can be reformulated as a replacement function giving this pleasing sequence:
"sub.fun<-" <- sub.fun
tt <- txt # make a copy so that we preserve the input txt
sub.fun(tt, "-", 7) <- "||"
sub.fun(tt, "-", 5) <- "|"
> tt
[1] "aaa-aaa-aaa-aaa-aaa|aaa-aaa||aaa-aaa-aaa"
(4) gsubfn Using gsubfn from the gsubfn package we can use a particularly simple regular expression (its just "-") and the code has quite a straight forward structure. We perform the substitution via a proto method. The proto object containing the method is passed in place of a replacement string. The simplicity of this approach derives fron the fact that gsubfn automatically makes a count variable available to such methods:
library(gsubfn) # gsubfn also pulls in proto
p <- proto(fun = function(this, x) {
if (count == 5) return("|")
if (count == 7) return("||")
x
})
> gsubfn("-", p, txt)
[1] "aaa-aaa-aaa-aaa-aaa|aaa-aaa||aaa-aaa-aaa"
UPDATE: Some corrections.
UPDATE 2: Added a replacement function approach to (3).
UPDATE 3: Added pat argument to sub.fun.

Answer 2

An alternative possibility is using Hadley's stringr package which builds the basis for the function I wrote:
require(stringr)
replace.nth <- function(string, pattern, replacement, n) {
locations <- str_locate_all(string, pattern)
str_sub(string, locations[[1]][n, 1], locations[[1]][n, 2]) <- replacement
string
}
txt <- "aaa-aaa-aaa-aaa-aaa-aaa-aaa-aaa-aaa-aaa"
txt.new <- replace.nth(txt, "-", "|", 5)
txt.new <- replace.nth(txt.new, "-", "||", 7)
txt.new
# [1] "aaa-aaa-aaa-aaa-aaa|aaa-aaa-aaa||aaa-aaa"

Answer 3

One way to do this is to use gregexpr to find the positions of the -:
posns <- gregexpr("-",txt)[[1]]
And then pasting together the relevant pieces and separators:
paste0(substr(txt,1,posns[5]-1),"|",substr(txt,posns[5]+1,posns[7]-1),"||",substr(txt,posns[7]+1,nchar(txt)))
[1] "aaa-aaa-aaa-aaa-aaa|aaa-aaa||aaa-aaa-aaa"