R - How to make a stringi str_extract pattern parametric in a loop

Question

I have strings that are segmented by forward-slashes, I am trying to generate this using a loop so I need to parametrize the regex so that I can use it inside a loop. I have 7 levels:

I want to extract the followings using the regex and stringi:

A
A/268
A/268/200
A/268/200/300
A/268/200/300/400

Here's what I have:

n=3
str_extract("A/268/200/300/400/500","(.*?/){n}"


str_extract("A/268/200/300/400/500","(.*?/){3}"

Answer 1

We can use glue::glue to interpolate the values

n <- 3
pat <- as.character(glue::glue("(.*?/){<-n-1->}([^/]+)", 
               .open = "<-", .close = "->"))
pat
#[1] "(.*?/){2}([^/]+)"
library(stringr)
str_extract("A/268/200/300/400/500", pat)
#[1] "A/268/200"

---

If we need it as a loop

v1 <- 1:7
lst1 <- vector('list', length(v1))
for(i in v1) {
   tmppat <- as.character(glue::glue("(.*?/){<-i-1->}([^/]+)",
                   .open = "<-", .close = "->"))
   lst1[[i]] <- str_extract("A/268/200/300/400/500", tmppat)
 }





head(lst1, 5)
#[[1]]
#[1] "A"

#[[2]]
#[1] "A/268"

#[[3]]
#[1] "A/268/200"

#[[4]]
#[1] "A/268/200/300"

#[[5]]
#[1] "A/268/200/300/400"