r - split string last delimiter

Question

I need help figuring out how to split strings in a column of a data frame based on the last delimiter when I have varying numbers of the same delimiter in R. For example,

col1 <- c('a', 'b', 'c')
col2 <- c('a_b', 'a_b_c', 'a_b_c_d')
df <- data.frame(cbind(col1, col2))

And I would like to split df$col2 to have a data frame that looks like:

col1 <- c('a', 'b', 'c')
col2 <- c('a', 'a_b', 'a_b_c')
col3 <- c('b', 'c', 'd')

Answer 1

Using the stringi package, you can also achieve your goal.stri_extract_last_regex() extract the last element of what you specify in a pattern. Here, I said "get the last small letter in a string." Likewise, you can use stri_replace_last_regex() to modify col2. Here I said "I want to replace the last pattern of _ and a small letter with nothing." That is, I said "I want to remove the last pattern of _ and a small letter."

library(dplyr)
library(stringi)

df %>%
mutate(col3 = stri_extract_last_regex(str = col2, pattern = "[a-z]"),
       col2 = stri_replace_last_regex(str = col2, pattern = "_[a-z]", replacement = ""))

#  col1  col2 col3
#1    a     a    b
#2    b   a_b    c
#3    c a_b_c    d