tidyr: separate column while retaining delimiter in the first column

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I have a column that I am trying to break up into two while retaining the delimiter. I got this far, but part of the delimiter is being dropped. I also need to do this split a second time, adding the delimiter to the first column which I cannot figure out how to do.

duplicates <- data.frame(sample = c("a_1_b1", "a1_2_b1", "a1_c_1_b2"))

duplicates <- separate(duplicates, 
                       sample, 
                       into = c("strain", "sample"),
                       sep = "_(?=[:digit:])")

using only the first name as an example, my output is a_1 and b1 while my desired output is a_1 and _b1.

I would also like to perform this split with the delimiter added to the first column as below.

sample batch
a_1_ b1
a1_2_ b1
a1_c_1_ b2

Edit: This post does not answer my question of how to retain the delimiter, or to control which side of the split it ends up on.

tidyr delimiter r
3 Answers
  1. Update: see request of OP in comments:
duplicates %>% 
    mutate(batch = sub(".*_", "_", sample)) %>%  
    mutate(sample = sub("_[^_]+$", "", sample))

output:

  sample batch
1    a_1   _b1
2   a1_2   _b1
3 a1_c_1   _b2

Update after clarification: see comments:

duplicates %>% 
    mutate(batch = sub(".*_", "", sample)) %>%  
    mutate(sample = sub("_[^_]+$", "_", sample))

Output:

   sample batch
1    a_1_    b1
2   a1_2_    b1
3 a1_c_1_    b2

First answer: We could use str_sub from stringr package:

library(stringr)
library(dplyr)

duplicates %>% 
    mutate(batch = str_sub(sample, -2,-1)) %>% 
    mutate(sample = str_sub(sample, end=-3))

Output:

   sample batch
1    a_1_    b1
2   a1_2_    b1
3 a1_c_1_    b2
user_133216470

Using separate

library(tidyr)
separate(duplicates, sample, into = c("strain", "sample"), 
        sep = "(?<=_)(?=[^_]+$)")

-output

    strain sample
1    a_1_     b1
2   a1_2_     b1
3 a1_c_1_     b2

For splitting the other way

separate(duplicates, sample, into = c("strain", "sample"), 
         sep = "(?<=[^_])(?=_[^_]+$)")
  strain sample
1    a_1    _b1
2   a1_2    _b1
3 a1_c_1    _b2
user_37322710

You can use tidyr::extract with capture groups.

tidyr::extract(duplicates, sample, c("strain", "sample"), '(.*_)(\\w+)')

#   strain sample
#1    a_1_     b1
#2   a1_2_     b1
#3 a1_c_1_     b2

The same regex can also be used with strcapture in base R -

strcapture('(.*_)(\\w+)', duplicates$sample, 
           proto = list(strain = character(), sample = character()))
user_39629140
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