Extract last regex match in R

Question

I'm using mutate() with str_extract() to condense a string column in a dataframe to just a single key word column. My problem is that one of the strings contains two keywords, and it's the second one that's more important to me. The regex, though, always takes the first hit it finds to the alternatives I tell it to look for. Is there a way to change this?

MWE (without mutate()):

teststring <- "abcdef"
str_extract(teststring, "b|c|a")

I'd like to be able to find patterns in the order I choose, not what's first in the test string.

Maybe `lapply(c("b","c","a"), stringr::str_extract, string=teststring)`? — GKi, Jul 08 '20 at 08:57
@WiktorStribiżew the first version is working great, thank you! — Lilith-Elina, Jul 08 '20 at 09:20
I could not find any duplicates using the "last match regex in R", so even though it is a duplicate it will be a good signpost for the future visitors. — Wiktor Stribiżew, Jul 09 '20 at 07:46
I didn't even think of it as looking for a last match. Luckily that is what the problem was, and not me trying to find something in the middle. — Lilith-Elina, Jul 09 '20 at 08:14

score 3 · Accepted Answer · answered Jul 08 '20 at 09:23

You may use either stringi::stri_extract_last or stringi::stri_extract_last_regex to get the last match:

stringi::stri_extract_last_regex('zzzayyyyc.', '[abc]')
stringi::stri_extract_last('zzzayyyyc.', regex=c('[abc]'))

See the R demo online, both return [1] "c".

Daniel O · Answer 2 · 2020-07-08T11:17:42.353

1

Building on Gki's answer in the comment, if you only want to return the 1st match of your chosen order. we can wrap the code with:

teststring <- "ccccbbbaaaaqwerty"

which.min(is.na((sapply(c("b","c","a"), stringr::str_extract, string=teststring))))

b 
1

edited Jul 08 '20 at 11:17

answered Jul 08 '20 at 11:11

Daniel O

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Thank you! Using this with `mutate()` will only give me the index number back, though. – Lilith-Elina Jul 08 '20 at 12:20
I'm not a `dplyr` expert, but if you wrap the code with `names(...)` you will get just the pattern back. perhaps you could implement this? – Daniel O Jul 08 '20 at 12:25
1

Good point, I would have expected that to work but it doesn't give me anything. – Lilith-Elina Jul 08 '20 at 13:03

Extract last regex match in R

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