Extracting first four digits after a hyphen using stringr

Question

I am trying to extract the first four digits after a hyphen in the following string: extract_public_2018_20190530180949469_58906_20110101-20111231Texas. I am using the following code:

stringr::str_extract(
"extract_public_2018_20190530180949469_58906_20110101-20111231Texas", 
"-[[:digit:]]{4}"
)

But I get -2011 instead of 2011. How can I only extract the four digits and not the hyphen?

score 5 · Accepted Answer · answered Aug 21 '19 at 00:07

5

Use regex's lookbehind, a non-greedy way of finding something before your pattern without consuming it:

stringr::str_extract(
  "extract_public_2018_20190530180949469_58906_20110101-20111231Texas", 
  "(?<=-)[[:digit:]]{4}"
)
# [1] "2011"

answered Aug 21 '19 at 00:07

r2evans

141,215
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score 2 · Answer 2 · answered Aug 21 '19 at 00:08

2

str_extract is behaving as expected i.e. it returns the complete match.

You can use str_match and include () in the pattern:

stringr::str_match(
  "extract_public_2018_20190530180949469_58906_20110101-20111231Texas", 
  "-([[:digit:]]{4})"
)

     [,1]    [,2]  
[1,] "-2011" "2011"

Then add [, 2] to return just the match:

stringr::str_match(
  "extract_public_2018_20190530180949469_58906_20110101-20111231Texas", 
  "-([[:digit:]]{4})"
)[, 2]

[1] "2011"

answered Aug 21 '19 at 00:08

neilfws

32,751
5
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Thanks, @neilfws! But I find @r2evans's answer to be relatively cleaner. – Ashirwad Aug 21 '19 at 00:19
1

I agree! It's a better answer. – neilfws Aug 21 '19 at 00:20
1

Your method would be preferred (necessary) when the preceding not-to-be-consumed pattern (`-` in this case) is non-fixed: lookaround doesn't deal well with varying sizes. – r2evans Aug 21 '19 at 15:57

score 2 · Answer 3 · answered Aug 21 '19 at 00:37

2

In base R, we can sub to extract 4 digits after hyphen.

string <- "extract_public_2018_20190530180949469_58906_20110101-20111231Texas"
sub(".*-(\\d{4}).*", "\\1", string)
#[1] "2011"

answered Aug 21 '19 at 00:37

Ronak Shah

377,200
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213

Extracting first four digits after a hyphen using stringr

3 Answers3

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