Extract last 4-digit number from a series in R using stringr

Question

I would like to flatten lists extracted from HTML tables. A minimal working example is presented below. The example depends on the stringr package in R. The first example exhibits the desired behavior.

years <- c("2005-", "2003-")
unlist(str_extract_all(years,"[[:digit:]]{4}"))

[1] "2005" "2003"

The below example produces an undesirable result when I try to match the last 4-digit number in a series of other numbers.

years1 <- c("2005-", "2003-", "1984-1992, 1996-")
unlist(str_extract_all(years1,"[[:digit:]]{4}$"))

character(0)

As I understand the documentation, I should include $ at the end of the pattern in order to request the match at the end of the string. I would prefer to match from the second example the numbers, "2005", "2003", and "1996".

Answer 1

The stringi package has convenient functions that operate on specific parts of a string. So you can find the last occurrence of four consecutive digits with the following.

library(stringi)

x <- c("2005-", "2003-", "1984-1992, 1996-")

stri_extract_last_regex(x, "\\d{4}")
# [1] "2005" "2003" "1996"

Other ways to get the same result are

stri_sub(x, stri_locate_last_regex(x, "\\d{4}"))
# [1] "2005" "2003" "1996"

## or, since these count as words
stri_extract_last_words(x)
# [1] "2005" "2003" "1996"

## or if you prefer a matrix result
stri_match_last_regex(x, "\\d{4}")
#      [,1]  
# [1,] "2005"
# [2,] "2003"
# [3,] "1996"