regex - 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

You can use base R sub for this quite easily:

sub('.*(\d{4}).*', '\1', years1)

## [1] "2005" "2003" "1996"

The pattern to be matched here is .* (zero or more of any character) followed by \d{4} (four consecutive numerals, which we capture by enclosing in parentheses), followed by zero or more characters.

sub replaces the matched pattern with the value in the second argument. In this case, \1 indicates that we want to replace the whole matched pattern with the first captured substring (i.e. the four consecutive numerals).

Here regex is greedy, so it will bypass early matches of \d{4}, consuming them with .*. Only the last sequence of four consecutive numerals is captured.