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".
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"
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.
The end of string $ anchor asserts the position at the end of the string.
Saying, match exactly four digits at the end of the string. Unfortunately, what happens is that the digits try to get matched then the regex engine advances trying to assert that position and fails because there not at this position and consecutively backtracks trying to match them.
To fix this, you can greedily consume all characters until the last set of digits.
years1 <- c('2005-', '2003-', '1984-1992, 1996-')
unlist(str_extract_all(years1, perl('.*\\K\\d{4}')))
# [1] "2005" "2003" "1996"
来源：https://stackoverflow.com/questions/28622174/extract-last-4-digit-number-from-a-series-in-r-using-stringr