Regex matching everything that's not a 4 digit number

Question

I match and replace 4-digit numbers preceded and followed by white space with:

str12 <- "coihr 1234 &/()= jngm 34 ljd"
sub("\\s\\d{4}\\s", "", str12)
[1] "coihr&/()= jngm 34 ljd"

but, every try to invert this and extract the number instead fails. I want:

[1] 1234

does someone has a clue?

ps: I know how to do it with {stringr} but am wondering if it's possible with {base} only..

require(stringr)
gsub("\\s", "", str_extract(str12, "\\s\\d{4}\\s"))
[1] "1234"

Answer 1

regmatches(), only available since R-2.14.0, allows you to "extract or replace matched substrings from match data obtained by regexpr, gregexpr or regexec"

Here are examples of how you could use regmatches() to extract either the first whitespace-cushioned 4-digit substring in your input character string, or all such substrings.

## Example strings and pattern
x <- "coihr 1234 &/()= jngm 34 ljd"          # string with 1 matching substring
xx <- "coihr 1234 &/()= jngm 3444  6789 ljd" # string with >1 matching substring
pat <- "(?<=\\s)(\\d{4})(?=\\s)"

## Use regexpr() to extract *1st* matching substring
as.numeric(regmatches(x, regexpr(pat, x, perl=TRUE)))
# [1] 1234
as.numeric(regmatches(xx, regexpr(pat, xx, perl=TRUE)))
# [1] 1234


## Use gregexpr() to extract *all* matching substrings
as.numeric(regmatches(xx, gregexpr(pat, xx, perl=TRUE))[[1]])
# [1] 1234 3444 6789

(Note that this will return numeric(0) for character strings not containing a substring matching your criteria).

Answer 2

It's possible to capture group in regex using (). Taking the same example

str12 <- "coihr 1234 &/()= jngm 34 ljd"
gsub(".*\\s(\\d{4})\\s.*", "\\1", str12)
[1] "1234"