Extract numbers that appear last in the string

This code extracts always the last numeric entries in a string:

(stringr::str_extract(asd, stringr::regex("(\\d+)(?!.*\\d)")))
"654"
(stringr::str_extract(asd1, stringr::regex("(\\d+)(?!.*\\d)")))
"65421"

If you want to get only the number when the very last character of the string is a number then you could implement a simple ifelse condition to check for that specifically, e.g.:

x<- c("asdf sfsfsd54 sdfsdfsdf sdfsdfsf654f")
ifelse(!is.na(as.numeric(substr(x, nchar(x), nchar(x)))), 
       (stringr::str_extract(x, stringr::regex("(\\d+)(?!.*\\d)"))),
       NA)
NA #returns NA because last entry of string is not numeric ("f")

A single regex is sufficient for your situation.

stringr::str_extract(asd, "(\\d+$)")

The $ anchors the capture group to the end of the string.

I would use sub combined with ifelse here:

x <- c("asdf sfsfsd54 sdfsdfsdf sdfsdfsf654", "abc", "123")
nums <- ifelse(grepl("\\d$", x), sub(".*?(\\d+)$", "\\1", x), "")
nums

[1] "654" ""    "123"

One solution which first splits the string based on whitespace, then gets the last substring and removes any letters. This should work as long as there is only letters and numbers in the strings.

library(stringr)

get_num = function(x) {
    str_remove_all(rev(unlist(str_split(x, " ")))[1], "[a-z]")
}

> get_num(asd)
[1] "654"
> get_num(asd1)
[1] "65421"

You can do something like this:

library(stringr)
val <- str_extract_all(asd1, "\\d+")[[1]]
tail(val, 1)
"65421"

OR

as.numeric(gsub("[^\\d]+", "", asd, perl=TRUE))
val <- regmatches(asd1, gregexpr("[[:digit:]]+", asd1))[[1]]
tail(val, 1)
"65421"

If string always ends with digits, then we can try gsub

> x <- c("asdf sfsfsd54 sdfsdfsdf sdfsdfsf654", "asdf sfsfsd54 sdfsdfsdf sdfsdfsf65421")

> gsub(".*\\D", "", x, perl = TRUE)
[1] "654"   "65421"

We can also use stri_extract_last_regex from stringi

> library(stringi)
> stri_extract_last_regex(asd, "\\d+")
[1] "654"
> stri_extract_last_regex(asd1, "\\d+")
[1] "65421"