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"

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"