x <- strsplit('dark_room',"_")
x[[1]][2]
[1] "room"

sub(".*_", "", x)

[1] "room"

library(stringr)
word('dark_room', -1, sep="_")
#[1] "room"

str_extract('dark_room', "[^_]+$")
#[1] "room"

sapply( strsplit('dark_room',"_"), tail, 1)
#[1] "room"

tail(scan(text = 'dark_room', what = "", sep="_"), 1)