We can use str_extract_all by specifying the pattern as one or more number ([0-9]+). The output will be a list of length 1, extract the vector with [[ and convert to numeric.
library(stringr)
as.numeric(str_extract_all(string, "[0-9]+")[[1]])
#[1] 2016 81 64 2017 18 36
If we are using strsplit, split by the non-numeric characters
as.numeric(strsplit(string, "\\D+")[[1]][-1])
#[1] 2016 81 64 2017 18 36
