ls <- do.call(c,lapply(sample(4:8,20,TRUE),function(i){
paste(sample(letters,i,TRUE),collapse = "")}))
ds <- do.call(c,lapply(1:20,function(i){
paste(sample(ls,1),i,sample(ls,1),"23",sep = "_")}))
str_extract(ds,"([0-9]+)")
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