Regex expression: filter files since 1970? Year number encoded in the file name string

Question

I have a set of files for each months and year stretching from 1881-2021. Names are in format month/year and look like:

• 01_Jan/193501asc.gz
• 09_Sep/188209asc.gz
• 01_Jan/197501asc.gz
• 07_Jul/202107asc.gz

I wonder how to write a regex expression to filter only the files that more then 1970? (period 1970-2021?) I have tried:

file_ls <- list.files(paste(myPath, "data", sep = "/"), 
                          pattern = "[>1970]",
                          #pattern = "[1970-2021]",
                          #pattern="*\\.gz$", # ending character
                          recursive=TRUE)

Expected files to return (years in period 1970-2021):

• 01_Jan/197501asc.gz
• 07_Jul/202107asc.gz

Answer 1

I don't think a regex is the best way to go here. If you want to do some numerical filtering, I would just process the file_ls after you list all the *.gz. For example you could use:

s <- c("193501asc.gz", "188209asc.gz", "197501asc.gz", "202107asc.gz")

f <- function(x, y = 1970) {
  first4 <- substr(x, 1, 4)
  year <- as.numeric(first4)
  year >= y
}

s[f(s)]
#> [1] "197501asc.gz" "202107asc.gz"

^{Created on 2022-07-27 by the reprex package (v2.0.1)}