a <- paste("string", 9:1, ".txt", sep = "")
nchar(a)
strsplit(a, "\\.")
substr(a, 1, 6) <- "Substr"; a
match("Substr9.txt", a)

a <- paste("string", 9:1, ".txt", sep = "")
grep("([1,3,5-7])", a)
b <- paste("string", 9:1, ".txt", sep = "", collapse = " ")
gsub("([1,3,5-7]).(txt)", "_\\1-\\2", b)