## Create an example file
fnm <- "foo.txt" # file name
sink(fnm)
cat("Hellon## ----worldn")
sink()

## Read the file 'fnm' one line at a time and write it back to 'fnm'
## if it does *not* contain the pattern 'pat'
pat <- "## ----" # pattern
while(TRUE) {
    rcon <- file(fnm, "r") # read connection
    line <- readLines(rcon, n = 1) # read one line
    close(rcon)
    if(length(line) == 0) { # end of file
        break
    } else {
        if(!grepl(pat, line)) {
            wcon <- file(fnm, "w")
            writeLines(line, con = wcon)
            close(wcon)
        }
    }
}

## Read the file 'fnm' one line at a time and write it back to 'fnm'
## if it does *not* contain the pattern 'pat'
pat <- "## ----" # pattern
rcon <- file(fnm, "r") # read connection
wcon <- file(fnm, "w") # write connection
while(TRUE) {
    line <- readLines(rcon, n = 1) # read one line
    if(length(line) == 0) { # end of file
        break
    } else {
        if(!grepl(pat, line)) {
            writeLines(line, con = wcon)
        }
    }
}
close(rcon)
close(wcon)