Oh, ddrudik, I just now realized what you meant ... ya, substrings would work perfectly fine.

But the problem is that the whole file itself is too large to hold in memory into a single concatenated string in the first place.

Sure - I can use perl to simply chomp each input line and then >> the lines to an empty file - in that way I will get 1 big file with one big line. But I am not sure how I could do a substring on that line after this?

I was just hoping - since grep itself can handle such large files, I could just include a "\n" with the indices as a pattern and just use it on my original file itself.