Narveson has asked for the wisdom of the Perl Monks concerning the following question:
My team was debating the idea of writing a large file using pipe characters to delimit the fields, and I pointed out that our data includes pipe characters. So the tilde was suggested, but I found some of those too.
So the obvious question arises, can I find a printable ASCII character that never appears in our big file?
I'd like to do this with just a single pass through the file. For locating a pipe character, all I had to say was
while (<>) {last if /\|/} printf "| seen at index %d in %s\n", index($_, '|'), $_;
Update: Should have mentioned that I'm going to run any executable solutions against a 2 GB file. So efficiency may be an issue.
Later: Thanks for the insights! I applaud almut's choice of an indicator array @seen, updated with $seen[ord $chr]++, which should be more efficient than the hash version $seen{$chr}++.
I think I will try kennethk's solution using \Q, both as written and then with the indicator array, as soon as my daughters are in bed and I can connect to my server again.
(Just a minute, sweetie, Daddy will be right with you ...)
It's true the indicator array doesn't offer a builtin keys function to list the seen items, but a grep should do just fine.