Re^2: Adding back missing newlines between records

perl -pe 's/(?<!\n)\n# file/\n\n# file/g' record_file
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I don't see how that is supposed to work. The -p flag creates a while(<>) loop around the code specified for the -e flag(with print; as the last line in the while loop). The s/// operator in your code is going to operate on the $_ variable, and the diamond operator(<>) will assign each line in the file to $_ one line at a time.

As far as I can tell, at some point $_ will be equal to the string "# file\n", and the previous string will have been "hello world\n" (i.e. not "\n" as desired). Your regex is looking for "\n# file" preceded by a "\n". First, because it seems to me that the diamond operator will produce the line "# file\n", your regex won't match because there is no "\n# file" in that line. Second, it looks to me like you are doing a negative lookbehind beyond the start of the string. How is that supposed to work?

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Re^3: Adding back missing newlines between records by johngg (Canon) on Nov 06, 2009 at 11:29 UTC
I don't see how that is supposed to work. It's supposed to work because, as ikegami pointed out, you use the `-0777` switch to make the interpreter slurp the whole file in one go, the equivalent of undefining `$/` in a script. Thus, the global replace operates on a single string which is the whole file and the `while` implied by `-p` only iterates once. I hope this is helpful. Cheers, JohnGG	[reply] [d/l] [select]
Re^3: Adding back missing newlines between records by Anonymous Monk on Nov 06, 2009 at 11:27 UTC
Its the magick `-0777` option that sets input record separator, so instead of reading lines, it reads records of no more than `oct(0777)` (511) bytes, or if your platform doesn't have record oriented files, it reads the whole file.	[reply] [d/l] [select]
Re^4: Adding back missing newlines between records by 7stud (Deacon) on Nov 06, 2009 at 12:20 UTC
Ah. Sorry, I thought my browser was rendering something wrong when I saw -0777. Thanks.	[reply]
Re^4: Adding back missing newlines between records by 7stud (Deacon) on Nov 06, 2009 at 12:38 UTC
The -0 flag is described differently here: http://affy.blogspot.com/p5be/ch17.htm According to them, the -0 option is not the number of bytes to read. Instead, it's the character(in octal format) that is to be considered the end of a "line". It says 0777 is not a valid character, therefore it is never found in the file and your whole file gets slurped as one line. Is that accurate?	[reply]
Re^5: Adding back missing newlines between records by Anonymous Monk on Nov 06, 2009 at 12:42 UTC
My mistake, I was referencing $/ in perlvar, not perlrun	[reply]
Re^3: Adding back missing newlines between records by 7stud (Deacon) on Nov 06, 2009 at 12:06 UTC
The -p flag creates a while(<>) loop around the code specified for the -e flag(with print; as the last line in the while loop) Actually, that's not quite accurate. According to what I read, the while loop looks like this: `LINE: while (<>) { # your code goes here } continue { print or die "-p destination: $!\n"; }` [download] A continue block gets executed the instant before the loop condition is evaluated. So 'redo' does not cause the continue block to execute, but 'next' does, and a normal iteration of the loop causes the continue block to execute as well. This works for me: `perl -pe 'if($_ eq "\n"){$n=1;next;} if($n){$n=0;next;}else{s/# file/\ +n# file/;}' data1.txt` [download]	[reply] [d/l] [select]