multi-line RE question

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I have a file containing the following content:

Line1
Line2
Line3

XYZ: Blah
[download]

I'm trying to get:

Line1
Line2
Line3
[download]

Why does the following test case not work?

printf "Line1\nLine2\nLine3\n\nXYZ: Blah\n" | perl -p -e "s|\nXYZ.+\n|
+|m"
[download]

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Re: multi-line RE question by moritz (Cardinal) on Aug 02, 2011 at 11:35 UTC
`perl -p` reads line by line, there can only be one \n at most inside a line, at the end of the line. Your regex requires two \n characters to match. Changing the first `\n` to `^` is necessary for it to match. But it seems what you really want is something like `grep -v ^Blah` or `grep ^Line`. See grep. Perl 6 - second systems done right	[reply] [d/l] [select]
Re^2: multi-line RE question by Anonymous Monk on Aug 02, 2011 at 11:47 UTC
But it should also remove the empty last line.	[reply]
Re: multi-line RE question by Eliya (Vicar) on Aug 02, 2011 at 11:47 UTC
The regex option `m` here does not do what you think. First, it's not required to match/replace multiple newlines in a string. Secondly, it has no influence on the behavior of the readline (`<>`) in the implicit `while` loop behind the command line option `-p`. In other words, all you ever have in `$_` is one line of text... One way around the problem would be to set the input record separator to `undef` to slurp in the whole file at once `$ printf "Line1\nLine2\nLine3\n\nXYZ: Blah\n" \| perl -p -e "$/=undef; +s\|\nXYZ.+\n\|\|" Line1 Line2 Line3` [download] (You can also say `BEGIN { $/=undef } ...` if you want to have it execute only once at the expense of a few more keystrokes.) Whether that is a good solution depends on the circumstances, e.g. how large the input file is, etc. In case it's huge, you might want to read it in single line mode, and have previous lines accumulate in a buffer up until the search pattern is found. In this case, you can trim the trailing newline in the buffer before printing it out. `$ printf "Line1\nLine2\nLine3\n\nXYZ: Blah\n" \| perl -n -e '$buf.=$_; +if ($buf=~s\|\nXYZ.+\n\|\|) {print $buf; $buf=""}' Line1 Line2 Line3` [download]	[reply] [d/l] [select]
Re^2: multi-line RE question by jwkrahn (Abbot) on Aug 02, 2011 at 12:08 UTC
There is a simpler solution to setting $/ to undef as described in perlrun: `$ printf "Line1\nLine2\nLine3\n\nXYZ: Blah\n" \| perl -0777pe "s\|\nXYZ. ++\n\|\|" Line1 Line2 Line3` [download]	[reply] [d/l]
Re^3: multi-line RE question by Anonymous Monk on Aug 02, 2011 at 13:26 UTC
Wow, thanks. I try to understand the magic...	[reply]
Re^2: multi-line RE question by Anonymous Monk on Aug 02, 2011 at 11:50 UTC
Thank you! That solves my problem.	[reply]
Re: multi-line RE question by ww (Archbishop) on Aug 02, 2011 at 11:37 UTC
Perhaps you intended to stick to Perl... but what you posted doesn't seem to do so as the `printf` seems to be a nix-ish shell command. Try, instead, this (and excuse the windows shell syntax and avoidance of file ops): `C:\_wo>perl -e "my $line = \"Line1\nLine2\nLine3\n\nXYZ: Blah\n\"; $li +ne =~ s\|\nXYZ.+\n\|\|m; print $line;" Line1 Line2 Line3 C:\_wo>` [download] OTOH, if I misunderstood your question, please correct me.	[reply] [d/l] [select]