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

Hi, I just hit the issue somewhere I wouldn't expect it. Let's say I want to remove new lines from file, but only some of them. It's obvious to me how to get rid of them all, and it's also what is described everywhere. But while it's easy to replace all Us with Os and all new lines with 'bla', how do I remove new lines from lines starting with U only? I tried following without any success:

perl -pe 's/\nU/U/g' /tmp/nejms.txt
perl -pe 's/^U/U/g' /tmp/nejms.txt

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Replies are listed 'Best First'.
Re: Remove SOME new lines
by hippo (Archbishop) on Jun 29, 2016 at 09:28 UTC

    The -p flag treats the file one line at a time so your first regex will never match. Overcome this with -0:

    $ cat in
foo
bar
Ubar
Ubaz
quux
$ perl -p0e 's/\nU/U/g' in
foo
barUbarUbaz
quux
$

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[d/l]
      Oh boy, and I thought I was looking everywhere. Thanks a lot!
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Re: Remove SOME new lines
by haukex (Archbishop) on Jun 29, 2016 at 09:29 UTC

    Hi RenMcCourtey,

    Is this what you're looking for?

    $ cat in.txt 
AaX
ObU
UcU
XdR
ZeZ
UfH
UgU
FhA
OiG
UjX
UkZ
UlZ
xmT
$ perl -pe 'chomp if /^U/' in.txt 
AaX
ObU
UcUXdR
ZeZ
UfHUgUFhA
OiG
UjXUkZUlZxmT

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    Update: Explanation: When using the -p switch without the -l switch (perlrun), each line read into $_ still contains the newline, which is normally output when the lines are printed back out. So the chomp just chops the newline off, and those lines are printed without the final newline.

    Hope this helps,
    -- Hauke D

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Re: Remove SOME new lines
by BrowserUk (Patriarch) on Jun 29, 2016 at 09:30 UTC

    Add /s to your substitution:

    $s = "fred\nundone\nfred\nunknown\nfred\nuncaring\n";;
$t = $s; $s =~ s[\nu][u]g; print $t;;
fred
undone
fred
unknown
fred
uncaring

$t = $s; $s =~ s[\nu][u]sg; print $t;;
fredundone
fredunknown
freduncaring

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Re: Remove SOME new lines
by RenMcCourtey (Acolyte) on Jun 29, 2016 at 12:17 UTC
    And why this perl -p0e 's/\n^\s*$//g' nejms.txt doesn't remove blank lines? :-(
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[d/l]

      Hi RenMcCourtey,

      Because by default ^ and $ only match at the beginning and end of the string, not each line within the string (I'm simplifying a little here - for the details see ^, $, and the /m modifier in perlre). This works:

      perl -p0e 's/\n^\s*$//mg' input.txt

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      But requires you to read the entire file* into memory. I'd use:

      perl -ne 'print if /\S/' input.txt

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      Hope this helps,
      -- Hauke D

      * assuming it doesn't contain NUL characters

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        Oh, thank you. I guess I'll have to start completely new question, being more descriptive.
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