Perl match newline in `-0` mode - regex

Question
Suppose I have a file like this:
I've got a loverly bunch of coconut trees.
Newlines!
Bahahaha
Newlines!
the end.
I'd like to replace an occurence of "Newlines!" that is surrounded by blank lines with (say) NEWLINES!. So, ideal output is:
I've got a loverly bunch of coconut trees.
NEWLINES!
Bahahaha
Newlines!
the end.
Attempts
Ignoring "surrounded by newlines", I can do:
perl -p -e 's#Newlines!#NEWLINES!#g' input.txt
Which replaces all occurences of "Newlines!" with "NEWLINES!".
Now I try to pick out only the "Newlines!" surrounded with \n:
perl -p -e 's#\nNewlines!\n#\nNEWLINES!\n#g' input.txt
No luck (note - I don't need the s switch because I'm not using . and I don't need the m switch because I'm not using ^and $; regardless, adding them doesn't make this work). Lookaheads/behinds don't work either:
perl -p -e 's#(?<=\n)Newlines!(?=\n)#NEWLINES!#g' input.txt
After a bit of searching, I see that perl reads in the file line-by-line (makes sense; sed does too). So, I use the -0 switch:
perl -0p -e 's#(?<=\n)Newlines!(?=\n)#NEWLINES!#g' input.txt
Of course this doesn't work -- -0 replaces new line characters with the null character.
So my question is -- how can I match this pattern (I'd prefer not to write any perl beyond the regex 's#pattern#replacement#flags' construct)?
Is it possible to match this null character? I did try:
perl -0p -e 's#(?<=\0)Newlines!(?=\0)#NEWLINES!#g' input.txt
to no effect.
Can anyone tell me how to match newlines in perl? Whether in -0 mode or not? Or should I use something like awk? (I started with sed but it doesn't seem to have lookahead/behind support even with -r. I went to perl because I'm not at all familiar with awk).
cheers.
(PS: this question is not what I'm after because their problem had to do with a .+ matching newline).

Following should work for you:
perl -0pe 's#(?<=\n\n)Newlines!(?=\n\n)#NEWLINES!#g'

I think they way you went about things caused you to combine possible solutions in a way that didn't work.
if you use the inline editing flag you can do it like this:
perl -0p -i.bk -e 's/\n\nNewlines!\n\n/\n\nNEWLINES!\n\n/g' input.txt
I have doubled the \n's to make sure you only get the ones with empty lines above and below.

If the file is small enough to be slurped into memory all at once:
perl -0777 -pe 's/\n\nNewlines!(?=\n\n)/\n\nNEWLINES!/g'
Otherwise, keep a buffer of the last three lines read:
perl -ne 'push #buffer, $_; $buffer[1] = "NEWLINES!\n" if #buffer == 3 && ' \
-e 'join("", #buffer) eq "\nNewlines!\n\n"; ' \
-e 'print shift #buffer if #buffer == 3; END { print #buffer }'

Related

Perl script versus one-liner - differences in functionality with regex

I have a perl program that takes the STDIN (piped from another bash command). The output from the bash command is quite large, about 200 lines. I want to take the entire input (multiple lines) and feed that to a one-liner perl script, but so far nothing i've tried has worked. Conversely, if I use the following perl (.pl file):
#!/usr/bin/perl
use strict;
my $regex = qr/{(?:\n|.)*}(?:\n)/p;
if ( <> =~ /$regex/g ) {
print "${^MATCH}\n";
}
And execute my bash command like this:
<bash command> | perl -0777 try_m_1.pl
It works. But as a one-liner, it doesn't work with the same regex/bash command. The result of the print command is nothing. I've tried it like this:
<bash command> | perl -0777 -e '/{(?:\n|.)*}(?:\n)/pg && print "$^MATCH";'
and this:
<bash command> | perl -0777 -e '/{(?:\n|.)*}(?:\n)/g; print "$1\n";'
And a bunch of other things, too many to list them all. I'm new to perl and only want to use it to get regex output from the text. If there's something better than perl to do this (I understand from reading around that sed wouldn't work for this?) feel free to suggest.
Update: based on #zdim answer, I tried the following, which worked:
<bash command> | perl -0777 -ne '/(\{(?:\n|.)*\}(?:\n))/s and print "$1\n"'
I guess my regex needed to be wrapped in () and the { curly braces needed to be escaped.
A one-liner needs -n (or -p) to process input, so that files are opened, streams attached, and a loop set up. It still needs that even as the -0777 unsets the input record separator, so the file is read at once; see Why use the -p|-n in slurp mode in perl one liner?
That regex matches either a newline or any character other than a newline, and there is a modifier for that, /s, with which . matches newline as well. Then that need be inside curly braces, which you need to escape in newer Perls. The newline that follows doesn't need grouping.
So altogether you'd have
<bash command> | perl -0777 -ne'/(\{(.*)\}\n)/s and print "$1\n"'

Delete blank line before a pattern. What's wrong? Currently using Perl but open to sed/AWK

In a long document, I want to selectively delete the particular newlines that precede the exact string \begin{enumerate*}, ideally with a one-liner in bash or zsh.
That is, I want to convert test.tex:
Text in paragraphs.
More text
\begin{enumerate*} \item thing
to
Text in paragraphs.
More text \begin{enumerate*} \item thing
with a one-liner like
cat test.tex | perl -p -e 's/\n(?=(\\begin\{enumerate\*\}))/ /'
or
cat test.tex | perl -p -e 's/\n\\begin\{enumerate\*\}/\\begin{enumerate*}/'
but I must be missing something because it doesn't make any change.
I also clearly don't need a regular expression here. If there's a way to do this with exact string matching instead of regex, I'd rather use that way. For instance, in R I could do this with sub("\n\\begin{enumerate*}", "\\begin{enumerate*}", fixed = TRUE).
You can use the -0 (digit zero) switch with Perl to specify the line separator. Traditionally -0777 is used to read the entire file
You also need to be careful about regex metacharacters in your search string. Characters like *, {, } and \ mean something special within a regex pattern, and you should escape them — usually with a \Q ... \E construct
Taking these points into account, this should work for you
perl -0777 -pe' s/\n+(?=\Q\begin{enumerate*}\E)/ / ' myfile
perl -p processes a file string by string, so you can't expect this regex to match.
I would recommend something like
perl -e '$text = join "", <>; $text =~ s/your_regex_here//; print $text' test.txt
Mind that it loads the whole file to memory.
Also, if you want to modify file immediately, you can't just say > test.txt, see this question.
I found a solution with sed (number 25 on this page) that doesn't read the entire file into memory:
sed -i bak -n '/^\\begin{enumerate\*}/{x;d;};1h;1!{x;p;};${x;p;}' test.tex
The downside is that this doesn't actually join the two lines; instead it produces
Text in paragraphs.
More text
\begin{enumerate*} \item thing
which is good enough for what I need (latex treats single newlines the same as regular spaces)

pipe sed command to create multiple files

I need to get X to Y in the file with multiple occurrences, each time it matches an occurrence it will save to a file.
Here is an example file (demo.txt):
\x00START how are you? END\x00
\x00START good thanks END\x00
sometimes random things\x00\x00 inbetween it (ignore this text)
\x00START thats nice END\x00
And now after running a command each file (/folder/demo1.txt, /folder/demo2.txt, etc) should have the contents between \x00START and END\x00 (\x00 is null) in addition to 'START' but not 'END'.
/folder/demo1.txt should say "START how are you? ", /folder/demo2.txt should say "START good thanks".
So basicly it should pipe "how are you?" and using 'echo' I can prepend the 'START'.
It's worth keeping in mind that I am dealing with a very large binary file.
I am currently using
sed -n -e '/\x00START/,/END\x00/ p' demo.txt > demo1.txt
but that's not working as expected (it's getting lines before the '\x00START' and doesn't stop at the first 'END\x00').
If you have GNU awk, try:
awk -v RS='\0START|END\0' '
length($0) {printf "START%s\n", $0 > ("folder/demo"++i".txt")}
' demo.txt
RS='\0START|END\0' defines a regular expression acting as the [input] Record Separator which breaks the input file into records by strings (byte sequences) between \0START and END\0 (\0 represents NUL (null char.) here).
Using a multi-character, regex-based record separate is NOT POSIX-compliant; GNU awk supports it (as does mawk in general, but seemingly not with NUL chars.).
Pattern length($0) ensures that the associated action ({...}) is only executed if the records is nonempty.
{printf "START%s\n", $0 > ("folder/demo"++i)} outputs each nonempty record preceded by "START", into file folder/demo{n}.txt", where {n} represent a sequence number starting with 1.
You can use grep for that:
grep -Po "START\s+\K.*?(?=END)" file
how are you?
good thanks
thats nice
Explanation:
-P To allow Perl regex
-o To extract only matched pattern
-K Positive lookbehind
(?=something) Positive lookahead
EDIT: To match \00 as START and END may appear in between:
echo -e '\00START hi how are you END\00' | grep -aPo '\00START\K.*?(?=END\00)'
hi how are you
EDIT2: The solution using grep would only match single line, for multi-line it's better use perl instead. The syntax will be very similar:
echo -e '\00START hi \n how\n are\n you END\00' | perl -ne 'BEGIN{undef $/ } /\A.*?\00START\K((.|\n)*?)(?=END)/gm; print $1'
hi
how
are
you
What's new here:
undef $/ Undefine INPUT separator $/ which defaults to '\n'
(.|\n)* Dot matches almost any character, but it does not match
\n so we need to add it here.
/gm Modifiers, g for global m for multi-line
I would translate the nulls into newlines so that grep can find your wanted text on a clean line by itself:
tr '\000' '\n' < yourfile.bin | grep "^START"
from there you can take it into sed as before.

Perl regex: remove everything (including line breaks) until a match is found

Apologies for the simple question. I don't clean text or use regex often.
I have a large number of text files in which I want to remove every line until my regex finds a match. There's usually about 15 lines of fluff before I find a match. I was hoping for a perl one-liner that would look like this:
perl -p -i -e "s/.*By.unanimous.vote//g" *.txt
But this doesn't work.
Thanks
Solution using the flip-flop operator:
perl -pi -e '$_="" unless /By.unanimous.vote/ .. 1' input-files
Shorter solution that also uses the x=!! pseudo operator:
per -pi -e '$_ x=!! (/By.unanimous.vote/ .. 1)' input-files
Have a try with:
If you want to get rid until the last By.unanimous.vote
perl -00 -pe "s/.*By.unanimous.vote//s" inputfile > outputfile
If you want to get rid until the first By.unanimous.vote
perl -00 -pe "s/.*?By.unanimous.vote//s" inputfile > outputfile
Try something like:
perl -pi -e "$a=1 if !$a && /By\.unanimous\.vote/i; s/.*//s if !$a" *.txt
Should remove the lines before the matched line. If you want to remove the matching line also you can do something like:
perl -pi -e "$a=1 if !$a && s/.*By\.unanimous\.vote.*//is; s/.*//s if !$a" *.txt
Shorter versions:
perl -pi -e "$a++if/By\.unanimous\.vote/i;$a||s/.*//s" *.txt
perl -pi -e "$a++if s/.*By\.unanimous\.vote.*//si;$a||s/.*//s" *.txt
You haven't said whether you want to keep the By.unanimous.vote part, but it sounds to me like you want:
s/[\s\S]*?(?=By\.unanimous\.vote)//
Note the missing g flag and the lazy *? quantifier, because you want to stop matching once you hit that string. This should preserve By.unanimous.vote and everything after it. The [\s\S] matches newlines. In Perl, you can also do this with:
s/.*?(?=By\.unanimous\.vote)//s
Solution using awk
awk '/.*By.unanimous.vote/{a=1} a==1{print}' input > output

How do I replace multiple newlines with a single one with Perl's Regular Expressions?

I've got a document containing empty lines (\n\n). They can be removed with sed:
echo $'a\n\nb'|sed -e '/^$/d'
But how do I do that with an ordinary regular expression in perl? Anything like the following just shows no result at all.
echo $'a\n\nb'|perl -p -e 's/\n\n/\n/s'
You need to use s/^\n\z//. Input is read by line so you will never get more than one newline. Instead, eliminate lines that do not contain any other characters. You should invoke perl using
perl -ne 's/^\n\z//; print'
No need for the /s switch.
The narrower problem of not printing blank lines is more straightforward:
$(input) | perl -ne 'print if /\S/'
will output all lines except the ones that only contain whitespace.
The input is three separate lines, and perl with the -p option only processes one line at time.
The workaround is to tell perl to slurp in multiple lines of input at once. One way to do it is:
echo $'a\n\nb' | perl -pe 'BEGIN{$/=undef}; s/\n\n/\n/'
Here $/ is the record separator variable, which tells perl how to parse an input stream into lines.