newlines in regular expressions

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

I have a file in which I am trying to parse out any blank lines with a subsitition. The problem is I'm not sure how to get rid of the blank lines.
the file looks something like this

#fred
#barney


#wilma
#betty
[download]

So how can I get rid of the blank line(s)in the file when they are just a newline within the text file. Thanks

Comment on newlines in regular expressions Download Code

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Re: newlines in regular expressions by tachyon (Chancellor) on Dec 07, 2004 at 06:29 UTC
The keys as it were are to note that \n\n is a blank line or if you read a file line by line then a blank line will match m/^\s$/ ~~perl -pi.bak -e 's/\n{2,}/\n/' file.txt~~ `perl -pi -e 's/^\s\z//' file.txt perl -ne 'print unless m/^\s*$/' file.txt > noblanks.txt` [download] Update Updated as per sgifford's comments. cheers tachyon	[reply] [d/l]
Re^2: newlines in regular expressions by sgifford (Prior) on Dec 07, 2004 at 06:57 UTC
A few nits: The first answer requires that Perl read in slurp mode, and needs a `g` modifier to replace all occurences: `perl -p -e 'BEGIN { undef $/ } s/\n{2,}/\n/g'` [download] Also, the two examples behave differently in the face of lines that contain only blank characters, like space and tab. The second will delete them, while the first will leave them intact. You can make the second leave them intact by using `m/^$/`; to make the first delete whitespace-only lines I think `s/\n\s*\n/\n/g` will work, but I haven't thought it through yet.	[reply] [d/l] [select]
Re^3: newlines in regular expressions by eyepopslikeamosquito (Bishop) on Dec 07, 2004 at 07:22 UTC
`perl -p -e 'BEGIN { undef $/ } s/\n{2,}/\n/g'` [download] Which can be shortened to: `perl -p0777e 's/\n{2,}/\n/g'` [download] See `perldoc perlrun` for details of perl's `-0` option.	[reply] [d/l] [select]
Re^3: newlines in regular expressions by tachyon (Chancellor) on Dec 07, 2004 at 12:09 UTC
`perl -pi -e 's/^\s*\z//' file.txt` deals with blank lines with other whitespace.	[reply] [d/l]
Re^3: newlines in regular expressions by tachyon (Chancellor) on Dec 07, 2004 at 11:59 UTC
It is as you say.	[reply]
Re: newlines in regular expressions by conrad (Beadle) on Dec 07, 2004 at 10:29 UTC
None of the non-`<>-print` solutions I see here will cope with a single opening blank line since they all search for multiple `\n` characters. This does the trick using the `/m` modifier, which allows `^` to match the beginning of a line anywhere in your string: `perl -p0777e 's/^\n+//mg'`	[reply] [d/l] [select]
Re: newlines in regular expressions by Crian (Deacon) on Dec 07, 2004 at 10:18 UTC
Perhaps it could be useful (in case that these lines are not really empty because they contain whitespace) to use a script like this: `while (<>) { print unless m~^\s*\n$~; }` [download] This is surely not the most elegant solution, but it should do the task.	[reply] [d/l]

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