Re: newlines in regular expressions

[tachyon]

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

Update

Updated as per sgifford's comments.

[sgifford]

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'

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.

[eyepopslikeamosquito]

perl -p -e 'BEGIN { undef $/ } s/\n{2,}/\n/g'

Which can be shortened to:

perl -p0777e 's/\n{2,}/\n/g'

See perldoc perlrun for details of perl's -0 option.

[tachyon]

It is as you say.

[tachyon]

perl -pi -e 's/^\s*\z//' file.txt deals with blank lines with other whitespace.