Regexps in emacs / sed - delete every other line of a file?

[Zorgoth]

Exactly as the title suggests, I am trying to find a way to delete every other line of a file using either emacs or sed. If you know how to do it in both emacs and sed I would appreciate it if you could show me both ways so I can understand regexps better.

[xsinsx]

simple enough.

Macintosh:Desktop stephenjackson$ cat text.txt
1
2
3
4
5
6
7
8
9
Macintosh:Desktop stephenjackson$ sed -n 'p;N' text.txt
1
3
5
7
9

[DaithiF]

Hi,
also, in gnu sed you can do:

sed '2~2d' testfile

where the pattern is firstline~step, ie. starting from line 2 delete every 2nd line

[Zorgoth]

I know little about sed so this may not be very elegant but to delete three of every four lines in a file



sed '1~4p;1~1d' /path/to/file > /path/to/modified/file


should work

[Zorgoth]

oops wrong thread

[jpkotta]

You don't need regexps to do what you want. (This is not tested very well.)



(defun delete-even-lines ()
(interactive)
(save-excursion
(beginning-of-buffer)
(while (not (eobp))
(forward-line 1)
(when (not (eobp))
(delete-region (line-beginning-position)
(1+ (line-end-position)))))))