Sed : Replace pattern on every second occurence?

Question

Is there a way to tell sed to replace the pattern on every second occurrence? Or at least on every second line? (Sure it's possible with a script, but I was asking myself if sed can do it to).

Thanks.

Edit

I found

sed -e "s/pattern/replacement/g;n"

But it replaced every first occurence, not the second.

Answer 1

see http://stackoverflow.com/questions/5858200/sed-replace-every-nth-occurrence

The solution uses awk rather than sed, but "use the right tool for the job". It may or may not be possible to do in sed but, even if it is, it will be a lot easier in a tool like awk or perl.

Answer 2

sed 's/pattern/replacement/2'

Will replace the second occurrence on every line that has the pattern.

---

if you have GNU sed:

sed '1~2N ; s/pattern/replacement/2' Starting from line one 1, the line after it will be added to the pattern space N then the s command will replace the second occurrence of the pattern. then sed will move two lines down ~2 and repeat.

Answer 3

One symbol pattern within the one line: sed 's/(a[^a]*)a/\1c/g'

Every second pattern (only one pattern match per line may appear. if two or more matches per one line will appear, only the second one will be replaced)

echo -e "test\nreplace_me\ntest\ntest\nreplace_me\nreplace_me\nreplace_me" | \
sed '/replace_me/{:a;N;/replace_me.*replace_me/!Ta;s/replace_me/replaced/2;}'