How to replace the nth occurrence of a string using sed

Question

Is there any way to replace the nth occurrence of a string in a file using sed?

I'm using sed -i '0,/jack.*/ s//jill/' to replace the first occurrence.

How can i change it so that it replaces the nth occurrence?

My file contents the following lines:

first line
second line
third line
jack=1
fifth line
jack=
seventh line

I don't know the value after jack=, it can be anything or nothing.

I want to replace the 2nd occurrence of jack= and anything that follows it with jill.

Answer 1

First replace all the newlines with a unique character that does not occur anywhere else in your file (e.g. ^) using tr. You need to do this in order to create a single string for sed. Then pass it to sed and tell it to replace the nth occurrence of your string. Finally, pass the output back through tr to recreate the newlines.

For n=2, the command is:

$ tr '\n' '^' < file | sed 's/jack/jill/2' | tr '^' '\n'
first line
second line
third line
jack
fifth line
jill
seventh line

Update:

It can also be done with sed, WITHOUT changing the newlines first, using the following command:

$ sed ':a;N;$!ba;s/jack/jill/2' file

Alternatively, use awk:

$ awk '/jack/{c+=1}{if(c==2){sub("jack","jill",$0)};print}' file