Replace every n'th occurrence in huge line in a loop

Question

I have this line for example:

1,2,3,4,5,6,7,8,9,10

I want to insert a newline (\n) every 2nd occurrence of "," (replace the 2nd, with newline) .

Answer 1

This might work for you (GNU sed):

sed 's/,/\n/2;P;D' file

Answer 2

If I understand what you're trying to do correctly, then

echo '1,2,3,4,5,6,7,8,9,10' | sed 's/\([^,]*,[^,]*\),/\1\n/g'

seems like the most straightforward way. \([^,]*,[^,]*\) will capture 1,2, 3,4, and so forth, and the commas between them are replaced with newlines through the usual s///g. This will print

1,2
3,4
5,6
7,8
9,10