sed for print two different word alernatively

I have a requirement to print two different words in alternative white spaces in the file.

For example,

ABCD
EFGH
IGKL
MNOP

The above scenario, I want print ab and /ab alternatively like below:

ab
ABCD
/ab
ab
EFGH
/ab
ab
IGKL
/ab
ab
MNOP
/ab

*I want this one by one in a line by line format(Not horizontal format).*I know sed 's|^[[:blank:]]*$|</ab>|' this command is almost near to my case. But I don’t know how to apply this. Please, someone, help me.

Answer

If you are ok with awk then following may help you here.

awk -v start="ab" -v end="/ab" '{print start ORS $0 ORS end}' Input_file

In case you need to save output into Input_file itself then append > temp_file && mv temp_file Input_file in above code too.