sed: insert a string after every N lines

Question

I want to insert a string after every 30 lines in my large file. I\'m using mini-sed, which doesn\'t support ~ (tilde) range operator. I\'m looking for sed-only solution please.

Answer 1

sed 'n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;n;s/$/\ 
string/' filename