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

This inserts a line every 3 lines;

seq 1 10 | sed ': loop; n; n; a insert
n; b loop'

Producing

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

adjust the number of n; commands before the a command accordingly