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 after every 3 lines.

[STEP 101] # cat insert.sed
# add one more 'x' into the hold space
x
s/^/x/
t reset_t_cond
: reset_t_cond
# check if there are 3 'x' chars now
s/x\{3\}//
x
t insert
b

: insert
a\
INSERT HERE
[STEP 102] # seq 10 | sed -f insert.sed
1
2
3
INSERT HERE
4
5
6
INSERT HERE
7
8
9
INSERT HERE
10
[STEP 103] #