"
1
2
3
<1...n # of lines content>
4
5
6
"

"<1..n # of lines content>"

s/^(?:.*\n){1,3}//;
s/(?:.*\n){1,3}\z//;

s/\n?\z/\n/;
s/^(?:.*\n){1,3}\z//;

s/^(?:.*\n){1,3}//;
s/(?:.*\n){0,2}.*\n?\z//;