Raw Input

Desired Output

1 PAT print this
2
3 --- print this
4 PAT print this
5 PAT print this
6
7 --- print this
8 PAT print this
9 PAT print this
10

1 PAT print this
3 --- print this
4 PAT print this
5 PAT print this
7 --- print this
8 PAT print this
9 PAT print this

Script and Comments

Script₁

[ 1] $!N
[ 2] /PAT/P
[ 3] D

Comments

A line must be printed if it contains PAT or the next one contains PAT.
The Pattern Space is abbreviated to `PS'.
If the current line is not the last one of the datafile, Step [1] will append the next line to PS.
After Step [1], PS have two lines in it. The regular expression PAT matches PS if
- the first line of PS contains PAT, or
- the second line of PS contains PAT, or
- both of them contain PAT.
In these cases, the first line of PS must be printed. That's why command `P' must be performed if PAT matches PS.
Command `D' of Step [3] deletes the first line of PS and then makes sed jump to Step [1].