Description

The following example use an empty string as PAT₁, and `this' as PAT₂. Speaking in another way, we want to delete a blank line and the following line if it contains `this'.

Raw Input		Desired Output
keep this delete this keep this keep this delete this keep this keep this		keep this keep this keep this keep this keep this

Script and Comments

Script₁

[ 1] $!N
[ 2] /^\n.*this/d
[ 3] P
[ 4] D

Comments

Pattern Space is abbreviated to `PS'.
Step [1] will append the next line to PS.
After Step [1], PS contains two physical lines:
- if the first line is a blank one and the second one contains `this', they will be deleted via step [2] and then sed will start a new cycle;
- otherwise,
  - The first logical line in PS (i.e., the blank one) will be printed by command `P'([3]), then deleted by command `D'([4]);
  - Command `D' will make sed branch to Step [1] to examine the second logical line.