Howto use sed to remove only triple empty lines?

Question

Howto use sed to remove only triple empty lines?

For example:

MyText.txt

line1

line2


line3



line4

with the use of sed i want the result to look like this
MyText.txt

line1

line2


line3
line4

I was able to delete double empty lines with

sed -i '/^$/{
N
/^\n$/D
}' MyText.txt

However my goal is to delete triple empty lines and only triple empty lines.

Any help would be much appreciated.

Answer 1

It's as simple as:

sed '1N;N;/^\n\n$/d;P;D'

Answer 2

It's not sed, but it's a whole lot shorter than what you can do with sed:

$ printf 'a\nb\n\nc\n\n\nd\n' | 
  perl -e 'undef $/; $_ = <>; s/\n\n\n/\n/g; print'
a
b

c
d