replace two newlines to one in shell command line

Question

There are lot of questions about replacing multi-newlines to one newline but no one is working for me.
I have a file:

first line
second line MARKER

third line MARKER
other lines

many other lines

I need to replace two newlines (if they exist) after MARKER to one newline. A result file should be:

first line
second line MARKER
third line MARKER
other lines

many other lines

I tried sed ':a;N;$!ba;s/MARKER\n\n/MARKER\n/g' Fail.
sed is useful for single line replacements but has problems with newlines. It can't find \n\n

I tried perl -i -p -e 's/MARKER\n\n/MARKER\n/g' Fail.
This solution looks closer, but it seems that regexp didn't reacts to \n\n.

Is it possible to replace \n\n only after MARKER and not to replace other \n\n in the file?
I am interested in one-line-solution, not scripts.

Answer 1

I think you were on the right track. In a multi-line program, you would load the entire file into a single scalar and run this substitution on it:

s/MARKER\n\n/MARKER\n/g

The trick to getting a one-liner to load a file into a multi-line string is to set $/ in a BEGIN block. This code will get executed once, before the input is read.

perl -i -pe 'BEGIN{$/=undef} s/MARKER\n\n/MARKER\n/g' input