How do I insert a newline/linebreak after a line using sed

Question

It took me a while to figure out how to do this, so posting in case anyone else is looking for the same.

Answer 1

Another possibility, e.g. if You don't have an empty hold register, could be:

sed '/pattern/{p;s/.*//}' file

Explanation:
/pattern/{...} = apply sequence of commands, if line with pattern found,
p = print the current line,
; = separator between commands,
s/.*// = replace anything with nothing in the pattern register,
then automatically print the empty pattern register as additional line)