Remove newline depending on the format of the next line

Question

I have a special file with this kind of format :

title1
_1 texthere
title2
_2 texthere

I would like all newlines starting with \"_\" to be

Answer 1

This might work for you (GNU sed):

sed ':a;N;s/\n_/ /;ta;P;D' file

This avoids slurping the file into memory.

or:

sed -e ':a' -e 'N' -e 's/\n_/ /' -e 'ta' -e 'P' -e 'D' file