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

Try following solution:

In sed the loop is done creating a label (:a), and while not match last line ($!) append next one (N) and return to label a:

:a
$! {
  N
  b a
}

After this we have the whole file into memory, so do a global substitution for each _ preceded by a newline:

s/\n_/ _/g
p

All together is:

sed -ne ':a ; $! { N ; ba }; s/\n_/ _/g ; p' infile

That yields:

title1 _1 texthere
title2 _2 texthere