sed substitution including newlines

Question

I want to change a text file so that any line beginning with "Length:" is appended to the previous line.

I'm aware that sed '/\nLength:/ Length:/' isn't going to work because sed is line based.

Googling for "How to match newlines in sed" did turn up a complex sed method for joining a pattern to the next line but I couldn't figure out how to adapt it.

Help would be appreciated.

Answer 1

In awk you can use something like:

awk '/^/&&!/^Length/{printf "\n"}{printf "%s",$0}' infile

Will only print \n when line start ^ is matched. Exception: Length is found at that beginnig.