Merge lines between two patterns using sed - sed

Question

I have an output file that looks like this:
HEADER 1:
server1 server2 server3
server4 server5 server6
server7 server8 server9
HEADER 2:
HEADER 1:
server10 server11 server12
server13 server14 server15
server16 server17 server18
HEADER 2:
I need to merge everything between the two headers so the updated output file looks like this:
HEADER 1:
server1 server2 server3 server4 server5 server6 server7 server8 server9
HEADER 2:
HEADER 1:
server10 server11 server12 server13 server14 server15 server16 server17 server18
HEADER 2:
The header names always remain constant.
I'm using UnxUtils in Windows, so unfortunately I don't have awk to do the manipulation. How do I go about it using sed?

Answer 1

Using sed:
sed '/HEADER 1/{n;:l N;/HEADER 2/b; s/\n//; bl}' input
n skips/prints the current line (HEADER 1), clears buffer
l is a label for looping (goto label)
N adds (appends) lines to buffer (preserving newlines)
/HEADER 2/b, where b is branch (without the label, it is break), break out when HEADER2 is seen
s/\n// removes the newlines in the buffer
bl jumps back to label l

Answer 2

This shows how it could be done using awk
You can download Gawk for Windows
awk '/HEADER/ {printf "\n%s\n",$0;next} {printf "%s ",$0}' file
HEADER 1:
server1 server2 server3 server4 server5 server6 server7 server8 server9
HEADER 2:
HEADER 1:
server10 server11 server12 server13 server14 server15 server16 server17 server18
HEADER 2: