bash - Extract text between two strings on different lines

Question

I have a big email file with the following random hosts:

......
HOSTS: test-host,host2.domain.com,
host3.domain.com,another-testing-host,host.domain.
com,host.anotherdomain.net,host2.anotherdomain.net,
another-local-host, TEST-HOST

DATE: August 11 2015 9:00
.......

The hosts are always delimited with a comma but they can be split on one, two or multiple lines (I can't control this, it's what email clients do, unfortunately).

So I need to extract all the text between the string "HOSTS:" and the string "DATE:", wrap it, and replace the commas with new lines, like this:

test-host
host2.domain.com
host3.domain.com
another-testing-host
host.domain.com
host.anotherdomain.net
host2.anotherdomain.net
another-local-host
TEST-HOST

So far I came up with this, but I lose everything that's on the same line with "HOSTS":

sed '/HOST/,/DATE/!d;//d' ${file} | tr -d '
' | sed -E "s/,s*/
/g"

Answer 1

Something like this might work for you:

sed -n '/HOSTS:/{:a;N;/DATE/!ba;s/[[:space:]]//g;s/,/
/g;s/.*HOSTS:|DATE.*//g;p}' "$file"

Breakdown:

-n                       # Disable printing
/HOSTS:/ {               # Match line containing literal HOSTS:
  :a;                    # Label used for branching (goto)
  N;                     # Added next line to pattern space
  /DATE/!ba              # As long as literal DATE is not matched goto :a
  s/.*HOSTS:|DATE.*//g; # Remove everything in front of and including literal HOSTS:
                         # and remove everything behind and including literal DATE 
  s/[[:space:]]//g;      # Replace spaces and newlines with nothing
  s/,/
/g;              # Replace comma with newline
  p                      # Print pattern space
}