I think sed (or ed) is not the right approach here, just implement the thing in your favorite scripting language, but if you want to see the disgusting thing I wrote:

─ cat fileA      
hello
world
how are you doing on this fine day
I am doing quite START fine 
what about you?
oh yeah whatever
blabla END
honestly i dont care i am a meanie beanie muhaahahaha
END
damn, another end huh
─ cat fileB
this is the 
new content
in between the tags :)
─ ed -s fileA
# This is all typed in, you can save it in a file and do ed -s < edscript
# Add newlines after STARTs and before ENDs
g/START/s/START/START\
/
g/END/s/END/\
END/
# Delete everything between the first START and the next END
1;/START/+1;/END/-1d
# Read in the other file in between
-1r fileB
# Get rid of newlines after STARTs and before ENDs
g/START/.;+1j
g/END/-1;+1j
w
q
─ cat fileA
hello
world
how are you doing on this fine day
I am doing quite STARTthis is the 
new content
in between the tags :)END
honestly i dont care i am a meanie beanie muhaahahaha
END
damn, another end huh

Anyways if you don't want to write your own script and would rather have a shell script with sed and such, you should probably do something similar to what I did:
1. add extra newlines similarly to what I did, but use some sed s/START/START\n/ (idk if that \n is correct)
2. use awk to filter out the unwanted content or perhaps split the file in pre-head and post-head parts, I'm not sure sed is able to store enough context to do it
3. cat pre-head new-head post-head to join them together
4. optionally if it's important, run a sed to remove extra newlines added in step #1 (for HTML head tags it doesn't matter)

CC: @apophis@kill-corporations.enterprises

I'm sitting in #1 and keep obnoxiously correcting him from "Open Source" to "Free Software" or from "Linux" to "GNU/Linux" until he moves to #2

screenshot #1 seems like someone doing tulpomancy or another kind of forceful "birthing" of new identities, I wouldn't know if it's helpful for them (it can be! if you're actually "discovering" hidden identities created or hidden by trauma or such) or if their life is too boring so they want to fuck it up

screenshot #2 seems like maybe it makes sense if it's a reply or a subpost to something

screenshot #3 is like 3 layers of "don't mention his name"

you do not know how many people i had to kill to become the #1 snac user

you do not know how many people i had to kill to become the #1 snac user

you do not know how many people i had to kill to become the #1 snac user

you may be best on fedi but don't forget i am #1 top rated 5 star snac user in the entire world (cannot be defeated)

you may be best on fedi but don't forget i am #1 top rated 5 star snac user in the entire world (cannot be defeated)

you may be best on fedi but don't forget i am #1 top rated 5 star snac user in the entire world (cannot be defeated)

i am #1 top rated 5 star snac user in the world (cannot be ban or warn)

i am #1 top rated 5 star snac user in the world (cannot be ban or warn)

i am #1 top rated 5 star snac user in the world (cannot be ban or warn)

#1 official source for top quality lemmy information

CC: @rat@brain.worm.pink

#1 official source for top quality lemmy information

CC: @rat@brain.worm.pink

#1 official source for top quality lemmy information

CC: @rat@brain.worm.pink