How to replace part of a text file between markers with another text file?

Question

Say I have a text file like this:

# custom content section
a
b

### BEGIN GENERATED CONTENT
c
d
### END GENERATED CONTENT

I'd like to replace the portion between the GENERATED CONTENT tags with the contents of another file.

What's the simplest way to do this?

Answer 1

lead='^### BEGIN GENERATED CONTENT$'
tail='^### END GENERATED CONTENT$'
sed -e "/$lead/,/$tail/{ /$lead/{p; r insert_file
        }; /$tail/p; d }"  existing_file

Answer 2

newContent=`cat new_file`
perl -0777 -i -pe "s/(### BEGIN GENERATED CONTENT\\n).*(\\n### END GENERATED CONTENT)/\$1$newContent\$2/s" existing_file

Answer 3

Warning: This is definitely not the simplest way to do it. (EDIT: bash works; POSIX grep is fine too)

If the main text is in file "main" and the generated content is in file "gen", you could do the following:

#!/bin/bash
BEGIN_GEN=$(cat main | grep -n '### BEGIN GENERATED CONTENT' | sed 's/\(.*\):.*/\1/g')
END_GEN=$(cat main | grep -n '### END GENERATED CONTENT' | sed 's/\(.*\):.*/\1/g')
cat <(head -n $(expr $BEGIN_GEN - 1) main) gen <(tail -n +$(expr $END_GEN + 1) main) >temp
mv temp main