How to split long sed expression into multiple lines?

sed 's'/\
'[long1]'\
'[long2]'\
'/'\
'[long3]'\
'[long4]'\
'/' file.txt

Splitting on several lines with backslash does work if new lines are not indented.

$ echo "a,b" | sed 's/\(.'\
> '\),\(.\)/\2-\1/'
b-a

Tested on Cygwin with GNU sed 4.2.2

Another aspect:

How maintainable are the [long] items above?

The shorter and more distinct you can keep the clauses, the easier they will be to change and enhance as time passes and (sed-external) facts change around the the function they provide.

e.g.

sed \
-e 's/[minimal-s1]/[minimal-r1]/' \
-e 's/[minimal-s2]/[minimal-r2]/' \
-e 's/[minimal-s3]/[minimal-r3]/' \
-e 's/[minimal-s4]/[minimal-r4]/' 

... might be something to strive for and prefer.

You can save long string to bash variables, then use in sed command:

string=[long1][long2]
replace=[long3][long4]
sed -e 's/'"$string"'/'"$replace"'/' file

If you can use perl, you can break long pattern with x modifier:

perl -e 's/
[long1]
[long2]
/[long3][long4]/x' file