can you use a string as a replacement with sed code example

Example 1: sed replace in file

sed -i 's/foo/bar/g' hello.txt

Example 2: sed replace all occurances?

echo dog dog dos | sed -e 's:dog:log:g'

#You can also use the above answer with appropriate slashes like
echo "grepper is a nice grepper" | sed "s/grepper/blow/g"