Use double quotes to make the shell expand variables while preserving whitespace:

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sed -i "s/$var1/ZZ/g" "$file"

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

#on my mac

sed -i -e 's/old-text/new-text/g' text.txt

Just concatenate var '"$var"' to replace values string in sed command:

$ sed -i 's/string_to_replace/'"$var"'/g' file_to_replace.txt

Just use one of following commands for replacing a number of:

sed "s/[0-9]//" f.txt   #One single digit as 1

sed "s/[0-9]\+//" f.txt #Further digits as 837, specifying match repetition "\+"

sed "s/[0-9]\+\.[0-9]\+]//" file.txt #Float type with decimals

sed -i "s/$var1/ZZ/g" "$file"

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