SHELL

bash sed with variable

# If you want to store your regex in a bash variable 
# and use it in sed, you might need the following
BASE_REGEX="[0-9]{4}.[0-9]+"
REGEX_ESCAPED=$(sed 's/[}{)(+]/&/g; s/^/^/g' <<< "$BASE_REGEX")

# One variable can be used with sed
sed -i "s/${REGEX_ESCAPED}/REPLACEMENT/g" file.txt
# The other can be used with bash internal regex operator
if [[ "2022.42" =~ ^${BASE_REGEX}$ ]]
then
	echo 'Matches !'
fi

sed with variable

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

sed with variables in shell script

Use double quotes for the sed expression.

sed "s/$replace_string/$replace_with/"`

passing bash variable to sed

rr=abcd # in shell

sed -i "s/aaa/$rr/g" file.dat 

# -i will update thefile,