How can I include any content in the sed replace command?

I want to be able to handle any type of content stored into the bash variable ${CONTENT}, to be used as sed replacement text into another content, no matter if there are quotation marks, single quotes or other special characters that an HTML file with CSS can have, without needing to create temporary files. CONTENT=$(cat "${HTML_FILE}") HTML=$(cat "parent_file.html" | tr -d '\n' | sed -E "s/(]*>).*()/\1\n${CONTENT}\n\2/") But this will error out with an error like: sed: -e expression #1, char XXX: unterminated `s' command Is what I'm asking for even possible?