# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/###(?:[^,]*,){8}\K([\s\S]*?)(?=,[^,]*,[^,]*###)/"\1"/g;t' <<< " ###element1,element2,element3,element4,element5,element6,element7,element8, \"element9,
element9,\"\"element9\"\",element9,
element9,element9,\"\"element9\",element10,
###
###BLAHBLAH,BLAHBLAH,BLAHBLAH,BLAHBLAH,BLAHBLAH,BLAHBLAH,BLAHBLAH,BLAHBLAH,BLAHBLAH,
BLAHBLAH,
BLAHBLAH,
BLAHBLAH, BLAHBLAH,
BLAHBLAH, BLAHBLAH,
BLAHBLAH,
\"BLAHBLAH\"\"\",E,
###"
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for SED, please visit: https://www.gnu.org/software/sed/manual/html_node/The-_0022s_0022-Command.html