# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/^(?<!\) end).*?\(([^()]+?)\) end/\1/gm;t' <<< "There are some other sentences before (some otherwords which I am not interested in) all these.This is a sample string (something which I am interested in) end. This is another repeated string (with some otherwords) end."
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