# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/\.zip\b[^,]*(*SKIP)(*F)|\s/%20/gm;t' <<< "http://abcd.com/old file/anotherfile.zip/some contents/sample.txt,http://abcd.com/new file/anotherfile.zip/some more contents/new sample.txt,http://abcd.com/newer file/another file.zip/some more contents/old sample.txt,http://abcd.com/newer file/another file.txt
"
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