# If you'd like to omit non-matching lines from the result; add ';d' to the end of the expression.
sed -E 's/.*_([^.[:space:]]+).*/\1/gm;t' <<< "617912568590104527563-Congress-Dem-Packages_Nomination-DC2019-08-08.xlsx
517912568590504527553-Dem-Plans-Packages_Debate2019-08-08.xlsx
47912568590104527523-Congress-Dem-Packages_House2019-08-08 (1).xlsx"
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