#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m).*_([^.[:space:]]+).*"
Local $sString = "617912568590104527563-Congress-Dem-Packages_Nomination-DC2019-08-08.xlsx" & @CRLF & _
"517912568590504527553-Dem-Plans-Packages_Debate2019-08-08.xlsx" & @CRLF & _
"47912568590104527523-Congress-Dem-Packages_House2019-08-08 (1).xlsx"
Local $sSubst = "\1"
Local $sResult = StringRegExpReplace($sString, $sRegex, $sSubst)
MsgBox($MB_SYSTEMMODAL, "Result", $sResult)
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 AutoIt, please visit: https://www.autoitscript.com/autoit3/docs/functions/StringRegExp.htm