#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "\[Link\][\s\S]*?\.pdf\s"
Local $sString = "[Link]" & @CRLF & _
"2017_07_11_RM_4b.pdf" & @CRLF & _
"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua."
Local $sSubst = ""
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