#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)\A(?:.*\R){15}\K\X+"
Local $sString = "1" & @CRLF & _
"2" & @CRLF & _
"3" & @CRLF & _
"4" & @CRLF & _
"5" & @CRLF & _
"6" & @CRLF & _
"7" & @CRLF & _
"8" & @CRLF & _
"9" & @CRLF & _
"10" & @CRLF & _
"11" & @CRLF & _
"12" & @CRLF & _
"13" & @CRLF & _
"14" & @CRLF & _
"15" & @CRLF & _
"16" & @CRLF & _
"17" & @CRLF & _
"18" & @CRLF & _
"19" & @CRLF & _
"20"
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