#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)([^/\n]+)/(?=[^/\n]+$)"
Local $sString = "https://d2ynliea65eb6o.cloudfront.net/6100052500-STXMLOPEN/sub_1.m3u8" & @CRLF & _
"https://d2ynliea65eb6o.cloudfront.net/6100052499-STXMLOPEN/sub_1.m3u8" & @CRLF & _
"https://d2ynliea65eb6o.cloudfront.net/6100052498-STXMLOPEN/sub_1.m3u8"
Local $sSubst = "$1/$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