#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)(?:(START)(?=.*END)|\G(?!^))((?:(?!START|END)(?>\\+\"|[^\r\n\"]))*)\""
Local $sString = "This " is START an " example input END string """ & @CRLF & _
"START This is a "" second example END" & @CRLF & _
"This" is "a START third example END " "" & @CRLF & _
"" & @CRLF & _
"START This is a ""\""\\\\\""" test "" test\" second example END " and more """" END ""
Local $sSubst = "$1$2\\""
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