#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)((?:P2 x=\")(.*?[^\"]+)(?=.+((?:P3 x=\")(.*?[^\"]+))))"
Local $sString = "<P1 x="-0,36935" y="0,26315"/><P2 x="4,29731" y="0,26315"/><P3 x="5,29731" y="-0,40351"/><P4 x="-0,36935" y="-0,40351"/>" & @CRLF & _
"<P1 x="4,64065" y="0,26315"/><P2 x="5,97398" y="0,26315"/><P3 x="5,30731" y="-0,40351"/><P4 x="4,64065" y="-0,40351"/>"
Local $sSubst = "P2 x="\4"
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