#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)(([A-F][0-9]|[0-9]{2}|[0-9][A-F]|[A-F]{2})|(\?))(\s|)"
Local $sString = "81 F9 00 00 00 00 75 1D F3 0F 10 05 00 00 00 00 F3 0F 11 44 24 00 8B 44 24 18 35 00 00 00 00 89 44 24 0C EB 0B"
Local $sSubst = "\\x$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