#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?s)<body>\K\s*<tag(?:(?!<inner_block=25>).)+</tag>"
Local $sString = "<body>" & @CRLF & _
"<tag>" & @CRLF & _
"<number=1>" & @CRLF & _
"<inner_block=5>" & @CRLF & _
"</tag>" & @CRLF & _
"<tag>" & @CRLF & _
"<number=12>" & @CRLF & _
"<inner_block=15>" & @CRLF & _
"</tag>" & @CRLF & _
"<tag>" & @CRLF & _
"<number=21>" & @CRLF & _
"<inner_block=25>" & @CRLF & _
"</tag>" & @CRLF & _
"<tag>" & @CRLF & _
"<number=29>" & @CRLF & _
"<inner_block=35>" & @CRLF & _
"</tag>" & @CRLF & _
"</body>"
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