#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)<h1>(.*?)<\/h1>((?!<h1>))*"
Local $sString = "<h1>Foo</h1><h2>Bar</h2><h1>Baz</h1><h3>Test</h3><h1>ghj</h1>zuio"
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