#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?s)\[((\w+)\b[^]]*)\]((?>(?!\[\2\b).|(?R))*)\[\/\2]"
Local $sString = "[html style="font-size: 18px;" dir="ltr"][div style="font-size: 18px;" dir="ltr"][p style="font-weight: bold;"]Hello,[/p][p]You have got a new message from [a href="https://www.example.com/"]Example.com[/a][br][br].You could check your message on [a href="https://www.example.com/en/manager/inbox.html"]Manager[/a][/p][p][img src="https://www.example.com/assets/images/logo-default-120x50.png" width="120px" height="80px"][div style="color: #D4192D; font-weight: bold;"]Example.com Team[/div][/p][/div][/html]" & @CRLF & _
"" & @CRLF & _
"[div style="font-size: 18px;" dir="ltr"][p style="font-weight: bold;"]Hello,[/p][p]You have got a new message from [a href="https://www.example.com/"]Example.com[/a][br][br].You could check your message on [a href="https://www.example.com/en/manager/inbox.html"]Manager[/a][/p][p][img src="https://www.example.com/assets/images/logo-default-120x50.png" width="120px" height="80px"][div style="color: #D4192D; font-weight: bold;"]Example.com Team[/div][/p][/div]" & @CRLF & _
"" & @CRLF & _
"You have got a new message from [a href="https://www.example.com/"]Example.com[/a][br][br].You could check your message on [a href="https://www.example.com/en/manager/inbox.html"]Manager[/a]"
Local $sSubst = "<$1>$3</$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