RegExp syntax in "highlight items" stash syntax specification?

I want to highlight all items that do not has "unidentified" in their description

According to e.g. https://www.regextester.com/15 the sctring should look like

(^((?!unidentified).)*)

However it simply does not work at all.

however the expression !(unidentified) works

Where can I find full specification of REGEX syntax supported by game engine?

Or have I missed something in syntax?

The regex engine always performs a single line match
it's not possible to switch the poe regex engine to multiple row mode, typical (?m) \m doesn't do anything, (*CRLF) and so on doesn't match anything.

So you can't do multiline match, so it's not possible to exclude an item that has a line of specific text because in order to do that you have to capture the whole item text block.

This sound logical. Thanks.

Obviously if the question isn't about regex but rather about usability — you can always use "!" as literal NOT keyword.

!unidentified

works the same way as if that multiline regex would work.