https://help.libreoffice.org/Common/List_of_Regular_Expressions doesn't say anything about regexp lookaround patterns (see https://www.regular-expressions.info/lookaround.html).
Steps to Reproduce:
I'd love to add this information but https://help.libreoffice.org/Main_Page says there's no way to participate.
Lookaround patterns do in fact work
=SEARCH("(?<=a)b", "cab") // 3
User Profile Reset: No
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.89 Safari/537.36
help.libreoffice.org is not editable because it is a transformation of the local help system written in XML.
However, if you are willing to write about the issue, please just write in a LibreOffice Writer document, and attach to this bug. I'll review and convert it in XML for the help system, with your authorship.
Please be factual and concise, your contents will be translated into >60 languages.
Actually, LibreOffice's RegEx support is via ICU libraries, our documentation comes from there including coverage of ICU RegEx Operators to lookahead and lookbehind and their negatives--the Lookarounds
The Help now links to the Wiki https://wiki.documentfoundation.org/Documentation/HowTo/Writer/Regular_Expressions or to https://wiki.documentfoundation.org/Documentation/HowTo/Calc/Regular_Expressions both of which correctly defer to the ICU content .
It is a little thin, but authoritative, and should be basis for addition to our Help and Documentation--with some practical examples included, preferably on our Wiki. The OOo era content at AOO should probably go away.