"Pcrecompat man page" says:
5. The following Perl escape sequences are not supported: \l, \u, \L, \U, and \N. In fact these are implemented by Perl's general string-handling and are not part of its pattern matching engine. If any of these are encountered by PCRE, an error is generated.
\N semantics is described in "pcrepattern man page" and this escape sequence is not generate an error.
Thanx. --## List details at https://lists.exim.org/mailman/listinfo/pcre-dev
