UCNUNSUPP

Message        An apparent UCN escape sequence was encountered, but
               UCNs are not supported in this language mode.  The
               backslash will be ignored.

Description    Universal Character Name (UCN) escape sequences were
               added to C in the C99 standard.  The language mode of
               the current compilation does not process UCNs, so
               they will be treated as unrecognized escape
               sequences, which ignore the backslash.

User Action    Compile in a mode that processes UCNs (C99, or the
               default "relaxed" mode), or remove the backslash.
               Relying on apparent escape sequences to be
               unrecognized is not good practice.