Andrew Dunstan <andrew@dunslane.net> writes:
>                       addlitchar(unescape_single_char(yytext[1]));
> +                     if (IS_HIGHBIT_SET(literalbuf[literallen]))
> +                         saw_high_bit = true;
Isn't that array subscript off-by-one?  Probably better to put the test
inside unescape_single_char(), anyway.
Otherwise it looks sane, though maybe shy a comment or so.
            regards, tom lane