Defining a replacement of a unicode character in lualatex

Question

In the past I asked a question about a replacement character for a unicode character (FFFD) in case of and xelatex (Defining a replacement of a unicode character), this works nicely. Now I need it for lualatex as well and found: Implementing \DeclareUnicodeCharacter in LuaLaTeX an XeLaTeX

So I tried to implement it (note there is no document yet as it crashes beforehand):

\documentclass{book}
\usepackage{ifluatex}

\newif\ifunicode
\ifluatex\unicodetrue\fi

\ifunicode
  \usepackage{fontspec}
  \usepackage{newunicodechar}
  \newcommand{\DeclareUnicodeCharacter}[2]{%
    \begingroup\lccode`|=\string"#1\relax
    \lowercase{\endgroup\newunicodechar{|}}{#2}%
  }
\else
  \usepackage[utf8]{inputenc}
\fi

\def\ucr{\adjustbox{width=\CodeWidthChar,height=\CodeHeightChar}{\stackinset{c}{}{c}{-.2pt}{%
   \textcolor{white}{\sffamily\bfseries\small ?}}{%
   \rotatebox{45}{$\blacksquare$}}}}

\ifunicode
  \DeclareUnicodeCharacter{FFFD}{\ucr}
\else
  % some other stuff
\fi

\begin{document}
\end{document}

though this gives the error message:

! String contains an invalid utf-8 sequence.
\newunicodechar #1#2->\if \relax \detokenize {#1}
                                                 \relax \nuc@emptyargerr \el...

l.23   \DeclareUnicodeCharacter{FFFD}{\ucr}

Any solution?

EDIT, based on @DavidCarlisle (used edit because of readability): I tried \newunicode{?}{\ucr} and get:

! Undefined control sequence. l.24 \newunicode {�}{\ucr}

(and with \newunicode{\ucr}{?} in get

! Undefined control sequence. l.25 \newunicode {\ucr}{�}.)

Answer 1

Like David Carlisle mentioned in a comment, \newunicodechar works for all engines. You use it like this:

\documentclass{book}
\usepackage{ifluatex}

\newif\ifunicode
\ifluatex\unicodetrue\fi

\ifunicode
\else
  \usepackage[utf8]{inputenc}
\fi
\usepackage{newunicodechar}

\def\ucr{X}

\newunicodechar{�}{\ucr}

\begin{document}
Hallo �.
\end{document}

The character U+FFFD is special through: LuaTeX uses it internally to mark invalid Unicode, so every time LuaTeX finds U+FFFD in your input, LuaTeX shows an error message. If you tell LuaTeX to continue, your document should still work.

This is a good example, why your source code should never contain characters like this. If you still want to do this, you can use a Lua callback to replace the U+FFFD character before LuaTeX sees it:

\documentclass{book}
\usepackage{ifluatex}
\usepackage{newunicodechar}

\ifluatex
\usepackage{luacode}
\else
  \usepackage[utf8]{inputenc}
\fi

\def\ucr{X}

\ifluatex
  \begin{luacode*}
    luatexbase.add_to_callback('process_input_buffer', function(buf)
      return buf:gsub(string.utfcharacter(0xFFFD), [[\ucr ]])
    end, 'replace U+FFFD')
  \end{luacode*}
\else
  \newunicodechar{�}{\ucr}
\fi

\begin{document}
Hallo �.
\end{document}