# How to replace default missing reference symbol

Could one make the ? which appears for an unfound bibtex key into some other arbitrary string or symbol? I have a large document and it is slow searching through the ? to look for missing bibliographic references. (MWE below, but assume I'm using a referencing style which does not output [?] for missing references, but simply ?. I decided to keep the MWE simple however).

Alternatively if this is not possible, or if this is not wise, I'd be interested to learn why.

\documentclass{article}

\begin{document}

\section{Immanuel Kant}

\cite{Kint} said it all far better than I could.

\begin{thebibliography}{1}
\bibitem{Kant} Kant, I.\ \textit{My Booky Wook} 2004.
\end{thebibliography}
\end{document}

• That document gives a warning "LaTeX Warning: Citation Kint' on page 1 undefined on input line 7.". Instead of looking in the compiled document you can use these warnings. – pst Jul 15 '15 at 21:16
• Especially if the environment you use can parse those warnings. For example with AucTeX for Emacs you have to turn on option "Debug Warnings", and then TeXing your document the TeX-next-error command will put you at \cite{Kint} showing the warning in another buffer. – pst Jul 16 '15 at 16:18

You could basically use any symbol for this, I decided for a bunch of + characters, which do not occur very often and stored this to a command, which is then used in a redefined \@citex command, which is called by \cite.

The \@citex command is taken from latex.ltx, i.e. the LaTeX kernel. Of course, any package using a different \cite definition will break this 'hack'.

\documentclass{article}

\newcommand{\myunknownrefsymbol}{++++++}
\makeatletter

\def\@citex[#1]#2{\leavevmode
\let\@citea\@empty
\@cite{\@for\@citeb:=#2\do
{\@citea\def\@citea{,\penalty\@m\ }%
\edef\@citeb{\expandafter\@firstofone\@citeb\@empty}%
\if@filesw\immediate\write\@auxout{\string\citation{\@citeb}}\fi
\@ifundefined{b@\@citeb}{\hbox{\reset@font\bfseries \myunknownrefsymbol}%
\G@refundefinedtrue
\@latex@warning
{Citation \@citeb' on page \thepage \space undefined}}%
{\@cite@ofmt{\csname b@\@citeb\endcsname}}}}{#1}}
\makeatother
\begin{document}

\section{Immanuel Kant}

\cite{Kint} said it all far better than I could.

\cite{Kant}

\cite{Kont}

\cite{Einstein}

\begin{thebibliography}{1}
\bibitem{Kant} Kant, I.\ \textit{My Booky Wook} 2004.
\end{thebibliography}
\end{document}

• A less scary code for the novice: \usepackage{xpatch} \newcommand{\myunknownrefsymbol}{++++++} \makeatletter \xpatchcmd{\@citex} {\bfseries} {\bfseries\myunknownrefsymbol} {} {} \makeatother – Gonzalo Medina Jul 15 '15 at 21:45
• @GonzaloMedina: I've tried \xpatchcmd as I always did, but I used the \if@... which is useless in this case – user31729 Jul 15 '15 at 21:49