# How to customize the style of citekey if the bib item does not exist

The is a MWE:

\documentclass{article}
\usepackage[]{biblatex}

\begin{document}
test \autocite{not-exist-bib-citekey}
\end{document}


In the resulting pdf, you can see that "not-exist-bib-citekey" is in bold font.

I want to style it, for example, with red color, so as to make it more distinct.

Following Wanted: question mark instead of bibtex key for missing citations (biblatex) the macro \abx@missing@entry was introduced. \abx@missing@entry is used to typeset missing entry keys, it defaults to \abx@missing, which is used for all kinds of missing things (mainly bibstrings).

So if you only want to influence missing entry keys, go with

\documentclass{article}
\usepackage{xcolor}
\usepackage{biblatex}

\makeatletter
\protected\def\abx@missing@entry#1{%
\mbox{\reset@font\color{red}??#1??}}
\makeatother

\begin{document}
test \autocite{missing-key}
\end{document}


For most intents and purposes, this will give the same results as changing \abx@missing and one may even want to make missing bibstrings and other similar issues pop with red colour as well...

BibLaTeX uses \abx@missing to print missing citation entries. You just need to redefine it:

\documentclass{article}
\usepackage{xcolor}
\usepackage{biblatex}