definecolorset of xcolor --- problem with color values starting with "f"?

Question

I am trying to write a script (that will be easy) to create a xkcd color set for the package xcolor. Look at this MWE:

\documentclass{article}
\usepackage{xcolor} %
\definecolorset{HTML}{xkcd}{}{%
    OneColor,fcc2d9;%
    LastColor,56ae57%
}
\begin{document}    
\textcolor{xkcdOneColor}{OneColor}
\textcolor{xkcdLastColor}{LastColor}
\end{document}

...this will fail with:

This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013/Debian)
[...]
! Missing number, treated as zero.
<to be read again> 
                   f
l.6 }

? 

If I change the first f in OneColor to a, for example:

\definecolorset{HTML}{xkcd}{}{%
    OneColor,acc2d9;%
    LastColor,56ae57%
}

...now it works ok.

I understand I can circumvent this easily (seems that uppercase F does not create any problem), but I am curious: is it a bug or a driver (/me) error?

Answer 1

It's a bug in xcolor.sty, where we find

283 \def\@hex@@Hex#1%
284  {\if a#1A\else \if b#1B\else \if c#1C\else \if d#1D\else
285   \if e#1E\else \if f#1f\else #1\fi\fi\fi\fi\fi\fi \@hex@Hex}

and it's clear that it should be \if f#1F. So no f gets converted to F, which is expected when hexadecimal digits conforming to TeX's syntax are later needed (as arguments for \number").

Indeed the input

\documentclass{article}
\usepackage{xcolor}

\makeatletter
\def\@hex@@Hex#1%
 {\if a#1A\else \if b#1B\else \if c#1C\else \if d#1D\else
  \if e#1E\else \if f#1F\else #1\fi\fi\fi\fi\fi\fi \@hex@Hex}
\makeatother

\definecolor{xk}{HTML}{fcc2d9}
\definecolor{XK}{HTML}{FCC2D9}

\begin{document}

A \textcolor{xk}{BCD} E

A \textcolor{XK}{BCD} E

\end{document}

produces the expected

Of course, expressing the HTML numbers in uppercase will circumvent the bug, too.

---

A possibly better patch is using etoolbox, that might be used in packages or classes (with \RequirePackage{etoolbox} in this case).

\documentclass{article}
\usepackage{xcolor}
\usepackage{etoolbox}
\makeatletter
\patchcmd{\@hex@@Hex}{f\else}{F\else}{\typeout{Patching xcolor}}{}
\makeatother

\definecolor{DHBW-blue}{HTML}{026f93}

\begin{document}
Hello World
\end{document}