Inital letter of chapter title as title background with KOMA-Script

Question

I’m trying to recreate the following style for chapter titles for scrbook.

I tried to implement this by redefining \chapterlinesformat and using the package xstring to extract the first character of the chapter title. I managed to cobble together the code below, but it gives the following error:

mwe.tex|22 error| Missing number, treated as zero. 

If one replaces #3 in \firstchar{#3} by an explicit text, then the code works and gives the desired result. I suspect that the command \firstchar doesn’t work because the argument #3 of \chapterlinesformat is already formatted. But I don’t know how to circumvent this problem, or how to approach the task differently.

\documentclass{scrbook}
\usepackage{xcolor}
\usepackage{xstring}
% getting the first character of a string using xparse
\newcommand{\firstchar}[1]{\StrChar{#1}{1}}
\renewcommand{\chapterlinesformat}[3]{%
  \makebox(0,20)[l]{%
      \hspace{-20pt}%
      \fontsize{60}{6}%
      \selectfont%
      \color{black!15!white}%
      \firstchar{#3}%
    }%
  #3
}
\begin{document}
\chapter*{Digital Services of the University}
Here is some text.
\end{document}

The output if one replaces \firstchar{#3} by \firstchar{Digital Services of the University}:

Answer 1

If you insert \showtokens{#3} at the beginning of your redefined \chapterlinesformat, you'll see this on the terminal and in the log file:

> \interlinepenalty \@M Digital Services of the University\@@par .
\chapterlinesformat #1#2#3->\showtokens {#3}
                                            \myExtractFirstLetter {#3}\makeb...l.32 \chapter*{Digital Services of the University}

So, your #3 does not start with the first letter of the title because KOMA-Script inserted a very high \interlinepenalty (10000) to make sure the title can't be broken between two pages without your explicit consent.

Robust method

Simplest setup without hyperref nor nameref:

\documentclass{scrbook}
\usepackage[T1]{fontenc}        % for the non-ASCII demo title
\usepackage{lmodern}            % can be arbitrarily scaled
\usepackage{xcolor}
\makeatletter
\long\def@my@extract@first@char#1\MyFirstChar#2#3\my@endTitle{#2}
\newcommand{\myExtractFirstChar}[2]{%
  \edef#2{\unexpanded\expandafter{@my@extract@first@char #1\my@endTitle}}%
}
\let\MyFirstChar@firstofone
\makeatother
\renewcommand{\chapterlinesformat}[3]{%
  \myExtractFirstChar{#3}{\MyExtractedFirstChar}%
  \makebox(0,20)[l]{%
      \hspace{-20pt}%
      \fontsize{60}{6}\selectfont
      \color{black!15!white}%
      \MyExtractedFirstChar
    }%
  #3% <--------- don't forget!
}
\begin{document}
\chapter{\MyFirstChar{Œ}uvres complètes}
Here is some text.
\end{document}

If you use hyperref or nameref, it is not strictly necessary but might be nicer, for some applications, to explicitly tell gettitlestring that the \MyFirstChar command in titles does not matter:

\documentclass{scrbook}
\usepackage[T1]{fontenc}        % for the non-ASCII demo title
\usepackage{lmodern}            % can be arbitrarily scaled
\usepackage{xcolor}
\usepackage[expand]{gettitlestring}
\usepackage{nameref}
\usepackage{hyperref}
\makeatletter
\long\def@my@extract@first@char#1\MyFirstChar#2#3\my@endTitle{#2}
\newcommand{\myExtractFirstChar}[2]{%
  \edef#2{\unexpanded\expandafter{@my@extract@first@char #1\my@endTitle}}%
}
\let\MyFirstChar@firstofone
\GetTitleStringDisableCommands{\let\MyFirstChar@firstofone}
\makeatother
\renewcommand{\chapterlinesformat}[3]{%
  \myExtractFirstChar{#3}{\MyExtractedFirstChar}%
  \makebox(0,20)[l]{%
      \hspace{-20pt}%
      \fontsize{60}{6}\selectfont
      \color{black!15!white}%
      \MyExtractedFirstChar
    }%
  #3% <--------- don't forget!
}
\begin{document}
\chapter{\MyFirstChar{Œ}uvres complètes\label{chap}}
Here is some text. The chapter title is ``\nameref{chap}.''
% You need \usepackage{refcount} in order to test this.
%
% \edef\zzz{\unexpanded\expandafter\expandafter\expandafter{%
%   \getrefbykeydefault{chap}{name}{dflt}}}%
% \show\zzz % > \zzz=macro:->Œuvres complètes.
\end{document}

The left side shows the PDF bookmark:

More automatic but less robust solution

Another, however less robust way to solve the problem, consists in using a simple regular expression in order to find the first letter in \chapterlinesformat's third argument. The command \myExtractFirstLetter, defined below, assigns its second argument locally, but this could easily be made global if you prefer.

\documentclass{scrbook}
\usepackage{lmodern}            % can be arbitrarily scaled
\usepackage{xcolor}
\usepackage{xparse}
\ExplSyntaxOn
\seq_new:N \l__jendrik_match_seq
% The second argument must be a macro or expl3 token list variable
\NewDocumentCommand \myExtractFirstLetter { m m }
  {
    % The (?i) makes the [a-z] class case-insensitive
    \regex_extract_once:nnNTF { (?i)[a-z] } {#1} \l__jendrik_match_seq
      { \seq_get_left:NN \l__jendrik_match_seq #2 }
      { \tl_clear:N #2 }        % no match -> clear the #2 macro / tl var
  }
\ExplSyntaxOff
\renewcommand{\chapterlinesformat}[3]{%
  \myExtractFirstLetter{#3}{\myFirstLetter}%
  \makebox(0,20)[l]{%
      \hspace{-20pt}%
      \fontsize{60}{6}\selectfont
      \color{black!15!white}%
      \myFirstLetter
    }%
  #3% <--------- don't forget!
}
\begin{document}
\chapter*{Digital Services of the University}
Here is some text.
\end{document}