How to determine current font style

Question

It is quite common in film studies to print film titles in small caps. In my documents I use the command \film{} to do this. I currently have a document which mixes both serif and sans serif fonts. But since the sans serif font does not come with small caps, I have to rely on fake small caps (using this solution).

What I would like to achieve is that \film{} automatically uses either \textsc{} or fake small caps depending on the font being used. In other words, I need a way to determine whether the current font is sans serif or not.

Answer 1

You have to compare the current value of \f@family and of \sfdefault: if they match, then you're typesetting in sans serif type.

Basically you do

\ifnum\pdf@strcmp{\f@family}{\sfdefault}=\z@
   <we are using sans serif>
\else
   <we are not using sans serif>
\fi

Example:

\usepackage{pdftexcmds}
\makeatletter
\DeclareRobustCommand{\film}[1]{%
  \ifnum\pdf@strcmp{\f@family}{\sfdefault}=\z@
    \fakesc{#1}%
  \else
    \textsc{#1}%
  \fi
}
\makeatother

Just for the fun of it, here's an implementation that uses l3regex:

\documentclass{article}

\usepackage{fontspec}
\usepackage{xparse}

\setmainfont{Linux Libertine O} % has small caps
\setsansfont{Alegreya Sans} % has no small caps

\ExplSyntaxOn
\NewDocumentCommand{\film}{ m }
 {
  \simifilm_film:n { #1 }
 }

\tl_new:N \l__simifilm_fakesc_tl

\cs_new_protected:Npn \simifilm_film:n #1
 {
  \str_if_eq_x:nnTF { \use:c { f@family } } { \sfdefault }
   {
    \simifilm_fakesc:n { #1 }
   }
   {
    \textsc{ #1 }
   }
 }

\cs_new_protected:Npn \simifilm_fakesc:n #1
 {
  \use:c { protected@edef } \l__simifilm_fakesc_tl { #1 }
  \regex_replace_all:NnN
   \c_simifilm_lowercase_regex % search any run of lowercase letters 
   { \c{simifilm_reduce:n}\cB\{\1\cE\} } % replace it with \simifilm_reduce:n{<run>}
   \l__simifilm_fakesc_tl
  \tl_use:N \l__simifilm_fakesc_tl
 }

\cs_new_protected:Npn \simifilm_reduce:n #1
 {
  { \fontsize{ \fp_eval:n { .8 * \use:c { f@size } } }{0}\selectfont \text_uppercase:n {#1} }
 }

% update the constant for accommodating the accented characters you need
\regex_const:Nn \c_simifilm_lowercase_regex { ( [ a-z é ]+ ) }

\ExplSyntaxOff

\begin{document}

\film{Stagecoach} is a great movie

\sffamily

\film{La Chevauchée fantastique} is the French title

\end{document}

Accented characters are supported, but you need to add the needed ones in the search regex.