How to modify overlay specifications in a new command in LaTeX Beamer?

Question

I would like to modify the overlay specification that is passed to a custom command, but for some reason, < and > are interpreted as ¡ and ¿ when they are included in a new command.

MWE:

\documentclass[12pt,t,aspectratio=169]{beamer}

\usepackage[utf8]{inputenc} % Ensure the correct input encoding
\usepackage{xstring}

% named colors
\definecolor{offwhite}{RGB}{249,242,215}
\definecolor{background}{RGB}{24,24,24}
\definecolor{hilight}{RGB}{255,69,0}

% use those colors
\setbeamercolor{titlelike}{fg=hilight}
\setbeamercolor{normal text}{fg=offwhite,bg=background}
\setbeamercolor{item}{fg=offwhite} % color of bullets

% define macros
\newcommand<>{\hilight}[1]{{\color#2{hilight} \textbf#2{#1}}}
\newcommand<>{\onlyhilight}[1]{%
\StrBetween{#2}{<}{>}[\spec]% Extract the number between < and >
\edef\modifiedspec{<\spec->}% Create the new overlay specification
% \edef\modifiedspec{<3->}% Create the new overlay specification
\only\modifiedspec{{\color#2{hilight} \textbf#2{#1}}}%
}

\newcommand{\test}{<5->}

\begin{document}

    \begin{frame}{\textbf{Introduction}}
    
        \vspace{60pt}
        \begin{itemize}\itemsep10pt
            \item<2-> \hilight<2>{First entry}, \onlyhilight<3>{second entry}, \onlyhilight<4>{third entry}, \onlyhilight<5>{fourth entry} \test
        \end{itemize} 
    \end{frame}

\end{document}

Answer 1

It seems unnecessary to parse the overlay specifications, you could instead use \temporal:

\documentclass[12pt,t,aspectratio=169]{beamer}

% named colors
\definecolor{offwhite}{RGB}{249,242,215}
\definecolor{background}{RGB}{24,24,24}
\definecolor{hilight}{RGB}{255,69,0}

% use those colors
\setbeamercolor{titlelike}{fg=hilight}
\setbeamercolor{normal text}{fg=offwhite,bg=background}
\setbeamercolor{item}{fg=offwhite} % color of bullets

\setbeamerfont{frametitle}{series=\bfseries}

% define macros
\newcommand<>{\onlyhilight}[1]{\temporal#2{}{{\color#2{hilight} \textbf#2{#1}}}{#1}}

\begin{document}

    \begin{frame}
    \frametitle{Introduction}
    
        \vspace{60pt}
        \begin{itemize}\itemsep10pt
            \item<2-> \onlyhilight<2>{First entry,} \onlyhilight<3>{second entry,} \onlyhilight<4>{third entry,} \onlyhilight<5>{fourth entry} 
        \end{itemize} 
    \end{frame}

\end{document}

Answer 2

You can use \NewExpandableDocumentCommand (from recent LaTeX or from xparse) to define the \extractoverlayspecs macro:

\NewExpandableDocumentCommand\extractoverlayspecs{R<>{}m}{\edef#2{#1}}

Your modified MWE:

\documentclass[12pt,t,aspectratio=169]{beamer}

\usepackage[utf8]{inputenc} % Ensure the correct input encoding
\usepackage[T1]{fontenc}
\usepackage{xparse} % use with old LaTeX

% named colors
\definecolor{offwhite}{RGB}{249,242,215}
\definecolor{background}{RGB}{24,24,24}
\definecolor{hilight}{RGB}{255,69,0}

% use those colors
\setbeamercolor{titlelike}{fg=hilight}
\setbeamercolor{normal text}{fg=offwhite,bg=background}
\setbeamercolor{item}{fg=offwhite} % color of bullets

% define macros
\NewExpandableDocumentCommand\extractoverlayspecs{R<>{}m}{\edef#2{#1}}
\newcommand<>{\hilight}[1]{{\color#2{hilight} \textbf#2{#1}}}
\newcommand<>{\onlyhilight}[1]{%
  \extractoverlayspecs#2\spec%
  \edef\modifiedspec{\spec-}% Create the new overlay specification
  \only<\modifiedspec>{{\color#2{hilight} \textbf#2{#1}}}%
}

\newcommand{\test}{<5->}

\begin{document}

    \begin{frame}{\textbf{Introduction}}
    
        \vspace{60pt}
        \begin{itemize}\itemsep10pt
            \item<2-> \hilight<2>{First entry}, \onlyhilight<3>{second entry}, \onlyhilight<4>{third entry}, \onlyhilight<5>{fourth entry} \test
        \end{itemize} 
    \end{frame}

\end{document}

Note: Just to work around ¡ and ¿, add \usepackage[T1]{fontenc}.

Answer 3

Ok, I just found a simple solution. I only need to move the < > outside \modifiedspec and move them to \only<\modifiedspec>:

\newcommand<>{\onlyhilight}[1]{%
\StrBetween{#2}{<}{>}[\spec]% Extract the number between < and >
\edef\modifiedspec{\spec-}% Create the new overlay specification
\only<\modifiedspec>{{\color#2{hilight} \textbf#2{#1}}}%
}

But I would still like to understand why it doesn't work the other way around.

Answer 4

Based on this post, I just came up with another, even better solution using \NewDocumentCommand from the xparse package, which allows to pass on useful optional arguments:

\NewDocumentCommand{\onlyhilight}{ d<> O{} m O{}}{%
    \only<#1->{#2{\color<#1>{hilight} \textbf<#1>{#3}}#4}%
}

MWE:

\documentclass[12pt,t,aspectratio=169]{beamer}

% Only required for older LaTeX versions
% \usepackage[utf8]{inputenc}
% \usepackage{xparse}

% named colors
\definecolor{offwhite}{RGB}{249,242,215}
\definecolor{background}{RGB}{24,24,24}
\definecolor{hilight}{RGB}{255,69,0}

% use those colors
\setbeamercolor{titlelike}{fg=hilight}
\setbeamercolor{normal text}{fg=offwhite,bg=background}
\setbeamercolor{item}{fg=offwhite} % color of bullets

% define macros
\newcommand<>{\hilight}[1]{{\color#2{hilight} \textbf#2{#1}}}
\NewDocumentCommand{\onlyhilight}{ d<> O{} m O{}}{%
    \only<#1->{#2{\color<#1>{hilight} \textbf<#1>{#3}}#4}%
}

\begin{document}

    \begin{frame}{\textbf{Introduction}}
    
        \vspace{60pt}
        \begin{itemize}\itemsep10pt
            \item<2-> \hilight<2>{First entry}\onlyhilight<3>[, ]{second entry}\onlyhilight<4>[, ]{third entry}\onlyhilight<5>[, ]{fourth entry}
        \end{itemize} 
    \end{frame}

\end{document}

I still don't understand though why < > are not correctly interpreted when inside a custom command.