Automatic upright subscripts in pdflatex

Question

For some time I used Micos solution to the problem of automatically typesetting subscripts upright based on lualatex. Now I want to switch back to pdflatex (because of compile times and the better preformat/mylatexformat support) but preserve the syntax provided by this solution.

In this this answer @Svend Tveskæg showed a solution which works also with pdflatex, but with a slightly different syntax. Now I want to modify this solution to behave like Micos solutions but works with pdflatex.

In particular it should use { and } as delimiters (instead of |) and also switches back to italic if I leave a gap like $v_ {x,\max}$.

I tried to replace the delimiters in @Svend Tveskæg like this, but it doesn't work:

\documentclass{article}

\makeatletter
 \begingroup
  \catcode`\_=\active
  \protected\gdef_{\@ifnextchar{\subtextup\sb}
 \endgroup
\def\subtextup\catcode{#1}{\sb{\textnormal{#1}}}
\AtBeginDocument{\catcode`\_=12 \mathcode`\_=32768}
\makeatother

\begin{document}

$A_1 A_p A_{p} A_ {p} $

\end{document}

Error message:

This is pdfTeX, Version 3.141592653-2.6-1.40.23 (TeX Live 2021) (preloaded format=pdflatex)
 restricted \write18 enabled.
entering extended mode
(./Upright.tex
LaTeX2e <2021-11-15> patch level 1
L3 programming layer <2022-02-21>
(/usr/local/texlive/2021/texmf-dist/tex/latex/base/article.cls
Document Class: article 2021/10/04 v1.4n Standard LaTeX document class
(/usr/local/texlive/2021/texmf-dist/tex/latex/base/size10.clo))
! Illegal parameter number in definition of _.
<to be read again> 
                   1
l.8 \def\subtextup\catcode{#1
                             }{\sb{\textnormal{#1}}}

Here is what I used so far:

% Automatically upright subscripts with lualatex
\NeedsTeXFormat{LaTeX2e}[1994/06/01]
\ProvidesPackage{uprightsub}
  [2021/09/06 v1.0 upright math via lualatex]


\RequirePackage{amsmath,luacode}

%%See https://tex.stackexchange.com/questions/156641/typeset-subscript-material-automatically-in-upright-font-shape

\begin{luacode}
function up_subs ( buff )
    return ( string.gsub ( buff , "_(%b{})" , "_{\\textnormal%1}" ) )
end
\end{luacode}
\AtBeginDocument{\directlua{luatexbase.add_to_callback(
"process_input_buffer", up_subs, "up_subs")}}

\endinput
%%
%% End of file `uprightsub.sty'.

If there is another pdflatex solution which shows the desired behavior it would be also welcome.

Edit

If I compile use the lualatex solution like this

\documentclass{article}

\usepackage{uprightsub}

\begin{document}

$A_1 A_p A_{p} A_ {p} $

\end{document}

The output is

which would be the desired output for this example.

Answer 1

If you don't understand TeX, you're going to have a bad time trying to modify TeX code.

Anyway, just do this (use nospace version to "respect" spaces)

%! TEX program = pdflatex
\documentclass{article}
\usepackage{amsmath}
\usepackage{ltxcmds}

\makeatletter
 \begingroup
  \catcode`\_=\active
  \protected\gdef_{\ltx@ifnextchar@nospace\bgroup\subtextup\sb}
 \endgroup
\def\subtextup#1{\sb{\textnormal{#1}}}
\AtBeginDocument{\catcode`\_=12 \mathcode`\_=32768}
\makeatother

\begin{document}

$A_1 A_p A_{p} A_ {p} $

\end{document}

If I understood correctly this output should be what you want.