Cfrtjryk

Is there a way to set indices (subscripts) to non-italic globally? I'm aware of the case by case

X_{text{text goes here}}

But I'm having to change that across a whole document is becoming a pain.

It is possible to overload _ to set all subscripts in roman type, but that seems like a bad idea since it might break something unexpected.

You may instead want to consider defining a macro that produces an upright subscript. I've defined such a macro (subtxt) below.
Since it seems unlikely that you'll need underscores in math mode, I've also redefined _ to expand to subtxt whenever it is used in math mode (and to produce an underscore otherwise, like normal).

documentclass{article}



usepackage{amsmath} %% <- necessary for correct scaling of subscripts



begin{document}



newcommand*subtxt[1]{_{textnormal{#1}}}

DeclareRobustCommand_{ifmmodeexpandaftersubtxtelsetextunderscorefi}



[

    X_i + Xsubtxt{i} + X_i + X_{text goes here}

]



end{document}

Notes:

• I'm using textnormal instead of text because the font of subscripts created with the latter command changes based on the surrounding text. You for instance probably wouldn't want all of your subscripts inside theorem environments to be in italics. See e.g. this answer for more info.




• I'm using DeclareRobustCommand to redefine _ because the original version of this macro is also defined like that. It isn't too important, but more information can be found here.

Unless you're using underscores for some other purpose in your document, you can now do a search-and-replace to change every _ into _.

documentclass{article}



begin{document}

sbox0{$$}



scriptfont1=scriptfont0





$X_{abc}+y_{max}$





end{document}

Note this affects all math uses of the script size font not just subscripts.

Here's a LuaLaTeX-based solution. Subscripts enclosed by curly braces are typeset with upright ("roman") letters, as long as there's no space between the _ (underscore) character and the material enclosed in curly braces. If a subscript term is not enclosed in curly braces, e.g., $x_i$, it's not processed by the Lua code.

If, for some reason, you do not want to the Lua function to operate on a subscript term encased in curly braces, just make sure that there are one or more spaces between _ and the subscript term. An obvious reason for wanting to suspend operation of the Lua function would be the fact that the subscript term contains math material which should be processed in math mode.

To activate operations, issue the instruction upsubOn. To terminate them completely, execute upsubOff.

% !TEX TS-program = lualatex

documentclass{article}



usepackage{amsmath,unicode-math}

usepackage{luacode}

%% Lua-side code:

begin{luacode}

function sub_up ( s )

  return ( s:gsub ( "_(%b{})" , "_{\textnormal%1}" ) )

end

end{luacode}

%% LaTeX-side code:

newcommandupsubOn{directlua{luatexbase.add_to_callback( 

    "process_input_buffer" , sub_up , "subup" )}}

newcommandupsubOff{directlua{luatexbase.remove_from_callback( 

    "process_input_buffer" , "subup" )}}

AtBeginDocument{upsubOn}  % activate the Lua function by default



begin{document}

$X_{text goes here}$quad $x_ {ab cd}$quad $x_ {i_j},x_k^u$



upsubOff

$X_{text goes here}$quad $x_ {ab cd}$quad $x_ {i_j},x_k^u$

end{document}

I suggest to use a different character for those subscripts. One way could be to use ? that's very rarely used in math mode. Another uses ↓ (maybe you can find a way to type it easily).

documentclass{article}

usepackage{amsmath}

usepackage{newunicodechar} % for using ↓



% the main command

newcommand{uprightsubscript}[1]{_{textnormal{#1}}}



% this sets up the use of ?

begingrouplccode`~=`?lowercase{endgrouplet~}uprightsubscript

AtBeginDocument{mathcode`?="8000 }



% this sets up the use of ↓

newunicodechar{↓}{uprightsubscript}



textheight=2cm % just for making a smaller picture



begin{document}



$x?{max}$ worksfootnote{Also here $x?{max}$} and again $x?{max}$?



$x↓{max}$ worksfootnote{Also here $x↓{max}$} and again $x↓{max}$?



end{document}