Highlight inline formulas

Question

I need either to add a color background or border around each inline formula. How?

I want to ensure every formula is no more wide that about the 1/2 of the width of text, in order to publish them in Kindle, as I describe here.

Answer 1

Here's a LuaLaTeX-based solution. It consists of two LaTeX macros called \InlineMathColorOn and \InlineMathColorOff, which act as switches to activate and deactivate the Lua function called inlinemath_yellow. The Lua function first sets aside cases of \$ and $$, then encases all instances of $...$ and \(...\) in yellow boxes, and finally restores the cases of \$ and $$. The Lua function operates through the process_input_buffer callback, which does its job at a very early stage of processing, well before TeX gets to apply its eyes, mouth, etc.

What this code doesn't do is allow automatic line breaking inside inline-math formualas. But then, you're not going to allow many such constructs in your book, are you?

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{xcolor}  % for '\colorbox' macro
\usepackage{luacode} % for 'luacode' environment

\begin{luacode}

function inlinemath_yellow ( s )
  s = s:gsub ( "\\%$" , "@@@@@@@@@@@@" )  -- instances of "\$"
  s = s:gsub ( "%$%$" , '""""""""""""' )  -- instances of "$$"
  s = s:gsub ( "(%$..-%$)" , function ( u )
           return "\\colorbox{yellow}{$"..u:sub(2,-2).."\\strut$}" 
      end )
  s = s:gsub ( "(\\%(..-\\%))" , function ( u )
         return "\\colorbox{yellow}{\\("..u:sub(3,-3).."\\strut\\)}" 
      end )  
  s = s:gsub ( "@@@@@@@@@@@@" , "\\$" ) -- revert instances of "\$"
  s = s:gsub ( '""""""""""""' , "$$" )  -- revert instances of "$$"
  return s
end

\end{luacode}

%% LaTeX-side code:
\newcommand\InlineMathColorOn{\directlua{%
  luatexbase.add_to_callback ( "process_input_buffer" , 
  inlinemath_yellow , "InlineMathYellow" )}}
\newcommand\InlineMathColorOff{\directlua{%
  luatexbase.remove_from_callback ("process_input_buffer" , 
  "InlineMathYellow" )}}
\AtBeginDocument{\InlineMathColorOn} % activate by default

\begin{document}
$1+1+1\ne2$, 
$1$, 
\(a^2+b^2=c^2\), 
\(\$12.34\), 
\$12.34
$$a^2+b^2=c^2$$ % not inline math
\end{document}