Macro finding a forgotten \command

Question

I do not know if this question has more to do with the editor I use (which is LaTeXShop, on Mac) or some clever LaTeX package, but I am typesetting some notes for a course and I would like to systematically use a command, say

\variable

for the variable of functions so that I can write

$f(\variable)$

and get $f(x)$ or $f(Y)$ consistently in the whole document by simply choosing once and for all in the preamble the value of \variable. The reason is that in a first version $f(x)$ might be good, but I think I will eventually want to move to different variable names. The problem is that I have been typing $f(x)$ automatically for ages now, and I fear that here and there I will write $f(x)$ instead of $f(\variable)$ getting the same visual effect if \variable is set to $x$, so that I will never catch the error while proof reading, but creating a mess the day I will rename my \variable. So comes my question: is there a systematic way to ask LaTeX to warn me whenever I use $x$ "although I wanted to use \variable"? Of course, I do not want it to warn me each time I use the letter x. (I will be writing in French, and there are a lot of x's here and there in text) nor to warn me about $f(x)$ and forget about $g(x)$.)

Answer 1

You can make x math active:

\documentclass{article}
\usepackage{xcolor}

%%% The following four lines will make all
%%% explicit `x' in math mode being printed red
\edef\variable{\mathchar\the\mathcode`\x\relax} % this indirectly defines \variable to print an x
\begingroup\lccode`~=`x
\lowercase{\endgroup\def~}{{\textcolor{red}{\variable}}}
\AtBeginDocument{\mathcode`x=\string"8000 }

%%% When you're through, remove the lines
%%% above and use the following one
%\newcommand{\variable}{x}


\begin{document}

This is good: $f(\variable)$

This is bad: $f(x)\ne g(x)$.

\end{document}