In TikZ, is it possible to isolate every character of an expression in a node as its own node?

Question

I often find myself typing equations into a node of a tikz picture. I then often find that I want to isolate certain characters in the equation and define them as a node.

For instance, consider the following figure.

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \node {$e^{i\pi}+1=0$};
\end{tikzpicture}
\end{document}

I'd like it if in this figure I could define each character e, i, pi, +, 1, =, and 0 as its own node. Is this possible?

Answer 1

The \subnode command from tikzmark is designed for this sort of situation. It fakes a node around text that's inside a \node. It does need a few compilation runs to settle (it took 3 for this code for me) and you must remember the remember picture key on the tikzpicture.

\documentclass{article}
%\url{https://tex.stackexchange.com/q/582127/86}
\usepackage{tikz}
\usetikzlibrary{tikzmark, arrows.meta}
\begin{document}
\begin{tikzpicture}[>=Latex,remember picture]
  \node (eipi) at (0,0) {(e^{i\pi}+1=0)};
\node at (0,-1) {(\subnode{e}{e}^{\subnode{i}{i}\subnode{pi}{\pi}}+\subnode{one}{1}=\subnode{zero}{0})};
\draw[<-] (e) to[out=-90, in=90] ++(-2,-1) node[below] {Base of natural logarithm};
\draw[<-] (i) to[out=135, in=0] ++(-2,1) node[left] {A square root of (-1)};
  \draw[<-] (pi) -- ++(0,-2) node[below] {Area of a unit circle};
  \draw[<-] (one) to[out=-45, in=180] ++(2,-1) node[right] {Unity};
\draw[<-] (zero) -- ++(2,0) node[right] {Zilch};
\draw[<-] (eipi) -- ++(0,1) node[above] {An ugly equation};
\end{tikzpicture}
\end{document}