catbox.tex

\documentclass{article}
% Copyright (C) 2001 by Martin Scharrer <martin.scharrer@web.de>, Nov 12th 2011
% This is free code under the LPPL v1.3 or later version OR the CC BY-SA 3.0

\usepackage{tikz}
\usepackage{graphicx}
\usepackage{collectbox}

\makeatletter
\newcommand\catcodebox[1]{%
    \@catcodebox#1\@nnil
}
\def\@catcodebox{%
    \futurelet\@let@token\@@catcodebox
}
\def\@@catcodebox{%
    \ifx\@let@token\@sptoken
        \expandafter\@@catcodebox@space
    \else
        \expandafter\@@@catcodebox
    \fi
}
\def\@@catcodebox@space{%
    \afterassignment\@@catcodebox@@space
    \let\@let@token= %
}
\def\@@catcodebox@@space{%
    \@@@catcodebox{ }%
}
\def\@@@catcodebox#1{%
    \ifx\@nnil#1\relax\else
    \begingroup
    \if\noexpand#1\relax
        \edef\thecatcode{16}%
    \else
        \edef\thecatcode{\the\catcode`#1}%
    \fi
    \sbox\collectedbox{\ifnum\thecatcode=11\else\ifnum\thecatcode=12\else\normalfont\ttfamily\fi\fi\string#1}%
    \begin{tikzpicture}[>=latex,semitransparent,ultra thin,baseline=(BOX.base)]
        % Save example text in box
        \def\HEIGHT{\the\ht\collectedbox}
        \def\TOTALHEIGHT{\the\totalheight}
        \def\WIDTH{\the\wd\collectedbox}
        \def\DEPTH{\the\dp\collectedbox}
        % Text node:
        \node [inner sep=0pt,anchor=base west,opacity=1] (BOX) {\usebox\collectedbox};
        \useasboundingbox;
        % Baseline
        \draw (0,0) -- (\WIDTH,0);
        % Box
        \draw (0,-\DEPTH) rectangle (\WIDTH,\HEIGHT);
        % Origin
        \fill (0,0) circle (2\pgflinewidth);
        \node [inner ysep=.2pt,inner xsep=0pt,anchor=north east] at (\WIDTH,-\DEPTH)
            {\scalebox{.2}{\tiny\thecatcode}};
    \end{tikzpicture}%
    \endgroup
    \expandafter\@catcodebox
    \fi
}
\makeatletter

\begin{document}

\catcodebox{ABC x cyi}

\itshape
\catcodebox{ABC x c\relax yi}

\itshape
\catcodebox{ABC$x$&cy~i_\_}

\end{document}