Conditional expression to convert figure to table. Is it possible?

Question

I have a though one for you:
I'm writing in MultiMarkdown and all my figures, tables, charts are usually pictures. This allows me to write something like this:

The MultiMarkdown input

![This is the caption.][table1]

[table_sometable]: tab_pic1.png

… and end up with:

Compiled LaTeX

\begin{figure}[htbp]
\centering
\includegraphics[keepaspectratio,width=\textwidth,height=0.75\textheight]{tab_pic1.png}
\caption{This is the caption.}
\label{table_sometable}
\end{figure}

I thought about writing a shell script that simple exchanges \begin{figure} to \begin{table} (same for \end{figure}) before I compile my .tex file with pdflatex to a PDF.

However, I'm looking for a pure LaTeX solution since it's more portable and more people at my University could use it.

After reading LaTeX programming: how to implement conditionals I thought that it might be possible and decided to ask my question here since I'm only 4 days old in the world of LaTeX.

I also read "How to form "if ... or ... then" conditionals in TeX?" and "LaTeX conditional expression", but I'm not sure if parsing for a string is supported.

I imagine the scipt to work like:

IF a string called \label{table_} is found between \begin{figure} and \end{figure} THEN replace {figure} with {table}

I also found Search & Replace Script for TeXworks but maybe it's possible to handle this all on the compile level, so that I don't have to run anything extra and just can start compiling in Sublime Text when I'm done writing.

---

So why would this be cool…

• obviously it's kind of a must-have for people who write in (Multi)Markdown
• a filter would also allow to split all figures further into catogories (photographies/images, charts, …)
• other geeky stuff which depends on parsing the content before compiling a pdf.

I'm open for ideas, solutions, links.

Answer 1

Here is an implementation using environ and expl3. With environ we absorb the whole environment's contents and with l3regex we check whether the environment contains the tokens

\label{table_

via the regular expression

\c{label}\cB.table_

Where \c{<string>} matches the control sequence with name <string> and \cB. matches one open brace.

\documentclass{article}
\usepackage{environ,xparse,l3regex}

% environ has problems in renewing
% existent environments
\let\figure\relax
\let\endfigure\relax

\NewEnviron{figure}{\CheckForTable}

\makeatletter
\ExplSyntaxOn
\cs_new_protected:Npn \pattulus_check:n #1
 {
  \regex_match:nnTF { \c{label}\cB.table_ } { #1 }
   {
    \@float{table}#1\end@float
   }
   {
    \@float{figure}#1\end@float
   }
 }
\cs_generate_variant:Nn \pattulus_check:n { V }
\NewDocumentCommand{\CheckForTable}{}
 {
  \pattulus_check:V \BODY
 }
\ExplSyntaxOff
\makeatother

\begin{document}
\begin{figure}[htbp]
\centering
THIS SHOULD BE A TABLE
\caption{This is the caption.}
\label{table_sometable}
\end{figure}

\begin{figure}[htbp]
\centering
THIS SHOULD BE A FIGURE
\caption{This is the caption.}
\label{figure_somefigure}
\end{figure}

\end{document}

Update March 2019

After several updates of expl3 and xparse, the code can be reduced:

\documentclass{article}
\usepackage{xparse}

\makeatletter
\ExplSyntaxOn

% use the default placements htp
\RenewDocumentEnvironment{figure}{O{htp} +b}
 {\pattulus_check:nn { #1 } { #2 }}
 {}

\cs_new_protected:Nn \pattulus_check:nn
 {
  \regex_match:nnTF { \c{label}\cB.table_ } { #2 }
   {
    \@float{table}[#1]#2\end@float
   }
   {
    \@float{figure}[#1]#2\end@float
   }
 }

\ExplSyntaxOff
\makeatother

\begin{document}
\begin{figure}
\centering
THIS SHOULD BE A TABLE
\caption{This is the caption.}
\label{table_sometable}
\end{figure}

\begin{figure}
\centering
THIS SHOULD BE A FIGURE
\caption{This is the caption.}
\label{figure_somefigure}
\end{figure}

\end{document}