In LuaLaTeX, how do I pass the content of an environment to Lua verbatim?

Question

In LuaLaTeX, how would I go about capturing the content of an environment for (verbatim) processing by Lua? For example, consider something like

\begin{foobar}
  Hello {World}
\end{foobar}

I'd like to be able to take the content of the environment and pass it to Lua, as in

foobar("  Hello {World}\n")

(and Lua would then eventually pass the processed version back to LaTeX, but I can presumably handle that).

Answer 1

Start the "recording" mode when you enter \begin{foobar} and end it when you close the environment. That way you get the pure buffer contents.

\documentclass{article}
\usepackage{luacode}
\begin{luacode*}
do 
  local mybuf = ""
  function readbuf( buf )
    mybuf = mybuf .. buf .. "\n" 
  end
  function startrecording()
    luatexbase.add_to_callback('process_input_buffer', readbuf, 'readbuf')
  end

  function stoprecording()
    luatexbase.remove_from_callback('process_input_buffer', 'readbuf')
    local buf_without_end = mybuf:gsub("\\end{foobar}\n","")
    print(string.format("Lua: %q", buf_without_end))
  end
end
\end{luacode*}
\begin{document}
  \newenvironment{foobar}{\directlua{startrecording()}}{\directlua{stoprecording()}}
\begin{foobar}
    Hello {World}
\end{foobar}

\end{document}

Answer 2

In ConTeXt, you can simply use buffers for this.

\def\startfoobar
    {\definebuffer[foobar][startfoobar][stopfoobar]}

\def\stopfoobar
    {\ctxlua{userdata.luafunction(buffers.getcontents('foobar'))}

The function userdata.luafunction will then receive the contents of the environment.

See the ConTeXt wiki for more examples.

Answer 3

A bit late to the party, but this fix to https://tex.stackexchange.com/users/243/topskip's answer does the job:

in response to question in comments:

In topskip's answer Hello World is still written to the pdf file, which is not typically what one would want. So I've taken his answer and made the change that if the line read does not contain "\end{foobar}" then nothing is returned to the tex interpreter, but if it does contain "\end{foobar}" it gets spit back (via the nil return value) to the tex interpreter which then ends the foobar environment. If the "\end{foobar}" line would not get returned then tex would never end the environment, which is clearly not what you want.

The example is still imperfect in that it doesn't give the desired result if there is text before "\end{foobar}" on the same line.

\documentclass{article}
\usepackage{luacode}
\begin{luacode*}
do 
  local mybuf = ""
  function readbuf( buf )
   print("buf=" .. buf)
    i,j = string.find(buf,"\\end{foobar}")
    print(i,j)
    if i==nil then
     mybuf = mybuf .. buf .. "\n" 
     print("[" .. mybuf .. "]")
     return ""
    else 
    return nil
    end
   end

  function startrecording()
    luatexbase.add_to_callback('process_input_buffer', readbuf, 'readbuf')
  end

  function stoprecording()
    luatexbase.remove_from_callback('process_input_buffer', 'readbuf')
    local buf_without_end = mybuf:gsub("\\end{foobar}\n","")
    print(string.format("Lua: %s", buf_without_end))
  end
end
\end{luacode*}

\begin{document}
  \newenvironment{foobar}{\directlua{startrecording()}}{\directlua{stoprecording()}}
\begin{foobar}
    Hello {World}
\end{foobar}

\end{document}