Lualatex code to select enclosed text from another file

Question

I am trying to write a function that would select a text delimited by some symbols (*) from another file and print it. The following gives an error. What is the right way to pass 'text' to string.gsub?

\documentclass{scrartcl}
\usepackage{luatextra}
\usepackage{filecontents}

\begin{filecontents}{testdata.dat}

  a *string enclosed in* stars

\end{filecontents}

\begin{luacode*}
  function readtxt()
    file = io.open("testdata.dat", "r")
    text = file:read("*all")
    file:close()
    for x in string.gsub(text,"*(.*)*","") do
        tex.print(x)
    end
  end
\end{luacode*}

\begin{document}
  \directlua{readtxt()}
\end{document}

Answer 1

You need to use gmatch instead of gsub. And probably non greedy regexp (.-) if there are more then one match:

\documentclass{scrartcl}
\usepackage{luatextra}
\usepackage{filecontents}

\begin{filecontents}{testdata.dat}

  a *string enclosed in* stars *two* 

\end{filecontents}

\begin{luacode*}
  function readtxt()
    file = io.open("testdata.dat", "r")
    text = file:read("*all")
    file:close()
    for x in string.gmatch(text,"*(.-)*") do
        tex.print(x)
    end
  end
\end{luacode*}

\begin{document}
  \directlua{readtxt()}
\end{document}