Converting TeX into a plaintext format?

If you're free to use LuaLaTeX, the following solution may be of interest to you. It sets up a LaTeX macro called TexToText which calls a Lua function called tex2text to do most of the work. Four separate steps are performed by tex2text:

• convert frac notation to inline-math notation
• drop sizing instructions for math "fence" symbols
• remove the (backslash) character in front of other TeX macro names (e.g, exp and ln)
• encase the final expression in a pair of parentheses.

% !TEX TS-program = lualatex
documentclass{article}
usepackage{luacode} % for 'luacode' env. and 'luastringN' macro
begin{luacode}

-- Aux. function to trim first and last char. of a string:
function trim ( s )
  return s:sub(2,-2) 
end
-- Aux. function to get rid of frac wrapper
function frac2text ( s )
  s =  s:gsub ( "frac%s-(%b{})%s-(%b{})", 
                function (u,v)
                   return "("..trim(u)..")/("..trim(v)..")" 
                end ) 
  return s
end
-- The main Lua function:
function tex2text ( s )
   -- Call frac2text function:
   s = frac2text ( s )
   -- Drop fence-sizing macros:
   s = s:gsub ( "left" , "" )
   s = s:gsub ( "right" , "" )
   s = s:gsub ( "[Bb]igg?[lmr]?" , "" )
   -- Remove leading backslash char from all other macros:
   s = s:gsub ( "(%a+)" , "%1ignorespacesafterend" )
   -- Encase the result in a pair of parentheses:
   return "(" .. s .. ")"
end

end{luacode}
newcommandTexToText[1]{directlua{tex.sprint(tex2text(luastringN{#1}))}}

begin{document}
TexToText{ln(exp(0))} quad
TexToText{frac{-x}{y}} quad
TexToText{frac{1}{xln(b)}}
end{document}