Pass a regexp to an engine to produce highlighting in the pdf output

Question

Is there a way to pass a regexp to an engine (pdflatex, xelatex... I don't know what could be the right choice) to produce a highlighting of the matches in the pdf output? I need the .tex file not be modified.
Let me explain. If I have the file:
 documentclass[11pt]{article}
usepackage{amsmath}
pagestyle{empty}
begin{document}

The words foo and bar should be higlighted in the pdf output of this
document, but I need this file not being modified.

An example for a more complex string, e.g. ``highlight this string'',
would be helpfull.

end{document}

I'd like to have something like (this is a conceptual example):
pdflatex --highlight "(foo|bar)" file.tex

and have:

Please note that I'm familiar with emacs regexp only so, please, specify what kind of regexp you are using.
Another usefull thing should be if I could store the regexps in a file to be passed to the engine.

Mico · Accepted Answer

Lua provides various pattern-matching capabilities. It's not "full" regexp, but it's quite similar. LuaTeX lets you assign functions to the process_input_buffer callback, where they can act as preprocessors, "on the fly", on the input stream, before TeX commences its usual processing.
I suggest you create a separate tex file called, say, highlight.tex, as follows:
AtBeginDocument{%
usepackage{xcolor}  % for 'textcolor' macro
usepackage{luacode} % for 'luaexec' macro
luaexec{
function colorize ( u )
   return ( "textcolor{red}{" .. u .. "}" )
end
function highlight ( s )
   s = s:gsub ( "foo" , colorize )
   s = s:gsub ( 'bar' , colorize )
   s = s:gsub ( "``highlight this string''" , colorize )
   return s
end
luatexbase.add_to_callback ( "process_input_buffer" , highlight , "highlight" ) 
}}

Then the only modification you'd have to apply to the "main" tex file is to insert the instruction input highlight in the preamble.
documentclass[11pt]{article}
input highlight  % <-- new

begin{document}
The words foo and bar should be highlighted in the pdf output of this
document, but I need this file not being modified.

An example for a more complex string, e.g., ``highlight this string'',
would be helpful.
end{document}

Pass a regexp to an engine to produce highlighting in the pdf output

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