Here's a LuaLaTeX-based solution. It defines a Lua function, called hidedelims, that extracts the material surrounded by the delimiters " {L" and "L} ", and it sets up two LaTeX macros, called \switchon and \switchoff, that enable and disable the operation of the Lua function. The term "whitespace" is understood in its generic sense, i.e., to comprise not only space but also "tab" characters. (In Lua, %s denotes a generic whitespace character.)
Remark: The write-up of your requirements is a bit confusing. At first, you seem to state that everything between the pair of delimiters should be hidden. Later on, you seem to state that the material between the delimiters -- but not the delimiters themselves -- should, in fact, be shown. In my answer, I've implemented the second requirement. If you do want to hide all material between the delimiters (as well as, presumably, the delimiters themselves), just change the line
return ( string.gsub ( line , '%s{L(.-)L}%s', '%1' ) )
to
return ( string.gsub ( line , '%s{L.*L}%s', '' ) )
In the following screenshot, the vertical line to the left and the dot to the right of all instances of the strings " {LHelloL} " and " {LGoodbyeL} " are used to delimit visually the output that's produced when hiding the delimiters is either switched on or off. The \verb+ ... + macro is used to display the { and } characters (if not removed by hidedelims).
% !TEX TS-program = lualatex
\documentclass{article}
%% Lua-side code: A function called "hidedelims"
\usepackage{luacode}
\begin{luacode}
function hidedelims ( line )
return ( string.gsub ( line , '%s{L(.-)L}%s', '%1' ) )
end
\end{luacode}
%% TeX-side code: Macros to enable/disable the Lua function
\usepackage{luatexbase}
\newcommand\switchon{\directlua{%
luatexbase.add_to_callback ( "process_input_buffer",
hidedelims, "hidedelims" )}}
\newcommand\switchoff{\directlua{%
luatexbase.remove_from_callback( "process_input_buffer",
"hidedelims" )}}
%% Just for this example...
\usepackage{showframe}
\setlength\parindent{0pt}
\begin{document}
\verb+ {LHelloL} +.
\switchon
\verb+ {LHelloL} +.
\switchoff
\verb+ {LHelloL} +.
\medskip
\verb+ {LGoodbyeL} +.
\switchon
\verb+ {LGoodbyeL} +.
\switchoff
\verb+ {LGoodbyeL} +.
\end{document}
Here's a version of the code that always hides the delimiters, and either shows or hides the material within the delimiters, depending on whether \HideOff or \HideOn is in effect. The following screenshot shows the result of two passes -- first with \HideOff, then with \HideOn -- over a paragraph that contains material delimited by {L and L}.
% !TEX TS-program = lualatex
\documentclass{article}
%% Lua-side code:
%% - Boolean variable "check_hide_stuff"
%5 - Function "hide_stuff", assigned to "process_input_buffer" callback
\usepackage{luacode,luatexbase}
\begin{luacode}
check_hide_stuff = false
function hide_stuff ( line )
if check_hide_stuff then
return ( string.gsub ( line , '%s{L.-L}%s', '' ) )
else
return ( string.gsub ( line , '%s{L(.-)L}%s', '%1' ) )
end
end
luatexbase.add_to_callback ( "process_input_buffer",
hide_stuff, "hidestuff" )
\end{luacode}
%% TeX-side code: Macros to modify value of "check_hide_stuff"
\newcommand\HideOn{\directlua{check_hide_stuff = true}}
\newcommand\HideOff{\directlua{check_hide_stuff = false}}
\begin{document}
\HideOff %% show material inside delimiters
\noindent
A long time ago {L in a galaxy far, far awayL} , a {L beautifulL} princess lived in a {LmajesticL} castle close to a {Lmysterious andL} dark forest {LL} .
\HideOn %% hide material inside delimiters
\bigskip\noindent
A long time ago {L in a galaxy far, far awayL} , a {L beautifulL} princess lived in a {LmajesticL} castle close to a {Lmysterious andL} dark forest {LL} .
\end{document}
\hidemesometimes{Hello}instead, you can just define\newcommand{\hidemesometimes}[1]{#1}if you want to see the text or\newcommand{\hidemesometimes}[1]{\relax}(or\newcommand{\hidemesometimes}[1]{\leavevmode\unskip}) if you don't. (There might be some rough edges here.) LuaTeX can do very fancy things as well, see Macro: Replace all occurrences of a word for something that comes close. – moewe Aug 22 '15 at 14:13{L ... L}if I might ask? Is there any way to change that?) – moewe Aug 28 '15 at 13:27{LandL}, only repeating words. It thus seems I have to stick to your first solution, as Lua scripting is too time consuming for me to learn right now - but I'm going to open a bounty, maybe slicker solutions will come along, or someone in an ideal case will perhaps provide a ready-made script. – l7ll7 Aug 30 '15 at 13:58{L ... L}you want replaced. This is not something the preprocessor seems to be able to handle (it can deal with replaces of textLOREMor functionsFOO(a,b)out of the box, so your structure of{L ... L}doesn't naturally fit). The most obvious way would be to use TeX macros as in my first comment, I guess, but I'm sure there are some other solutions out there, maybe some that come closer to your liking. – moewe Aug 31 '15 at 18:35;-)– moewe Sep 01 '15 at 12:44