Re: Lpeg for one who is regexp-poisoned...

[meino.cramer@...]

Sean Conner <sean@conman.org> [14-11-12 18:50]:
> It was thus said that the Great Andrew Starks once stated:
> > On Tue, Nov 11, 2014 at 9:58 PM, <meino.cramer@gmx.de> wrote:
> > 
> > > I looked (and tried to understand ...) Robertos video about the
> > > concepts of lpeg/peg on Youtube and I read the lpeg tutorial
> > > lpeg pages.
> > >
> > > But still I am running against an inner wall...years of using regexps
> > > could not be nullified that fast... ;)
> > >
> > > One sentence of Robertos video stick in my head: Lpeg do not search.
> > >
> > > How can I recognize the appearance of a certain pattern in a string
> > > then?
> > >
> > > For example:
> > >
> > > mystinrg="CHaskellLispFortranBPCLAssemblerForthLuaSchemePerlCHILLTeXJavaJavascript"
> > >
> > > Is it possible to check with LPeg and without missusing it , wheter "Lua"
> > > is in that string and where (I know, that there are other lpegless methods
> > > to
> > > do that ... thats why this is an example ;) ?
> > >
> > > If this not possible or only can be "tricked"...how can LPeg react on
> > > only partly known input formats?
> > >
> > > Or is that a typical question of someone, who is still under the bad
> > > influence of regexps?
> >
> > Yes you can search. With lpeg, there are at least two methods that I can
> > think of. The one that doesn't use grammars can be described thusly:
> > 
> > zero or more characters that do not equal "Lua" followed by "Lua"
> > 
> > or
> > 
> > (untested)
> > 
> > local P = lpeg.P
> > 
> > local contains_lua_pat = (P(1) - "Lua")^01 * P("Lua")
> > 
> > print(contains_la_pat:match(your_string))
> > --> position of the match
> 
>   That actually returns the position *just past* the match, so there's
> little indication of what you matched, just that you did.
> 
> > If it succeeds, it returns the position of... i believe the first (and
> > maybe also last) position of the match. If you use "lpeg.C", you'll get the
> > capture.
> 
>   Here's a sample with captures:
> 
> -- ----------------------------------------------------------------
> -- load LPeg, and grab some local references to two LPeg functions:
> --
> -- C()	- Return the text comprising the pattern [1]
> -- Cp() - Return the current position in the string
> --
> -- [1] It can return more than just the pattern text, but for now, 
> --     this explanation is Good Enough.
> -- ----------------------------------------------------------------
> 
> lpeg     = require "lpeg"
> local Cp = lpeg.Cp
> local C  = lpeg.C
> 
> -- -------------------------------------------------------------------------
> -- Try to match against a list of languages.  Becuase of the way LPeg works,
> -- the search will first try "Haskell", then "Lisp", then the next one.  In
> -- this example, it's best to search for longer terms before shorter ones. 
> -- If you had "Java" then "Javascript", then searching for "Javascript" will
> -- return "Java", since "Java" will be found first.  To avoid this, use the
> -- order "Javascript", "Java".
> --
> -- It's this reason that "C" is searched for last in the list.
> --
> -- This will also return the position just past the match so we can resume
> -- searching the string past what we've matched.
> -- -------------------------------------------------------------------------
> 
> lang = (
>          C("Haskell")
>        + C("Lisp")        
>        + C("Fortran")
>        + C("BPCL")
>        + C("Assembler")
>        + C("Forth")
>        + C("Lua")
>        + C("Scheme")
>        + C("Perl")
>        + C("CHILL")
>        + C("TeX")
>        + C("Javascript")
>        + C("Java")
>        + C("C")
>      ) * Cp()
> 
> test = "CHaskellLispFortranBPCLAssemblerForthLuaSchemePerlCHILLTeXJavaJavascript"
> 
> -- -------------------------------------------------------------------------
> -- Start at the first position in the string (remember: Lua is 1-based). Get
> -- the language at that position, plus the position past the language name. 
> -- Print the language, then resume searching for other language names.
> -- -------------------------------------------------------------------------
> 
> pos = 1
> while pos <= #test do
>   local name,newpos = lang:match(test,pos)
>   if not name then break end
>   print(name)
>   pos = newpos
> end
> 
>   -spc 
> 

Hi Andrew, hi Sean,

thank you so much for the examples! :)

LPeg and Lua (of course) are really fascinating things!
I always LOVE """small""" things, which such richness!
Why should I use imore than just one nail to hang a picture? ;)

Best regards,
Meino