Re: Lpeg query
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- Subject: Re: Lpeg query
- From: Sean Conner <sean@...>
- Date: 2014年1月19日 17:02:39 -0500
It was thus said that the Great Gavin Wraith once stated:
>
> What is the most elegant way to amend WORDS so that { WORDS:match " " }
> has the value { }? Or is that not possible seeing that the operation
> of composing values has no unit; i.e. no value @ so that @,x and x,@ are
> just x?
>
> Does it make any sense to ask for a variant of Lpeg in which lpeg.match
> does not return a number but nil by default? The capture lpeg.Cp could
> then be used to produce the current behaviour.
You can do what you ask with LPeg now. I take it that you want to return
either captures of words, or just nil if there are none. But, what if you
change it up a bit, and return an optional list of words, followed by a nil?
local Cc = lpeg.Cc
local WORDS = BLANK*(C(WORD)*BLANK)^0*END*Cc(nil)
Cc() will match the empty string and return its parameter, in this case,
nil. And that does what you are asking:
print(WORDS:match "one two three")
one two three nil
print(WORDS:match " ")
nil
LPeg is quite powerful, and yes, it does take time to work through all it
can do.
-spc (You could also add the call to Cc() in the defintion of END with
the same results ... )