Re: LPEG 0.12 bug?

[Sean Conner <sean@...>]

It was thus said that the Great Solra Bizna once stated:
> I'm not sure this is the right place to send this, or even that it's a
> bug in LPEG and not in my understanding of LPEG. I've got a simplified
> test case that reproduces an (apparent) bug I encountered while using
> LPEG to write a Base64-like decoder.
> 
> As in this test case, I was able to get my by decoder working by
> adding a dummy capture, but it doesn't seem like it should be
> necessary. I could be wrong... if I am, where's the flaw in my
> understanding?

  It's your understanding.  Try adding the following to the script you sent
to the list:

	local Cg = lpeg.Cg

	local fixed_pattern = P{
	  "fixed_pattern",
	  fixed_pattern = Cg(V"capture_A_or_B" * V"capture_A_or_B")
	                / do_something,
	  capture_A_or_B = C(R"AB"),
	}

	fixed_pattern:match("AA")

lpeg.Cg() does a group capture, which is what you are trying to do here,
trying to group multiple captures of "capture_A_or_B" and passing them all
into a single function.

  You could also write this as:

	capture_A_or_B = C(R"AB")
	fixed_pattern  = Cg(capture_A_or_B * capture_A_orB)
	               / do_something

  Unless you are writing recursive patterns [1] you don't really need the
extra baggage of using grammars (in my opinion).

  -spc

[1]	Like (not LPeg grammar; more like a pseudoBNF format):

		expr   = term '+' term
		term   = factor '*' factor
		factor = NUMBER | '(' expr ')'

	Note that factor referrs to expr, making this a recursive
	pattern.