From: Clifford Heath <no@...>
Date: 2008-01-29T14:29:56+09:00
Subject: Re: Treetop parser (or PEG in general?) questions

Clifford Heath wrote:
> The keyword rule should only be called when you know you've just seen
> something that can't be part of a word - never after an arbitrary 
> character.
> rule vhdl_file_wth_port
>    non_port* port_decl non_port*
> end
> 
> rule non_port
>    comment / string / white / !port alphanumeric+
> end

Hmmm, it looks like I broke my own rule here, but I didn't.

There's no need to worry about non_port choosing the alphanumeric
alternative, but stopping just in time to see that the AN string
contains the word "port", because the + is greedy. If "alphanumeric+"
sees the "port" in "supportable", it'll walk on by. The only time the
negative assertion is needed is at the start of an alphanumeric string.

In other words, "port_decl" will only ever get called here either at
the start of the input, or after a comment, string, white or a
*non-alphanumeric* (because an alphanumeric would have been eaten).

Hope that clarifies. In this case greediness makes things easier :-).

Clifford Heath.