Re: option: /<option/i /\w+/ optarg(?) '>'

On Thu, 20 May 2004, [email protected] wrote:

> why doesn't the question mark in the optarg(?)
> below work as I expect? In the trace I see that
> optarg consumes the ">" character and then the
> option rule fails, since the final ">" is missing.
> 
> Isn't P:RD supposed to backtrack and say 
> "ok, the optarg(?) didn't match anything here"?
> 
> Of course the script below works when I change
> the optarg from /\S+/ to /\w+/, but I'm curious,
> why doesn't optarg(?) mean "ZERO or one" here?

Your basic problem is this:

> optarg: /\S+/

Because P::RD relies on the Perl 5 regex engine, it can't backtrack
regular expressions on its own.  It just asks Perl 5 to match an
optarg and that's used subsequently.  Maybe the Perl 6 grammars will
do this (I think they can), but for now you have to be careful with
the regular expressions you specify.  This example, in particular,
should exclude '>' from the possible things optarg will match - so \w+
is a solution, but you are being unnecessarily strict on the optarg
rule.  You may like /[^>]+/ better.  But then, depending on how
SGML-like your language is, you may have embedded '>' characters in
your option arguments.  In your case, it seems like what I suggest
would be OK.

Ted