develooper Front page | perl.perl5.porters | Postings from September 1999

Re: Regular Expression Bug

Thread Previous | Thread Next
From:
horos
Date:
September 7, 1999 21:20
Subject:
Re: Regular Expression Bug
Message ID:
199909080418.VAA04172@earth.he.net
> $brackets = qr{
>             {
>                 (?:
>                     (?> [^{}]+  | (?p{ $qr }) )
>                 )*
>             }
>         }x;
> 
> [ I will investigate why one cannot drop these (?:). ]

I just sent you an example of a regular expression that cores when ?: turns
into ?> in the same regex, and an example of the infinite loop problem.

> 
> And probably you want m{\G(?p{ $brackets })}, which would avoid
> recompile of the REx.

How about  m"\G$brackets"o? Is this equivalent? (hope so)

> > Come to think of it, perhaps 'all or nothing' is a better term for the operator
> > - ie, if something fails to match, the engine rewinds all the way back
> > to the beginning, bumps up its position by 1 from the beginning point
> > and then starts again.
> 
> Why this cannot be implemented by nested (?>) ?

I would hope that it would be. I'd also assume that this would be a good way
to describe the regex inside of perlre.

> > So matching the above without the \G modifier would look like:
> > 
> > tries '{.... (goes to end without success)
> > tries 't' (fails immediately)
> > tries 'h' (fails immediately)
> > tries 'i' (fails immediately)
> > tries 's' (fails immediately)
> > tries '{........ slowly } (succeeds.)
> 
> Why do you think it is not doing it like this?

Well, like I said, I'd hope so. All I'm doing is making sure that we are talking
about the same thing. Describing it this way in perlre would be a good idea,
IMO.

Mentioning 'use re 'debugcolor';' would be a good idea as well. 'Tis extremely
useful... Planning an interface with it and emacs? ;-)

Ed

Thread Previous | Thread Next


nntp.perl.org: Perl Programming lists via nntp and http.
Comments to Ask Bjørn Hansen at ask@perl.org | Group listing | About