Re: [Gnu-arch-users] Preventing matches in regular expressions

[Jan Hudec]

On Tue, Aug 10, 2004 at 20:42:25 +0100, Andrew Suffield wrote:
> On Mon, Aug 09, 2004 at 03:29:53PM -0400, Aaron Bentley wrote:
> > That would add or update the regexes in the .arch-inventory.  But since 
> > backup is tried before unrecognized, the rule for backup must match 
> > every zip file except 'fox.zip'
> > 
> > Here's what I've finally come up with, using the example 'fox.zip':
> > 
> > backup ^(.{0,2}|.{4}.*|[^f]..|.[^o].|..[^x])\.zip$
> 
> Yes, that's right. It's an old and rather evil trick.
> 
> > But man, oh man, that's an ugly regex, and it'll only get less readable 
> > with longer filenames and multiple filenames.
> > 
> > Does anyone know a better general solution?
> 
> Well, you can implement regexp extensions that construct these
> things. They can get a bit memory-hungry though.
> 
> More normally you just step outside the bounds of regexps though. Perl

Lookahead/lookback assertions are perfectly regular. They definitely can
be expressed as a NFA. And negative lookahead assertion is all you need.
Using the vim syntax:

^\(foo\.zip$\)address@hidden

(well, it's actualy not exactly what I'd want. I'd want a negation per
se -- say ! is negation and & is conjunction (vim has conjunction, btw).
Then it whould simply be:

^((foo)!&.*)\.zip$

And it would still be regular expression. That is, it would still
describe a regular grammar (L3).

Obviously though, the regular expression must be in hackerlab rx syntax.

> does it by having regexps that aren't; perlre cannot be expressed as
> an NFA, although it still uses one to do *most* of the work.

Perl expression are not regular, but for completely different reasons.
Mainly because they can be recursive, because they can do backreferences
and because they can include arbitrary code.

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                                                 Jan 'Bulb' Hudec 
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