From: Kristof Bastiaensen <kristof@...>
Date: 2004-05-15T03:58:52+09:00
Subject: Re: Regexp Error?

On Fri, 14 May 2004 10:41:20 -0600, Ara.T.Howard wrote:

> On Fri, 14 May 2004, Kristof Bastiaensen wrote:
> 
>> On Fri, 14 May 2004 08:54:22 -0600, Ara.T.Howard wrote:
>> > 
>> > what guy is saying here is that __unless__ both '(.*)' __and__ '.*'
>> > match in the above - you don't have match.  we all agree that the
>> > '(.*)' matches the entire 'abc' and we all agree that the pattern
>> > '(.*).*' matches 'abc' - therefore we all agree that it's correct
>> > that certain pattern match zero width positions in strings and
>> > consume no chars whilst still matching.
>> 
>> Agree until here.
>> 
>> > therefore we
>> > all agree that
>> > 
>> >   '123-456'.gsub /.*$/, 'X' => 'XX'
>> > 
>> > because if it __didn't__ '(.*).*' could not match 'abc'
>> > 
>> > 
>> > -a
>> 
>> I don't agree.  The things is, you can't really consume 'nothing'. If I
>> follow your reasoning, the above should give 'XXX', because 123-456
>> matches /(.*)(.*)(.*)/.  How much empty spaces are there between two
>> characters?  One, three, infinite? There have to be rules, and I would
>> find the following the most logical one:
>> 
>> * when the empty space is already matched before, don't match again,
>> unless explicitly against ? or *.  (gawk behaviour?)
>> 
>> Kristof
> 
> but then
> 
>   /^$/ would not match ''
> 
> empty tokens must not consume and must stack
> 
> -a

I would say it does match, because the empty token wouldn't have been
matched at that moment.  Perhaps a better way to formulate it would be
this:
*  consume the empty token when matching greedily, but still allow it
   to be matched against * and ?.
This way the empty token will treated as part of any string (not a
seperate entity).
I don't want to say it is the only way, but it seems the most logical to
me.

(But maybe it is best to just avoid such a regexp :-)

k