Re: What should /\08/ match and do?

Abigail wrote:
> On Thu, Jun 03, 2010 at 10:02:43AM -0600, karl williamson wrote:
>> I'm still digesting what people have written.  But I
>>
>> What I'm hearing is that it's not ok to add warnings on long-standing  
>> possible usages that don't make sense at least to the person proposing  
>> the message, such as me with empty {}, and Ævar with the single  
>> character sub names.  FWIW, I think that it's a mistake to not warn on  
>> using a reserved term; I don't think that putting it into the docs alone  
>> suffices.  And there would be more unhappy people when the construct  
>> they were using suddenly goes away than there would be to get a  
>> deprecation warning in advance that they are using a reserved term.
>>
>> But that's what I'm hearing.  But, Abigail originally said that it was  
>> ok for \08 to warn; and Nicholas agreed; but then I hear in the latest  
>> post on the subject from Abigail that "I'd say that pointing this  
>> potential problem out in the docs should be enough."
> 
> I made that remark regarding to the more general concatenation problem:
> 
>     $a = '(.)\1';
>     $p = qr /${a}1/;  # Equivalent to /(.)\011/
> 
> I do not think this should warn - it's not warning now, and, IMO, it's 
> uncommon enough to not introduce a new warning. [*]
> 
> The warning on use of \18 already exists - regardless whether the \18
> appears as is, or comes from a concatenation [+]. The only change I suggested
> was the wording of the warning (as the '8' isn't ignored).

OK.  What about '\0a'  It doesn't warn currently.  It is analogous to 
\08 but has a non-octal hex digit instead of a non-octal decimal digit. 
  Should it warn?

> 
>>                                                       I can go either  
>> way.  The patch I had prepared simply silenced the warning; I can add  
>> text to the documents, perhaps using his sly examples, as Yves  
>> suggested.  And I think it is a good idea to change them to stress using  
>> \g{} for backreferences.
>>
>> But I'm not sure we have consensus.
> 
> 
> [*]  Unless one can manage to have a very accurate detection - that is, 
>      concatenating a variable whose value ends in \\[0-7] with something
>      that starts with [0-7]. My worry for new warnings for existing constructs
>      is always false positives.
> 
> [+]  Although, when using C<< $a = '\1'; /${a}8/ >> Perl will warn, but
>      is likely to do what the programmer intends. Eliminating that case 
>      would be nice.
> 
> Abigail
>