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

On 2 June 2010 12:22, Nicholas Clark <[email protected]> wrote:
> On Tue, Jun 01, 2010 at 10:38:10PM +0200, demerphq wrote:
>> On 1 June 2010 22:30, Abigail <[email protected]> wrote:
>> > On Tue, Jun 01, 2010 at 11:52:38AM -0600, karl williamson wrote:
>> > This one is sneaky as well:
>> >
>> >    $a = '(.)\1';
>> >    "aa" =~ /${a}/;      # True
>> >    "aa0" =~ /${a}0/;    # False!
>> >    "aa\x8" =~ /${a}0/;  # True!
>>
>> Wow. Those are gnarly.  Thats a great example of why using old style
>> back refs is dangerous. Might even be worth adding to the docs?
>
> Or, alternatively, that interpolation is happening too early in regexp
> compilation, such that it's possible to produce syntactically valid
> constructions by co-incidence.

Yeah, i thought that at first too, in reference to comments Dave M
made years ago in relation to embedding a qr// object in a larger
pattern.

But in terms of /strings/ I don't think it is actually correct. How
could we tell when its deliberate? People construct complex patterns
all the time out implicitly concatenated vars wheras a compiled
pattern is already compiled, so it makes sense to treat it as distinct
unit.

> (A bit like, IIRC, \N{} escapes being interpolated, and then treated as
> metacharacters)

I think thats a little different.

The problem there is that what \N{} escapes can expand to can vary
both at compile time in terms lexical-scoping AND at run time too,
with a whole tangle of subtle issues involved, like what happens when
you embed a pattern with one behaviour into a pattern that has the
same symbol mapped to a different behaviour. The fact that currently
this information is thrown away before run time made it even worse. I
think maybe Dave's idea would have solved this one, but I'm happy Karl
managed to fix it in the way he did.

cheers,
Yves


-- 
perl -Mre=debug -e "/just|another|perl|hacker/"