From: Logan Capaldo <logancapaldo@...>
Date: 2006-07-15T03:59:43+09:00
Subject: Re: Help with a regexp


On Jul 14, 2006, at 2:16 PM, dblack@wobblini.net wrote:

> Hi --
>
> On Thu, 13 Jul 2006, Daniel Martin wrote:
>
>> Daniel Schierbeck <daniel.schierbeck@gmail.com> writes:
>>
>>> I'm trying to write a regular expression that matches bencoded
>>> strings, i.e. strings on the form x:y, where x is the numeric length
>>> of y.
>>>
>>> This is valid:
>>>
>>>   6:foobar
>>>
>>> while this is not:
>>>
>>>   4:foo
>>
>> I don't think that what you want to do is possible with a mere  
>> regular
>> expression.
>>
>> It might be possible using perl's special
>> evaluate-code-while-in-regexp (??{ code }) feature, but not with any
>> language that doesn't allow regular expression evaluations to escape
>> back into the host language.
>
> Is ??{ code } in Perl different from #{...} in Ruby?  (Not that I was
> able to solve Daniel's problem with #{...}, but I'm just curious about
> the comparison.)
>

According to _Programming Perl_ yes indeedy it is. ??{ } is "Match  
Time Pattern Interpolation", and it lets you do all sorts of evil  
(like matching nested parens with a regexp).

So in perl his code would be something like:

% cat mtpi.pl
$s1 = "3:abc";
$s2 = "24:abc";

print "Good\n" if ( $s1 =~ /(\d+):(??{'\w{' . $1 . '}'})/);
print "Bad\n" if ( $s2 =~ /(\d+):(??{'\w{' . $1 . '}'})/);

% perl mtpi.pl
Good

I apologize to any perlers if this isn't idiomatic (or clean) perl, I  
never had to use this kind of magic in my perl days and I had  
difficulty getting it to work when I stored the regexp in a variable.  
But the point is, is that it does work. Which is kind of scary.


>
> David
>
> -- 
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>                                     Slashdot, 7/12/2006!)
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> dblack@wobblini.net              => me
>