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Postings from July 2001
From: Mark-Jason Dominus
July 30, 2001 05:04
Message ID: firstname.lastname@example.org
> Eradicate-$^N-named-captures-reminder to sit in your inbox until
> you feel guilty enough.
On Thursday I had a long conversation with Autrijius Tang, who says
he is using $^N. I tried to understand why, and if an alternative
solution would do. My conclusion was that $^N is the correct solution
to his problem.
For Friedl's suggestion, $^N is unnecessary, because it's not hard to
transform a regex like
where the 37 can be generated at compile time. A module which
overloads regex constants can do a lexical analysis of the incoming
regex, count the parentheses, and generate the correct $37 variable.
$^N would make this simpler, but is not necessary.
In Autrijius's example, this is not enough. Autrijius is taking a
Template Toolkit template specification, and the output from the
template operation, and trying to reconstruct the values that were
substituted for the escape sequences in the template. A simple
example is: Given the template
<a href='[% URL %]'>[% TEXT %]</a>
and the output
<a href='http://www.perl.com'>Click Here</a>
He wants to generate the data structure:
( URL => "http://www.perl.com/",
TEXT => "Click Here",
To do this, he transforms the template into a dynamic regex something
For this simple example, it's easy to generate the appropriate
numerals for the $1, $2, and so on. But templates may also include
'for' loops, and nested 'for' loops:
[% foreach i %]
[% foreach j %]
i/j: <a href='[% URL %]'>[% TEXT %]</a>
[% endfor %]
[% endfor %]
1/1: <a href='http://www.perl.com'>Perl</a>
1/2: <a href='http://www.plover.com'>Plover Systems</a>
2/1: <a href='http://www.pm.org>Perl Mongers</a>
2/2: <a href='http://www.google.com'>Google</a>
In this case the dynamic pattern is much more complicated and it
becomes difficult to keep track of the numbers while generating it.
Being able to use $^N in place of $1, $2, ... is a major convenience.
On the other hand, I still wonder why there needs to be a new variable
for this. Could $^N be eliminated, and the functionality were
transferred to $+? That seems to me like a natural extension of what
$+ already does.
by Mark-Jason Dominus
by Jeffrey Friedl