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Date: Tue, 1 Jun 2004 16:47:02 +0200
From: "A. Pagaltzis" <pagaltzis@gmx.de>
To: fwp@perl.org
Subject: Re: Regular expression for years after a given one
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* Winter Christian <Christian.Winter@coc-ag.de> [2004-06-01 16:31]:
> Another approach:
> 
> use re 'eval';
> my $re = qr/(\d{4})(??{ $1>$year?'':'x{1000}' })$/;
> 
> Assuming, of course, that $_ never contains a smaller year
> number followed by thousand 'x'es. :-)

You can remove that limitation by replacing 'x{1000}' by '(?!)',
a zero-width negative lookahead, which *always* fails.

Your approach requires multiple regex compilation, too -- the
regex returned by your code is compiled in at run time.

If you do want to go down that route, I'd rather do it like this:

    my $re = qr/(\d{4})(?(?{ $1 <= $year })(?!))/;

which uses a conditional subexpression (?(cond)if-true), where
the condition is a Perl expression (?{ $1 <= $year }), to force
failure using the aforementioned (?!), if the condition is true
(which means I must test for less-than-or-equal, where you test
for greater-than).

-- 
Regards,
Aristotle
 
"If you can't laugh at yourself, you don't take life seriously enough."

]