Regular expressions in URLs are frowned up in Cat (and I agree; it’s harder to get right than other approaches). The package that does it was even split out of the core last year. I have done this kind of date thing before and this is what I’d recommend.

• Explicitly take 3 args for year, month, date (you can pare this out into other controllers too; one for year index, one for month index, and the original for a day’s view/index).
• Validate that is it a real date with Date::Calc’s check_date. Invalid date yields a 404.
• Then use the known real date for whatever model lookup (DB/file/etc), then present results, no results yields a 404.

package MyApp::Controller::WhatEver;
use warnings;
use strict;
use parent "Catalyst::Controller";
use Date::Calc "check_date";

sub by_date : Path("d") Args(3) {
    my ( $self, $c, $yyyy, $mm, $dd ) = @_;
    $c->detach( YOUR_404_HANDLER ) unless eval { check_date($yyyy,$mm,
+$dd) };
    my $some_rs = $c->model("Your::Schema")->search({ ...use dates... 
+});
    $some_rs->count || $c->detach( YOUR_404_HANDLER );
}
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Update (2014-07-26): Sloppy. check_date is fatal with bad input, added eval, See also: Re: Script to validate date fails.