DamnDirtyApe's solution does not account for the fact that each tier may be optional, though. Additionally, Quantum::Superpositions is not really intended for serious work.

A good way to go about this is to programmatically construct a regex that will only match valid names.

Your specification of the problem is insufficient, though. I will assume that of each tier, only one kind of segment may appear, if one appears at all, ie a file may have aa or ab or neither, but not both. The note that bb doesn't have to immediatly follow aa in particular makes this somewhat questionable. In any case, the code would look something like this:

#!/usr/bin/perl
use strict;
use warnings;

use Test::More;

my @tier = (
    [ qw( aa ab ) ],
    [ qw( bb bc ) ],
    [ qw( cc cd ) ],
    [ qw( dd de ) ],
    [ qw( ee ef ) ],
    [ qw( ff fg ) ],
    [ qw( gg gh ) ],
    [ qw( hh hi ) ],
    [ qw( ii ij ) ],
    [ qw( jj jk ) ],
);

my $rx = do {
    local $" = ' | ';
    my $tiers = join "\n", (
        "\\A",
        map( "(?: (?: \\A | (?<! \\A ) _ ) (?: @$_ ) \\d* )?", @tier )
+,
        "\\z",
    );
    qr/$tiers/x;
};

my %testcase = (
    aa_bb_cc       => 1,
    aa_bb123_cc_dd => 1,
    aa_cc          => 1,
    cc_ee123_jj    => 1,
    bb_aa          => 0,
    bb12_aa43      => 0,
    1              => 0,
    _              => 0,
    aa_            => 0,
    _ee            => 0,
    _12            => 0,
);

plan tests => keys( %testcase ) + 0;

while( my( $test, $expect_match ) = each %testcase ) {
    if( $expect_match ) {
        like( $test, $rx, "match $test" );
    }
    else {
        unlike( $test, $rx, "reject $test" );
    }
}
__END__
1..11
ok 1 - reject bb12_aa43
ok 2 - match cc_ee123_jj
ok 3 - match aa_bb123_cc_dd
ok 4 - reject bb_aa
ok 5 - reject _ee
ok 6 - match aa_cc
ok 7 - reject _
ok 8 - reject _12
ok 9 - reject aa_
ok 10 - match aa_bb_cc
ok 11 - reject 1

[download]

Put a print "$rx\n"; in there somewhere to see what pattern the code produces.

Update: added more degenerate test cases and adjusted regex slightly to catch case 5 (simply added (?<! \\A ) to make sure the underscore cannot match at the start of the string), thanks to shenme.

Makeshifts last the longest.

In reply to Re: hash keys and regex by Aristotle
in thread hash keys and regex by state-o-dis-array

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