Here's a small bench to highlight the speed difference in some of the chosen strategies. 
#! /usr/bin/perl -wl

use Benchmark 'cmpthese';

my (@v, @b);
@v[0 .. 19] = 'a' .. 'z';

for (1..1e6) {
    my %r;
    undef $r{$v[rand @v]} until keys %r == 4;
    push @b, join '', keys %r;
}

my @AoA = map [map ord()-97, split //,$_], @b;
my @vectors;
for my $t (@v) {
    push @vectors, pack "b*", pack "c*", map !!/$t/, @b;
}

my $AoA = pack 'C*', map @$_, @AoA;

cmpthese -5, {
    bitmap => sub {
        my $res = '';
        $res |= $vectors[$_] for @{$AoA[0]};
        int(@AoA) - unpack "%32b*", $res;
    },
    regex1 => sub {
        my $omit = $AoA[0];
        my $pat = '[^'.( join '', map quotemeta, pack 'C*', @$omit ).'
+]{4}';
        my $re = qr/\G(?:.{4})*?($pat)/s;
        int(() = $AoA =~ /$re/g);
    },
    regex2 => sub {
        my $pat = qr/[$b[0]]/;
        my $cnt = 0;
        $cnt += !/$pat/ for @b;
        $cnt;
    },
};

[download]

In reply to Re: Efficient selection mechanism? by oiskuu
in thread Efficient selection mechanism? by BrowserUk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.