alternations are slower, but tr is faster for simple expressions:

               Rate alternate    single        tr
alternate  631309/s        --      -80%      -86%
single    3198780/s      407%        --      -27%
tr        4411077/s      599%       38%        --

use Benchmark qw(cmpthese);

cmpthese(-1, {
    alternate => \&alternate,
    single => \&single,
    tr => \&tr_op
});

my $s;
BEGIN{ $s = 'this is a string'; }

sub alternate {
    return $s =~ /[a-b]|[c-d]|[e-f]/ ;
}

sub single {
    return $s =~ /[a-f]/;
}

sub tr_op {
    return $s =~ tr/a-f/a-f/;
}
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