Rate brackets  slash i
brackets 719424/s       --      -7%
slash i  775194/s       8%       --

##</code><code>##

            Rate brackets  slash i
brackets  3508/s       --     -66%
slash i  10434/s     197%       --

##</code><code>##

#!/usr/bin/perl -w
use strict;
use Benchmark 'cmpthese';

our $s = "HELLOWORLD" x 1024;
cmpthese(1_000_000, {
                     'slash_i' => sub { $s =~ m/hello/i; $s =~ m/jello/i },
                     'brackets' => sub { $s =~ m/[Hh][Ee][Ll][Ll][Oo]/;
                                         $s =~ m/[Jj][Ee][Ll][Ll][Oo]/; },
                    });

##</code><code>##

            Rate brackets  slash_i
brackets  5633/s       --     -66%
slash_i  16683/s     196%       --