use strict;
use warnings;

my $str = '[][ab]c[]d';

for my $chr (split '', $str) {
    my $result = $chr eq '[' .. $chr eq ']';
    printf "%-2s %s\n", $chr, $result;
}

##</code><code>##

[  1
]  2E0
[  1
a  2
b  3
]  4E0
c  
[  1
]  2E0
d