'okoK'=~ /^(o|k)(k|i)(O|K)(K)/

##</code><code>##

'okoK'=~ /
   ^
   (?: (o|k)
       (?: (k|i)
           (?: (O|K)
               (?: (K)
               )?
           )?
       )?
   )?
/x;

my $n = length($&);

#  -or-

use List::Util qw( first );

my $n = first { defined($-[$_]) } 1..4;