sub using_index {
   our $seq; *seq = \$_[0];
   my @groups;

   my $pos = -1;
   my $start = -1;

   for (;;) {
      my $new_pos = index($seq, 'M', $pos+1);

      if ($new_pos < 0) {
         if (defined($start)) {
            push(@groups, [ $start, $pos ]);
         }
         last;
      }

      if ($start < 0) {
         $start = $new_pos;
      }
      elsif ($new_pos - $pos > 1) {
         push(@groups, [ $start, $pos ]);
         $start = $new_pos;
      }

      $pos = $new_pos;
   }

   return @groups;
}

##</code><code>##

use strict;
use warnings;

use Benchmark qw( cmpthese );

sub using_index {
   our $seq; *seq = \$_[0];
   my @groups;

   my $pos = -1;
   my $start = -1;

   for (;;) {
      my $new_pos = index($seq, 'M', $pos+1);

      if ($new_pos < 0) {
         if (defined($start)) {
            push(@groups, [ $start, $pos ]);
         }
         last;
      }

      if ($start < 0) {
         $start = $new_pos;
      }
      elsif ($new_pos - $pos > 1) {
         push(@groups, [ $start, $pos ]);
         $start = $new_pos;
      }

      $pos = $new_pos;
   }

   return @groups;
}

sub using_regexp {
   our $seq; *seq = \$_[0];
   my @groups;
   push(@groups, [ $-[0], $+[0]-1 ]) while $seq =~ /M+/g;
   return @groups;
}

{
   my $seq = "IIIIIMMMMMMMMMMMOOOOOOOOOOOOOOOOMMMMMMMMMMMMMIIIIIMMMMMMMMMOOOOOOOOOOOOOOMMMMMMMMMMMMMIIIMMMMMMMMMMMOOOOOOOOOOOOOOOMMMMMMMMMMMIIIIIIMMMMMMMMMMMMMOOOOOOOOOOOOOOOOOOOOOOOMMMMMMMIIIMMMMMMMMMOOOOOOOOOOOOOOOOOOOOOOOOOOOMMMMMMMIIIIMMMMMMMMMMMOOOOOOOOOOOOOOOOOOOOOMMMMMMMIIIMMMMMMMMMOOOOOOOOOOOOOOOOOOOOOOOOOMMMMMMMMMIIIMMMMMMMMMMMOOOOOOOOOOOOOOOOOMMMMMMMMI";

   print("using_index\n");
   print("-----------\n");
   printf("%d to %d\n", @$_) foreach using_index($seq);

   print("\n");

   print("using_regexp\n");
   print("------------\n");
   printf("%d to %d\n", @$_) foreach using_regexp($seq);

   print("\n");

   cmpthese(-3, {
      using_index  => sub { my @groups = using_index  $seq; 1; },
      using_regexp => sub { my @groups = using_regexp $seq; 1; },
   });
}

##</code><code>##

               Rate  using_index using_regexp
using_index  2039/s           --         -63%
using_regexp 5467/s         168%           --