use strict;
use warnings FATAL => 'all';
 
my $string = "aacbbbcac";
 
my $re1 = qr/((a+)?(b+)?(c))/;
while ($string =~ /$re1/g) {
  foreach (0..$#-) {
    printf "Group %d: <%s>\n", $_, defined($-[$_]) ? substr($string, $-[$_], $+[$_] - $-[$_]) : '';
  }
  print "\n";
}

##</code><code>##

Group 0: <aac>
Group 1: <aac>
Group 2: <aa>
Group 3: <>
Group 4: <c>

Group 0: <bbbc>
Group 1: <bbbc>
Group 2: <>
Group 3: <bbb>
Group 4: <c>

Group 0: <ac>
Group 1: <ac>
Group 2: <a>
Group 3: <>
Group 4: <c>