use strict;
my $str = 'PolaBexar'; # Missing letter, extra letter, two capitals
my @checkpoints = ();
sub stack_checkpoints {
  my $val = shift;
  # Remove all checkpoint markers higher than $val
  @checkpoints = grep { $val>$_ } @checkpoints;
print "Backtracking to $val\n";
  push @checkpoints, $val;
}
print "Matched <$&> with ".@checkpoints." misses\n" if
$str =~ /(?:p|.{0,1}?(?{stack_checkpoints 0}))
         (?:o|.{0,1}?(?{stack_checkpoints 1}))
         (?:l|.{0,1}?(?{stack_checkpoints 2}))
         (?:a|.{0,1}?(?{stack_checkpoints 3}))
         (?:r|.{0,1}?(?{stack_checkpoints 4}))
         (?:b|.{0,1}?(?{stack_checkpoints 5}))
         (?:e|.{0,1}?(?{stack_checkpoints 6}))
         (?:a|.{0,1}?(?{stack_checkpoints 7}))
         (?:r|.{0,1}?(?{stack_checkpoints 8}))
        /x;