use strict;
use warnings;

my $orig_string = 'abababa';

my @tests = ( qr/(b)(?!.*b)$/ , 
              qr/b(?!.*b)$/   ,
              qr/b(?!.*b.*$)/ ,
              qr/b(?=[^b]*$)/   );

foreach my $test_re ( @tests ) {
    my $scratch = $orig_string;
    print "$test_re\tdid",
          $scratch =~s/$test_re/x/ ? '    ' : "n't " ,
          "match '$orig_string', yielding '$scratch'\n";
}