I was curious about whether the anchoring really reduced the work, so I ran a benchmark of the various suggested methods. I found some of the results surprising -- like the fact that xeger was not a lot slower than the anchored match. And anchoring the 2-step solution slowed it down. 
@strings = ( 'There was a bright man from Nantucket'
        ,'Abba babbles about bubble blowers'
        ,'Enough with the excess bs already'
        ,'None at all in this one');
my @c;

use Benchmark 'cmpthese';

%methods = (
  'anch'  => sub { s/b(?=[^b]*$)/x/ for @c=@strings},
  'xeger' => sub { $_ = reverse, s/b/x/, $_ = reverse for @c=@strings 
+},
  'plain' => sub { s/(.*)b/$1x/ for @c=@strings},
  '2part' => sub { /.*(?=b)/g and s/\Gb/x/ for @c=@strings },
  '2part-A' => sub { /b(?=[^b]*$)/g and s/\Gb/x/ for @c=@strings },
  'substr' => sub { substr($_, rindex($_, 'b'), 1) = 'x' for @c=@strin
+gs},
  'neglook' => sub { s/b(?!.*b)/x/ for @c=@strings }
);

cmpthese(-3, \%methods );

[download]
Results: 
          Rate   plain 2part-A   2part neglook   xeger    anch  substr
plain   3208/s      --    -31%    -35%    -44%    -47%    -51%    -64%
2part-A 4630/s     44%      --     -6%    -20%    -23%    -29%    -48%
2part   4948/s     54%      7%      --    -14%    -18%    -24%    -45%
neglook 5767/s     80%     25%     17%      --     -4%    -12%    -36%
xeger   6029/s     88%     30%     22%      5%      --     -8%    -33%
anch    6536/s    104%     41%     32%     13%      8%      --    -27%
substr  8985/s    180%     94%     82%     56%     49%     37%      --

[download]
Caution: Contents may have been coded under pressure.

In reply to Re^2: RE - match from right by Roy Johnson
in thread RE - match from right by bangers

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