You want to add the $ anchor in that pattern (=~ s/(b)(?!.*b)$/x/; Another approach is to use a sexeger 
my $string = "abababa";
$string = reverse $string;
$string =~ s/b/x/;
$string = reverse $string;
print $string,$/;

[download]
or if you're dealing exact strings, substr/rindex 
my $string = "abababa";
my $ri  = rindex( $string, 'b' );
substr( $string, $ri, 1) =  'x';
print $string,$/;

[download]
I'm not sure which is better (and that may vary from different perl versions, and/or size of string), so whats left is to benchmark.

update: and here is my benchmark. sexeger comes out on top in two different versions of perl 

use Benchmark 'cmpthese';

my $sref = {
    neglook => sub {
        my $string = "abababa";
        $string =~ s/(b)(?!.*b)/x/;
        return;
    },
    rindex => sub {
        my $string = "abababa";
        my $ri  = rindex( $string, 'b' );
        substr( $string, $ri, 1) =  'x';
        return;
    },
    sexeger => sub {
        my $string = "abababa";
        $string = reverse $string;
        $string =~ s/b/x/;
        $string = reverse $string;
        return;
    },
};

cmpthese( 3_000_000, $sref );

__END__

perl5.8.4
            Rate neglook  rindex sexeger
neglook 523195/s      --    -21%    -36%
rindex  659776/s     26%      --    -19%
sexeger 813670/s     56%     23%      --


perl5.6.2
            Rate neglook  rindex sexeger
neglook 719252/s      --    -19%    -20%
rindex  888889/s     24%      --     -1%
sexeger 901442/s     25%      1%      --

[download]

UPDATE: And of course, I hadn't taken my own advice. Here is the updated benchmark: 

use Benchmark 'cmpthese';
use strict;
use warnings;

my $sref = {
    neglook => sub {
        my $string = "abababa";
        $string =~ s/(b)(?!.*b)/x/;
        return;
    },
    neganch => sub {
        my $string = "abababa";
        $string =~ s/(b)(?!.*b)$/x/;
        return;
    },
    rindex => sub {
        my $string = "abababa";
        my $ri  = rindex( $string, 'b' );
        substr( $string, $ri, 1) =  'x';
        return;
    },
    sxedni => sub {
        my $string = "abababa";
        $string = reverse $string;
        my $ri  = index( $string, 'b' );
        substr( $string, $ri, 1) =  'x';
        $string = reverse $string;
        return;
    },
    sexeger => sub {
        my $string = "abababa";
        $string = reverse $string;
        $string =~ s/b/x/;
        $string = reverse $string;
        return;
    },
};

$|=1;
print "Revving up my CPU";
for( 1 .. 3 ){
    my $foo = 0;
    $foo++ for 1 .. 3_000_000;
    $foo-- for 1 .. 3_000_000;
    print '.';
}
print "\n";


cmpthese( 3_000_000, $sref );

__END__

perl5.8.4
             Rate  sxedni neglook  rindex sexeger neganch
sxedni   505221/s      --     -6%    -26%    -39%    -79%
neglook  534855/s      6%      --    -22%    -35%    -77%
rindex   685714/s     36%     28%      --    -17%    -71%
sexeger  824176/s     63%     54%     20%      --    -65%
neganch 2369668/s    369%    343%    246%    188%      --



perl5.6.2
             Rate  sxedni neglook  rindex sexeger neganch
sxedni   596303/s      --    -17%    -33%    -33%    -80%
neglook  716503/s     20%      --    -19%    -19%    -76%
rindex   884695/s     48%     23%      --      0%    -71%
sexeger  884695/s     48%     23%      0%      --    -71%
neganch 3003003/s    404%    319%    239%    239%      --

[download]

MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"
I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).
** The third rule of perl club is a statement of fact: pod is sexy.

In reply to Re: RE - match from right by PodMaster
in thread RE - match from right by bangers

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