That's a good description: b not followed by a string containing a b. You could also do it with "sexeger" (using a regex on the reverse of the string): 
$string = reverse $string;
$string =~ s/b/x/;
$string = reverse $string;

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though that seems a little excessive for this case. The other usual way to do it is: 
$string =~ s/(.*)b/$1x/;

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or (at least as clunky as your original): 
$string =~ m/.*(?=b)/g and $string =~ s/\Gb/x/;

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Finally, you can do it without using a regex at all: 
substr($string, rindex($string, 'b'), 1) = 'x';

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Caution: Contents may have been coded under pressure.

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

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