$foo = reverse $foo;
$foo =~ s/b/X/;
$foo = reverse $foo;

[download]
there's probably a more succinct way to write this.

Update: i haven't tested this (or the other one actually), but something like this might work... find a 'b' that isn't followed by any more 'b's: 
s/b[^b]*$/X/;

[download]

Update:
After reading Transient's post (below), i realized my second regex is wrong. should be s/b([^b]*)$/X$1/, but now i feel like i just ripped it off from him.

In reply to Re: matching operator question by mrborisguy
in thread matching operator question by redss

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