Anchor to the right with the $ metacharacter. By anchoring to the right, and using a negative character class, you minimize the amount of work needed to find the last occurrence even if it's not the last thing in the string.

s/b([^b]*)$/x$1/

Update:
And the following should yield better performance by using a positive lookahead assertion (eliminating the need for capturing parens), and still anchoring to the right.

s/b(?=[^b]*$)/x/

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Dave

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

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