return if ! ( /\.[jJ][pP][eE]?[gG]$/ );
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The brackets mean "one of these characters" while the question mark means "the preceeding element is optional" or "0 or 1 of the preceeding element", depending on how you want to look at it.

Of course, I'd switch out the "if !" for "unless"... but there be another religious war ;-)

(PS - unless performance matters, just use the i modifier - /\.jpe?g/i is just about as fast as the above.)