Hi , The code for the variable width look_ahead assertion is

 $product = "MOTOROLAx|xx|xx|x01-01-1900x|xYx|xx";
 $product =~ s/(?<=\|)xx(?=(\||$))/x x/g;
 print "[ $product ]\n";
 __END__


[ MOTOROLAx|x x|x x|x01-01-1900x|xYx|x x ]
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The paranthesis was not there ,thanks Enlil for your help on finding this .