s/(.*(?=right.*)|(.*))/\

echo "left"|perl -ne 'print if s/(.*(?=right.*)|(.*))/\1 <> \2/'        left <> left
echo "right"|perl -ne 'print if s/(.*(?=right.*)|(.*))/\1 <> \2/'       <> right
echo "leftright"|perl -ne 'print if s/(.*(?=right.*)|(.*))/\1 <> \2/'   left <> right