left          \1=left \2=
right         \1=     \2=right
rightabc      \1=     \2=rightabc
leftright     \1=left \2=right
leftrightabc  \1=left \2=rightabc

##</code><code>##

s/(.*)(right)?/\1 <> \2/

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

##</code><code>##

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