$ gsed -f sum.sed
1+2
RESULT: 3
10+99
RESULT: 109
9992 + 19
RESULT: 10011
6456+ 435234
RESULT: 441690
789 + 123467001
RESULT: 123467790
1+2
RESULT: 3
10+99
RESULT: 109
9992 + 19
RESULT: 10011
6456+ 435234
RESULT: 441690
789 + 123467001
RESULT: 123467790