I can't believe I may one day die without the ability to transmit to all humanity the utility of this:

cat a | grep -o "[0-9][0-9][0-9][0-9]:" | cut -d : -f 1 > x ;
sort x > xs ; cut -d , -f 1 b > y ; sort y > ys ;
diff xs ys > xydiff ;

(Let alone myself.)