IFS=X
eval set 0 \{-13..{31,29,31,30,31,30,31,31,30,31,30,31}} 0 0
set ${@/-*/0}
printf %\ 3.d ${@:13}|while read -n21 i;do
[[ $i$j =~ ^.{,20}[0-9] ]];k=$BASH_REMATCH
[[ $i$j =~ [0-9] ]]&&echo "$k"
j=$i
done
Note that non-ascii characters in the above source code will be escaped (such as \x9f).