Replace a directory string with a ".." using sed

Question

How can I do these in sed?

#input                 #output
file.txt              "nothing" 
dir1/                 ../
dir1/file.txt         ../
dir1/dir2/            ../../
dir1/dir2/file.txt    ../../

Let's say #input is placed to $var1

sed "do something" <<< $var1
echo $var1

Answer 1

You can try this GNU sed

sed "s#dir[0-9]\+/*#\.\./#g; s#file\.txt##g"