Your sed syntax says: "replace every instance of 1-12 characters, followed by a space". I don't get the same results on my system.
How about this:
sed 's/[[:alpha:] ]//g' inputfile > outputfile
(Note that "cat" is not required.)(replaces all letters or spaces)
sed can also number the lines, but you need an extra command to get the line numbers on the same line.
Really good SED tutorial: