Bash function named with the question mark char does not work anymore

Bash function named with the question mark char does not work anymore

In Ubuntu 11.10 I had a bash command/alias/function like:
function ?() {
xxx $1
}
This worked like a charm.
Now in Ubuntu 13.04 it doesn't work anymore. When I try to execute the
command with some argument I get:
0: command not found
What has changed that I cannot use this abbreviation anymore?
P.S. Of course it's possible to change the alias name, it's just a
question out of curiousity.