I think eiii meant yes, you will be able to, not yes, you won't be allowed to.
alphanumeric means alpha characters (a-z) and numeric characters (0-9). You can still use non-alphanumeric characters in your name, they will just be ignored (except in clan tags).
Say, for a realistic example, I'm in crossbones (which has the .xb. tag), and I have "jitspoe" registered. I could log in as .xb.[jitspoe]. The clan tag, .xb., would be stripped off, leaving [jitspoe], and the []'s would be ignored, since they're not alphanumeric, leaving just "jitspoe", which is registered on the login server.
If somebody else came along and tried to use -jitspoe- or **jitspoe** or some variant like that, he would not be able to (unless he had my password).