Alphanumeric username javascript

Hello,

I am trying to write some javascript code to validate a registration form. I use the match function of the string library, so for example Quote:

"someusername".match(reg);

I know that there is plenty of resources on the Internet but unfortunately I have tried too many things and still validation does not work. Can someone please tell me how the regular expression should look like??
The regular expression is for the username which needs to be only alphanumeric.

Thank you.