Android : Alphanumeric Input In An EditText View (how To Use "digits")?

I need to create a form to insert a username for my application, so I want the user to be able to insert only letters, numbers and some symbols.

I noticed that there is not a simple way to do it (while if I needed a numeric fields the implementation would be easy).

The most easy way that I found is based on the usage of the attribute "digits" of the EditText View. Hence, I inserted a string with the list of characters to be accepted (example: 1234abce).

The filter works as expected. When the keyboard appears, the user can insert only the characters that I specified in the "digits" field. The problem is that the keyboard does not accept the "Enter" key, so there is no way for the user to click/touch Enter so to confirm their Input and make the on-screen keyboard to disappear.