New to Java

Hello, i'm having and html form and i would like the user in
the username field to type ONLY ascii characters.

For example, in other fields of the form i
would like the user to type his mother language but
as far as the username and password fields are concerned
the characters have to be ascii.

How am i supposed to check when the username is accepted/correct (*consists of ascii characters*)?
and which are the desirable characters a username must have (e.g. *?* is a desirable character in a username , *:* this one?)

Thanks, in advance!