Valid characters for URI schemes?

Question

I was thinking about Registering an Application to a URL Protocol and I\'d like to know, what characters are allowed in a scheme?

Some examples:

• h323

Answer 1

Quoth RFC 2396:

Scheme names consist of a sequence of characters beginning with a lower case letter and followed by any combination of lower case letters, digits, plus ("+"), period ("."), or hyphen ("-").