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

According to RFC 2396, Appendix A:

  scheme        = alpha *( alpha | digit | "+" | "-" | "." )

Meaning:

The scheme should start with a letter (upper or lower case), and can contains letters (still upper and lower case), number, "+", "-" and ".".

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Note: in the case of

paparazzi:http:[//<host>[:[<port>][<transport>]]/

the scheme is only the "paparazzi" part.

Answer 2

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 ("-").

Answer 3

The scheme according to RFC 3986 is defined as:

scheme      = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

So the scheme must begin with an alphabetic character (A–Z, a–z) and may be followed by any number of alphanumeric characters, +, -, or ..