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 “.”.
Note: in the case of
paparazzi:http:[//<host>[:[<port>][<transport>]]/
the scheme is only the “paparazzi” part.