URI Schemes are in an IANA Registry at Uniform Resource Identifier (URI) Schemes
All URIs and Absolute URI references are formed with a scheme name, followed by a colon character (":"), and the remainder of the URI often called the scheme-specific part.
The syntax and semantics of the scheme-specific part are left largely to the specifications governing individual schemes, subject to certain constraints such as reserved characters and how to "escape" them.
The scheme name consists of a sequence of characters beginning with a letter and followed by any combination of: