The extensibility requirement is met by allowing an arbitrary (but registered) string to be used as a prefix. A prefix is chosen as left to right parsing is more common than right to left. The choice of a colon as separator of the prefix from the rest of the URI was arbitrary.
The decoding of the rest of the string is defined as a function of the prefix. New prefixed are introduced for new schemes as necessary, in agreement with the registration authority. The registration of a new scheme clearly requires the definition of the decoding of the URI into a given name space, and a definition of the properties and, where applicable, resolution protocols, for the name space.
The completeness requirement is easily met by allowing particularly strange or plain binary names to be encoded in base 16 or 64 using the acceptable characters.
The printability requirement could have been met by requiring all schemes to encode characters not part of a basic set. This led to many discussions of what the basic set should be. A difficult case, for example, is when an ISO latin 1 string appears in a URL, and within an application with ISO Latin-1 capability, it can be handled intact. However, for transport in general, the non-ASCII characters need to be escaped.
The solution to this was to specify a safe set of characters, and a general escaping scheme which may be used for encoding "unsafe" characters. This "safe" set is suitable, for example, for use in electronic mail. This is the canonical form of a URI.
The choice of escape character for introducing representations of non-allowed characters also tends to be a matter of taste. An ANSI standard exists in the C language, using the back-slash character "\". The use of this character on unix command lines, however, can be a problem as it is interpreted by many shell programs, and would have itself to be escaped. It is also a character which is not available on certain keyboards. The equals sign is commonly used in the encoding of names having attribute=value pairs. The percent sign was eventually chosen as a suitable escape character.
There is a conflict between the need to be able to represent many characters including spaces within a URI directly, and the need to be able to use a URI in environments which have limited character sets or in which certain characters are prone to corruption. This conflict has been resolved by use of an hexadecimal escaping method which may be applied to any characters forbidden in a given context. When URLs are moved between contexts, the set of characters escaped may be enlarged or reduced unambiguously.
The use of white space characters is risky in URIs to be printed or sent by electronic mail, and the use of multiple white space characters is very risky. This is because of the frequent introduction of extraneous white space when lines are wrapped by systems such as mail, or sheer necessity of narrow column width, and because of the inter-conversion of various forms of white space which occurs during character code conversion and the transfer of text between applications. This is why the canonical form for URIs has all white spaces encoded.