Update "Character encoding" and related provisions #438

PURL-SPECIFICATION.rst

@@ -114,9 +114,11 @@ Rules for each ``purl`` component
 
 A ``purl`` string is an ASCII URL string composed of seven components.
 
-Some components are allowed to use other characters beyond ASCII: these
-components must then be UTF-8-encoded strings and percent-encoded as defined in
-the "Character encoding" section.
+Except as expressly stated otherwise in this section, each component:
+
+- MAY be composed of any of the characters defined in the "Permitted
+  characters" section
+- MUST be encoded as defined in the "Character encoding" section
 
 The rules for each component are:
 
@@ -219,30 +221,24 @@ The rules for each component are:
   - The ``subpath`` MUST be interpreted as relative to the root of the package
 
 
-Character encoding
-~~~~~~~~~~~~~~~~~~
-
 Permitted characters
---------------------
-
-A canonical ``purl`` is an ASCII string composed of these characters:
+~~~~~~~~~~~~~~~~~~~~
 
-- alphanumeric characters ``A to Z``, ``a to z``, ``0 to 9``,
-- the ``purl`` separators ``:/@?=&#`` (colon ':', slash '/', at sign '@',
-  question mark '?', equal sign '=', ampersand '&' and pound sign '#'), and
-- these punctuation marks ``%.-_~`` (percent sign '%', period '.', dash '-',
-  underscore '_' and tilde '~').
+A canonical ``purl`` is composed of these permitted ASCII characters:
 
-All other characters MUST be encoded as UTF-8 and then percent-encoded.
-In addition, each component specifies its permitted characters and
-its percent-encoding rules.
+- the Alphanumeric Characters: ``A to Z``, ``a to z``, ``0 to 9``,
+- the Punctuation Characters: ``.-_~`` (period '.',
+  dash '-', underscore '_' and tilde '~'),
+- the Plus Character: ``+`` (plus '+'),
+- the Percent Character: ``%`` (percent sign '%'), and
+- the Separator Characters ``:/@?=&#`` (colon ':', slash '/', at sign '@',
+  question mark '?', equal sign '=', ampersand '&' and pound sign '#').
 
 
 ``purl`` separators
--------------------
+~~~~~~~~~~~~~~~~~~~
 
-These ``purl`` separator characters MUST NOT be percent-encoded when used as
-``purl`` separators:
+This is how each of the Separator Characters is used:
 
 - ':' (colon) is the separator between ``scheme`` and ``type``
 - '/' (slash) is the separator between ``type``, ``namespace`` and ``name``
@@ -256,17 +252,34 @@ These ``purl`` separator characters MUST NOT be percent-encoded when used as
 - '#' (number sign) is the separator before ``subpath``
 
 
-Percent-encoding rules
-----------------------
-
-When applying percent-encoding or decoding to a string, use the rules of RFC
-3986 section 2 (https://datatracker.ietf.org/doc/html/rfc3986#section-2).
-
-Each component defines when and how to apply percent-encoding and decoding to
-its content.
+Character encoding
+~~~~~~~~~~~~~~~~~~
 
-When percent-encoding is required, all characters MUST be encoded except for
-the colon ':'.
+- In the "Rules for each ``purl`` component" section, each component
+  defines when and how to apply percent-encoding and decoding to its content.
+- When percent-encoding is required by a component definition, the component
+  string MUST first be encoded as UTF-8.
+- In the component string, each "data octet" MUST be replaced by the
+  percent-encoded "character triplet" applying the percent-encoding mechanism
+  defined in RFC 3986 section 2.1 (https://datatracker.ietf.org/doc/html/rfc3986#section-2.1),
+  including the RFC definition of "data octet" and "character triplet",
+  and using these definitions for RFC's "allowed set" and "delimiters":
+
+  - "allowed set" is composed of the Alphanumeric Characters and the
+    Punctuation Characters
+  - "delimiters" is composed of the Separator Characters
+
+- The following characters MUST NOT be percent-encoded:
+
+  - the Alphanumeric Characters,
+  - the Punctuation Characters,
+  - the Separator Characters when being used as ``purl`` separators,
+  - the colon ':', whether used as a Separator Character or otherwise, and
+  - the percent sign '%' when used to represent a percent-encoded character.
+
+- Where the space ' ' is permitted, it MUST be percent-encoded as '%20'.
+- With the exception of the percent-encoding mechanism, the rules regarding
+  percent-encoding are defined by this specification alone.
 
 
 How to build ``purl`` string from its components