Fix #9507 Describe accurately acceptable package names

doc/cabal-package-description-file.rst

@@ -318,20 +318,33 @@ describe the package as a whole:
     tools require the package-name specified for this field to match
     the package description's file-name :file:`{package-name}.cabal`.
 
-    Package names are case-sensitive and must match the regular expression
-    (i.e. alphanumeric "words" separated by dashes; each alphanumeric
-    word must contain at least one letter):
-    ``[[:digit:]]*[[:alpha:]][[:alnum:]]*(-[[:digit:]]*[[:alpha:]][[:alnum:]]*)*``.
+    A valid package name comprises an alphanumeric 'word'; or two or more
+    such words separated by a hyphen/minus character (``-``). A word cannot be
+    comprised only of the characters ``0`` to ``9``.
 
-    Or, expressed in ABNF_:
+    An alphanumeric character is one in one of the Unicode Letter categories
+    (Lu (uppercase), Ll (lowercase), Lt (titlecase), Lm (modifier), or
+    Lo (other)) or Number categories (Nd (decimal), Nl (letter), or No (other)).
+
+    Package names are case-sensitive.
+
+    Expressed as a regular expression:
+
+    ``[0-9]*[\p{L}\p{N}-[0-9]][\p{L}\p{N}]*(-[0-9]*[\p{L}\p{N}-[0-9]][\p{L}\p{N}]*)*``
+
+    Expressed in ABNF_:
 
     .. code-block:: abnf
 
         package-name      = package-name-part *("-" package-name-part)
-        package-name-part = *DIGIT UALPHA *UALNUM
+        package-name-part = *DIGIT UALPHANUM-NOT-DIGIT *UALNUM
 
-        UALNUM = UALPHA / DIGIT
-        UALPHA = ... ; set of alphabetic Unicode code-points
+        DIGIT = %x30-39 ; 0-9
+
+        UALNUM = UALPHANUM-NOT-DIGIT / DIGIT
+        UALPHANUM-NOT-DIGIT = ... ; set of Unicode code-points in Letter or
+                                  ; Number categories, other than the DIGIT
+                                  ; code-points
 
     .. note::
 
@@ -1480,13 +1493,13 @@ system-dependent values for these fields.
     Version constraints use the operators ``==, >=, >, <, <=`` and a
     version number. Multiple constraints can be combined using ``&&`` or
     ``||``.
-    
+
     .. Note::
 
        Even though there is no ``/=`` operator, by combining operators we can
        skip over one or more versions, to skip a deprecated version or to skip
        versions that narrow the constraint solving more than we'd like.
-       
+
        For example, the ``time =1.12.*`` series depends on ``base >=4.13 && <5``
        but ``time-1.12.3`` bumps the lower bound on base to ``>=4.14``.  If we
        still want to compile with a ``ghc-8.8.*`` version of GHC that ships with