I don't think that you can directly.
The values for an enumeration in an ATTLIST
have to be either NOTATION
or NMTOKEN
.
https://www.w3.org/TR/REC-xml/#NT-AttlistDecl
[Definition: Enumerated attributes have a list of allowed values in their declaration ]. They must take one of those values. There are two kinds of enumerated attribute types:
Enumerated Attribute Types
[57] EnumeratedType ::= NotationType | Enumeration
[58] NotationType ::= 'NOTATION' S '(' S? Name (S? '|' S? Name)* S? ')' [VC: Notation Attributes]
[VC: One Notation Per Element Type]
[VC: No Notation on Empty Element]
[VC: No Duplicate Tokens]
[59] Enumeration ::= '(' S? Nmtoken (S? '|' S? Nmtoken)* S? ')' [VC: Enumeration]
[VC: No Duplicate Tokens]
A /
isn't allowed in the value of an NMTOKEN
.
It is allowed as the value of a NOTATION
.
<!NOTATION hw PUBLIC "h/w">
<!NOTATION sw PUBLIC "s/w">
<!ATTLIST item
item_category NOTATION (hw|sw) #REQUIRED>
]>