1  <?xml version="1.0"?>
 2  <!DOCTYPE doc [
 3  <!ELEMENT doc (#PCDATA)>
 4  <!ATTLIST doc At NMTOKEN "$$$">
 5  <!ATTLIST doc Ok NMTOKEN "1abc_2">
 6  <!ATTLIST doc bad IDREF "1abc_2">
 7  <!ATTLIST doc ok2 IDREF "abc_2">
 8  <!ATTLIST doc bad2 IDREFS "abc:1 1abc_2">
 9  <!ATTLIST doc ok3 IDREFS "abc:1 a1bc_2">
10  ]>
11  <doc val="v1"/>
12