DTD on ID field

Question

I have the following basic XML and DTD:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE items [
    <!ELEMENT items (item*)>
    <!ELEMENT item (name, age*)>
    <!ATTLIST item id ID #IMPLIED>
    <!ELEMENT name (#PCDATA)>
    <!ELEMENT age (#PCDATA)>
]>
<items>
    <item id="1">
        <name>Tom</name>
        <age>20</age>
    </item>
    <item id="2">
        <name>mike</name>
        <age>20</age>
    </item>
    <item id="3">
        <name>erik</name>
        <age>20</age>
    </item>
</items>

This validates if I change ATTLIST item id ID to ATTLIST item id CDATA, but currently I get the following error:

What's the issue here with the ID field? How should this be used properly?

Answer 1

ID must match the name constraint, so needs to start with a "NameStartChar", i.e., not a number. For example, this will work:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE items [
    <!ELEMENT items (item*)>
    <!ELEMENT item (name, gender, age*)>
    <!ATTLIST item id ID #IMPLIED>
    <!ELEMENT name (#PCDATA)>
    <!ELEMENT gender (#PCDATA)>
    <!ELEMENT age (#PCDATA)>
]>
<items>
    <item id="a1">
        <name>Tom</name>
        <gender>M</gender>
        <age>20</age>
    </item>
    <item id="a2">
        <name>Sarah</name>
        <gender>F</gender>
        <age>20</age>
    </item>
</items>

Though if you need, for example, an auto-incrementing ID, just use CDATA as the type instead.