Docbook / XML restrictions for ID / IDREF

In my (generated) docbook files I got an ID / IDREF pair that started with a number (say 015_code with the attributes xml:id / linkend and consequently a warning was emitted (XMLMind in this case):

bad value for attribute "id" from namespace "http://www.w3.org/XML/1998/namespace"

In the book "XML in a nutshell" I read:

ID numbers are tricky because a number is not an XML name and therefor not a legal XML ID. The normal solution prefixes the values with an underscore or common letter.

This solution works for me, but when looking at my example 015_code this is not a number so my questions:

is my interpretation / quote incorrect and it be that an ID / IDREF cannot start with a digit
is this possible a bug in XMLMind

Solution

Your first bullet point is correct; ID/IDREF can’t start with a number. The value must be a valid Name...

Names and Tokens

[4]   NameStartChar ::=   ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
[4a]  NameChar      ::=   NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
[5]   Name          ::=   NameStartChar (NameChar)*
[6]   Names         ::=   Name (#x20 Name)*
[7]   Nmtoken       ::=   (NameChar)+
[8]   Nmtokens      ::=   Nmtoken (#x20 Nmtoken)*

From https://www.w3.org/TR/REC-xml/#sec-attribute-types ...

Validity constraint: ID

Values of type ID must match the Name production. A name must not appear more than once in an XML document as a value of this type; i.e., ID values must uniquely identify the elements which bear them.

and

Validity constraint: IDREF

Values of type IDREF must match the Name production, and values of type IDREFS must match Names; each Name must match the value of an ID attribute on some element in the XML document; i.e. IDREF values must match the value of some ID attribute.