DOCBOOK: Language codes with countries

p. 120 of the Docbook book gives en_US as an example of an ISO 3166 country code for the lang attribute. However, these are normally written using hyphens rather than underscores; e.g. en-US. Which is correct for Docbook?

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+-----------------------+------------------------+-------------------+
| Elliotte Rusty Harold |  | Writer/Programmer |
+-----------------------+------------------------+-------------------+ 
|          The XML Bible, 2nd Edition (Hungry Minds, 2001)           |
|              http://www.ibiblio.org/xml/books/bible2/              |
|   http://www.amazon.com/exec/obidos/ISBN=0764547607/cafeaulaitA/   |
+----------------------------------+---------------------------------+
|  Read Cafe au Lait for Java News:  http://www.cafeaulait.org/      | 
|  Read Cafe con Leche for XML News: http://www.ibiblio.org/xml/     |
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