The Nutch FAQ explains that OpenSearch includes "all fields that are available at search result time." However, some Lucene column names can start with numbers. Valid XML tags cannot. If Nutch is generating OpenSearch results for a document with a Lucene document column whose name starts with numbers, the underlying Xerces library throws this exception:
org.w3c.dom.DOMException: INVALID_CHARACTER_ERR: An invalid or illegal XML character is specified.
So I have written a patch that tests strings before they are used to generate tags within OpenSearch.
I hope you merge this, or a better version of the patch!