public static String getNamespacePrefix(String namespaceURI) {
        return registry.getNamespacePrefix(namespaceURI);
    }
Obtain the prefix for a registered namespace URI.
It is not an error if the namespace URI is not registered.

@param namespaceURI The URI for the namespace. Must not be null or the empty string.
@return Returns the prefix registered for this namespace URI or null.
tika-xmp/src/main/java/org/apache/tika/xmp/XMPMetadata.java