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

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