How Parsers Handle Namespaces

Namespaces are not part of XML 1.0. They were invented about a year after the original XML specification was released. However, care was taken to ensure backwards compatibility. Thus, an XML 1.0 parser that does not know about namespaces should not have any trouble reading a document that uses namespaces. Colons are legal characters in XML 1.0 element and attribute names. The parser will simply report that some of the names contain colons. Possible problems arise in the rare cases where different qualified names resolve to the same full name or where the same qualified name indicates a different full name in different parts of a document.

A namespace-aware parser does add a couple of checks to the normal well-formedness checks that a parser performs. Specifically, it checks to see that all prefixes are mapped to URIs. It will reject documents that use unmapped prefixes (except for xml and xmlns when used as specified in the XML 1.0 or Namespaces in XML specifications.) It will further reject any element or attribute names that contain more than one colon. Otherwise, it behaves almost exactly like a non-namespace-aware parser. Other software that sits on top of the raw XML parser, an XSLT engine for example, may treat elements differently depending on to which namespace they belong. However, the XML parser itself mostly doesn't care as long as all well-formedness and namespace constraints are met.

A possible exception occurs in the unlikely event that elements with different prefixes belong to the same namespace. In this case, a namespace-aware parser will report the elements as being the same, while a non-namespace-aware parser will report them as different. About equally unlikely is the case where two elements or attributes with the same qualified name are in different namespaces because the common prefix is bound to different URIs in different places in the document. Slightly more likely is the case where two unprefixed names are placed in different default namespaces. In both these cases, a namespace-aware processor will report them as different, whereas a non-namespace-aware processor will treat them the same. Many parsers let you turn namespace processing on or off as you see fit.