How to Construct an XML QName with Prefix and Local Part

In XML, a Qualified Name (QName) resolves naming conflicts by binding an element or attribute name to a specific Uniform Resource Identifier (URI) namespace. This article explains the structural syntax of a QName, detailing how the namespace prefix and the local part are combined using a colon delimiter and resolved by an XML parser.

Anatomy of a QName

A QName consists of two main lexical components defined by the W3C XML Namespaces specification:

  1. Prefix: An identifier that acts as a compact proxy for a full namespace URI.
  2. Local Part: The specific name of the XML element or attribute (technically defined as an NCName, or Non-Colon Name).

When combined, the syntax follows this strict format:

Prefix:LocalPart

If no namespace prefix is used, the QName consists solely of the LocalPart (an unprefixed QName), which belongs either to the default namespace (if declared) or to no namespace.

Step-by-Step Construction Process

1. Declare the Namespace URI

Before a prefix can be used in a QName, it must be mapped to a URI using an xmlns attribute within the XML document:

<library xmlns:bk="https://example.org/bookstore">

In this declaration, bk is established as the prefix associated with the URI https://example.org/bookstore.

2. Define the Local Part

The local part represents the standard element or attribute name. It must be a valid XML identifier (must not contain whitespace or colons, and must begin with a letter or underscore). For example: title.

3. Join with a Colon Delimiter

To construct the final QName, place the prefix and the local part together, separated directly by a single colon (:) with no spaces:

<bk:title>XML Fundamentals</bk:title>

Here, bk:title is the complete QName.

How Parsers Resolve a QName into an Expanded Name

While the lexical representation in the document is Prefix:LocalPart, an XML processor resolves the QName into an Expanded Name (an internal two-part identifier):

Often represented conceptually in James Clark notation as:

{https://example.org/bookstore}title

The prefix itself serves only as a temporary syntactic placeholder within the document markup to keep files readable, while the combination of the resolved URI and the local part ensures global uniqueness.