XML Validator (Well-Formedness Checker)

An XML file must be well-formed before any program will read it: one root element, every tag closed in the right order, attribute values quoted, & written as &, no illegal control characters. This validator parses the text against the XML 1.0 rules and, if something is wrong, tells you the exact line and column and what to fix. It also checks namespace prefixes, which XML parsers used for SOAP, RSS, SVG and Office files enforce.

How to use this tool

1. Paste the XML (a whole document, not a fragment).
2. Leave Check namespaces on unless the file deliberately uses prefixes without declarations.
3. Read the verdict; if it is not well-formed, go to the line and column shown and use the ^ in the context line.
4. Fix the problem — the check runs again as you type.

Formula

document = [XML declaration] misc* [DOCTYPE] misc* element misc*

Exactly one root element; misc means comments, processing instructions and whitespace. Inside elements: start and end tags must match, attribute names must be unique, values quoted and free of <, references must be &lt; &gt; &amp; &quot; &apos;, a numeric reference, or an entity declared in the DOCTYPE. Comments cannot contain --, and the XML declaration may only appear at the very start.

Example

The sample catalog is well-formed: 7 elements, 7 attributes (two of them xmlns declarations), maximum depth 3, and two declared namespaces — the default urn:example:books and dc for Dublin Core. Change </price> to </prices> and the result becomes Not well-formed, pointing at the mismatched closing tag and the line where <price> was opened.