Is there any difference between 'valid xml' and 'well formed xml'?

Question

I wasn\'t aware of a difference, but a coworker says there is, although he can\'t back it up. What\'s the difference if any?

Answer 1

Taken from Extensible Markup Language (XML) 1.0 (Fifth Edition) - W3C Recommendation 26 November 2008 :

[Definition: A data object is an XML document if it is well-formed, as defined in this specification. In addition, the XML document is valid if it meets certain further constraints.]

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For those who prefer psuedo-code to paragraphs upon paragraphs of text... :)

IF is_well_formed() THEN
    # It is well-formed, and can be parsed
    IF is_valid() THEN
        # Well-formed and ALSO valid. Hurray! 
        # **A valid XML doc, is a well-formed doc!**
    ELSE
        # Only well-formed, NOT valid
    END IF
ELSE
    # Not well-formed, or valid!
END IF

FUNCTION is_well_formed
    IF  THEN
        RETURN TRUE
    ELSE 
        RETURN FALSE
    END IF
END FUNCTION 

FUNCTION is_valid
    IF  THEN
        # Standards used to validate XML could be a DTDs or XML Schemas, referenced within the XML document
        RETURN TRUE
    ELSE 
        RETURN FALSE
    END IF
END FUNCTION

Based on the theory: "Well Formed" vs. Valid