VAT Information Exchange System (VIES)

VIES VAT number validation (online)
You can verify the validity of a VAT number issued by any Member State by selecting that Member State from the drop-down menu provided, and entering the number to be validated. http://ec.europa.eu/taxation_customs/vies/vatRequest.html

FAQ
http://ec.europa.eu/taxation_customs/vies/faqvies.do#item_13

WSDL file
http://ec.europa.eu/taxation_customs/vies/checkVatService.wsdl

European VAT ID number directory
You can search here for European Value Added Tax identification numbers (often called “VAT registration numbers” in the UK) for making international payments as required by law. This service is free of charge and you do not need to register.
http://www.vat-reg-no.org/

Link to xPath tutorial

XPath is used to navigate through elements and attributes in an XML document.

• Expressions
• Standard Functions
• xPath and XSLT

Tutorial is here on this page:

http://www.w3schools.com/xsl/xpath_intro.asp