Custom validations in XML Schema

Hi,

XML Schema validates an XML data for checking place, optional/required and
data type etc. Thats fine, but is there any where to give one or more Custom
validation method names in an XML schema at the element level so that
whenever the parser iterates the Schema while validating an XML file, checks
these customs validations and exposes/enables the user to define and
implement custom validations to be perfumed during this normal usual
validation process?
Regards,
Durga Prasad Perumalla
Software Architect
ACENTA
Work Phone: 479-709-7470 Ext 6341
E-mail:  prasad@acenta.com