Validation API Module for JS/PHP

For more information or to ask questions about this proposal, see http://groups.drupal.org/node/9911.

This idea was inspired by Benjamin Keen's Really Simple Validation and PHP Validation pair of scripts. Those two scripts use identical rules to perform both client-side and server-side form validation using Javascript and PHP respectively.

The idea here would be to create similar validation functionality for adding rules to Drupal forms. Adding a rule to a field once would add both Javascript and PHP validation. To eliminate duplicate effort, the PHP side might simply leverage the existing rule set of the jQuery Validation plugin. Also, there should be a way to send to the content to be validated back to the server with AHAH. Ideally, the module would function both as a validation API, to be called from custom modules, and as a CCK add-on, for beginners to more easily add validation to CCK forms.