Our web forms are really complex. What's a great solution for extensible form validation, preferably one that works with jQuery?

Background:

Our site has a bit of Ajax, but the real focus is on user experience through about 20 multi-page forms or "wizards." These forms are complicated.

  • Presentation: Some fields are floats or ints. Validation means stripping non-decimal characters, but we also want to make sure that, if a user enters 5 into a price field, the field is updated to 5.00.
  • Side effects: Some fields have side effects when updated. E.g., updating the price or quantity of an item needs to update a subtotal field.
  • Widget-driven elements: Some fields are hidden and have values populated by widgets. E.g., a map widget lets you point to a location and a hidden field is updated with latitude-longitude coordinates, but the location must be within a certain region.
  • Groups: Some fields are groups, like address/city/state/zip, and should only be validated when all of the fields have bee populated.
  • Server-side validation: Validation of some fields requires back-end checking via Ajax requests
  • Multiple forms per page: Sometimes a user needs to fill out one form before a dialog opens with another form. A framework has to be more versatile than binding to onSubmit — we sometimes post multiple forms in order from the same page using Ajax. (E.g., we let users sign up and create a widget in one swoop but, due to legacy systems, that flow requires two POST requests.)
  • Customizable error display: Sometimes errors appear above fields, sometimes the field style changes, and our new designs call for tooltip-like popups (ala qTip) for some errors.
  • Snappiness:
    javascript jquery html validation forms
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