what's a good form-checking library with conditional validation?

I had been using jsvalidate, but it doesn't fit my requirement of conditional validation for what I want to do; I want to make a couple of fields required only if开发者_JS百科 a select is previously marked as yes.

I have been looking into mootools's formcheck, it seems a bit fancier and nicer than jquery's validation plugin, but I don't know if it would allow me to do a conditional validation. Which should I pick here to make my job easier?

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I've used Ketchup, another jQuery form validation plug-in. You can write your own validators with jQuery to use custom conditions.