[jQuery] variable validation rules for the same input field depending on other fields

[jonathan]

I have the following html, field xxx requires a number from 0 to 100
when percentage is selected, and 0 to inifinity when dollar is
selected.  what's the best practice to setup validation for this?  the
key is to control the maximum value.  I have tried depends expression
in a max rule, but it doesn't seem that it can satisfy my requirement.
form.validate({
                                xxx:{
                                        number:true,
                                        max:{
                                                depends:function(el){
                                                        var unit = 
$('#xxx_unit').val();
                                                        if(unit=='percentage'){
                                                                return 100;
                                                        }
                                                        return 
MAX_DOLLAR_AMOUNT;
                                                },

                                        },
                                        min:0
                                }
});

                                                                <input id="xxx" 
name="xx" type="text" maxlength="10" value=""/
>

                                                                <select 
id="xxx_unit" name="xxx_unit">
                                                                        <option 
value="percent">Percentage</option>
                                                                        <option 
value="dollar">Dollar Amount</option>
                                                                </select>