Reply To: What's form validation process when showing portion of form depending on select?

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Just use a branch in your logic. For example, if you can select “letters” to choose options “a”, “b”, and “c”, or select “numbers” to select “1”, “2”, and “3”, then your validation structure might look something like this (pseudocode):

if( $_POST['select'] === 'letters' ){
    validate( $_POST['a'] );
    validate( $_POST['b'] );
    validate( $_POST['c'] );
}
elseif( $_POST['select'] === 'numbers ){
    validate( $_POST['1'] );
    validate( $_POST['2'] );
    validate( $_POST['3'] );
}

You can follow a similar approach in javascript, though the specific plugin you choose might already have a way to deal with such things (you should look into that).