Is it possible to validate a field to a specific set of numbers using javascript? Say for example in a postcode field you can only enter four numbers to be be accepted e.g. 2747,2750, 2753, 2760, 2777. Any other number will be returned as false. I have done a bit of searching around but i can't seem to find anything of use for this specific situation and i'm quite knew to javascript so any sort of help would be great.