I am having some trouble with the most simple of scripts.

I have a textbox and a submit button, thats all. What I want to happen is; a user enters a string into a textbox and clicks a submit button. I want a dialogue box to appear to say either area 1 or area 2. I have an array of postcodes that fall into area 1, all others are area 2. the user will enter the 2, 3 or 4 character UK postcode prefix into the textbox. the string will be compared to values in the array. If the post code is in the array, then this is displayed; else display that.