gistfile1.js

var DecisivZipcodeInput = Class.create({

  initialize: function(input) {
    this.input = input;
    this.fivePattern = /^\d{5}$/;
    this.fiveFourPattern = /^\d{5}-\d{4}$/;
    this.maxLength = 10;
    this.message = "Zipcode must be a 5-digit number with an optional 4-digit suffix seperated by a hyphen.";
    this.input.writeAttribute('maxlength',this.maxlength);
    this._initEvents();
  },
  
  enforceFormat: function() {
    var value = this.input.getValue();
    var isBlank = value.blank();
    var matched = !isBlank && (value.match(this.fivePattern) || value.match(this.fiveFourPattern));
    if (isBlank || matched) { 
      return true;
    } else {
      alert(this.message);
      this.input.select();
    };
  },
  
  _initEvents: function() {
    this.input.observe('blur',this.enforceFormat.bindAsEventListener(this));
  }

});