var default_email_value = 'you@yours.com';
var error_message = 'Please enter a valid email address.';

var email = document.getElementById('email');
email.value = default_email_value;
email.onfocus = function(event) {
  if (email.value == default_email_value)      
	this.value = '';
};
email.onblur = function(event) {
  if (!email.value.match(/[a-zA-Z0-9._-]/))
	this.value = default_email_value;
};

var form = document.forms[0];

form.onsubmit = function(event) {
  if (email.value == default_email_value ||
		!email.value.match(/^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$/)) {              
	alert(error_message);
	return false;
  }           
};