<form class="newsletter">
 <input type="email" value="" placeholder="Enter your email address" />
 <input type="submit" value="OK" />
</form>   

$( function( $, window, document, undefined )
{
 'use strict';
 
 var form  = '.newsletter',
  className = 'newsletter--active',
  email  = 'input[type="email"]';
 
 $( form ).each( function()
 {
  var $form = $( this ),
   $email = $form.find( email ),
   val  = '';
 
  $email.on( 'keyup.addClassWhenEmail', function()
  {
   val = $email.val();
   $form.toggleClass( className, val != '' && /^([\w-\.]+@([\w-]+\.)+[\w-]{2,12})?$/.test( val ) );
  });
 });
})( jQuery, window, document ); 

$( '.newsletter input[type="email"]' ).off( '.addClassWhenEmail' );     

;( function( window, document, undefined )
{
 'use strict';
 
 var form  = '.newsletter',
  className = 'newsletter--active',
  email  = 'input[type="email"]',
 
  addEventListener = function( element, event, handler )
  {
   element.addEventListener ? element.addEventListener( event, handler ) : element.attachEvent( 'on' + event, function(){ handler.call( element ); });
  },
  forEach = function( elements, fn )
  {
   for( var i = 0; i < elements.length; i++ ) fn( elements[ i ], i );
  },
  addClass = function( element, className )
  {
   element.classList ? element.classList.add( className ) : element.className += ' ' + className;
  },
  removeClass = function( element, className )
  {
   element.classList ? element.classList.remove( className ) : element.className += element.className.replace( new RegExp( '(^|\\b)' + className.split( ' ' ).join( '|' ) + '(\\b|$)', 'gi' ), ' ' );
  };
 
 forEach( document.querySelectorAll( form ), function( $form )
 {
  var $email = $form.querySelectorAll( email );
 
  if( $email.length )
  {
   $email = $email[ 0 ];
   addEventListener( $email, 'keyup', function()
   {
    $email.value != '' && /^([\w-\.]+@([\w-]+\.)+[\w-]{2,12})?$/.test( $email.value ) ? addClass( $form, className ) : removeClass( $form, className );
   });
  }
 });
})( window, document );  

var form  = '.newsletter',   // form selector
className = 'newsletter--active',  // class name for form when correct email is entered
email  = 'input[type="email"]', // email input field selector   

.newsletter:not( .newsletter--active ) input[type='submit']
{
 display: none;
}