preg_match Warning on Email

hi guys,... I'm getting this preg_match Warning on Email issue no matter what I do,.... 

can anyone help me please,.... 

Warning (2): preg_match() [function.preg-match]: Delimiter must not be alphanumeric or backslash [APP/controllers/invitations_controller.php, line 83]

and this is what I have in a Model: 

    var $validate = array(

      'email' => array(

      'rule' => 'alphaNumeric',

      'required' => true,

      'allowEmpty' => false,

     ),

 );

thanks in advance,... 

chris

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