// A generic function for checking to see if an input element has
  // a Phone Number entered in it
  function checkPhone( elem ) {
      // Check to see if we have something that looks like
      // a valid phone number
      var m = /(\d{3}).*(\d{3}).*(\d{4})/.exec( elem.value );
                          
      // If it is, seemingly, valid - force it into the specific
      // format that we desire: (123) 456-7890
      if ( m ) obj.value = "(" + m[1] + ") " + m[2] + "-" + m[3];
                                  
      return !elem.value || m;
  }
  
  // Get an input element to check
  var elem = document.getElementById("phone");
  
  // Check to see if the field contains a valid phone number
  if ( ! checkPhone( elem ) ) {
      alert( "Field does not contain a phone number." );
  }