codice:
  <html>

    <head>

      <script type="text/javascript" language="JavaScript">
      <!--

        function control( __objForm )
        {
          if ( !__objForm.elements['Password'].value.length )
          {
            alert( "Inserire la password." );

            __objForm.elements['Password'].select( );
            __objForm.elements['Password'].focus( );

            return false;
          }

          else if ( !(/^\d{5}$/).test( __objForm.elements['Password'].value ) )
          {
            alert( "La password deve essere di 5 cifre." );

            __objForm.elements['Password'].select( );
            __objForm.elements['Password'].focus( );

            return false;
          }

          else
            return true;
        }

      //-->
      </script>

    </head>

    <body>

      <form onsubmit=" return control( this ); ">
        <input type="text" maxlength="5" name="Password" /> <input type="submit" />
      </form>

    </body>

  </html>