[Deff]

<script>
var str = '1вфыааSDSSSSSSSSD  2DFS  3DSSDFSDFSD   4DFSDFFDS   5DFSDFFDS';

function replacer(str, p1, p2, offset, s)
  {
     if( p1) return str;
     return str.toLowerCase();
   }

var str = str.replace(/(^\S*)|(\S{5,})(?=\s|$)/g, replacer);
alert(str);
</script>