If you are only _searching_ for standard (non AUR) packages, then you can simply use `pacman`. This will search for packages only in `core`, `extra` and `community`. (When searching `yaourt`'s only additional functionality is to search `aur`)
You can then install these packages with `yaourt` if you like.
If you really want to use `yaourt` for searching, you can also filter with grep, of course:
yaourt --color -Ss query | grep '^\S*[ce]\w*/' -A1
Explanation:
* `--color` forces colorized output (would be normally gone, as it detects its output goes to a pipe)
* The `\S*` skips the escape-codes used for colouring, and ensures only the package names are matched (package descriptions always start with four spaces)
* The `[ce]\w*/` matches the prefixes `core/`, `extra/` and `community/` in front of package names, but nor `aur/`
* The `-A1` switch also prints the package description in the line following the match