altering search terms

You can use parse_query:

function wpa64292_parse_query( $query ) {
    if( isset( $query->query_vars['s'] ) && !empty( $query->query_vars['s'] ) )
        $query->set( 's', prepSearchStr( $query->query_vars['s'] ) );
}

add_action( 'parse_query', 'wpa64292_parse_query' );

Note that it won’t alter the query string you see in the browser, but if you echo get_query_var('s') in your template you’ll see it’s been altered.