location ~* /collections.*?products/([^/]+)/?$ {
    rewrite ^/collections.*?products/([^/]+)/?$ /$1.html;
    rewrite ^([^_]*)_([^_]*)_(.*)$ $1-$2-$3;
    rewrite ^([^_]*)_(.*)$ $1-$2 permanent;
}

"/collections/products/someproduct/" to "/someproduct.html"
"/collections/products/some_product/" to "/some-product.html"
"/collections/products/some_other_product/" to "/some-other-product.html"