Simple Regex to drop .html and add /product/

I’m trying to follow your regex expamples here
https://redirection.me/support/redirect-regular-expressions/

but my old site product links like
/this-is-a-product-link.html
are changed to
/product/this-is-a-product-link

so I created at regex type redirect with

source: ^/(.*)\.html$

target: /product/$1

But the out put redirect in the browser is

/product/(.*

What have I done wrong?