Filtering query strings in Laravel using regex

I'm trying to filter products based on query string. My goal is to get products from a collection if it's given, otherwise get every product. Could someone help me what's wrong with the following code?

$products = \App\Product::where([
'collection' => (request()->has('collection')) ? request('collection') : '[a-z]+',
'type' => (request()->has('type')) ? request('type') : '[a-z]+'
])->get();

PS.: I've also tried with 'regex:/[a-z]+', it's not working...

$products = \App\Product::where(['collection' => (request()->has('collection')) ? request('collection') : 'regex:/[a-z]+'])->get();

via Chebli Mohamed