Laravel Validation for integers or specific word

I'm creating a validation rule where location attribute can have any integer or a word "all" value.
For integer validation I use this rule:

'location' => 'required|integer'

and for a particular word I can use this rule:

'location' =>   ['required',
                    Rule::in([all]),
                ]

How can apply both of rules together so that location can either be any integer or the word "all"? Can regex be of any help here?

via Chebli Mohamed