I've got the folling function which returns a RegexMatch!string:

> auto extract_list(const string entry)
> {
>     // check if entry is valid
> 
>     return matchAll(entry, some_regex);
> }

I call this function using enforce(extract_list(some_argument)). I think, that enforce is quite useless in this version. But I'd like to add a check, if entry is valid in the extract_list-function, something like:

> if (!matchFirst(entry, other_regex)) return null;

That doesn't work, because I cannot cast null to RegexMatch!string (I don't understand why). What works is to create a string[] from the result of matchAll (using map and array) and use that array instead of the range. Now I can return null too.

Is there a way to accomplish this?