How to allow null as an option when using regexp_matches?

We can do this:
select count(*) from regexp_matches('Great London', 'Great
London|Information Centre|Department for Transport', 'g');

Is it possible to allow null as an option?  something like this
select count(*) from regexp_matches('Great London', 'null|Great
London|Information Centre|Department for Transport', 'g');

Regards,

David