How to allow null as an option when using regexp_matches?

[Shaozhong SHI <shishaozhong@xxxxxxxxx>]

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