How to apply multipe regular expression on single field

Currently I have regular expression for zip code for US

validates :zip,        presence: true, format: { with: /\A\d{5}(-\d{4})?\z/ }

I want to use different regular expression for other country on the same field zip code so the regular expression should be use according to country

For Australia 4 digits

For Canada 6 digits alphanumeric

For UK 6-7 digits alphanumeric

Can some suggest to me how can I full fill my requirement. What are the way to achieve this.

Thank you in Advance for any kind of help and suggestion.