Numbers in Project Keys

What happens, or how bad is it, if I allow numbers within Jira Project keys?
I found a properties file where the project keys are tested for capital letters only:
File: jira-application.properties
jira.projectkey.pattern = ([A-Z][A-Z]+)

I would like to do something like
jira.projectkey.pattern = ([A-Z][A-Z0-9]+)

Thanks for your answer, Peter