What characters are allowed in a Google App Engine Key?

While testing my Google App Engine application, I search for links which include app engine keys. For example:

/story/ag5yZXBsaWUtdGVzdGluZ3IMCxIFU3RvcnkY-w0M/

What characters are allowed in these keys? I had been using th开发者_StackOverflow中文版e regex [a-zA-Z0-9], but it seems "-" is also allowed. What else?

Is there a reference on this? I can't find it in the App engine docs.

Google App Engine keys are encoded using URL safe base 64 encoding (see The source code for the Key datastore type).

it's very probably all digits, upper- and lowercase letters, underscore and hyphen (try to base64.urlsafe_b64decode this string ;) )