What characters are allowed in twitter hashtags?

Question

In developing an iOS app containing a twitter client, I must allow for user generated hashtags (which may be created elsewhere within the app, not just in the tweet body).

Answer 1

Twitter allows letters, numbers, and underscores.

I checked this by generating tweets via their API. For example, tweeting

Hash tag test #foo[bar

resulted in "#foo" being marked as a hash tag, and "[bar" being unformatted text.