Regex for timestamp

I'm terrible at regex apparently, it makes no sense to me...

I'd like an expression for matching a time, like 01:23:45 within a string.

I tried this

(r'(([0-9]*2)[:])*2([0-9]*2)

but it's not working. I need to be able to get the entire timestamp. Others I've tried have only found like 2 digits in it.