regex line start in character set

Let's say I want to search for a 'b' that occurs either at start of line or followed by 'a'.

Why doesn't re.match('[\^a]b','b') match while re.match('^b','b') does?

Update: I realised I should have been using search instead of match. I want it to be a positive for things like 'b', 'cab', 'ab', 'bc', and 'abd'.