Just note the difference between these two regular expressions:

[^a]at
[download]

Your regex, will match any expression with three characters where the first one is not 'a' and the rest is 'at'

[^a]?at
[download]

The regex you would expect, will match any expression ending with 'at' and starting AT MOST (repetition operator ?) by any character not being 'a'

Not the suitable place to post it since it seems I'm answering to Loops, sorry. The response was directed to anaconda