I need to check inside a given string, if a character is contained while another not e.g. It needs to contain ‘a’ but not ‘b’.

I tried with something like:

echo preg_match('/(?:a)(?!b)*$/','hi a good day')

I tried with the lookahead, but I am missing something … the first part matches the second does not because it should be valid for the length of all the string and not just for some.
So I tried also with

echo preg_match('/^(?:a)[^b]*$/','hi a good day');

and it does not match, what am I missing ?
( I know I could use strpos but I would like to use a regex approach ).