There are two cases to take care of.
1) Only one search pattern
2) More than one search pattern
((^\*\.[a-zA-Z0-9]+\,\s*)*(\*\.[a-zA-Z0-9]+\s*)$)
The first part
(^\*\.[a-zA-Z0-9]+\,\s*)*
matches for
zero or more patterns followed by comma and the second part
(\*\.[a-zA-Z0-9]+\s*)
for only one or the last pattern.
^ and $
are matching beginning end of string.
[Edit]
Regarding your comment I tested on RegExr. With omitting ^ and $ it worked fine. But I also found a shorter one
(\*\.[a-zA-Z0-9]+\s*\,?\s*)+
Almost the same that you came up with but this one also matches whitespace in front
and behind comma.
[/Edit]