Code:

man regexp

gives you a comprehensive description of what is in the libc (that C-compiled programs use).
The trailing g* means "g zero or more times" and is useless unless there is a $ anchor (line end):
g*$ forces any number of g until the line end.
The g? in the meaning of "g zero or one time" is only defined for ERE (extended regular expression), used with egrep or grep -E and awk and most modern languages.