/[^(][a-zA-Z0-9]+(?=\))/g

Look at it like this

/\(([^()]+)\)/g

> s='dskf(AAA)_8hjk(CCC)dsk(BBB)' > p=/\(([^()]+)\)/g > while (r=p.exec(s)){console.log(r[1])} AAA CCC BBB

/[^()]+(?=))/g, after personal testing, it can meet the needs of the question owner

The return value of the

1. matchfunction is related to whether the regular expression used contains thegflag;
   If there is nogflag, if the string matches, the returned result is an array, and the elements of the array are respectively It is the complete substring matched by,the content of the first capturing bracket,the content of the second capturing bracket,the content of the third capturing bracket... so the length of the array isThe number of capturing brackets + 1;If there is the
   gflag and if the string matches, the return result is an array. The elements of the array are the first complete substring matched byand the second matched byComplete substring,matches the third complete substring...so the length of the array isthe number of matches;If there is no match, return null;
   

So, after using
2. g

   , the result will only return the complete substring matched by, and will not include the content of the capturing brackets. For your needs, the match function should not be able to do it.