in any regex [] creates a grouping of characters to match whereas {} tells the regex how many times to match something
will match a or b exactly 3 times.
anything in the [] will be matched, so
will actually match, a, (, a, c, or ). In your case you probable want the following regex
a good tool to use to test regex on linux is kregexpeditor or kodos, and for windows there a program called The Regulator, which can be found on sourceforge.
If this does not work, please post a couple of examples of strings you need matched for more clarity.
Good luck.
-Dizz