regular expression
 

so I have to write a reg expression for
the set of strings whose first character is a and ends in either a
or ac. In the middle the strings have any combinations of b's and c's,
including no b's nor c's.

Now, what I came up with is
{a}.{b, c}*.{a, ac}

apparently, this is not right (my solution doesnt match with the answer given) Now, can you tell me what you see wrong in my solution?

Not sure what type of syntax that is. In perl I'd do something like
^a[bc]*[a(ac)] haven't tested it

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

kodos is good, it what i use to help me figure out my regular expressions.

Since you are specifying the end as well, I'll just borrow Dizzutch's for a sec: