Need help figuring out a regular expression for these strings

I'm just learning regex and I've been trying to figure out a pattern which only matches the following strings:

-a
-A
--add
--Add
a
A
add
Ad开发者_JS百科d

I'm using this in Perl if it matters. If you answer, could you explain your regex so I can try and learn what I've been doing wrong?

^(-?[aA]|(--)?[aA]dd)$

• ^ matches the beginning of the string
• -?[aA]:
  • -? matches - or nothing
  • [aA] matches a or A
• (--)?[aA]dd:
  • (--)? matches -- or nothing
  • [aA] matches a or A
  • dd matches dd
• (x|y) matches x or y
• $ matches the end of the string

You should invest some time carefully reading http://perldoc.perl.org/perlrequick.html. I still need a reference handy when I write regular expressions.