In the first challenge of 2013 after the holidays, you have to match valid regular expressions. This challenge will be in several parts - in the first, you only have to match literal characters, character classes, and a few other minor features, and in future weeks I will be adding more features until we can accurately match nearly every regular expression.
To see what regex features must be supported, it would probably be better to look at the test cases. The features that must be matched are literal characters, character classes, and the following operators:
.?+*. The global and case insensitive operators should also be matched.