If you are sure, then the only other thing I can think of is that you might need to use "( )" brackets to protect your original patterns when you combine them - I didn't need to, but maybe your answers to the earlier parts are different.
For example, just as in arithmetic expressions, the following regular expressions have completely different meanings:
(a+b)*(c+d)and:
a+(b*c)+dbut, for regular expressions, neither of the above is the same as:
a+b*c+dFor example, even though all three can match "abcd", only the first can match "cd", and only the second can match "abcbcd".