Find regular expressions that define the following languages:

1. All strings over the alphabet {a,b} that start and end with the substring bab.
2. All strings over the alphabet {a, b} that do not contain the substring aaa.
3. All strings over the alphabet {a, b, c} for which the number of "a" is a multiple of 3 (including zero).
4. All strings over the alphabet {a, b} that do not contain the substring aab and end with bb.