Regular expressions short exercise

Find a regular expression for each of the following languages. In each case, let the alphabet be {a, b}.

• Strings that have at least three as.
• Strings with length at least 3, and its third symbol is a.
• Any strings except bb and bbb.
• Strings that contain at least two as and at most one b.

These exercises are taken from Michael Sipser, Introduction to the Theory of Computation, Second Edition, Thomson Course Technology, 2006. Pg 84 and 86.