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 neither aa nor bb as substrings.
• Strings that have an odd number of as and an even number of bs. (Remember that 0 is an even number.)
• Strings that have both ab and ba as substrings. (Make sure that your RE will accept a string like aaaabaaa; ab and ba are both substrings, even though they overlap.)

These exercises are taken from Lewis and Papadimitriou, Elements of the Theory of Computation, Prentice Hall, 1998. There's a copy in the CSCI 335 library.