My attempt is
We can fairly easily build an expression containing no a, one a, or one aa:
(b+c)(€+a+aa)(b+c)
but if we want to repeat this, we need to be sure to have at least one non-a between repetitions:
(b+c)*(€+a+aa)(b+c)*((b+c)(b+c)*(€+a+aa)(b+c)*)*
Therefore the final answer is
(b+c)*(€+a+aa)(b+c)*((b+c)(b+c)*(€+a+aa)(b+c)*)*
Is this ok?
