1. [b-zA-Z]* | [b-zA-Z]*a[a-z]*

   [b-zA-Z]*(a[a-z]*)?

2. [A-Z]?([a-z][A-Z])*[a-z]?

   [a-z]?([A-Z][a-z])*[A-Z]?

3. [a-zA-Z][a-zA-Z][a-zA-Z]?
   [a-zA-Z]{2,3}

4. [b-zA-Z]*a[b-zA-Z]*a[b-zA-Z]*

5. [a-zA-Z]


AAaaa 1
     AaAAAaAAAA 4
               aAa 2
                  aAAaA 4
                       aA 2