1. letter? ((b|...|z) letter?)*
2. letter letter letter? letter?
3. ((b|...|z)* a (b|...|z)* a)+ (b|...|z)*

bzw.

1. (a (b|...|z)|(b|...|z))* a?
2. letter letter (letter | letter letter)?
3. ((b|...|z)* a (b|...|z)* a (b|...|z)*)+