Give the Regular-Expression (NFA) with specific Separation Patterns
I'll assume that no $a$s can appear together without interruption by a group of $(bb)^+$s, even if they form a group of odd $a$s. In that case
$$
a(bb)^+ (a(bb)^+ a(bb)^+)^*
$$
should suffice, where $x^+$ is defined simply as $x^+ = x x^*$.