REGEX HELP

All words in which a is tripled and b is tripled, but not both. This means each word contains the substring aaa or the substring bbb but not both

These strings should have FULL matching

aaa

bbb

baaab

abbba

abbb

bbba

baaa

aaab

abaaaab

aaaaaaa

bbbbbbb

bbaaabb

These strings should NOT have FULL matching:

bbbaaa

aaabbb

abba

a

b

ab

ba

abab

aabb

aaababababbb

bbbabababaaa