What regex matches strings of consecutive 'a' and 'b'? -

i need regex matches strings of consecutive a , b, e.g.:

ababa bab

edge case (smallest):

ab ba

(no upper limit.)

...and shouldn't match:

abba bbab bbaabb

i've tried several regex 1 kind of tricky. can throw me hints?

my tries:

if want find matches within text (potentially several words per line):

\b(((ab)+a?)|((ba)+b?))\b

\b word boundary.

XPATH