<human>:  How to capture the entire string while using 'lookaround' with chars in regex?
<bot>: Here is some code to find all substrings in string `mystring` composed only of letters `a` and `b` where each `a` is directly preceded and succeeded by `b`

re.findall('\\b(?:b+a)+b+\\b', mystring)

#Warning: this code generation is experimental. Please inspect the code for any bugs before executing.