^ # Assert we're at the beginning of the string.
(
a # Match one more 'a' per each iteration of this loop.
# Lookahead assertion - when this finishes, the regex engine's "cursor"
# "cursor" will jump back to the same position as it was in when it entered the
# lookahead here. It's atomic, so the regex engine can't backtrack into
# the lookahead once it enteredhas completed its match or non-match. This means
# the main loop will exit immediately if this lookahead herefails to match
# (due to running out of 'b' characters to match), or if the 'a' above
# fails to match.
(?=
a* # Skip over as many 'a' as necessary to get to the first 'b'.
# Capture \2, embedding within it the previous contents of itself.
# If this is the first iteration, \2 will be unset, so capture a
# single 'b' in \2. Otherwise, append one more 'b' to the previous
# contents of \2.
(
\2?+ # Optionally match \2, but if it does match, lock that
# match in (don't backtrack to here and try not making the
# match, in the case that a non-match occurs after this
# point). This is guaranteed to fail to match if and only if
# we're on the first iteration, due to \2 being unset. On
# subsequent iterations, since \2 will have been captured
# starting at the first non-'a' character, and always starts
# its attempted match there, it will be guaranteed to match.
b # Match one 'b'.
)
)
)+
\2 # Match however many 'b' we captured in \2.
$ # Assert we're at the end of the string.
