You used "(?<!a)" and "(?!b)" as anchors, so your solution was designed to match a substring, but it doesn't work for 'a_abb' (false negative) and 'a_abbb' (false positive). 

for (qw[ ab abb abbb aabb aabbb aabbbb a_abb a_abbb ]) {
   m[
      (?<!a)
      (?>
         (a+) (?{ length($^N) })
         (b+)
      )
      (?(?{ length($^N)-$^R != 1 })(?!))
   ]x
      and print("$_\n");
}

[download]
abb
aabbb
a_abb

[download]

In reply to Re^2: Integrating match counts into regex matching by ikegami
in thread Integrating match counts into regex matching by pat_mc

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