KISS?

while ( <DATA> ){
  print if /(a+)(b+)/ and length( $1 ) == length( $2 ) - 1;
}

__DATA__
a
b
x

ab
abb
abbb
aabb
aabbb
aabbbb
...

[download]

In reply to Re: Integrating match counts into regex matching by Not_a_Number
in thread Integrating match counts into regex matching by pat_mc

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