Hmmm? 

our( $a, $b )=(0,0),  
    m[
      ( 
        (?<!a) 
        ( a (?{++$a}) )+ 
        ( b (?{++$b}) )+ 
        (?!b) 
      )
      (?(?{ $a+1 == $b }) (?=) | (?!) )
    ]x and print "'$1'" 
    for qw[ ab abb abbb aabb aabbb aabbbb ];;
'abb'
'aabbb'

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
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In reply to Re: Integrating match counts into regex matching by BrowserUk
in thread Integrating match counts into regex matching by pat_mc

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