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'