Perl> 'aaabb' =~ m[ 
    (?{ local( $a, $b ) }) 
    ( 
        (?<!a) 
        ( a (?{++$a}) )+ 
        ( b (?{++$b}) )+
    )
    (?(?{ $a+1 == $b }) (?=) | (?!) ) 
]x and print "'$1'";;

Perl> 'aaabb' =~ m[ 
    (?{ local( $a, $b ) }) 
    ( 
        ( a (?{++$a}) )+ 
        ( b (?{++$b}) )+ 
    )
    (?(?{ $a+1 == $b }) (?=) | (?!) ) 
]x and print "'$1'";;
'abb'