[grep 2.5.1] odd output for some patterns combining backreference, alternation, and repetition

[Alex Lamey]

Hi,
I think grep (2.5.1) is misbehaving on me.  Here are twelve examples,  
wherein I apply six patterns (differing only in the repetition  
argument) to two inputs (differing only in their fifth character).   
Five of the outputs (marked '!') seem to be wrong to me.  But I'm  
quite new to grep so maybe I've misunderstood something.

> $ echo 88-88 | egrep -o '([0-9])([0-9])-(\1|\2){1}'
> 88-8

> $ echo 88-88 | egrep -o '([0-9])([0-9])-(\1|\2){2}'
> 88-88

> $ echo 88-88 | egrep -o '([0-9])([0-9])-(\1|\2){3}'
> 88-88
    !

> $ echo 88-88 | egrep -o '([0-9])([0-9])-(\1|\2){4}'
> 88-8
    !!

> $ echo 88-88 | egrep -o '([0-9])([0-9])-(\1|\2){5}'
> 88-88
    !

> $ echo 88-88 | egrep -o '([0-9])([0-9])-(\1|\2){6}'
> 88-88
    !

> $ echo 88-81 | egrep -o '([0-9])([0-9])-(\1|\2){1}'
> 88-8

> $ echo 88-81 | egrep -o '([0-9])([0-9])-(\1|\2){2}'
> 88-8
    !

> $ echo 88-81 | egrep -o '([0-9])([0-9])-(\1|\2){3}'

> $ echo 88-81 | egrep -o '([0-9])([0-9])-(\1|\2){4}'

> $ echo 88-81 | egrep -o '([0-9])([0-9])-(\1|\2){5}'

> $ echo 88-81 | egrep -o '([0-9])([0-9])-(\1|\2){6}'

If this isn't a bug, but a mistake on my part, an explanation would  
be appreciated.
Thanks,
-Alex