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

[Bauke Jan Douma]

Alex Lamey wrote on 30-09-07 22:03:
> 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

Not a bug -- assuming the ! is and envisualization of your surprise.

The [0-9] means just one digit in that range.  So each of these
two constructs matches at most just one digit.  Hence \1 can only be
a reference to at most one digit.  Same for \2.  And hence the same
for \1|\2.
Therefore {1} gives you just one digit on the right side of the dash,
{2} gives you 2, and anything else just doesn't match, because you're
asking too many: {n} means /exctly/ n matches.

bjd