Re: grep -e '\(a\)\1' -e '\(b\)\1'

[Tapani Tarvainen]

address@hidden (Hans-Bernhard Broeker) writes:

> Tapani Tarvainen <address@hidden> wrote:
> 
> > The grep command in the subject line doesn't work
> > correctly with Gnu grep 2.4: it fails to match 'bb'.
> 
> Looks like pilot error, to me:
> 
> $ echo bb | grep -e '\(a\)\1' -e '\(b\)\2'
> bb

That's not the way it should work: multiple -e arguments
should be treated as independent patterns and back references
should not refer to previous ones.

At least that's how I read POSIX.2, and that's the way
HP-UX grep works.

Same applies when multiple patterns are separated by newline:

grep '\(a\)\1
\(b\)\1'

should match aa and bb, not ba.

Incidentally,

echo ba | grep -e '\(a\)\1' -e '\(b\)\1'

doesn't print anything either.

> > Same problem with egrep '(a)\1|(b)\1'.
> 
> $ echo bb | egrep '(a)\1|(b)\2'
> bb

There you are correct, now they are in the same pattern
(and that's a nonportable Gnu extension anyway).
But again,

echo ba | egrep '(a)\1|(b)\1'   

doesn't print ba either.