Re: lowercase letters for binaries

[Benno Schulenberg]

Andreas Schwab wrote:
> Benno Schulenberg <address@hidden> writes:
> > Hmm...  I'm not sure that backreferences are working correctly.
> > Shouldn't
> >
> >   echo "cbaa" | egrep '(aa)cb|cb\1'
> >
> > give output?
>
> Since (aa) does not match anything, what should \1 match?  Note
> that a back-reference does not repeat the matching of the
> referenced subexpression, but only matches the exact string that
> was previously matched.

Ah.  That "previously matched" is what I hadn't understood -- and 
this is precisely what point 13 tries to explain.  So maybe the 
following is a better text for 'info grep usage':

13. Why is this back-reference failing?

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

    This gives no output, because the first alternate '(a)\1' does
    not match, as there is no "aa" in the input, so the '\1' in the
    second alternate has nothing to refer back to, meaning it will
    never match anything.  (The second alternate in this example
    can only match if the first alternate has matched -- making
    the second one superfluous.)

Benno