BUG #3645: regular expression back references seem broken

The following bug has been logged online:

Bug reference: 3645
Logged by: Eric Haszlakiewicz
Email address: erh+pgsql(at)swapsimple(dot)com
PostgreSQL version: 8.2.5
Operating system: NetBSD
Description: regular expression back references seem broken
Details:

I was attempting to create a simple regular expression that uses back
references and I noticed some very odd behaviour. This regexp is supposed
to match a string where all the characters are the same:

^(.)\1*$

If I try it, it doesn't work. I would expect this to return false:

template1=# select 'xyz' ~ E'^(.)\\1*$';
?column?
----------
t
(1 row)

But adding some extra parens does:
template1=# select 'xyz' ~ E'^(.)(\\1)*$';
?column?
----------
f
(1 row)

As does changing the "." to an "x":

template1=# select 'xyz' ~ E'^(x)\\1*$';
?column?
----------
f
(1 row)

As does forcing it to be a extended regular expression:

template1=# select 'xyz' ~ E'(?e)^(.)\\1*$';
?column?
----------
f
(1 row)

The docs claim: "A single non-zero digit, not followed by another digit, is
always taken as a back reference." (The note at the end of 9.7.3.3)

It's relatively easy to work around the problem, but it certainly led to a
fair bit of head scratching while trying to debug some code. :)