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BUG #3645: regular expression back references seem broken

From: "Eric Haszlakiewicz" <erh+pgsql(at)swapsimple(dot)com>
To: pgsql-bugs(at)postgresql(dot)org
Subject: BUG #3645: regular expression back references seem broken
Date: 2007-10-01 00:43:49
Message-ID: (view raw or whole thread)
Lists: pgsql-bugs
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

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:


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

template1=# select 'xyz' ~ E'^(.)\\1*$';
(1 row)

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

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

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

As does forcing it to be a extended regular expression:

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

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. :)


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Next:From: Tom LaneDate: 2007-10-01 15:34:54
Subject: Re: BUG #3645: regular expression back references seem broken
Previous:From: Tom LaneDate: 2007-09-29 18:45:40
Subject: Re: Nasty tsvector can make dumps unrestorable

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