From: Craig Ringer <craig@postnewspapers.com.au>
To: pgsql-sql@postgresql.org
Subject: select regexp_matches('a a a', '([a-z]) a','g');
Date: Fri, 08 May 2009 13:26:14 +0800
Message-ID: <4A03C276.9050007@postnewspapers.com.au> (raw)
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Marc Mamin wrote:
> I have a string that contains a serie of chars, separated by single
> spaces.
>
> e.g 'a b x n r a b c b'
>
> Having such a string, I d'like to get a list of all predecessors of a
> given character.
> In the example, the predecessors of b are a,a,c.
OK, so wherever `b' occurs, you want the character at index `b -2'.
> select regexp_matches('a a a', '([a-z]) a','g');
> => {"a "} only
The issue is that regular expressions don't like to overlap matches. The
first match consumes _two_ leading `a' characters.
What you need is a zero-width lookahead assertion, available in
Perl-style extended regular expressions. Handily, recent PostgreSQL
versions support these, so you can write:
test=> select regexp_matches( 'a a a', '([a-z]) (?=a)', 'g');
regexp_matches
----------------
{a}
{a}
(2 rows)
--
Craig Ringer
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To: pgsql-sql@postgresql.org
Cc: craig@postnewspapers.com.au
Subject: Re: select regexp_matches('a a a', '([a-z]) a','g');
In-Reply-To: <4A03C276.9050007@postnewspapers.com.au>
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