Обсуждение: Stripping a prefix

Greetings.

Is there a function available to strip a string from the start of a
string?  trim and ltrim can strip any occurrence of a set of characters
from the start of a string, but I want to be able to strip an exact
string.  In particular, if the string starts with "www." (including the
dot), then it should be stripped.

Thanks.
--
Bruce Guenter <bruceg@em.ca>                       http://em.ca/~bruceg/

>  Date: Fri, 25 Aug 2000 09:35:43 -0600
>  From: Bruce Guenter <bruceg@em.ca>
>  To: pgsql-general@postgresql.org
>  Subject: Stripping a prefix
>
>  Is there a function available to strip a string from the start of a
>  string?  trim and ltrim can strip any occurrence of a set of characters
>  from the start of a string, but I want to be able to strip an exact
>  string.  In particular, if the string starts with "www." (including the
>  dot), then it should be stripped.

trim(leading 'www.' from foo)

-- Mike

On Fri, Aug 25, 2000 at 09:28:59PM -0700, Ian Turner wrote:
> If the string is like `^www\.', then trim `w' and `.'.

But this trims all 'w's and all '.'s, even if the string doesn't start
with 'www.':

select trim(leading 'www.' from 'wfoo');
 ltrim
-------
 foo
(1 row)

I ended up writing a function to do it, and for the data sets I'm
dealing with, it's fast enough:

CREATE FUNCTION strip_www (text ) RETURNS text AS
'BEGIN
  IF position(''www.'' IN $1) = 1 THEN
    RETURN substring($1 FROM 5);
  ELSE
    RETURN $1;
  END IF;
END;' LANGUAGE 'plpgsql';
--
Bruce Guenter <bruceg@em.ca>                       http://em.ca/~bruceg/