RE: RE: Re: select substr???

The references to REs in the docs are a little hard to find (maybe it's just
me).  Bruce's book gives a good coverage, with the benefit of an index to
find where that is.  AFAICT, the implementation is "extended regular
expressions", like egrep.
My choice of characters to match was a little arbitrary: it would depend on
the specification.
You are correct, the check for "$1 is null" is not required.  I was
attempting an optimisation, as in "don't do anything else if this is null".
The gain would depend on how much further processing the function would
attempt before recognizing that it was dealing with a null value, which is
something that I don't know enough to tell.

> -----Original Message-----
> From:    Albert REINER [SMTP:areiner@tph.tuwien.ac.at]
> Sent:    Thursday, April 12, 2001 10:44 AM
> To:    pgsql-sql@postgresql.org
> Subject:    Re: RE: Re: select substr???
> 
> Thanks, interesting. I did not find any mention of regular expressions
> in the 7.0.2 docs I installed locally.
> 
> BTW, your code does not do exactly the same, as it removes any
> whitespace while the other one only collapses consecutive blanks. But,
> of course, regular expressions in PL/pgSQL make this much easier.
> 
> As a further aside, I think that in both versions of the function the
> check for `$1 IS NULL' is not necessary; I got the impression that
> passing NULL as an argument to a function will automatically return
> NULL as the result, doesn't it?
> 
> Albert.
> 
> 
> On Tue, Apr 10, 2001 at 05:41:26PM -0500, Jeff Eckermann wrote:
> > Regular expressions make this much easier.  The below could be shortened
> to:
> > 
> >         create function ComparisonString(text) returns text  as '
> >       declare
> >         t alias for $1;
> >         r text;
> >         c char;
> >     begin   
> >         if t is null  or t !~ ''[^a-zA-Z0-9]''
> >        then
> >           return t;
> >         end if;
> >         r = '''';
> >         for i in 1 .. char_length(t) loop
> >           c = substring(t from i for 1);
> >           if c ~ ''[a-zA-Z0-9]''
> >           then
> >               r = r || c;
> >           end if;
> >         end loop;
> >         return r;
> >       end;
> >     ' language 'plpgsql' with (IsCachable);
> > 
> > > -----Original Message-----
> > > From:    Albert REINER [SMTP:areiner@tph.tuwien.ac.at]
> ...
> > > 
> > > somewhere I use the following, which might be adapted to do what you
> > > want.  I am sure there are more elegant ways of doing this, though.
> > > 
> > >      create function ComparisonString(text) returns text  as '
> > >   declare
> > >     t text;
> > >     r text;
> > >     c char;
> > >     ns bool;
> > >   begin
> > >     if $1 is null then
> > >       return NULL;
> > >     end if;
> > >     t = lower(trim(both $1));
> > >     r = '''';
> > >     ns = false;
> > >     for i in 1 .. char_length(t) loop
> > >       c = substring(t from i for 1);
> > >       if c = '' '' then
> > >         if ns then
> > >           r = r || '' '';
> > >         end if;
> > >         ns = false;
> > >       else
> > >         if position(c in ''abcdefghijklmnopqrstuvwxyz0123456789'') > 0
> > > then
> > >           r = r || c;
> > >           ns = true;
> > >         end if;
> > >       end if;
> > >     end loop;
> > >     return trim(both r);
> > >   end;
> > > ' language 'plpgsql' with (IsCachable);
> > > 
> 
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