is_numeric() or extract_numeric() functions?

I can't find an is_numeric() or extract_numeric() function in postgres
so I decided I would make my own. However, when I try to create the
following function:

CREATE OR REPLACE FUNCTION getnumber(varchar(1)) RETURNS integer AS '
BEGIN
return ($1 ~ '[0-9]');
END;
' LANGUAGE 'plpgsql';

I get the following error:

parse error at or near "["

Anyone know how to fix this, or if there is an is_numeric() or
extract_numeric() function available?

Thanks

--
Ron St.Pierre
Syscor R&D
tel: 250-361-1681
email: rstpierre(at)syscor(dot)com