FreeBSD's portversion does:

=
<  for older
>  installed version is newer (such as testing or out-of-date pkgsrc)
?  unknown

A man page is here
http://www.gsp.com/cgi-bin/man.cgi?section=1&topic=portversion 

FreeBSD's pkg_version is similar:
http://www.freebsd.org/cgi/man.cgi?query=pkg_version&apropos=0&sektion=0&manpath=FreeBSD%206.0-stable&format=html

Examples:

% pkg_version
Hermes                              <
a2ps-letter                         <

% pkg_version -v
Hermes-1.3.2                        <   needs updating (port has 1.3.3_2)
a2ps-letter-4.13_1                  <   needs updating (port has 4.13b_3)

And "-l" can be used to only list matching status flags.


I could upgrade the following is: 
  pkg_version -l '<'

So my suggestion is to use a "<".

devel/readline - readline-5.0 < readline-5.1pl1nb1
 
(Including category is good too.)

 Jeremy C. Reed

echo ':6DB6=88>?;@69876tA=AC8BB5tA6487><' | tr '4-F' 'wu rofIn.lkigemca'