trimming functions.

Hi all.
I've got a table with a field called "code". This field is a code of
asociated crops and vegetation in one area. As this code is too complex for
our purposes. I need to trim the text to the main crop in every row.
Sometimes the first character is a '('. So I need to remove this first '('
and extract the first code. This first code is formed by alphabet character
and can be up to 3 haracters in length.
So, at the moment I've done:

 SELECT cod_grass,code,substring(ltrim(code,'(') FROM 1 FOR 3) AS
code_trimmed FROM landuses WHERE code LIKE '(%';

An extract of the result is:

 cod_grass |         code                       | code_trimmed
-----------+-----+-------------------------+----------------
      1539     | (NJ/LI)+NJ{10:}+LI{10:} | NJ/
      1847     | (AG/L)+AL{10:}              | AG/
      2767     | (OL/AL)+L{20:}              | OL/
        19       |  LI+NJ                            | LI+
        20       |  I                                    | I
        29       | NJ                                  | NJ
       106      | PH{:LZ40}                      | PH{
       111      | AG^                               | AG^
       112      | PD                                 | PD
       187      | L+AL                             | L+A
       189      | M                                   | M
       195 |   1 | MD                              | MD
       196 |   2 | L+AL{40:}                    | L+A
...

So I still need to improve the SELECT to remove all possible symbols after
the first group of alphabet characters to get a 'code_trimmed' column with
just the characters:(NJ, AG, OL, LI, I, NJ, PH, AG, PD, L, M, MD, L).
Possible 'non alphabet' symbols are '{+/^('
Any idea?

Thanks and regards,
Javier