Exception handling from trigger

Hello,

I would like to handle the exceptions that a database returns. For example,
if a "mycolumn" column is defined as varchar(64), I'd like to catch
if the new.mycolumn is larger than 64 with my own trigger
(for cutoms, internationalized messages, etc ..). So I did,
but the database catch this error before my 'before insert'-trigger
(using character_length(new.mycolumn) ...).

Is this way of handling exceptions possible in postgres ?
If so, what is the normal way to handle this exceptions, from a
plpgsql/trigger(rule??) perspective ?

Thank you ...

Vlad Dimitriu