Functions and triggers can anyone help

I'm having a problem with setting up a function and a trigger for one of me tables.  I have just started using Postgresql and I'm trying to convert my Oracle database over to see how it runs.

Here is my Oracle Functions

Create Or Replace FUNCTION SET_USERNAME

  ( eventtype IN varchar2,

    message IN varchar2)

  RETURN  varchar2 IS

username varchar2(30);

pos1  number(7);

pos2 number(7); 

BEGIN

    if eventtype = 'AUDIT_SUCCESS' then

      pos1 := instr(message, 'User Name:') + 10;

      pos2 := instr(message, 'Domain');

      return substr(message, pos1, pos2-pos1);  

    elsif eventtype = 'AUDIT_FAILURE' then

      pos1 := instr(message, 'User Name:') + 10;

      pos2 := instr(message, 'Domain:');

      if (pos1 = 0 or pos2 = 0) then

        pos1 := instr(message, 'Account Name:') + 13;

        pos2 := instr(message, 'Target Domain:');

      end if; 

      return substr(message, pos1, pos2-pos1);  

    end if;       

    return '' ;

EXCEPTION

   WHEN others THEN

       return '' ;

END;

And Here is my Postgresql function but when I try to set the trigger it says it can't find the function (it does not exist)

CREATE  FUNCTION set_username4(varchar, varchar) RETURNS opaque AS '

DECLARE

eventtype varchar;

message varchar;

username varchar(30);

pos1  number(7);

pos2 number(7); 

BEGIN

    if eventtype = ''AUDIT_SUCCESS'' then

      pos1 := instr(message, ''User Name:'') + 10;

      pos2 := instr(message, ''Domain'');

      return substr(message, pos1, pos2-pos1);  

    elsif eventtype = ''AUDIT_FAILURE'' then

      pos1 := instr(message, ''User Name:'') + 10;

      pos2 := instr(message, ''Domain:'');

      if (pos1 = 0 or pos2 = 0) then

        pos1 := instr(message, ''Account Name:'') + 13;

        pos2 := instr(message, ''Target Domain:'');

      end if; 

      return substr(message, pos1, pos2-pos1);  

    end if;       

    return '' ;

EXCEPTION

   WHEN others THEN

       return '' ;

END;

'  LANGUAGE 'plpgsql';