converting Oracle to postgres

Here is a copy of my function in oracle.

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;

How do I get it to work in postgres

Please help me with the SQL commands

Thanks