SQL & PL/SQL

hi

how can i use REVERSE function in my udf?

i have got an error

PLS-00201: identifier 'REVERSE' must be declared

my function is as below

 CREATE OR REPLACE FUNCTION GEN_15_DIGIT_NO(BRANCH NUMBER,TYP NUMBER,AC VARCHAR2)
 RETURN VARCHAR2 IS
 VVAR VARCHAR2(20):=' ';
 BEGIN
    BEGIN
            if REGEXP_INSTR (AC, '[-|/|*|@|#|$|%|]')>0 then
                    for x in
                            1..length(AC)
                            loop
                                    if ascii(SUBSTR(REPLACE(AC,' '),x,1))   between 48 and 57 then
                                            vvar:=vvar||SUBSTR(REPLACE(AC,' '),x,1);
                                    end if;
                                    end loop;
                                    vvar:=lpad(replace(vvar,' '),8,'0');
                                    elsIF
                                             LENGTH(REPLACE(ac,' '))<15 THEN
                                                    vvar:=reverse(SUBSTR(reverse(ac), 1, 8));
                                    elsif
                                            LENGTH(REPLACE(ac,' '))>=15 THEN
                                                    vvar:=substr(ac,-8);
                                    ELSE
                                            VVAR:=ac;
                                    end if;
                    VVAR:=LPAD(SUBSTR(BRANCH,-3),3,'0')||LPAD(TYP,3,'0')||'1'||LPAD(VVAR,8,'0');
    RETURN VVAR;
    EXCEPTION
            when NO_DATA_FOUND THEN
            VVAR:='SOMETHING_WRONG';
            RETURN VVAR;
    END;
    END GEN_15_DIGIT_NO;

i am working on

BANNER

----------------------------------------------------------------

Oracle Database 11g Release 11.2.0.1.0 - 64bit Production

PL/SQL Release 11.2.0.1.0 - Production

CORE    11.2.0.1.0      Production

TNS for 64-bit Windows: Version 11.2.0.1.0 - Production

NLSRTL Version 11.2.0.1.0 - Production

please help