create or replace function test_num(v_ip in number) return varchar2 is
v_text varchar2(18);
v_num1 number;
v_num2 number;
v_num3 number;
v_num4 number;
Begin
v_num1:=substr(v_ip,1,1);
v_num2:=substr(v_ip,2,1);
v_num3:=substr(v_ip,3,1);
v_num4:=substr(v_ip,4,1);
If v_num1=v_num2 or v_num1=v_num3 or v_num1=v_num4 or v_num2 =v_num3 or v_num2=v_num4 or v_num3=v_num4 then
v_text :='Invalid Number';
else
v_text :='Valid Number';
end if;
return v_text;
end;
/

SQL> sho err
No errors.
SQL> select test_num(1223) from dual;

TEST_NUM(1223)
--------------------------------------------------------------------------------
Invalid Number

SQL> select test_num(12341) from dual;
TEST_NUM(12341)
---------------------------------------
Valid Number

SQL> create or replace function test_num (p in number) return varchar2 is
  2    type numbers is table of number(1) index by binary_integer;
  3    tab  numbers;
  4    val  number := 
  5      abs(p)*power(10,nvl(length(substr(to_char(p),nullif(instr(to_char(p),'.'),0)))-1,0));
  6  begin
  7    loop
  8      case 
  9        when val is null then return null;
 10        when val < 10 then 
 11          if tab.exists(val) then return 'Invalid number';
 12          else return 'Valid number';
 13          end if;
 14        else 
 15          if tab.exists(mod(val,10)) then return 'Invalid number';
 16          else 
 17            tab(mod(val,10)) := 1; 
 18            val := trunc(val/10);
 19          end if;
 20      end case;
 21    end loop;
 22  end;
 23  /

Function created.

SQL> col t format a20
SQL> with data as (
  2    select null val from dual
  3    union all
  4    select 0 val from dual
  5    union all
  6    select 10 val from dual
  7    union all
  8    select 11 val from dual
  9    union all
 10    select -1234.5 val from dual
 11    union all
 12    select 12.341 val from dual
 13    )
 14  select val, test_num(val) t from data
 15  /
       VAL T
---------- --------------------

         0 Valid number
        10 Valid number
        11 Invalid number
   -1234.5 Valid number
    12.341 Invalid number

SQL> select test_num(12345.6789) from dual;

TEST_NUM(12345.6789)
---------------------------------------------------
Valid number

SQL> 

create or replace
  function test_num(
                    p in number
                   )
    return varchar2
      is
          tab sys.OdciNumberList;
      begin
          case
            when abs(p) < 1234 then return 'Invalid number';
            when abs(p) > 9876 then return 'Invalid number';
            when p != trunc(p) then return 'Invalid number';
          end case;
          tab(substr(p,1,1)) := 1;
          tab(substr(p,2,1)) := 1;
          tab(substr(p,3,1)) := 1;
          tab(substr(p,4,1)) := 1;
          if tab.count != 4 then return 'Invalid number'; end if;
          return 'Valid number';
end;
/

orcl> create or replace
  2    function test_num(
  3                      p in number
  4                     )
  5      return varchar2
  6        is
  7            tab sys.OdciNumberList;
  8        begin
  9            case
 10              when abs(p) < 1234 then return 'Invalid number';
 11              when abs(p) > 9876 then return 'Invalid number';
 12              when p != trunc(p) then return 'Invalid number';
 13            end case;
 14            tab(substr(p,1,1)) := 1;
 15            tab(substr(p,2,1)) := 1;
 16            tab(substr(p,3,1)) := 1;
 17            tab(substr(p,4,1)) := 1;
 18            if tab.count != 4 then return 'Invalid number'; end if;
 19            return 'Valid number';
 20  end;
 21  /

Function created.

orcl> select test_num(1023) from dual;

TEST_NUM(1023)
----------------------------------------------------------------------

Invalid number

orcl>

orcl> select test_num(1023) from dual;

TEST_NUM(1023)
------------------------------------------------------------------------

Invalid number

orcl> select test_num(2023) from dual;
select test_num(2023) from dual
       *
ERROR at line 1:
ORA-06592: CASE not found while executing CASE statement
ORA-06512: at "SCOTT.TEST_NUM", line 8


orcl>

create or replace
  function test_num(
                    p in number
                   )
    return varchar2
      is
          tab sys.OdciNumberList;
      begin
          case
            when abs(p) < 1000 then return 'Invalid number';
            when abs(p) > 9999 then return 'Invalid number';
            when p != trunc(p) then return 'Invalid number';
          end case;
          tab(substr(p,1,1)) := 1;
          tab(substr(p,2,1)) := 1;
          tab(substr(p,3,1)) := 1;
          tab(substr(p,4,1)) := 1;
          if tab.count != 4 then return 'Invalid number'; end if;
          return 'Valid number';
end;
/

create or replace
  function test_num(
                    p in number
                   )
    return varchar2
      is
          type xtype 
            is table of number(1) index by binary_integer;
          tab xtype;
      begin
          case
            when abs(p) < 1023 then return 'Invalid number';
            when abs(p) > 9876 then return 'Invalid number';
            when p != trunc(p) then return 'Invalid number';
            else null;
          end case;
          tab(substr(p,1,1)) := 1;
          tab(substr(p,2,1)) := 1;
          tab(substr(p,3,1)) := 1;
          tab(substr(p,4,1)) := 1;
          if tab.count != 4 then return 'Invalid number'; end if;
          return 'Valid number';
end;
/

Function created.

SQL> select test_num(1023) from dual;

TEST_NUM(1023)
-------------------------------------
Valid number

SQL> 

create or replace function test_num(v_ip in number) return varchar2 is
v_text varchar2(18);
v_num1 number;
v_num2 number;
v_num3 number;
v_num4 number;
Begin

v_num1:=substr(v_ip,1,1);
v_num2:=substr(v_ip,2,1);
v_num3:=substr(v_ip,3,1);
v_num4:=substr(v_ip,4,1);
  begin
  if length(v_ip)=4 and to_number(v_ip)>0 then
   v_text :='Valid Number';
  end if;
  
  exception when others then 
  v_text :='Invalid Number';
  end;

dbms_output.put_line('result= '|| v_num1|| v_num2||v_num3|| v_num4);
if ( regexp_count(v_ip,v_num1)>=2 or regexp_count(v_ip,v_num2)>=2 
or regexp_count(v_ip,v_num3)>=2  or regexp_count(v_ip,v_num4)>=2 ) then 
v_text :='Invalid Number';
else
v_text :='Valid Number';
end if;
return v_text;
end;
/