SELECT 1 FROM DUAL
WHERE  decode(DECODE( TRANSLATE('-123.45','-0.123456789',''), NULL, 1,0), 1,substr('-123.45',1,1) ,' ' ) = '-'  

SQL> with
  2    data as (
  3      select '-123.45' v from dual
  4      union all
  5      select '123.45' v from dual
  6      union all
  7      select 'michel' v from dual
  8      union all
  9      select '-123' v from dual
 10      union all
 11      select '123' v from dual
 12    )
 13  select v, 
 14         case
 15           when     regexp_like(v, '^[+-]{0,1}\d+\.{0,1}\d*$') -- is a number
 16                and substr(v,1,1) = '-'
 17             then 'Negative number'
 18           else 'Other stuff'
 19         end
 20  from data
 21  /
V       CASEWHENREGEXP_
------- ---------------
-123.45 Negative number
123.45  Other stuff
michel  Other stuff
-123    Negative number
123     Other stuff

orcl> select dump(1) from dual;

DUMP(1)
------------------
Typ=2 Len=2: 193,2

orcl> select dump('1') from dual;

DUMP('1')
----------------
Typ=96 Len=1: 49

orcl>

 with
  data as (
      select '-123.45' v from dual
       union all
       select '123.45' v from dual
       union all
       select 'michel' v from dual
       union all
       select '-123' v from dual
      union all
      select '123' v from dual
      union all
      select '0' v from dual
      union all
      select '000.00' v from dual
    )
   select v, 
          case
            when  regexp_like(v, '^[+-]{0,1}\d+\.{0,1}\d*$') -- is a number
                 and v/1 = 0
                then '0'
             when regexp_like(v, '^[+-]{0,1}\d+\.{0,1}\d*$') -- is a number
                 and substr(v,1,1) = '-'
               then 'Negative Number' 
                  
            else 'Other stuff'
          end
   from data
   

WITH data AS 
  (SELECT '-123.45' v FROM DUAL UNION ALL
   SELECT ' -12.23 '  FROM DUAL UNION ALL
   SELECT ' - 1.2'    FROM DUAL UNION ALL
   SELECT '+123.45'   FROM DUAL UNION ALL
   SELECT 'michel'    FROM DUAL UNION ALL
   SELECT '-123'      FROM DUAL UNION ALL
   SELECT '--345'     FROM DUAL UNION ALL
   SELECT '-01'       FROM DUAL UNION ALL
   SELECT ' 0'        FROM DUAL UNION ALL
   SELECT ' -0'       FROM DUAL UNION ALL
   SELECT '-'         FROM DUAL UNION ALL
   SELECT '-.345'     FROM DUAL UNION ALL
   SELECT '123'       FROM DUAL)
SELECT v,
       CASE
         WHEN REGEXP_LIKE (trim(v), '^-\d*\.*\d*$') 
          AND REGEXP_LIKE (trim(v), '^.*[1..9]+.*$')                   
           THEN to_number(trim(v))
           ELSE NULL
       END numc
  FROM data;

v       numc
-------------------
-123.45	-123.45
 -12.23  -12.23
 - 1.2	
+123.45	
michel	
-123	-123
--345	
-01	-1
 0	
 -0	
-	
-.345  -0.345
123	
 

when  regexp_like(v, '^[+-]{0,1}\d+\.{0,1}\d*$') -- is a number
                 and v/1 = 0